modular high temperature pebble bed reactor

Massachusetts Institute of TechnologyDepartment of Nuclear Engineering

Advanced Reactor Technology Pebble Bed Project

MPBR-1

Modular High Temperature Pebble Bed Reactor

Faculty AdvisorsAndrew C. Kadak

Ronald G. BallingerMichael J. Driscoll

Sidney YipDavid Gordon Wilson



MPBR-2

Student Researchers

• Fuel Performance- Heather MacLean- Jing Wang

• Core Physics- Julian Lebenhaft

• Thermal Hydraulics- Chunyun Wang- Tamara Galen

• Safety- Tieliang Zhai



MPBR-3

Project Objective

Develop a sufficient technical and economicbasis for this type of reactor plant to determinewhether it can compete with natural gas and stillmeet safety, proliferation resistance and waste disposal concerns.



MPBR-4

Generation IV Reactor

• Competitive with Natural Gas• Demonstrated Safety• Improved Proliferation Resistance• Readily Disposable Waste Form



MPBR-5

Modular High TemperaturePebble Bed Reactor

• 110 MWe• Helium Cooled• “Indirect” Cycle• 8 % Enriched Fuel• Built in 2 Years• Factory Built• Site Assembled

• On-line Refueling• Modules added to

meet demand.• No Reprocessing• High Burnup

>90,000 Mwd/MT• Direct Disposal of

HLW



MPBR-6

What is a Pebble Bed Reactor ?

• 360,000 pebbles in core• about 3,000 pebbles handled by

FHS each day• about 350 discarded daily• one pebble discharged every 30

seconds• average pebble cycles through

core 15 times• Fuel handling most

maintenance-intensive part of plant



MPBR-7

MPBR SpecificationsThermal Power 250 MWCore Height 10.0 mCore Diameter 3.0 mPressure Vessel Height 16 mPressure Vessel Radius 5.6 mNumber of Fuel Pebbles 360,000Microspheres/Fuel Pebble 11,000Fuel UO2Fuel Pebble Diameter 60 mmFuel Pebble enrichment 8% Uranium Mass/Fuel Pebble 7 gCoolant HeliumHelium mass flow rate 120 kg/s (100% power)Helium entry/exit temperatures 450oC/850oCHelium pressure 80 barMean Power Density 3.54 MW/m3

Number of Control Rods 6Number of Absorber Ball Systems 18



MPBR-8

TurbomachineryModule

IHX ModuleReactorModule

Conceptual Design Layout



MPBR-9

Competitive With Gas ?

• Natural Gas 3.4 Cents/kwhr• AP 600 3.62 Cents/kwhr• ALWR 3.8 Cents/kwhr• MPBR 3.3 Cents/kwhr

Levelized Costs (1992 $ Based on NEI Study)



MPBR-10



MPBR-11

Improved Fuel Particle Performance Modeling:

Chemical ModelingStudent: Heather MacLean

Advisor: R. Ballinger



MPBR-12

Objective• Model fuel particle chemical environment

– particle temperature distributions– internal particle pressure

• Model chemical interactions with coatings (SiC)– migration of fission products (Ag) through coatings

(SiC)– chemical attack (Pd) of SiC



MPBR-13

Accomplishments• Analyzed likely temperatures in fuel

particles in different core locations– ∆T across any particle < 20 °C– peak kernel temperature range: 500-1100 °C

• Diffusion experiment in progress– 2 diffusion couples heated, continuing– characterization in progress



MPBR-14

Coated Particle Fuel SystemFuel Kernel

BufferIPyC

SiCOPyC

Individual Microsphere780 µm nominal diameter

60 mmdiameter

~11,000 microspheres per fuel pebble~350,000 pebbles per core

Fuel Pebble



MPBR-15

Thermal ModelCalculate temperatures in a pebble:• Linear axial bulk He temperature rise (450-850 °C)• Packed-bed He heat transfer correlation

