application of astec, melcor, and maap computer...

Research ArticleApplication of ASTEC MELCOR and MAAP ComputerCodes for Thermal Hydraulic Analysis of a PWRContainment Equipped with the PCFV and PAR Systems

Siniša Šadek1 Davor GrgiT1 and Zdenko ŠimiT2

1University of Zagreb Faculty of Electrical Engineering and Computing 10000 Zagreb Croatia2European Commission Joint Research Centre Directorate G ndash Nuclear Safety and Security Westerduinweg 31755 LE Petten Netherlands

Correspondence should be addressed to Zdenko Simic zdenkosimiceceuropaeu

Received 20 December 2016 Accepted 12 March 2017 Published 14 May 2017

Academic Editor Tim Haste

Copyright copy 2017 Sinisa Sadek et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

The integrity of the containment will be challenged during a severe accident due to pressurization caused by the accumulation ofsteam and other gases and possible ignition of hydrogen and carbon monoxide Installation of a passive filtered venting systemand passive autocatalytic recombiners allows control of the pressure radioactive releases and concentration of flammable gasesThermal hydraulic analysis of the containment equipped with dedicated passive safety systems after a hypothetical station blackoutevent is performed for a two-loop pressurized water reactor NPP with three integral severe accident codes ASTEC MELCORandMAAP MELCOR andMAAP are twomajor US codes for severe accident analyses and the ASTEC code is the European codejoint property of Institut de Radioprotection et de Surete Nucleaire (IRSN France) andGesellschaft fur Anlagen und Reaktorsicherheit(GRS Germany) Codesrsquo overall characteristics physics models and the analysis results are compared herein Despite considerabledifferences between the codesrsquo modelling features the general trends of the NPP behaviour are found to be similar althoughdiscrepancies related to simulation of the processes in the containment cavity are also observed and discussed in the paper

1 Introduction

The accident at the Fukushima Dai-ichi nuclear power plant(NPP) made utilities review the plantsrsquo safety systems andprocedures Significant changes in nuclear safety approachoperational procedures introduction of new systems andmodifications of current ones took place after the accidentat the Three Mile Island NPP Activity in the severe accidentarea was intensified again after the accident at the ChernobylNPP due to public opinion even though there is no directconnection between the LWR and RBMK reactor types Afterthe accident at the Fukushima NPP more countries paidadditional attention to containment and containment safetysystemsrsquo performance

Preserving the containment integrity limits the radioac-tive material release even in the case of a core meltdown andthe reactor pressure vessel failure Installation of a ventingsystem coupled with appropriate filter devices may prevent

damage of the containment wall due to overpressure andreduce release of fission products Autocatalytic recombinerscan be used to lower combustible gases hydrogen and carbonmonoxide concentration by triggering chemical reactionwithoxygen in the containment building

Controlling containment conditions will be difficult if theplant relies solely on active systems while there is an interrup-tion in electrical power supply In that case passive systemsprovide high level of protection during a long period of timeThe passive containment filtered venting (PCFV) systemand passive autocatalytic recombiners (PAR) are speciallydesigned systems and equipment which operate in the mostsevere conditions without the need of an operator actionThereports and papers published in the open literature some ofthem listed in [1ndash4] support the interest of nuclear facilitiesfor such systems to restrict radioactive releases out of theNPPs

HindawiScience and Technology of Nuclear InstallationsVolume 2017 Article ID 8431934 16 pageshttpsdoiorg10115520178431934

2 Science and Technology of Nuclear Installations

Z

Figure 1 PCFV system layout taken from [9]

Influence of safety systems on containment behaviour isexamined by calculating a station blackout event without anyrecovery actions with three integral severe accident codesASTEC MELCOR and MAAP ASTEC [5] and MELCOR[6] are predominantly mechanistic computer codes and theMAAP code [7] on the other hand is a fast runningparametric code The code main features are given in aseparate chapter

The calculation is performed for a two-loop pressurizedwater reactor (PWR) nuclear power plant of a Westinghousetype The plant model is based on the NPP Krsko layout [8]The input decks for the three codes are made as similar aspossible and consistent initial and boundary conditions areplaced on all of the analyses The objectives of calculationsare to analytically confirm the ability of passive containmentsafety systems to mitigate accident consequences and tojustify the applicability of currently the most popular severeaccident codes for such analyses The analysis covers con-tainment thermal hydraulics releases of incondensable gasesbasemat concrete melting and all other phenomena relevantfor the study of containment integrity The fission productrelease and the transport of radio nuclides were calculated byall three codes but results are not presented It is our opinionthat thermal hydraulic differences should be addressed beforepaying attention to corresponding differences in prediction ofradio nuclides behaviour A comparison of codesrsquo calculationresults is reported herein

2 Description of Containment PassiveSafety Systems

21 Passive Containment Filtered Venting System The pas-sive containment filtered venting system a system speciallydesigned for light water reactor NPPs to work withoutexternal power supply is used to filter radioactive aerosolsgaseous iodine and iodine organic compounds and to controlthe containment pressure

The PCFV system consists of aerosol filters the iodinefilter the rupture disc valves expansion orifices instrumen-tation and associated piping Figure 1 The venting gas firstpasses aerosol filter modules leaves the containment via

piping through a containment penetration passes the iodinefilter and discharges to the environment through the stack

Aerosol filters (five units on the right hand side inFigure 1) remove solid particles from the vented gases bymechanical filtering using a metal fiber filterThe iodine filter(green boxes in the middle of Figure 1) removes radioactiveiodine and its organic compounds through chemical sorptionwith silver active material inside the iodine filter

System actuation occurs after the failure of the rupturedisk at the burst pressure of 06MPa Vented gases flow thenthrough the piping containing two isolation valves and thepressure relief valve which closes at the pressure 041MPaThe line containing the rupture disc isolation valves and therelief valve lies between the outlet of aerosol filters and theinlet to the iodine filter

22 Passive Autocatalytic Recombiners The aim of passiveautocatalytic recombiners is to prevent occurrence of flam-mable gas mixture in containment compartments by reduc-ing concentration of combustible gases hydrogen and carbonmonoxide

The recombination process is based on a self-actuatedcatalytic exothermic reaction between hydrogencarbonmonoxide and oxygen Products of the reaction are steamand CO2 respectively accompanied with release of heat Thereaction is supported by natural circulation of gases PARconsists of a stainless steel enclosure and plates with thecatalyst material The enclosure is opened at the bottom toallow the gases to enter the PAR unit and at the top for thegases to be discharged back in the containment It extendsabove the catalyst elevation to provide a chimney to yieldadditional lift to enhance the effect of buoyancy Catalystmaterial (palladium) is not consumed during the reaction

3 Overview of the Codes and TheirContainment Models

31 The ASTEC Code The ASTEC is a modular computercode It is being jointly developed since almost 20 yearsby IRSN France and GRS Germany The code versionV20R3p3 was used in the calculation

ASTEC consists of 13 coupled modules (CESAR ICARECPA MEDICIS RUPUICUV CORIUM COVI SYSINTELSA SOPHAEROS ISODOP IODE and DOSE) thatmodel different phenomena or different parts of a nuclearpower plant Some of the modules important for the pre-sented calculation are briefly described

The CESAR module [10] computes two-phase thermalhydraulics (TH) in the primary and secondary circuitsModelling is based on a 1D two-fluid five-equation approachcompleted by a phase slip model The finite volume methodis used for the space discretization by means of the staggeredmesh and the time discretization of the basic equations isdone using the fully implicit first-order backward differencescheme ensuring the numerical stability The ICARE modulemodels in-vessel core degradation and vessel rupture [11]Thethermal hydraulics in the core is based on a 1D swollen waterlevel approach completed by a 2D gas modellingThe coriumbehaviour in the lower plenum is based on a 0Dmodelling of

Science and Technology of Nuclear Installations 3

corium layers (oxide metallic and debris layers) with a 2Dmeshing of the RPV lower headThemolten corium concreteinteraction is simulated by the MEDICIS module [12]

The reactor containment thermal hydraulics aerosoland fission product behaviour is calculated by the CPAmodule [13] The containment is divided into zones thermalhydraulic volumes consisting of two parts liquid and gaseousalso named the sump and the atmosphere Gaseous zone partcan contain liquid particles (fog) and liquid zone part cancontain dissolved gaseous particles including incondensablegases (nitrogen oxygen hydrogen carbon monoxide andcarbon dioxide) The user can choose between equilibriumand nonequilibrium zone models If the zone is in equilib-rium thermodynamic (TD) state temperatures of the liquidand gaseous parts are equal If the nonequilibrium model isused temperatures can differThe latter model is usually usedin order to correctly evaluate energy distribution betweenphases due to differences in heat capacities of water steamand incondensable gases

Connections between the zones are realized by the twotypes of junctions atmospheric and drainage junctionsGaseous components which may carry liquid droplets aretransported through atmospheric junctions Water includingdissolved gases is transported through drainage junctionsWater drainagemay occur fromone zone to another or acrossthe wall in the case of steam condensation

The thermodynamic behaviour of walls and internalstructures in the containment is simulated by heat structuresOne-dimensional heat conduction models are used to calcu-late temperature profiles and energy transfer Heat transfercoefficients and condensation models are calculated usingappropriate correlations built in the code

32 The MELCOR Code The MELCOR is a modular engi-neering-level computer code whose primary purpose is tomodel the progression of severe accidents in light waterreactor nuclear power plants It is developed by the SandiaNational Laboratories USA for the US Nuclear RegulatoryCommission The MELCOR version 186 was used in thecalculation

Initially the MELCOR code was envisioned as beingpredominantly parametric but over the years as phenomeno-logical uncertainties have been reduced the models imple-mented into MELCOR have become increasingly best esti-mate in natureThe use of models that are strictly parametricis limited in general to areas of high phenomenologicaluncertainties where there is no consensus concerning anacceptable mechanistic approach

Unlike in ASTEC thermal hydraulic calculation inMEL-COR is performed by the same modules for all NPP systemsControl volume hydrodynamics (CVH) flow path (FL)and heat structure (HS) packages are used to model THbehaviour of the primary and secondary circuits as wellas of the containment The CVH package is concernedwith control volumes (CV) and their contents and the FLpackagewith connections that allow transfer of these contentsbetween control volumes That approach is analogous tothe ASTEC CPA module which uses concept of TH zonevolumes connected by junctions Furthermore the contents

of the MELCOR control volume may be divided between apool containing water which may be subcooled (liquid) orsaturated (two-phase) and an atmosphere containing watervapour liquid water fog and incondensable gases Govern-ing differential equations converted to linearized implicitfinite difference form are solved for each phase separatelyAlso like in the ASTEC code two thermodynamic optionsare available equilibrium and nonequilibrium Equilibriumthermodynamics assumes that the pool and the atmosphereare in thermal and mechanical equilibrium that is that theyhave the same temperatures and pressures Nonequilibriumthermodynamics on the other hand assumes mechanicalequilibriumbut not thermal equilibrium so that pressures areequal but temperatures may be different and there may be asubstantial driving force for condensation or evaporation