(∆T across He film surrounding a pebble)⇒ Pebble surface temperature for any core location• Homogenized (volumetric averages) pebble thermal

conduction model based on core average power• Thermal conductivity depends on temperature⇒ Temperature distribution inside a pebble⇒ Fuel particle thermal model



MPBR-16

Fuel Particle Thermal ModelCalculate temperatures in a particle:• Particle surface temperature determined by radial

location in a pebble• Fuel kernel fission product swelling data

1% volume swelling per 10 GWd/T• Buffer densification to accommodate kernel swelling• Kernel thermal expansion -- 10-6 K-1



MPBR-17

Bounding Thermal ExamplesTHe(inlet) = 450 °C

THe(outlet) = 850 °C

Low Temperature Case

High Temperature Case

core inletpower: average peak T: 513 °C∆T: 16 °C

core outlet power: average 2x averagepeak T: 968 °C 1074 °C∆T: 17 °C 19 °C



MPBR-18

Typical Particle TemperaturesPebble Power Average Average 2x AverageAxial Location Core Inlet Core Outlet Core OutletLoc. In Pebble Outer Edge Center Center

Bulk He Temp.(°K) 723 1123 1123

Temperatures

Kernel Center 786 1241 1347Kernel Outer Edge 779 1234 1340Buffer Outer Edge 771 1225 1329IPyC Outer Edge 770 1224 1329SiC Outer Edge 770 1224 1329OPyC Outer Edge 770 1224 1328

Particle ∆T 16 17 19



MPBR-19

Significance of Temperatures• Small ∆T across all particles

⇒ can use average temperature in a particle for chemical analyses

• Variation in particle peak temperature⇒ chemistry phenomena (diffusion) vary

exponentially with temperature• Gradient across particle

⇒ significant impact on mechanical phenomena



MPBR-20

Chemical Interactions• Palladium

– very destructive, local attack

– can open a pathway for fission product release

– limited data

• Silver– diffuses through

intact SiC– activity / maintenance

concern– limited data in MPBR

proposed temperature range



MPBR-21

SiC Diffusion Experiments• Limited data for specific

fission products and temperatures of interest

• Develop experimental foundation for future study of advanced fission product barrier materials 3/4 inch OD 30 mil thick wall



MPBR-22

Experimental Procedure

Ar gas

1. Sputter Coating

Metal Coating Deposited on

Inside of Shell

Ag or Pd target

Graphite Hemisphere

(hollow shell)

3. Heat Treatment

1000-1600 oC10-1000 hours

5. Data Analysis

DAg(T)DPd(T)DMFP(T)

r

CAg

4. Characterization

MicroscopySIMSESCAXRD

Metal Inner Coating

Diffusion Couple(cross section)

Graphite Shell

SiC or ZrC overcoating

2. SiC Overcoating

MTS & H2



MPBR-23

Status of Experiments• Experimental procedure defined• System operational• 2 diffusion heat treatments completed

– 1500 °C, 24 hours, Ag-SiC– 1400 °C, 44 hours, Ag-SiC– other temperatures in progress

• Characterization in progress



MPBR-24

Future Work• Continue diffusion couple heat treatments• Collect and analyze diffusion experiment

data• Update thermal and pressure models

– include UCO data– include radial and axial power peaking



MPBR-25

Fuel Performance Modeling— Mechanical Analysis

Student: Jing WangAdvisors: Prof. Ballinger & Prof. Yip



MPBR-26

Objective• Develop a mechanical model to predict the

mechanical behavior of TRISO fuel particles, including failure, that can be used as a tool in understanding past behavior and in developing advanced particles



MPBR-27

Mechanical ModelStress Analysis Failure Model

Analytical Solution

Finite Element

ABAQUS ‘FUEL’ code

• Anisotropy• Asphericity• Substructure

Specialized FEM

• Isotropy• Fast Speed• MC Sampling

Crack Induced Failure

Pure Pressure Vessel Failure

Fracture Mechanics

Failure Criteria

Fracture Strength ( σF )