The junction representation is different in the MELCORcode In the ASTEC code atmospheric and water junctionsare separated MELCOR junctions transport both phases atthe same time When preparing a NPP input database it iseasier for the user to use only one junction instead of two toconnect two volumes that exchange water and gases In thatcase there is no need to define all possible drainage flowpathsin complicated containment geometry

The heat structure package calculates one-dimensionalheat conduction within an intact solid structure and energytransfer across its boundary surfaces into a TH controlvolume

33 The MAAP Code The modular accident analysis pro-gram (MAAP) is a computer code that can simulate responseof light water reactor power plants both current designsand advanced reactors during severe accident sequencesincluding actions taken as part of the accident managementThe code is developed for the Electric Power ResearchInstitute (EPRI) by Fauske and Associates LLC The MAAPversion 405 was used herein

MAAP includes models for all of the important phenom-ena which might occur during accident transients involvingdegraded cores It utilizes simplified and fast running modelsfor thermal hydraulics description using a simple fixednodalization of the primary and secondary circuits in whichthe type and number of components and the geometry arepredetermined MAAP is a parametric code that includescombination of phenomenological and user defined paramet-ric models necessary to describe the important trends in thebehaviour of a NPP

The code solves a set of lumped parameter nonlinearfirst-order coupled ordinary differential equations in timeMost of differential equations express conservation of massor energy Momentum balances in MAAP are considered tobe quasi-steady which reduces them to algebraic expressionsTherefore there are no differential equations in MAAPfor the conservation of momentum Combination of phe-nomenological zero-dimensional models with nonexistenceof momentum equations makes MAAP a fast running codecalculating 10ndash100 times faster than ASTEC and MELCOR

Contrary to the primary and secondary circuit modelswhich are prearranged in a way that the user cannot make itsown nodalization but uses the default one the containment


Steamheader

FeedwaterFeedwater

Aux feed Aux feed

RPV

PRZ

SG1SG2

RCP1RCP2

ACC1ACC2

Figure 2 ASTEC and MELCOR nodalization schemes of the primary and secondary systems

model has a free format The modelling follows the samepattern as in the ASTEC and MELCOR codes the contain-ment is divided into thermal hydraulic volumes connected byjunctions with addition of heat structures which act as heatsinks

The code distinguishes two types of heat sinks distributedand lumped Both can be used at the same time Distributedheat sinks are one-dimensional structures with heat flow ratedirected through the wall They can be a wall or a floorbetween two compartments or an internal wall within acompartment Any significant masses of equipment suchas piping piping supports valves pumps and ironworkstructures within the compartment (internal structures) aremodelled as lumped heat sinks represented only with theirtotal masses and surface areas

4 Computational Model of the NuclearPower Plant

In order to conduct reliable comparison of the results theNPP input models for ASTEC MELCOR and MAAP areprepared to be as similar as possible Since processes in thecontainment depend on the mass and energy releases fromthe reactor coolant system (RCS) an integral analysis of theNPP behaviour is performed although the emphasis is puton containment results The primary secondary and con-tainment systems are all modelled to support such analysisBefore the accident simulation a steady state calculation wasperformed first to check model accuracy and to qualify it fortransient simulations

41 Models of the Primary and Secondary Systems ASTECand MELCOR codes allow the user to develop its owninput deck almost without any constraints regarding therepresentation of the nuclear equipment the reactor coolantsystem steam generators the containment and all othersystems important for the plant operationTheir nodalizationscheme of the primary and secondary circuits is shown inFigure 2 The primary circuit is marked with the orangecolour the secondary circuit with the green colour and thepart of the reactor pressure vessel below the upper core plate

with the blue colour Each box in the scheme representsa single control volume The reactor pressure vessel andsteam generators are modelled with a fine mesh of controlvolumes in order to better predict heat transfer across thefuel rods in the reactor core and across the U-tubes in thesteam generators The core fuel elements are divided intofive radial regions and twelve axial nodes Pressurizer safetyand relief valves as well as proportional and backup heatersare also modelled because they are used for the primarypressure regulation Many other systems like pressurizerspray RCS charging and letdown flows SG valves and RCPseal flows aremodelled as well but are not explicitly displayedin Figure 2 because it would make the scheme too complexAll those systems are important for the safe NPP operationand without them it would be difficult to obtain the steadystate However they will not work in the case of a stationblackout so they are switched off at the beginning of thetransient calculation

The MAAP default nodalization is much simplerFigure 3 The whole reactor coolant system is representedwith 15 control volumes The reactor pressure vessel isrepresented with four CVs the SG U-tubes with two CVsand the hot leg the cold leg the intermediate leg and thepressurizer each with one CV making it in total 15 controlvolumes taking into account the fact that there are two loopsin the RCS The reactor core is modelled in the same wayas in the ASTEC and MELCOR codes with five radial ringsand twelve axial nodes A single control volume is used tocalculate core thermal hydraulic behaviour The 15-noderepresentation is characteristic for the gas and fission productTH calculation Water mass balance calculation is performedwithin the water pools There are six water pools meaningthat more gas volumes are lumped into one water volumeFor example hot legs are lumped together with the RPVupper plenum and the reactor core volumes and the cold legswith the reactor downcomer

42TheContainmentModel All the three codes use the samecontainmentmodelThe containment nodalization scheme isshown in Figure 4


9

RPV

Upperhead

Upperplenum

Reactorcore

Lower Plenum

1

8

14

PRZ

15

3

4 5

6

7

11 10

12

13

SG2 SG1

RCP1RCP2

Downcomer +

2

Figure 3 MAAP nodalization of the primary system taken from [7]

1 2 910

1516 19 22

41123

5 12317

1813

67 148

21

20

DOM

BET

BET

SG1PRZ SG2RPO

ARV

CAV

SMP

24

ANL

ANL

DOM

SG1

SG2

PRZ

SMP CAV

ARV

RPO

BET

Figure 4 Containment nodalization scheme (representation of control volumes and junction connections)

The containment building is represented with 10 controlvolumes

(1) DOM (containment dome) cylindricalspherical airspace above the reactor pool steam generators andpressurizer compartments

(2) ANL (annulus) air space between the steel liner andthe containment building

(3) SG1 (steam generator 1 compartment) air space in theSG1 compartment that contains components SG1 andRCP1


(4) SG2 (steam generator 2 compartment) air space inthe SG2 compartment that contains components SG2and RCP2

(5) PRZ (pressurizer compartment) air space in thecompartment that contains pressurizer and primarysystem safety and relief valves

(6) BET (lower compartment) lower compartmentbelow the containment dome placed between SG1SG2 and PRZ compartments excluding the reactorpool and the reactor pressure vessel area

(7) RPO (reactor pool) air space above the reactor vesselfilled with water during the shutdown otherwiseempty

(8) ARV (around reactor vessel) air space between thereactor vessel and the primary shield walls

(9) CAV (reactor cavity) air space below the reactorvessel including the instrumentation tunnel

(10) SMP (containment sump) the lowest control volumebelow the SG1 compartment and the lower compart-ment that contains the recirculation and drainagesumps

An additional control volume with a large volume and fixedtemperature (308K) is used to represent the environmentThis volume is necessary for the code to accurately calculateheat losses from the containment building Heat transfercoefficient from the outside containment wall to the environ-ment is calculated by the code

The containment atmosphere is an air-vapour mixtureinitialized at the atmospheric pressure 1013 kPa and thetemperature 322K with 30 relative humidity

Control volumes are connected with 24 junctionsFigure 4 More than one opening is used between the samevolumes if they are located at different elevations to promoteinternal thermal mixing flow what can be important forlong term containment transients For example there arethree connections between the lower compartment CV-BETand SG1 and SG2 compartments respectively at floor levelsThere are also more connections between the containmentdome CV and steam generator and pressurizer compart-ments Pressurizer and steam generator compartments areopen and junction areas between these compartments and thedome are large between 6m2 and 35m2 Other connectionssuch as between ARV and SG1 and SG2 compartmentswhich are through cold and hot leg openings in the primaryshield walls are smaller their values are taken to be 1m2Connections between the cavity and the ARVBET volumesare established through small openings on the top of thecavity compartment The connection between the sump andthe cavity is based on cross section area of a 4-inch pipeThe largest connection area is between the reactor pool andthe dome 1085m2 The reactor sump is just below the SG1compartment with the connection area being 445m2

The outer containment concrete wall the steel linerinternal concrete walls and floors the polar crane fancoolers platforms the refuelling channel embedment andother miscellaneous stainless and carbon steel structures

are modelled as heat structures They act as heat sinks andexchange heat with water and gases inside and outside thecontainment

Passive autocatalytic recombiners are modelled usingcorrelations developed by German manufacturer NISIngenieurgesellschaft mbH (MAAP MELCOR) and GRS(ASTEC) depending on the available correlations in thecodes Twenty-two PAR units are installed across thecontainment There are 14 units in the containment domesix units in the lower compartment one unit in the SG1compartment and one unit in the SG2 compartment

The PCFV system is modelled as a simple pipe betweenthe containment upper dome and the environment Thejunction connecting the dome and the pipe contains therupture disc that breaks when the upstream pressure onthe containment side reaches 06MPa The other junctionconnecting the pipe with the environment contains thepressure relief valve which is modelled taking into account itsldquohysteresisrdquo characteristic the size of the flow area alternatesbetween being fully open and fully closed at the openingand closing set points 049MPa and 041MPa respectivelyThus the PCFV system is not modelled explicitly and theoperation of aerosol and iodine filters is not consideredOnly the systemrsquos function in controlling the containmentpressure and temperature by releasing excessive containmentinventory was taken into account

421 The Cavity Model In each code there is a package re-sponsible for calculation of the molten corium concreteinteraction (MCCI) The following concrete compositionincluding reinforcement is used in the calculation 35 CaO13 SiO2 4 H2O 215 CO2 25 Al2O3 1 Na2O 05MgO 05 Fe2O3 and 22 Fe Corium discharged fromthe reactor vessel will spread on the cavity floor A coriumspreading area is shown in Figure 5 Molten core materialsdischarged from the ruptured reactor vessel will react withconcrete at the bottom of the cavity The reaction results inmelting of the cavity floor and is accompanied with releasesof hydrogen carbon monoxide carbon dioxide and steamAccumulation of gases leads to containment pressurizationand the release of fission products from the melt causesheating of the atmosphere

Intensity of the containment pressurization and heatingdepends on the reactor cavity layout In the performed analy-sis which is based on the NPP Krsko containment design theconnection between the cavity and the containment dome isrestricted to several small openings on the top of the cavitycompartment In such configuration it is hard to expect asignificant dispersion of corium debris in the containmentafter the failure of the reactor pressure vessel lower headNevertheless steam and gases will freely exit through theseholes and cause containment pressure increase

5 Analysis of the NPP Behaviour duringthe Accident

51 Accident Description The analyzed transient is a stationblackout (SBO) which includes the loss of both off-site andon-site AC power The only systems available are passive