• Stress IntensityFactor ( )

• Strain EnergyRelease ( G )

KI

Chemistry



MPBR-28

Conclusion• Currently with closed form solutions,

the fracture mechanics based failure model, developed as part of this task, yields a good prediction of the failure probability of TRISO fuel particles



MPBR-29

Closed Form Solutions

• System: IPyC/SiC/OPyC• Assumptions– Spherical layers

– Elastically isotropic

– Creep and swellingare treated for IPyC/OPyC

– SiC is just elastic shell

– Three layers aretightly bonded

Layer Thickness(µm)

Outer PyC 43

SiC 35

Inner PyC 53

Buffer PyC 100

UCO 195

(diameter)

Total Diameter 0.66 mm

TRISO Fuel Particle



MPBR-30

Time Evolution of Radial Stress Distribution in TRISO Fuel Particles

197.

520

6.67

215.

8422

5.01

234.

18

243.

35

252.

52

261.

69

270.

86

280.

03

289.

2

298.

37

307.

54

316.

71

325.

88 0

0.20

4

0.40

8

0.61

2

0.81

6

1.02

1.22

4

1.42

8

1.63

2

-30

-20

-10

0

10

20

30

40

50

60

Rad

ial S

tres

s (M

Pas)

Radius (microns)

Fluence (10^21nvt)

50-6040-5030-4020-3010-200-10-10-0-20--10-30--20

IPyCSiC

OPyC



MPBR-31

Time Evolution of Tangential Stress Distribution in TRISO Fuel Particles

197.

520

9.29

221.

08

232.

87

244.

66

256.

45

268.

24

280.

03

291.

82

303.

61

315.

4

327.

19 0

0.20

4

0.40

8

0.61

2

0.81

6

1.02

1.22

4

1.42

8

1.63

2

-400

-300

-200

-100

0

100

200

Tang

entia

l Stre

ss (M

Pas)

Radius (microns)

Fluence (10^21nvt)

100-2000-100-100-0-200--100-300--200-400--300

IPyCSiC

OPyC



MPBR-32

Finite Element Method• Check with analytical solutions and other calculations• Study the non-ideal particles

– anisotropy of material– anisotropy of geometry

• Evaluate the effects of crack and debonding• Combine with analytical solutions to predict failure



MPBR-33

Fracture Mechanics based Failure Model

• Consider the impact of the cracking of IPyC to SiC• Stress distributions come from closed form solutions• Currently use stress intensity factor to evaluate local

stresses

• Predict failure probability with MC sampling process(Gaussian Distribution : Layer thickness, kernel & buffer densitiesWeibull Distribution : Fracture strength of IPyC, SiC and OPyCTriangular Distribution : Critical stress intensity factor of SiC )



MPBR-34

IPyCSiC

OPyC

PIPO

)(SiCKIC

aIPyCK TI πσ=)(

Material PropertyσT

The Sketch for FM based Failure Model



MPBR-35

Prediction by FM based Failure Model• Inputs

SiC layer –– (Sampled with triangular distribution)KIC(SiC) (mean) = 3300MPa·µm1/2 [1] Standard Deviation = 530MPa·µm1/2

IPyC layer –– (Sampled with Weibull distribution)σF(IPyC) (mean) = 384MPa [2] Weibull modulus = 8.6 [2]

End-of-life Burnup –– 75% FIMA

Particles Sampled –– N=1,000,000

• OutputsCases with IPyC Failure: 595 Failure Probability: 5.95×10-4

Cases with SiC Failure: 17 Failure Probability: 1.70×10-5

[1] Material Specification No. SC-001, Morton Advanced Materials, 185 New Boston Street, Woburn, MA 01801[2] J. L. Kaae, et al., Nucl. Tech., 35, 1977, p368



MPBR-36

Conclusions• The previous mechanical model, based on a pressure

vessel failure by overpressure, from INEEL predicts zero fuel failure

• New fracture mechanics based failure model gives reasonable prediction of the failure probability of TRISO fuel particles