Corium spreading area

Reactor

Instrumentation tunnelReactor

Coriumspreading area

vessel

vessel

Figure 5 Corium spreading area (a cross section and a floor plan)

Table 1 Time sequence of main events during the in-vessel phase

Event ASTEC MELCOR MAAPTwo-phase flow at the break 3800 s 2900 s 4000 sLoss of the SG heat sink 4310 s 3540 s 4500 sCore uncovery 5150 s 4050 s 4990 sOnset of fuel rod cladding oxidation 5350 s 4350 s 5250 sStart of the core melting 5960 s 4750 s 6330 sMelt relocation to the lower head 6380 s 6100 s 9630 sRPV failure 9440 s 9080 s 15020 s

safety systems accumulators and the pressurizer and steamgenerator safety valves Unavailability of electrical powermeans that reactor coolant pumps main and auxiliaryfeedwater pumps charging high-pressure and low pressuresafety injection pumps are disabled Containment safety sys-tems fan coolers and sprays are also inoperable Followingthe loss of power RCP seals will overheat due to lack ofcooling normally provided by charging pumps a breakwill beformed and coolant will be released from the reactor coolantsystem to the containment

Reactor coolant pumps are equipped with staged shaftseals which are provided with cooling system designed tomaintain seal integrity such that there is a low seal leakagerate at the nominal RCS pressure For accident sequences inwhich there is no cooling of the RCP seals (eg SBO) theleakage rate through the seals will increase due to degradationof seal materials when exposed to the coolant at elevated RCStemperatures The seal leakage rate is 003m3s a value thatcorresponds to a scenario of a total seal rupture in pumpswhich use a high temperature o-ring RCP seal package [14] atypical arrangement in Westinghouse PWR plants Leakageof the RCS fluid through the RCP seals combined withunavailability of electrical power is a small LOCA (loss ofcoolant accident) without makeup capability

52 In-Vessel Severe Accident Progression System thermalhydraulic behaviour and core damage progression are brieflydescribed as the focus of the paper is on the containment

analysis ASTEC results of the calculation of the in-vesselphase of a station blackout accident are reported in [15]

Shutting off the reactor coolant pumps leads to decreaseof the coolant mass flow rate Shortly afterwards the reactorand the turbine are tripped due to the low cold leg coolantflow The turbine trip means the closure of the turbinestop valve and isolation of the steam line Steam generatorpressure rises instantly as a consequence of the steam lineisolation forcing the opening of the SG safety valves andrelease of excess steam Since there is no auxiliary feedwatersupply steam generators dry out after about one hourdeteriorating heat transfer from the primary to the secondaryside across the SG U-tubes The insufficient cooling of theRCS in combination with generation of decay heat and theloss of coolant through the damaged RCP seals leads todecrease of the core water level production of steam andincrease of fuel elementsrsquo temperatures The core heat-upadditionally supported by oxidation of fuel rod cladding andother metallic materials causes the core to melt The meltingprocess propagates to formation of an in-core molten pooland ends up with relocation of molten material to the lowerhead of the reactor pressure vessel The RPV wall ultimatelyfails under thermal and mechanical stress and the corium isreleased in the containment cavity

Time sequence of main events during the in-vessel phaseis shown in Table 1 Calculated MELCOR events precede theother two by about 1000 s The water mass flow rate out ofthe RCS through the break during initial 2500 s is 10ndash15


Table 2 Main results of the in-vessel severe accident analysis important for the latter containment behaviour

Parameter ASTEC MELCOR MAAPMass of water released from the RCS before the RPV failure 128000 kg 105000 kg 130000 kgMean mass flow rate at the break 11 kgs 9 kgs 11 kgsTemperature of released watersteam 600ndash1100K 600ndash1100K 600ndash1000KMass of hydrogen produced in the RPV 268 kg 211 kg 265 kgRCS pressure at the time of the RPV failure 56MPa 78MPa 69MPaMass of material released from the RPV 85700 kg 87500 kg 88000 kgTemperature of released material 2400K 2120K 2330KLong term decay heat level in the material accumulated in the cavity 4ndash14MW

higher inMELCOR than in ASTEC orMAAPThe differenceis not large but affects the ensuing accident progression Thecore is thus uncovered earlier and the whole process of coredegradation begins before that calculated by the other twocodes A larger release of liquid causes earlier transition to atwo-phase flow In the long term the total coolant release inMELCOR is lower since the vapour flow rate is lower thanthe liquid flow rate Table 2 summarizes mass and energyreleases from the RCS into the containment as calculated byall three codes Masses and temperatures of released coolantand molten material are rather well reproduced Apart fromthe primary pressure whose influence is described later thebiggest discrepancies between codesrsquo predictions are for thehydrogenmass generated in the reactor vessel and the time ofthe RPV failure The hydrogen production depends on RCSthermal hydraulic conditions which as noted before differbetween the codersquos calculations It should be emphasized herethat such difference in hydrogen release is not a generaltrend only in this specific scenario was a lower amountof hydrogen calculated by MELCOR Regarding the totalhydrogen releases in the containment this behaviour only hasa limited effect since the unoxidized corium inMELCORwilleventually oxidize in the cavity and the hydrogen productionwill continue after the start of the MCCI process

The failure criteria employed in the MAAP code lead inmedium and low pressure accident sequences (LOCAs) tolater lower head failure times [16] Containment conditionsare considered at a larger time scale than RCS conditions dur-ing a severe accident Accident progression in the RCS andthe reactor core lasts for few hours and in the containmentthe accident sequence lasts for days Therefore differences inthe time of the reactor vessel failure are not very significantfor the presented containment analysis

53 Containment Behaviour and Comparison of Codesrsquo Results

531 Heat-Up and Pressurization Discharge of reactor cool-ant in the containment is responsible for the initial con-tainment pressure increase Figure 6 (Results of ASTECMELCOR and MAAP calculation are put together on thesame graphs) Mass and energy release from the RCS causesthe containment to heat up Figure 7 For the first 4000 s thereleased coolant is mainly water with a low void fraction ofsteam but as the pressure continues to decrease the steamfraction is increasing The air heat-up also contributes to the

SBO Sequence Code to Code Comparison

1 2 3 4 5 6 70Time (days)

15

20

25

30

35

40

45

50

55

Con

tain

men

t pre

ssur

e (M

Pa)

ASTECMELCORMAAP

Figure 6 Pressure in the containment upper dome

pressure rise but not as effective as the release of steam at theRCP seal break Figure 8

Discharge of hot molten corium (gt2100K) from the RPVto the containment cavity followed by the blow-down ofprimary circuit gases speeds up containment heating Massof corium released in the containment is about 90000 kgmeaning that almost all fuel elements and a large portion ofreactor internals have beenmelted and carried away out of thereactor vessel Initial decay heat generation inside the melt is14MW and during the next seven days it gradually decreasesto 4MWThe reactor nominal power is 2000MWt The totalcoolant inventory in the primary system during normal plantoperation is 133000 kg out of which 105000ndash130000 kg isreleased in the containment before the vessel break Thesmall discrepancy between code simulation results regardingthe released coolant inventory is mainly due to differencesin predictions of thermal hydraulic conditions in the RCSand timing of the reactor pressure vessel failure Coolantis released from the RCP breaks to steam generator com-partments and from there it drains into the containmentsump Pipe connection between the sump and the cavityenables water to enter and to flood the cavity Half of the



1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

Con

tain

men

t atm

osph

ere t

empe

ratu

re (K

)

ASTECMELCORMAAP

Figure 7 Temperature in the containment upper dome

SBO Sequence

0

05

10

15

20

25

30

35

40

45

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f gas

es

1 2 3 4 5 6 70Time (days)

Steam CON2

O2

CO2

H2

Figure 8 Gas partial pressures in the containment (ASTEC calcu-lation)

released water accumulates in the cavity (sim60000 kg) andthe other half evaporates Injection of corium leads to fastwater evaporation and containment pressurization Due tointensive evaporation the reactor cavity dries out in less thanone day Figure 9 The effect of drying out is also visible onFigure 8 as a sharp drop in the steampartial pressure increase

Conditions in the RCS before the vessel rupture influenceinitial increase of pressure and temperature The fastestearly pressurization rate is calculated by MELCOR becausethe primary system pressure is the highest when the RPVfailed The RCS pressure is rapidly decreasing in the periodbetween 9000 s and 10000 s due to uninterrupted loss ofcoolant through the break The ASTEC calculates pressure


1 2 3 4 5 6 70Time (days)

0

10

20

30

40

50

60

Wat

er m

ass i

n th

e cav

ity (times

1000

kg)

ASTECMELCORMAAP

Figure 9 Mass of water in the reactor cavity

that is 2MPa lower than MELCORrsquos result since ASTECsimulates the RPV failure 360 s later The ASTECrsquos con-tainment pressure increase rate is thus smaller than in theMELCOR case When the RCS pressure drops to 5MPaaccumulators discharge water in the cold legsTheMAAP in-vessel sequence is long enough to account for accumulatoractuation Evaporation of the injected water increases theprimary pressure and after the vessel breach causes contain-ment pressurization rate to surpass the pressure calculated byMELCOR

When the containment dome pressure reaches 06MPa(the first pressure peak) the rupture disc in the PCFV linewill break causing containment gases to be released in theenvironment The pressure drops fast to 041MPa promptingthe relief valve in the PCFV line to close Following thevalve closure the pressure rises once again After reaching049MPa the relief valve opens and again some containmentinventory is released Later the pressure continues to cyclebetween 041MPa and 049MPa by the operation of thePCFV pressure relief valve Figure 6 That kind of valvebehaviour is important for preserving containment integrityand minimizing radioactive releases Failure of the contain-ment wall is assessed by using fragility curves which deter-mine failure probabilities depending on the containmentpressure The containment fragility curve shows 5 failureprobability at sim06MPa and for pressures above 09MPa theprobability for containment failure is about 90ndash95 If therewere no pressure relief systems inside the containment (egPCFV) the pressure would reach critical value in less than aday (Figure 10)

After each cycle of the relief valve operation the new gasdistribution is established Concentrations of steam nitrogenand oxygen are being reduced while those of hydrogen COand CO2 products of the MCCI are going up (Figure 8)Apart from being released out of the containment steam isalso produced in the recombiners and by boiling of water



90 probability for

PCFV + PARsNo PCFV + PARs

containment failure

2 4 6 8 1 12 14 16 18 20Time (days)

2

3

4

5

6

7

8

91

1112

Con

tain

men

t pre

ssur

e (M

Pa)

Figure 10 Containment pressure behaviour and indication offailure criterion in the case without safety systems

bounded in the cavity concrete Its concentration thereforetends to stabilize Oxygen partial pressure drops to zeroalready during the first day because it reacts with hydrogenin PARs to produce steam Nitrogen is neither produced norconsumed and its concentration decreases steadily

532 Influence of the Molten Corium Concrete InteractionDecay heat generated in corium dissolves concrete basematat the bottom of the cavity

Concrete is a mixture of calcium carbonate waterand metal oxides predominately silica At temperatures873ndash1173 K calcium carbonate is decomposed into calciumoxide and carbon dioxide [17]

CaCO3 + 1637 kJkg(CaCO3)997888rarr CaO + CO2 (1)

The reaction is endothermic thus internal energy of thecorium is used to dissolve CaCO3 The released CO2 andsteam produced by evaporation of water from the concretewill react with free metals from the corium (Zr Cr and Fe)and iron from the concrete reinforcement (rebar)

Reactions between metals and steam are the following

Zr + 2H2O 997888rarr ZrO2 + 2H2 (2)

2Cr + 3H2O 997888rarr Cr2O3 + 3H2 (3)

Fe +H2O 997888rarr FeO +H2 (4)

2Fe + 3H2O 997888rarr Fe2O3 + 3H2 (5)

and reactions between metals and CO2 are

Zr + 2CO2 997888rarr ZrO2 + 2CO (6)

2Cr + 3CO2 997888rarr Cr2O3 + 3CO (7)

Fe + CO2 997888rarr FeO + CO (8)


ASTECMELCORMAAP

0

005

010

015

020

025

030

035

040

045

050

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f hyd

roge

n

1 2 3 4 5 6 70Time (days)

Figure 11 Partial pressure of hydrogen in the containment


ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

001020304050607080910

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f CO

Figure 12 Partial pressure of carbonmonoxide in the containment

Oxidation of zirconium and chromium in steam and CO2is an exothermic reaction while iron oxidation is a slightlyendothermic reaction The amounts of Zr and Cr are limitedas they are found only in the reactor coreThus the long termreleases of H2 and CO are due to oxidation of concrete rebarsince there are no elementary metals in the concrete itself

Intensity of incondensable gases production can bedemonstrated by their partial pressures shown in Figures11ndash13 Differences are substantial but a general trend can beidentified Considerable amounts of hydrogen and carbonmonoxide are released during the first two days owingmostly to oxidation of metals inside the corium At the sametime hydrogen concentration decreases due to operation ofrecombiners and that is why the partial pressure of H2 does



ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

00102030405060708091011

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

fCO2

Figure 13 Partial pressure of carbon dioxide in the containment

not go up like that of CO Carbon dioxide is largely consumedin that process so its release becomes significant only afterthe reinforcement remains the only material that can oxidizeAfter approximately 4-5 days CO2 is the most importantincondensable gas Figure 8 which causes containment pres-surization

Initial and final cavity temperature profiles as calculatedby the ASTEC code indicating concrete degradation duringthe MCCI are shown in Figure 14 The initial corium thick-ness is about 10 cm because spreading area is relatively large(382m2) Mass of eroded concrete is shown in Figure 15As the core-concrete interaction progresses concrete oxidesare dissolved and the molten debris pool and the surfacearea grow in size Hence volumetric heat rate and the melttemperature decrease

The surface of the concrete is ablated at a rate of 1-2centimetres per hour Gases released at the bottom of thepool are assumed to rise through it as bubbles The risingbubbles also promote production of aerosols containingfission products stripped from the fuel debris Removal offission products leads to decrease of decay heat level inthe pool Heat losses from the surface are due to melteruptions radiation and convection to containment gases orto an overlying water layer by means of water boiling Melteruptions and water evaporation are major mechanisms forcorium cooling in the early phase of the accident Later asthe corium surface stabilizes convection from the melt tocontainment atmosphere gases prevails over the heat transfercaused by melt eruption

Melt configuration is modelled to be homogenous thusthere is no melt separation on oxide and metallic materialsalthough that is not completely fulfilled for the MELCORcalculation MELCORrsquos CORCON module responsible forthe cavity simulation considers up to 15 possible debrisconfigurations depending on the extent of oxides and metalsentrainment into a molten corium mixture ASTEC and

MAAP codes also containmodels for layer separation but notas detailed as the MELCOR models

The cavity erosion progresses in axial and radial direc-tions The amount of liquefied concrete is calculated basedon the data of the latent heat of fusion liquids and solidstemperatures for corium concrete mixtures and the concretecomposition

The ablation rate of concrete is given by

Vabl = 119902119875120588conc119871conc (9)

where 119902119875 is the heat flux at the coriumconcrete interface120588concthe density of concrete and 119871conc the latent heat for concretemelting

Heat convection between the corium layer and concreteis enhanced by bubble formation at the corium concreteinterface Correlations [18ndash20] for the calculation of the heattransfer coefficient used by the ASTEC and MELCOR codesinclude superficial bubble transport velocities For examplethe Bali correlation that is used in the ASTEC calculationgives the following expression for the heat transfer coefficient

ℎ119888 = 120582119897Nu119903119887 (10)

where the Nusselt number is defined as

Nu = 205(1205881198971198953119892119892120583119897 )0105

Prminus025 (11)

In the equations above ℎ119888 is the heat transfer coefficient120582119897 120588119897 120583119897 are the thermal conductivity density and dynamicviscosity of the liquid debris respectively 119903119887 is the gas bubbleradius 119895119892 is the superficial gas rising velocity 119892 is the gravityacceleration and Pr is the Prandtl number

The heat transfer coefficient in the MAAP code is notdetermined by experimental correlations but it is directlyentered by the user It exponentially depends on the coriumsolid fraction where exponent is also a user defined valueMajority of MAAP models follow the similar approachmechanistic models are replaced with simple algebraic equa-tions whose parameters are selected by the user Althoughthe MAAP is relatively simple to use broad knowledge aboutsevere accident phenomena is necessary to correctly predictthe NPP behaviour

Mass of hydrogen removed by passive autocatalyticrecombiners is shown in Figure 16 Hydrogen productionduring the oxidation in the core and the molten coriumconcrete interaction is shown in Figure 17ThePARoperationstarts when hydrogenmole fraction reaches value of 002 andstops after oxygenmole fraction drops to 0005 Despite beingrather short about 15 days the process of recombinationis very efficient since 70ndash85 of hydrogen is removed Thetime interval when the PARs are active coincides with theearly phase of the MCCI process This is very important forthe severe accident management planning because duringthat period hydrogen production rate is the highestTherebyoperation of passive safety systems provides crucial time for


Temperature field in core and cavity Temperature field in core and cavity

600000

10000

15000

20000

25000

30000

604800001205495

minus108

minus719

minus357

00421

366

minus362minus723 362 7230600000

10000

15000

20000

25000

30000

minus108

minus719

minus357

00421

366

minus362minus723 362 7230

Figure 14 Initial and final cavity temperature profiles as calculated by the ASTEC code


ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

50

100

150

200

250

300

350

400

450

in th

e cav

ity (times

1000

kg)

Mas

s of e

rode

d co

ncre

te

Figure 15 Mass of eroded concrete in the cavity during the processof the MCCI

the members of a technical support centre and emergencyresponse organizations in taking preventive and mitigatingactions to restrict consequences of a severe accident

6 Discussion of Results

The most significant differences between the results impor-tant for the latter accident progression occur during the firsttwo days Figure 18 shows the temperature of the moltenmaterial The initial cool-down of corium is followed bya temperature increase lasting from 3000 s (MELCOR) to20000 s (ASTEC) Steam outflow to the neighbouring com-partments is limited by the cavity design and the temperatureincreases because of the reduced heat transfer rate andconvection heat flux The total temperature increase and


ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

050

100150200250300350400450500550600

Hyd

roge

n m

ass r

emov

ed b

y PA

Rs (k

g)

Figure 16 Mass of hydrogen removed by PAR operation

duration of that time period depend on the water mass in thecavity In the case with less water (MELCOR) the two-phaseflow regime is established earlier and the higher vapour voidfraction results in more efficient cavity ventilation UnlikeASTEC andMELCORpredictions there is a temperature risein the MAAP simulation after water in the cavity dries outThe mass of molten material is low Figure 15 and so is theheat capacity Degradation of the heat transfer to the cavityatmosphere causes heat-up of the melt and since the mass ofthe melt is low there is a considerable temperature increaseThe bulk of molten material in the analyses with ASTECand MELCOR has a heat capacity large enough to preventtemperature increase following the change in heat transferconditions on its upper surface In general MAAP calculatesslower concrete erosion at the beginning of the MCCI when



ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Prod

uctio

n of

hyd

roge

n (k

g)

Figure 17 Total hydrogen production


ASTECMELCORMAAP

0200400600800

10001200140016001800200022002400

Melt

tem

pera

ture

in th

e cav

ity (K

)

1 2 3 4 5 6 70Time (days)

Figure 18 Temperature of the molten material in the cavity

there is still water present in the cavity The other two codessimulate almost unperturbed concrete degradation regardlessof water layer floating on the debris Despite the cooling of theupper debris surface the downward and sideward heat flowremains strong enough to melt the concrete In MAAP theheat flow is concentrated to the upper surface water is rapidlyevaporating Figure 9 and the cavity heats up slightly faster inthat initial accident period Figure 19

Corium heats up water and gases in the cavity Tem-peratures of steam and incondensable gases rise sharply by20ndash80K after the water evaporates Figure 19 The lowestvalue is obtained by ASTEC which calculates that waterremains in the cavity for the longest time Both MELCORand MAAP calculate higher heat-up Temperature in the


ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

650

Cavi

ty at

mos

pher

e tem

pera

ture

(K)

Figure 19 Temperature in the cavity compartment


ASTECMELCOR

1 2 3 4 5 6 70Time (days)

0

1

2

3

4

5

6

7

8

9

10

Hea

t los

s to

conc

rete

(MW

)

Figure 20 Heat flow from the corium to concrete

containment dome Figure 7 follows the pattern of thecavity temperature profile The ASTEC temperature trendis different from the other two The reason is that the heatreleased from the corium surface to fluid is lower in the shortterm and larger in the long term (gt2 days) than calculated byMELCOR and MAAPThus the rate of temperature increasein ASTEC is steeper This is evident from Figure 20 showingthe heat losses to concrete They begin to decrease after thefirst day when at the same time cavity temperature starts toincrease (The MAAP curve is not shown because there is nosuch data in the MAAP plot files) The upper debris surfaceis much more active in ASTEC than in the other codes Melteruptions cause fission products and melt particles to carryaway fraction of the internal energy out from the debrisThatenergy output is heating up the cavity and the containment


Eruptions eventually subside after the first day but less energyremains in the melt to be transferred to concrete A largerheat transfer for the first two days corresponds to moreintensive concrete melting (Figure 15) The melting rate laterslows down and masses calculated by MELCOR and MAAPexceed the mass predicted by the ASTEC after seven days oftransient

The US codes (MELCOR and MAAP) show more simi-larities in results comparing to ASTEC The difference arisesfrom the fact that ASTEC is a new severe accident code itsphysical models and correlations are mostly based on thelatest modelling practices and experimental data whereasMELCOR and MAAP initial MCCI models were preparedin the early 1990s and occasionally updated afterwards Thecomplexity of the MCCI process itself the phenomenologyandmathematical representation of concretemelting coriumgrowth release of gases and so forth and differences in themethods of model implementation in the codes addition-ally contribute to modelling uncertainties and variations inresults