MPBR-37

Future Work• Use finite element method (ABAQUS) to incorporate

non-ideal particles• Improve the fracture mechanics based failure model• Build a more realistic pyrocarbon model• Use specialized finite element method



MPBR-38



MPBR-39

Chunyun Wang Advisors: Prof. R. G. Ballinger

Dr. H. C. No

(Draft) Presentation for INEEL Review Aug. 01, 00

Dynamic Modeling for MPBR



MPBR-40

Current Design Schematic

97°C 8.0MPa

30°C 2.0MPa

803 °C 7.6MPa

401°C 2.1MPa

445°C 7.7MPa

850°C 7.6MPa



MPBR-41

Objective

• Develop a dynamic model as the primary tool for– developing the control system– performing transient analysis– optimizing power conversion system



MPBR-42

Summary• Have developed dynamic models for core ,

heat exchanger. A simple steady state turbo-machinery model is used.

• Verification of core kinetic model• Verification heat exchanger model• Provide control methods and use PID

controller to implement them



MPBR-43

Model StructureMPBRSimMPBRSim

Core ModelCore Model

T-HT-H KineticKinetic PoisoningPoisoning

Turbo-machineryTurbo-machinery Heat exchangerHeat exchangerControl ModelControl Model

Gas-GasGas-Gas Gas-LiquidGas-Liquid



MPBR-44

• Nodalization• Assumptions

– Fuel region is treated as a homogeneous region

– Helium heat conduction is negligible – The heat loss from the reactor vessel

is negligible• Pebble bed effective thermal

conductivity (GE correlation, taking into account both conduction and radiation).

• Helium convection:KFA correlation

Core Thermal-Hydraulic Model



MPBR-45

Core neutronics, product poisoning and verification

• One effective group point kinetics equation (Fig. 1)

• Fission product poisoning (Fig.2)• Temperature coefficient of reactivity(Doppler effect)

σaφ

FissionγXγI

135I 135Xe

136Xe

λXλI 135Cs



MPBR-46

Fig. 1 Numerical result versus analytical result for one effective group kinetics equation

0

1

2

3

4

5

0 5 10 15 20 25Time (sec)

P/P0

Nor

mal

ized

Pow

er

Analytical (0.22% Reactivity)

Analytical(-0.22% Reactivity)

Numerical (0.22% Reactivity)

Numerical (-0.22% Reactivity)



MPBR-47

Fig.2 135Xe buildup following the core shutdown

0

1

2

3

4

5

6

0 20 40 60 80 100

Time (Hour)

Nor

mal

ized

Con

cent

ratio

n

Xe-135 Concentration(Normalized)

I-135 Concentration(Normalized)



MPBR-48

Heat Exchanger Model

• Heat exchanger model– For counter flow heat exchanger– Divide it into many equally length sections– In each section, for gas side, ignore the gas mass,

therefore ignore the gas stored energy



MPBR-49

Fig. 3 Recuperator steady state temperature distribution

Recuperator steady-state temperature distribution

50

100

150

200

250

300

350

400

0 2 4 6 8 10 12 14 16 18

Axial Positions (m)

Tem

pera

ture

(C)

Hot SideSteel WallCold Side



MPBR-50

Fig. 4 Recuperator transient response to a step temperature increase of 200 C at hot helium inlet side

0

100

200

300

400

500

600

700

0 500 1000 1500 2000 2500

Tiem (sec)

Tem

pera

ture

(C)

Hot helium inletHot helium outletCold helium inletCold helium out



MPBR-51

Fig. 5 Helium energy storage’s effect on the recuperator performance

23

23.2

23.4

23.6

23.8

24

24.2

24.4

24.6

24.8

25

0 2 4 6 8 10 12 14 16

Hehlium Pressure (MPa)

Tim

e Co

nsta

nt (

sec)