The largest differences in results are related to productionof gases hydrogen carbon dioxide and carbon monoxide(Figures 11ndash13) Even though the partial pressures of gasesare not representative for the chemical reaction kineticsduring the MCCI the amount of generated H2 CO andCO2 can be relatively correctly determined based on theirvaluesThe codes have extensive models of theMCCI when itcomes to heat transfer modes concrete erosion molten frontprogression in radial and axial directions layer formationand stratification but the chemistry data are rather scarce Itcan be found that equilibrium chemistry is assumed based onminimization of the total Gibbs function for oxidic metallicand gaseous phases [6] Corium and concrete are regardedas homogenous mixtures of metals and oxides which is nottrue for the cavity basemat reinforced concrete structureThereinforcement arrangement affects the progress of concreteerosion and iron oxidation with H2O and CO2 The massesof unoxidized zirconium and other coriummetals are limitedand the only permanent source of combustible gases is theoxidation of the cavity rebarThe impact of codesrsquo differencesinmodelling chemical reactions between debris and concreteis particularly emphasized for the hydrogen release Figure 17where ASTEC calculates 80 more hydrogen productionthan MELCOR Gaseous emission influences containmentpressurization and potential formation of flammable mixtureof combustible gases air and steam Figure 21 shows theternary diagram of air combustible (H2 and CO) and inertgases with respect to oxidation (steam and CO2) Flamma-bility limits for hydrogen [21] and CO [22] are also drawnThe oxygen is consumed by PARs early during the transientand hence the mixture of gases remains outside the limitsof combustion The accumulation of hydrogen and carbonmonoxide in the latter phase of the accident does not poseany threat to containment safety because there is no oxygento support the burning The aforementioned differences incodesrsquo predictions of gases concentrations are noticeable onthe bottom right end of the diagram The closest point tothe flammability limit curve is calculated by MAAP justbefore the first opening of the PCFV relief valve Some PCFV

1

09

08

07

06

05

04

03

02

01

01

09

08

07

06

05

04

03

02

01

0

Air

Steam + CO2

09 08 07 06 05 04 03 02 01 01H2 + CO

Hydrogen flammability limitCO flammability limitASTEC

MELCORMAAP

Figure 21 Flammability limits of the air ndash H2 + CO ndash steam + CO2mixture

systems are thus designed with additional line of manuallyoperated valves that enable operator to control containmentconditions and keep the atmosphere in an inert state byactively discharging containment inventory

The current code models of recombiners do not simulaterecombination of carbon monoxideThe results are thereforeconservative because the concentration of combustible gasesin reality is lower than that represented on the ternarydiagram Nonetheless the ternary diagram representation isvery sensitive on the accident scenario and no conclusioncould be drawn for a different course of the accident becausesmall variations in steam and air concentrations can movethe mixture properties closer to or further away from theflammability limits

7 Conclusions

Nuclear power plant behaviour during a hypothetical severeaccident event with emphasis on containment conditionswas analyzed with the ASTEC MELCOR and MAAP com-puter codes All three codes consist of models that enableintegral simulation of NPP phenomena The physics of anSA progression is interpreted through code routines withsignificant differences in the level of details of numericalmodelling and the representation of the incorporated exper-imental findings TheMAAP code for example uses simplerphenomenologicalmodels to reduce the running times whileMELCOR and ASTEC rely on more complicated numerics atthe expense of the duration of the analysis

The analyzed accident showed that installation of thepassive filtered venting system and autocatalytic recombinerspreserves containment integrity and keeps its atmosphere ininert conditions In the absence of the containment spray


system and the active heat removal the wall would bebreached after one day following the loss of complete powersupply if the relief valve of the PCFV system does notopen Recombination of hydrogen maintains the mixture ofgases beyond flammability limits The oxygen starvation inthe early part of the transient compensates production ofhydrogen and carbon monoxide during the MCCI Resultsare rather conservative since there are no codemodels for COrecombination in PAR units The releases of incondensablegases in the codes differ considerably which is especiallypronounced for the release of hydrogen Regarding theimportance for the NPP safety the chemistry data aboutoxidation processes taking place in the cavity with focus onthe concrete reinforcement behaviour is an area that couldbe improved in the codes The heat loss from the melt inthe cavity atmosphere intensity of concrete erosion andmelteruptions also differ between the codes and they influencethe release of gases temperature and pressure increase in thecontainment

Despite considerable differences between the codesrsquomod-elling features calculation results showed that the thermalhydraulic phenomena are in good agreement Overall trendswere similar between all three used codes Discrepanciesbetween code predictions were due to a number of reasonsunequal core and containment degradation models usageof parametric instead of mechanistic approach to simulatecertain phenomena differences in resolution of some thermalhydraulic issues and so forth The results were nonethelesscomparable and the differences can be attributed to uncer-tainties associated with complex SA processes and the scalingof the experimental to the real plant data

Comparison of calculations of different severe accidentcodes is the only possible practice for evaluating the codesrsquoaccuracy in the situationwhen integral severe accident exper-imental results are missing It also aids in improving existinginput database and selecting the appropriate physical modelsand correlations for various severe accident phenomenaContinuous code application in analyses of a broad spectrumof accidents can reveal possible deficiencies in the represen-tative models and thus help the code developers to improvecode routines based on existing experimental findings

Nomenclature

Latin Symbols

119892 Gravity acceleration [ms2]ℎ119888 Heat transfer coefficient [Wm2K]119895119892 Superficial gas rising velocity [ms]119871 Latent heat for melting [Jkg]Nu Nusselt number [mdash]Pr Prandtl number [mdash]119902119875 Heat flux [Wm2]119903119887 Gas bubble radius [m]V Velocity [ms]

Greek Symbols

120582 Thermal conductivity [WmsdotK]

120583 Dynamic viscosity [kgmsdots]120588 Density [kgm3]Subscripts

119886119887119897 Ablation119888119900119899119888 Concrete119897 Liquid debris

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

The authors would like to express their gratitude to the IRSNfor providing the ASTEC code and to the NPP Krsko forproviding the MAAP code and the plant data

References

[1] S-W Lee T-H Hong Y-J Choi M-R Seo and H-T KimldquoContainment depressurization capabilities of filtered ventingsystem in 1000MWe PWRwith large dry containmentrdquo Scienceand Technology of Nuclear Installations vol 2014 Article ID841895 10 pages 2014

[2] Y M Song H S Jeong S Y Park D H Kim and J H SongldquoOverview of containment filtered vent under severe accidentconditions at Wolsong NPP unit 1rdquo Nuclear Engineering andTechnology vol 45 no 5 pp 597ndash604 2013

[3] Y S Na K S Ha R-J Park J-H Park and S-W Cho ldquoThermalhydraulic issues of containment filtered venting system for along operating timerdquo Nuclear Engineering and Technology vol46 no 6 pp 797ndash802 2014

[4] E-A Reinecke I M Tragsdorf and K Gierling ldquoStudies oninnovative hydrogen recombiners as safety devices in the con-tainments of light water reactorsrdquo Nuclear Engineering andDesign vol 230 no 1ndash3 pp 49ndash59 2004

[5] P Chatelard N Reinke S Arndt et al ldquoASTEC V2 severeaccident integral code main features current V20 modellingstatus perspectivesrdquo Nuclear Engineering and Design vol 272pp 119ndash135 2014

[6] R O Gauntt J E Cash R K Cole et al MELCOR ComputerCode Manuals 1-2 Version 186 Sandia National LaboratoriesUS Nuclear Regulatory Commission 2005

[7] Fauske and Associates Inc MAAP4mdashModular Accident Analy-sis Program for LWR Power Plants Vols 1 to 4 Electric PowerResearch Institute 1994

[8] D Grgic V Bencik and S Sadek ldquoNEK RELAP5MOD33Post-RTDBE nodalization notebookrdquo Tech Rep NEKESD-TR-0213 FER-ZVNESADA-TR0313-1 NPP Krsko University ofZagreb FER 2013

[9] D Grgic V Bencik S Sadek and I Basic ldquoIndependentreview ofNPPmodifications and safety upgradesrdquo InternationalJournal of Contemporary Energy vol 1 no 1 pp 41ndash51 2015

[10] L Piar N Tregoures and A Moal ldquoASTEC V2 code CESARphysical and numerical modellingrdquo Tech Rep ASTEC-V2DOC09-10 DPAM-SEMCA-2010-380 Institut de Radiopro-tection et de Surete Nucleaire 2011


[11] P Chatelard N Chikhi L Cloarec et al ldquoASTEC V2 codeICARE physical modellingrdquo Tech Rep ASTEC-V2DOC09-04 DPAM-SEMCA-2009-148 Revision 0 Institut de Radiopro-tection et de Surete Nucleaire Gesellschaft fur Anlagen- undReaktorsicherheit 2009

[12] F Duval and M Cranga ldquoASTEC V2MEDICIS MCCI moduletheoretical manualrdquo Tech Rep DPAMSEMIC 2008-102 Revi-sion 1 Institut de Radioprotection et de Surete Nucleaire 2008

[13] W Klein-Heszligling and B Schwinges ldquoASTEC V0mdashCPAmodulemdashprogram reference manualrdquo Tech Rep ASTEC-V0DOC01-34 Institut de Radioprotection et de Surete NucleaireGesellschaft fur Anlagen- und Reaktorsicherheit 1998

[14] WOG ldquoWOG 2000 reactor coolant pump seal leakage modelfor Westinghouse PWRsrdquo Tech Rep WCAP-15603 Revision 1-A Westinghouse Electric Company 2003

[15] S Sadek M Amizic and D Grgic ldquoSevere accident analysis ina two-loop PWR nuclear power plant with the ASTEC coderdquoAtwmdashInternational Journal for Nuclear Power vol 58 no 12 pp694ndash700 2013

[16] K-I Ahn and D-H Kim ldquoA state-of-the-art review of thereactor lower head models employed in three representativeUS severe accident codesrdquo Progress in Nuclear Energy vol 42no 3 pp 361ndash382 2003

[17] Z P Bazant and M F Kaplan Concrete at High TemperaturesMaterial Properties and Mathematical Models Monograph andReference Volume Longman (Addison-Wesley) London UK1996

[18] J M Bonnet ldquoThermal hydraulic phenomena in corium poolsfor ex-vessel situations the Bali experimentrdquo in Proceedingsof the 8th International Conference on Nuclear EngineeringICONE-8 ICONE-8177 Baltimore Md USA 2000

[19] F A Kulacki and M E Nagle ldquoNatural convection in a hor-izontal fluid layer with volumetric energy sourcesrdquo Journal ofHeat Transfer vol 97 no 2 pp 204ndash211 1975

[20] M T Farmer J J Sienicki and B W Spencer ldquoCORQUENCHa model for gas sparging-enhanced melt-water film-boilingheat transferrdquo Transactions of the American Nuclear Society vol62 pp 646ndash647 1990 Proceedings of the American NuclearSociety (ANS) Winter Meeting Washington DC USA 1990

[21] Z M Shapiro and T R Moffette ldquoHydrogen flammability dataand application to PWR loss-of-coolant accidentrdquo Tech RepWAPD-SC-545 US Atomic Energy Commission 1957

[22] N Cohen ldquoFlammability and explosion limits of H2 andH2CO a literature reviewrdquo Aerospace Report TR-92(2534)-1Space andMissile Systems Center The Aerospace CorporationEl Segundo Calif USA 1992

TribologyAdvances in

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Submit your manuscripts athttpswwwhindawicom


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The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


Z

Figure 1 PCFV system layout taken from [9]

Influence of safety systems on containment behaviour isexamined by calculating a station blackout event without anyrecovery actions with three integral severe accident codesASTEC MELCOR and MAAP ASTEC [5] and MELCOR[6] are predominantly mechanistic computer codes and theMAAP code [7] on the other hand is a fast runningparametric code The code main features are given in aseparate chapter

The calculation is performed for a two-loop pressurizedwater reactor (PWR) nuclear power plant of a Westinghousetype The plant model is based on the NPP Krsko layout [8]The input decks for the three codes are made as similar aspossible and consistent initial and boundary conditions areplaced on all of the analyses The objectives of calculationsare to analytically confirm the ability of passive containmentsafety systems to mitigate accident consequences and tojustify the applicability of currently the most popular severeaccident codes for such analyses The analysis covers con-tainment thermal hydraulics releases of incondensable gasesbasemat concrete melting and all other phenomena relevantfor the study of containment integrity The fission productrelease and the transport of radio nuclides were calculated byall three codes but results are not presented It is our opinionthat thermal hydraulic differences should be addressed beforepaying attention to corresponding differences in prediction ofradio nuclides behaviour A comparison of codesrsquo calculationresults is reported herein

2 Description of Containment PassiveSafety Systems

21 Passive Containment Filtered Venting System The pas-sive containment filtered venting system a system speciallydesigned for light water reactor NPPs to work withoutexternal power supply is used to filter radioactive aerosolsgaseous iodine and iodine organic compounds and to controlthe containment pressure

The PCFV system consists of aerosol filters the iodinefilter the rupture disc valves expansion orifices instrumen-tation and associated piping Figure 1 The venting gas firstpasses aerosol filter modules leaves the containment via

piping through a containment penetration passes the iodinefilter and discharges to the environment through the stack

Aerosol filters (five units on the right hand side inFigure 1) remove solid particles from the vented gases bymechanical filtering using a metal fiber filterThe iodine filter(green boxes in the middle of Figure 1) removes radioactiveiodine and its organic compounds through chemical sorptionwith silver active material inside the iodine filter

System actuation occurs after the failure of the rupturedisk at the burst pressure of 06MPa Vented gases flow thenthrough the piping containing two isolation valves and thepressure relief valve which closes at the pressure 041MPaThe line containing the rupture disc isolation valves and therelief valve lies between the outlet of aerosol filters and theinlet to the iodine filter

22 Passive Autocatalytic Recombiners The aim of passiveautocatalytic recombiners is to prevent occurrence of flam-mable gas mixture in containment compartments by reduc-ing concentration of combustible gases hydrogen and carbonmonoxide

The recombination process is based on a self-actuatedcatalytic exothermic reaction between hydrogencarbonmonoxide and oxygen Products of the reaction are steamand CO2 respectively accompanied with release of heat Thereaction is supported by natural circulation of gases PARconsists of a stainless steel enclosure and plates with thecatalyst material The enclosure is opened at the bottom toallow the gases to enter the PAR unit and at the top for thegases to be discharged back in the containment It extendsabove the catalyst elevation to provide a chimney to yieldadditional lift to enhance the effect of buoyancy Catalystmaterial (palladium) is not consumed during the reaction

3 Overview of the Codes and TheirContainment Models

31 The ASTEC Code The ASTEC is a modular computercode It is being jointly developed since almost 20 yearsby IRSN France and GRS Germany The code versionV20R3p3 was used in the calculation

ASTEC consists of 13 coupled modules (CESAR ICARECPA MEDICIS RUPUICUV CORIUM COVI SYSINTELSA SOPHAEROS ISODOP IODE and DOSE) thatmodel different phenomena or different parts of a nuclearpower plant Some of the modules important for the pre-sented calculation are briefly described

The CESAR module [10] computes two-phase thermalhydraulics (TH) in the primary and secondary circuitsModelling is based on a 1D two-fluid five-equation approachcompleted by a phase slip model The finite volume methodis used for the space discretization by means of the staggeredmesh and the time discretization of the basic equations isdone using the fully implicit first-order backward differencescheme ensuring the numerical stability The ICARE modulemodels in-vessel core degradation and vessel rupture [11]Thethermal hydraulics in the core is based on a 1D swollen waterlevel approach completed by a 2D gas modellingThe coriumbehaviour in the lower plenum is based on a 0Dmodelling of

















Steamheader

FeedwaterFeedwater

Aux feed Aux feed

RPV

PRZ

SG1SG2

RCP1RCP2

ACC1ACC2











9

RPV

Upperhead

Upperplenum

Reactorcore

Lower Plenum

1

8

14

PRZ

15

3

4 5

6

7

11 10

12

13

SG2 SG1

RCP1RCP2

Downcomer +

2


1 2 910

1516 19 22

41123

5 12317

1813

67 148

21

20

DOM

BET

BET

SG1PRZ SG2RPO

ARV

CAV

SMP

24

ANL

ANL

DOM

SG1

SG2

PRZ

SMP CAV

ARV

RPO

BET



























Reactor



vessel

vessel


















1 2 3 4 5 6 70Time (days)

15

20

25

30

35

40

45

50

55

Con

tain

men

t pre

ssur

e (M

Pa)

ASTECMELCORMAAP






1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

Con

tain

men

t atm

osph

ere t

empe

ratu

re (K

)

ASTECMELCORMAAP


SBO Sequence

0

05

10

15

20

25

30

35

40

45

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f gas

es

1 2 3 4 5 6 70Time (days)

Steam CON2

O2

CO2

H2





1 2 3 4 5 6 70Time (days)

0

10

20

30

40

50

60

Wat

er m

ass i

n th

e cav

ity (times

1000

kg)

ASTECMELCORMAAP







90 probability for


containment failure

2 4 6 8 1 12 14 16 18 20Time (days)

2

3

4

5

6

7

8

91

1112

Con

tain

men

t pre

ssur

e (M

Pa)








Zr + 2H2O 997888rarr ZrO2 + 2H2 (2)

2Cr + 3H2O 997888rarr Cr2O3 + 3H2 (3)


2Fe + 3H2O 997888rarr Fe2O3 + 3H2 (5)


Zr + 2CO2 997888rarr ZrO2 + 2CO (6)

2Cr + 3CO2 997888rarr Cr2O3 + 3CO (7)



ASTECMELCORMAAP

0

005

010

015

020

025

030

035

040

045

050

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f hyd

roge

n

1 2 3 4 5 6 70Time (days)



ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

001020304050607080910

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f CO






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

00102030405060708091011

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

fCO2









Vabl = 119902119875120588conc119871conc (9)



ℎ119888 = 120582119897Nu119903119887 (10)


Nu = 205(1205881198971198953119892119892120583119897 )0105

Prminus025 (11)






600000

10000

15000

20000

25000

30000

604800001205495

minus108

minus719

minus357

00421

366

minus362minus723 362 7230600000

10000

15000

20000

25000

30000

minus108

minus719

minus357

00421

366




ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

50

100

150

200

250

300

350

400

450

in th

e cav

ity (times

1000

kg)

Mas

s of e

rode

d co

ncre

te






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

050

100150200250300350400450500550600

Hyd

roge

n m

ass r

emov

ed b

y PA

Rs (k

g)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Prod

uctio

n of

hyd

roge

n (k

g)



ASTECMELCORMAAP

0200400600800

10001200140016001800200022002400

Melt

tem

pera

ture

in th

e cav

ity (K

)

1 2 3 4 5 6 70Time (days)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

650

Cavi

ty at

mos

pher

e tem

pera

ture

(K)



ASTECMELCOR

1 2 3 4 5 6 70Time (days)

0

1

2

3

4

5

6

7

8

9

10

Hea

t los

s to

conc

rete

(MW

)







1

09

08

07

06

05

04

03

02

01

01

09

08

07

06

05

04

03

02

01

0

Air

Steam + CO2

09 08 07 06 05 04 03 02 01 01H2 + CO


MELCORMAAP




7 Conclusions







Nomenclature

Latin Symbols


Greek Symbols






Acknowledgments


References


























FuelsJournal of







Advances in



Journal of


Renewable Energy





RotatingMachinery


EnergyJournal of



Journal of






























Steamheader

FeedwaterFeedwater

Aux feed Aux feed

RPV

PRZ

SG1SG2

RCP1RCP2

ACC1ACC2











9

RPV

Upperhead

Upperplenum

Reactorcore

Lower Plenum

1

8

14

PRZ

15

3

4 5

6

7

11 10

12

13

SG2 SG1

RCP1RCP2

Downcomer +

2


1 2 910

1516 19 22

41123

5 12317

1813

67 148

21

20

DOM

BET

BET

SG1PRZ SG2RPO

ARV

CAV

SMP

24

ANL

ANL

DOM

SG1

SG2

PRZ

SMP CAV

ARV

RPO

BET



























Reactor



vessel

vessel


















1 2 3 4 5 6 70Time (days)

15

20

25

30

35

40

45

50

55

Con

tain

men

t pre

ssur

e (M

Pa)

ASTECMELCORMAAP






1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

Con

tain

men

t atm

osph

ere t

empe

ratu

re (K

)

ASTECMELCORMAAP


SBO Sequence

0

05

10

15

20

25

30

35

40

45

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f gas

es

1 2 3 4 5 6 70Time (days)

Steam CON2

O2

CO2

H2





1 2 3 4 5 6 70Time (days)

0

10

20

30

40

50

60

Wat

er m

ass i

n th

e cav

ity (times

1000

kg)

ASTECMELCORMAAP







90 probability for


containment failure

2 4 6 8 1 12 14 16 18 20Time (days)

2

3

4

5

6

7

8

91

1112

Con

tain

men

t pre

ssur

e (M

Pa)








Zr + 2H2O 997888rarr ZrO2 + 2H2 (2)

2Cr + 3H2O 997888rarr Cr2O3 + 3H2 (3)


2Fe + 3H2O 997888rarr Fe2O3 + 3H2 (5)


Zr + 2CO2 997888rarr ZrO2 + 2CO (6)

2Cr + 3CO2 997888rarr Cr2O3 + 3CO (7)



ASTECMELCORMAAP

0

005

010

015

020

025

030

035

040

045

050

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f hyd

roge

n

1 2 3 4 5 6 70Time (days)



ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

001020304050607080910

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f CO






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

00102030405060708091011

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

fCO2









Vabl = 119902119875120588conc119871conc (9)



ℎ119888 = 120582119897Nu119903119887 (10)