Model including heliumenergy storageModel excluding heliumenergy storage



MPBR-52

Control Model

• Control methods– Control rod motion– Primary circulator speed– Bypass control in the power conversion system– Inventory control in the power conversion

system• Controller: PID



MPBR-53

Summary• Have developed dynamic models for core ,

heat exchanger. A simple steady state turbo-machinery model is used

• Verification of core kinetic model• Verification of heat exchanger model• Provide control methods and use PID

controller to implement them



MPBR-54

Future Work• Use programmed turbo-machinery

characteristic maps (Cooperate with NREC) in the model

• Develop valve model• Simulate electric load ramp



MPBR-55

Comparison Between Air and Helium for Use as Working Fluids in the Energy-

Conversion Cycle of the MPBR

Student: Tammy GalenFaculty: D. G. Wilson



MPBR-56

Objective:

Comparisons:•Cycle efficiency•Component design

•Size•Efficiency•Possible development work required

To assess the relative advantages of using air and helium for the working fluid in the MPBR energy-conversion cycle.



MPBR-57

ConclusionUse of helium in a closed-cycle configuration is best suited to the modularity requirement of the MPBR. It results in the smallest sized components, high efficiency, and implements well established turbomachinery technology.



MPBR-58

Three cycles investigated

Closed Cycle

HX

HX

TbC

Open Cycle

Tb

HX

C

•Air open cycle•Air closed cycle•Helium closed cycle



MPBR-59

Influence of choice of working fluid

Component Design

Development Work

Cycle Efficiency

Component Cost

Component Size

Component Efficiency

Working Fluid



MPBR-60

Thermophysical Properties of Helium and Air at 700K and 2MPa

Air Helium

Molecular weight 28.97 4.003

Specific heat (J/kgK) 1074 5193

Sonic velocity (m/s) 590 1500

Thermal conductivity(W/mK)

.056 .273

Viscosity (10-5) 3.33 3.56

Density (kg/m3) 9.96 1.42

Turbines and Compressors↑ Specific heat↑ Number of stages

↓ Viscosity, ↑Re #↑ Efficiency

↑ Sonic velocity, ↑ Compressor blade speed ↓ Mean diameter

↑ Density↓ Mean diameter

Heat Exchangers↑ Thermal conductivity↓ Size



MPBR-61

Turbomachinery industrial experience• Air open-cycle experience

– Majority of gas-turbomachinery experience– Combustion-gas turbines: power and aircraft industries

• Air closed-cycle experience– Less popular demand and less industrial experience to date– First built in 1939 by Escher Wyss in Switzerland– 20 plants built in Europe following WW2

ranging in power under 20MWe • Helium experience:

– 2 facilities part of HHT international high temperature gas reactor project• Oberhausen II (50MWe, 2MPa), HHV(90MWe, 5MPa)

– Circulators from previous gas reactors



MPBR-62

Industrial experience conclusions•Open-cycle air plant

•Least amount of development work required for MPBRdeployment

•Closed-cycle helium and closed-cycle air plants•Comparable amount of development work required•Technology has been tested and judged successful*•Open-cycle turbomachinery design methodology and operating experience is directly applicable

*IAEA Summary report on technical experiences from high-temperature heliumturbomachinery testing in Germany, 1995



MPBR-63

Busbar Efficiency

• Deducts–Primary circulator work, 7.7 MW–3.5 MW station load–98% electric generator efficiency



MPBR-64

Baseline Thermodynamic ComparisonHelium Air closed Air open

Pressure ratio 3.0 4.7 4.6

Busbar efficiency 45.2% 46.9% 47.7%

Reactor Tin (K) 767 792 797

Turbine Tin (K) 1101 1101 1101

Reactor inlet temperature constrained to 718K*Helium Air closed Air open

Pressure ratio 3.7 7.4 7.3Busbar efficiency 44.8% 46.0% 46.8%Turbine Tin (K) 1101 1101 1101