Nu = 205(1205881198971198953119892119892120583119897 )0105

Prminus025 (11)






600000

10000

15000

20000

25000

30000

604800001205495

minus108

minus719

minus357

00421

366

minus362minus723 362 7230600000

10000

15000

20000

25000

30000

minus108

minus719

minus357

00421

366




ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

50

100

150

200

250

300

350

400

450

in th

e cav

ity (times

1000

kg)

Mas

s of e

rode

d co

ncre

te






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

050

100150200250300350400450500550600

Hyd

roge

n m

ass r

emov

ed b

y PA

Rs (k

g)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Prod

uctio

n of

hyd

roge

n (k

g)



ASTECMELCORMAAP

0200400600800

10001200140016001800200022002400

Melt

tem

pera

ture

in th

e cav

ity (K

)

1 2 3 4 5 6 70Time (days)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

650

Cavi

ty at

mos

pher

e tem

pera

ture

(K)



ASTECMELCOR

1 2 3 4 5 6 70Time (days)

0

1

2

3

4

5

6

7

8

9

10

Hea

t los

s to

conc

rete

(MW

)







1

09

08

07

06

05

04

03

02

01

01

09

08

07

06

05

04

03

02

01

0

Air

Steam + CO2

09 08 07 06 05 04 03 02 01 01H2 + CO


MELCORMAAP




7 Conclusions







Nomenclature

Latin Symbols


Greek Symbols






Acknowledgments


References


























FuelsJournal of







Advances in



Journal of


Renewable Energy





RotatingMachinery


EnergyJournal of



Journal of















9

RPV

Upperhead

Upperplenum

Reactorcore

Lower Plenum

1

8

14

PRZ

15

3

4 5

6

7

11 10

12

13

SG2 SG1

RCP1RCP2

Downcomer +

2


1 2 910

1516 19 22

41123

5 12317

1813

67 148

21

20

DOM

BET

BET

SG1PRZ SG2RPO

ARV

CAV

SMP

24

ANL

ANL

DOM

SG1

SG2

PRZ

SMP CAV

ARV

RPO

BET



























Reactor



vessel

vessel


















1 2 3 4 5 6 70Time (days)

15

20

25

30

35

40

45

50

55

Con

tain

men

t pre

ssur

e (M

Pa)

ASTECMELCORMAAP






1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

Con

tain

men

t atm

osph

ere t

empe

ratu

re (K

)

ASTECMELCORMAAP


SBO Sequence

0

05

10

15

20

25

30

35

40

45

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f gas

es

1 2 3 4 5 6 70Time (days)

Steam CON2

O2

CO2

H2





1 2 3 4 5 6 70Time (days)

0

10

20

30

40

50

60

Wat

er m

ass i

n th

e cav

ity (times

1000

kg)

ASTECMELCORMAAP







90 probability for


containment failure

2 4 6 8 1 12 14 16 18 20Time (days)

2

3

4

5

6

7

8

91

1112

Con

tain

men

t pre

ssur

e (M

Pa)








Zr + 2H2O 997888rarr ZrO2 + 2H2 (2)

2Cr + 3H2O 997888rarr Cr2O3 + 3H2 (3)


2Fe + 3H2O 997888rarr Fe2O3 + 3H2 (5)


Zr + 2CO2 997888rarr ZrO2 + 2CO (6)

2Cr + 3CO2 997888rarr Cr2O3 + 3CO (7)



ASTECMELCORMAAP

0

005

010

015

020

025

030

035

040

045

050

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f hyd

roge

n

1 2 3 4 5 6 70Time (days)



ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

001020304050607080910

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f CO






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

00102030405060708091011

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

fCO2









Vabl = 119902119875120588conc119871conc (9)



ℎ119888 = 120582119897Nu119903119887 (10)


Nu = 205(1205881198971198953119892119892120583119897 )0105

Prminus025 (11)






600000

10000

15000

20000

25000

30000

604800001205495

minus108

minus719

minus357

00421

366

minus362minus723 362 7230600000

10000

15000

20000

25000

30000

minus108

minus719

minus357

00421

366




ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

50

100

150

200

250

300

350

400

450

in th

e cav

ity (times

1000

kg)

Mas

s of e

rode

d co

ncre

te






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

050

100150200250300350400450500550600

Hyd

roge

n m

ass r

emov

ed b

y PA

Rs (k

g)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Prod

uctio

n of

hyd

roge

n (k

g)



ASTECMELCORMAAP

0200400600800

10001200140016001800200022002400

Melt

tem

pera

ture

in th

e cav

ity (K

)

1 2 3 4 5 6 70Time (days)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

650

Cavi

ty at

mos

pher

e tem

pera

ture

(K)



ASTECMELCOR

1 2 3 4 5 6 70Time (days)

0

1

2

3

4

5

6

7

8

9

10

Hea

t los

s to

conc

rete

(MW

)







1

09

08

07

06

05

04

03

02

01

01

09

08

07

06

05

04

03

02

01

0

Air

Steam + CO2

09 08 07 06 05 04 03 02 01 01H2 + CO


MELCORMAAP




7 Conclusions







Nomenclature

Latin Symbols


Greek Symbols






Acknowledgments


References


























FuelsJournal of







Advances in



Journal of


Renewable Energy





RotatingMachinery


EnergyJournal of



Journal of



































Reactor



vessel

vessel


















1 2 3 4 5 6 70Time (days)

15

20

25

30

35

40

45

50

55

Con

tain

men

t pre

ssur

e (M

Pa)

ASTECMELCORMAAP






1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

Con

tain

men

t atm

osph

ere t

empe

ratu

re (K

)

ASTECMELCORMAAP


SBO Sequence

0

05

10

15

20

25

30

35

40

45

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f gas

es

1 2 3 4 5 6 70Time (days)

Steam CON2

O2

CO2

H2





1 2 3 4 5 6 70Time (days)

0

10

20

30

40

50

60

Wat

er m

ass i

n th

e cav

ity (times

1000

kg)

ASTECMELCORMAAP







90 probability for


containment failure

2 4 6 8 1 12 14 16 18 20Time (days)

2

3

4

5

6

7

8

91

1112

Con

tain

men

t pre

ssur

e (M

Pa)








Zr + 2H2O 997888rarr ZrO2 + 2H2 (2)

2Cr + 3H2O 997888rarr Cr2O3 + 3H2 (3)


2Fe + 3H2O 997888rarr Fe2O3 + 3H2 (5)


Zr + 2CO2 997888rarr ZrO2 + 2CO (6)

2Cr + 3CO2 997888rarr Cr2O3 + 3CO (7)



ASTECMELCORMAAP

0

005

010

015

020

025

030

035

040

045

050

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f hyd

roge

n

1 2 3 4 5 6 70Time (days)



ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

001020304050607080910

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f CO






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

00102030405060708091011

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

fCO2









Vabl = 119902119875120588conc119871conc (9)



ℎ119888 = 120582119897Nu119903119887 (10)


Nu = 205(1205881198971198953119892119892120583119897 )0105

Prminus025 (11)






600000

10000

15000

20000

25000

30000

604800001205495

minus108

minus719

minus357

00421

366

minus362minus723 362 7230600000

10000

15000

20000

25000

30000

minus108

minus719

minus357

00421

366




ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

50

100

150

200

250

300

350

400

450

in th

e cav

ity (times

1000

kg)

Mas

s of e

rode

d co

ncre

te






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

050

100150200250300350400450500550600

Hyd

roge

n m

ass r

emov

ed b

y PA

Rs (k

g)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Prod

uctio

n of

hyd

roge

n (k

g)



ASTECMELCORMAAP

0200400600800

10001200140016001800200022002400

Melt

tem

pera

ture

in th

e cav

ity (K

)

1 2 3 4 5 6 70Time (days)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

650

Cavi

ty at

mos

pher

e tem

pera

ture

(K)



ASTECMELCOR

1 2 3 4 5 6 70Time (days)

0

1

2

3

4

5

6

7

8

9

10

Hea

t los

s to

conc

rete

(MW

)







1

09

08

07

06

05

04

03

02

01

01

09

08

07

06

05

04

03

02

01

0

Air

Steam + CO2

09 08 07 06 05 04 03 02 01 01H2 + CO


MELCORMAAP




7 Conclusions







Nomenclature

Latin Symbols


Greek Symbols






Acknowledgments


References


























FuelsJournal of







Advances in



Journal of


Renewable Energy





RotatingMachinery


EnergyJournal of



Journal of






















1 2 3 4 5 6 70Time (days)

15

20

25

30

35

40

45

50

55

Con

tain

men

t pre

ssur

e (M

Pa)

ASTECMELCORMAAP






1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

Con

tain

men

t atm

osph

ere t

empe

ratu

re (K

)

ASTECMELCORMAAP


SBO Sequence

0

05

10

15

20

25

30

35

40

45

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f gas

es

1 2 3 4 5 6 70Time (days)

Steam CON2

O2

CO2

H2





1 2 3 4 5 6 70Time (days)

0

10

20

30

40

50

60

Wat

er m

ass i

n th

e cav

ity (times

1000

kg)

ASTECMELCORMAAP







90 probability for


containment failure

2 4 6 8 1 12 14 16 18 20Time (days)

2

3

4

5

6

7

8

91

1112

Con

tain

men

t pre

ssur

e (M

Pa)








Zr + 2H2O 997888rarr ZrO2 + 2H2 (2)

2Cr + 3H2O 997888rarr Cr2O3 + 3H2 (3)


2Fe + 3H2O 997888rarr Fe2O3 + 3H2 (5)


Zr + 2CO2 997888rarr ZrO2 + 2CO (6)

2Cr + 3CO2 997888rarr Cr2O3 + 3CO (7)



ASTECMELCORMAAP

0

005

010

015

020

025

030

035

040

045

050

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f hyd

roge

n

1 2 3 4 5 6 70Time (days)



ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

001020304050607080910

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f CO






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

00102030405060708091011

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

fCO2









Vabl = 119902119875120588conc119871conc (9)



ℎ119888 = 120582119897Nu119903119887 (10)


Nu = 205(1205881198971198953119892119892120583119897 )0105

Prminus025 (11)






600000

10000

15000

20000

25000

30000

604800001205495

minus108

minus719

minus357

00421

366

minus362minus723 362 7230600000

10000

15000

20000

25000

30000

minus108

minus719

minus357

00421

366




ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

50

100

150

200

250

300

350

400

450

in th

e cav

ity (times

1000

kg)

Mas

s of e

rode

d co

ncre

te






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

050

100150200250300350400450500550600

Hyd

roge

n m

ass r

emov

ed b

y PA

Rs (k

g)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Prod

uctio

n of

hyd

roge

n (k

g)



ASTECMELCORMAAP

0200400600800

10001200140016001800200022002400

Melt

tem

pera

ture

in th

e cav

ity (K

)

1 2 3 4 5 6 70Time (days)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

650

Cavi

ty at

mos

pher

e tem

pera

ture

(K)



ASTECMELCOR

1 2 3 4 5 6 70Time (days)

0

1

2

3

4

5

6

7

8

9

10

Hea

t los

s to

conc

rete

(MW

)







1

09

08

07

06

05

04

03

02

01

01

09

08

07

06

05

04

03

02

01

0

Air

Steam + CO2

09 08 07 06 05 04 03 02 01 01H2 + CO


MELCORMAAP




7 Conclusions







Nomenclature

Latin Symbols


Greek Symbols






Acknowledgments


References


























FuelsJournal of







Advances in



Journal of


Renewable Energy





RotatingMachinery


EnergyJournal of



Journal of
















1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

Con

tain

men

t atm

osph

ere t

empe

ratu

re (K

)

ASTECMELCORMAAP


SBO Sequence

0

05

10

15

20

25

30

35

40

45

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f gas

es

1 2 3 4 5 6 70Time (days)

Steam CON2

O2

CO2

H2





1 2 3 4 5 6 70Time (days)

0

10

20

30

40

50

60

Wat

er m

ass i

n th

e cav

ity (times

1000

kg)

ASTECMELCORMAAP







90 probability for


containment failure

2 4 6 8 1 12 14 16 18 20Time (days)

2

3

4

5

6

7

8

91

1112

Con

tain

men

t pre

ssur

e (M

Pa)








Zr + 2H2O 997888rarr ZrO2 + 2H2 (2)

2Cr + 3H2O 997888rarr Cr2O3 + 3H2 (3)


2Fe + 3H2O 997888rarr Fe2O3 + 3H2 (5)


Zr + 2CO2 997888rarr ZrO2 + 2CO (6)

2Cr + 3CO2 997888rarr Cr2O3 + 3CO (7)



ASTECMELCORMAAP

0

005

010

015

020

025

030

035

040

045

050

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f hyd

roge

n

1 2 3 4 5 6 70Time (days)



ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

001020304050607080910

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f CO






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

00102030405060708091011

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

fCO2









Vabl = 119902119875120588conc119871conc (9)



ℎ119888 = 120582119897Nu119903119887 (10)


Nu = 205(1205881198971198953119892119892120583119897 )0105

Prminus025 (11)






600000

10000

15000

20000

25000

30000

604800001205495

minus108

minus719

minus357

00421

366

minus362minus723 362 7230600000

10000

15000

20000

25000

30000

minus108

minus719

minus357

00421

366




ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

50

100

150

200

250

300

350

400

450

in th

e cav

ity (times

1000

kg)

Mas

s of e

rode

d co

ncre

te






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

050

100150200250300350400450500550600

Hyd

roge

n m

ass r

emov

ed b

y PA

Rs (k

g)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Prod

uctio

n of

hyd

roge

n (k

g)



ASTECMELCORMAAP

0200400600800

10001200140016001800200022002400

Melt

tem

pera

ture

in th

e cav

ity (K

)

1 2 3 4 5 6 70Time (days)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

650

Cavi

ty at

mos

pher

e tem

pera

ture

(K)



ASTECMELCOR

1 2 3 4 5 6 70Time (days)

0

1

2

3

4

5

6

7

8

9

10

Hea

t los

s to

conc

rete

(MW

)







1

09

08

07

06

05

04

03

02

01

01

09

08

07

06

05

04

03

02

01

0

Air

Steam + CO2

09 08 07 06 05 04 03 02 01 01H2 + CO


MELCORMAAP




7 Conclusions







Nomenclature

Latin Symbols


Greek Symbols






Acknowledgments


References


























FuelsJournal of







Advances in



Journal of


Renewable Energy





RotatingMachinery


EnergyJournal of



Journal of
















90 probability for


containment failure

2 4 6 8 1 12 14 16 18 20Time (days)

2

3

4

5

6

7

8

91

1112

Con

tain

men

t pre

ssur

e (M

Pa)








Zr + 2H2O 997888rarr ZrO2 + 2H2 (2)

2Cr + 3H2O 997888rarr Cr2O3 + 3H2 (3)


2Fe + 3H2O 997888rarr Fe2O3 + 3H2 (5)


Zr + 2CO2 997888rarr ZrO2 + 2CO (6)

2Cr + 3CO2 997888rarr Cr2O3 + 3CO (7)



ASTECMELCORMAAP

0

005

010

015

020

025

030

035

040

045

050

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f hyd

roge

n

1 2 3 4 5 6 70Time (days)



ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

001020304050607080910

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

f CO






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

00102030405060708091011

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

fCO2









Vabl = 119902119875120588conc119871conc (9)



ℎ119888 = 120582119897Nu119903119887 (10)


Nu = 205(1205881198971198953119892119892120583119897 )0105

Prminus025 (11)






600000

10000

15000

20000

25000

30000

604800001205495

minus108

minus719

minus357

00421

366

minus362minus723 362 7230600000

10000

15000

20000

25000

30000

minus108

minus719

minus357

00421

366




ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

50

100

150

200

250

300

350

400

450

in th

e cav

ity (times

1000

kg)

Mas

s of e

rode

d co

ncre

te






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

050

100150200250300350400450500550600

Hyd

roge

n m

ass r

emov

ed b

y PA

Rs (k

g)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Prod

uctio

n of

hyd

roge

n (k

g)



ASTECMELCORMAAP

0200400600800

10001200140016001800200022002400

Melt

tem

pera

ture

in th

e cav

ity (K

)

1 2 3 4 5 6 70Time (days)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

650

Cavi

ty at

mos

pher

e tem

pera

ture

(K)



ASTECMELCOR

1 2 3 4 5 6 70Time (days)

0

1

2

3

4

5

6

7

8

9

10

Hea

t los

s to

conc

rete

(MW

)







1

09

08

07

06

05

04

03

02

01

01

09

08

07

06

05

04

03

02

01

0

Air

Steam + CO2

09 08 07 06 05 04 03 02 01 01H2 + CO


MELCORMAAP




7 Conclusions







Nomenclature

Latin Symbols


Greek Symbols






Acknowledgments


References


























FuelsJournal of







Advances in



Journal of


Renewable Energy





RotatingMachinery


EnergyJournal of



Journal of
















ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

00102030405060708091011

in th

e con

tain

men

t (M

Pa)

Part

ial p

ress

ure o

fCO2









Vabl = 119902119875120588conc119871conc (9)



ℎ119888 = 120582119897Nu119903119887 (10)


Nu = 205(1205881198971198953119892119892120583119897 )0105

Prminus025 (11)






600000

10000

15000

20000

25000

30000

604800001205495

minus108

minus719

minus357

00421

366

minus362minus723 362 7230600000

10000

15000

20000

25000

30000

minus108

minus719

minus357

00421

366




ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

50

100

150

200

250

300

350

400

450

in th

e cav

ity (times

1000

kg)

Mas

s of e

rode

d co

ncre

te






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

050

100150200250300350400450500550600

Hyd

roge

n m

ass r

emov

ed b

y PA

Rs (k

g)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Prod

uctio

n of

hyd

roge

n (k

g)



ASTECMELCORMAAP

0200400600800

10001200140016001800200022002400

Melt

tem

pera

ture

in th

e cav

ity (K

)

1 2 3 4 5 6 70Time (days)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

650

Cavi

ty at

mos

pher

e tem

pera

ture

(K)



ASTECMELCOR

1 2 3 4 5 6 70Time (days)

0

1

2

3

4

5

6

7

8

9

10

Hea

t los

s to

conc

rete

(MW

)







1

09

08

07

06

05

04

03

02

01

01

09

08

07

06

05

04

03

02

01

0

Air

Steam + CO2

09 08 07 06 05 04 03 02 01 01H2 + CO


MELCORMAAP




7 Conclusions







Nomenclature

Latin Symbols


Greek Symbols






Acknowledgments


References


























FuelsJournal of







Advances in



Journal of


Renewable Energy





RotatingMachinery


EnergyJournal of



Journal of
















600000

10000

15000

20000

25000

30000

604800001205495

minus108

minus719

minus357

00421

366

minus362minus723 362 7230600000

10000

15000

20000

25000

30000

minus108

minus719

minus357

00421

366




ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

50

100

150

200

250

300

350

400

450

in th

e cav

ity (times

1000

kg)

Mas

s of e

rode

d co

ncre

te






ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

050

100150200250300350400450500550600

Hyd

roge

n m

ass r

emov

ed b

y PA

Rs (k

g)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Prod

uctio

n of

hyd

roge

n (k

g)



ASTECMELCORMAAP

0200400600800

10001200140016001800200022002400

Melt

tem

pera

ture

in th

e cav

ity (K

)

1 2 3 4 5 6 70Time (days)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

650

Cavi

ty at

mos

pher

e tem

pera

ture

(K)



ASTECMELCOR

1 2 3 4 5 6 70Time (days)

0

1

2

3

4

5

6

7

8

9

10

Hea

t los

s to

conc

rete

(MW

)







1

09

08

07

06

05

04

03

02

01

01

09

08

07

06

05

04

03

02

01

0

Air

Steam + CO2

09 08 07 06 05 04 03 02 01 01H2 + CO


MELCORMAAP




7 Conclusions







Nomenclature

Latin Symbols


Greek Symbols






Acknowledgments


References


























FuelsJournal of







Advances in



Journal of


Renewable Energy





RotatingMachinery


EnergyJournal of



Journal of
















ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Prod

uctio

n of

hyd

roge

n (k

g)



ASTECMELCORMAAP

0200400600800

10001200140016001800200022002400

Melt

tem

pera

ture

in th

e cav

ity (K

)

1 2 3 4 5 6 70Time (days)





ASTECMELCORMAAP

1 2 3 4 5 6 70Time (days)

350

400

450

500

550

600

650

Cavi

ty at

mos

pher

e tem

pera

ture

(K)



ASTECMELCOR

1 2 3 4 5 6 70Time (days)

0

1

2

3

4

5

6

7

8

9

10

Hea

t los

s to

conc

rete

(MW

)







1

09

08

07

06

05

04

03

02

01

01

09

08

07

06

05

04

03

02

01

0

Air

Steam + CO2

09 08 07 06 05 04 03 02 01 01H2 + CO


MELCORMAAP




7 Conclusions







Nomenclature

Latin Symbols


Greek Symbols






Acknowledgments


References


























FuelsJournal of







Advances in



Journal of


Renewable Energy





RotatingMachinery


EnergyJournal of



Journal of


















1

09

08

07

06

05

04

03

02

01

01

09

08

07

06

05

04

03

02

01

0

Air

Steam + CO2

09 08 07 06 05 04 03 02 01 01H2 + CO


MELCORMAAP




7 Conclusions







Nomenclature

Latin Symbols


Greek Symbols






Acknowledgments


References


























FuelsJournal of







Advances in



Journal of


Renewable Energy





RotatingMachinery


EnergyJournal of



Journal of


















Nomenclature

Latin Symbols


Greek Symbols






Acknowledgments


References


























FuelsJournal of







Advances in



Journal of


Renewable Energy





RotatingMachinery


EnergyJournal of



Journal of





























FuelsJournal of







Advances in



Journal of


Renewable Energy





RotatingMachinery


EnergyJournal of



Journal of














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