*Permissible using 9Cr-1-Mo-V as the vessel material according to ASME Class 2 &3 pressure-vessel in Codes Case N-47



MPBR-65

Turbine Design Choices

Maximum blade speed at tip (m/s) 550

Work Coefficient (Ψ) 1 and 2

Flow coefficient (φ) 0.5

Reaction (%) 50

Hub shroud ratio at turbine outlet 0.6



MPBR-66

Component volumes (m3)Helium Air-closed Air-open

IHX 17 86 260HP turbine 0.27 0.06 2.2LP turbine 0.58 0.22 13LP compressor 0.35 0.13 2.7Recuperator 13 61 180Precooler 50 97 ---Intercooler 1 41 80 180Intercooler 2 41 80 180Sum 160 400 820



MPBR-67

Final Cycle ConclusionsHelium Air-closed Air-open

Pressure ratio 3.7 7.1 7.1Busbar efficiency 45.3% 47.1% 47.3%Turbine Tin (K) 1101 1101 1101Component volume (m3) 160 400 820Number of flatbed trucks* 1 2 N/A

*Based on volume analysis, carries all components except IHX and ducting



MPBR-68

ConclusionUse of helium in a closed-cycle configuration is best suited to the modularity requirement of the MPBR. It results in the smallest sized components, high efficiency, and implements well established turbomachinery technology.



MPBR-69



MPBR-70

Pebble-Bed Reactor PhysicsResearch at MIT

Julian Lebenhaft Professor Michael Driscoll



MPBR-71

OverviewStatement of progressProgram goalsNeutronic design methodology• MCNP/VSOP model of PBMR• Preliminary results

Proliferation resistance of PBMRPath forward



MPBR-72

ProgressMCNP4B benchmark of HTR-10 completedVSOP94 fuel management code implementedProcedure established for linking MCNP and VSOP for core neutronic calculationsMCNP/VSOP model of PBMR developed and preliminary analysis performedProliferation resistance of PBMR investigated



MPBR-73

Program Goals•• Establish a MonteEstablish a Monte--Carlo based methodology for Carlo based methodology for

modeling neutron and photon transport in PBRsmodeling neutron and photon transport in PBRs

•• Ability to model cores with fuel burnupAbility to model cores with fuel burnup

•• Integrated computerIntegrated computer--aided design toolsaided design tools

•• Perform preliminary design calculationsPerform preliminary design calculations

•• Investigate fuel cycles and assess their proliferation Investigate fuel cycles and assess their proliferation resistanceresistance

•• Validation of codes and methodsValidation of codes and methods



MPBR-74

Neutronic Design Methodology

• Monte Carlo method for accurate analysis of:– control rod worth– void effects– geometry changes– shielding– heat deposition

• Diffusion-theory code for burnup calculation

diffusion code

Monte Carlocode

compositionof

fuel

A versatile design tool!



MPBR-75

Why MCNP?

• Solves the Boltzmann equation by the Monte-Carlo method

• Exact representation of neutron and photon transport

• Continuous-energy cross sections• Detailed geometry specification• Millions of n-histories used

to generate ensemble averages

accurate calculations



MPBR-76

VSOPVery Superior Old Programs

• Widely used for PBRs• Diffusion theory• Comprehensive code suite:

- Neutronics- Thermal hydraulics- Fuel cycles- Economics

• Installed at MIT• Verification in progress



MPBR-77

MCNP/VSOP Model of PBMR

Detailed MCNP4B model of ESKOMPebble Bed Modular Reactor:• reflector and pressure vessel• 18 control rods (HTR-10) • 17 shutdown sites (KLAK) • 36 helium coolant channels

Core idealization based on VSOPmodel for equilibrium fuel cycle:• 57 fuel burnup zones• homogenized compositions



MPBR-78

MCNP4B Model of Core

reflectorcoolantcontrolreflectorfuel

shutdownpressurevessel



MPBR-79

MCNP4B Model of Control Rod

graphite

s/s

B4C

helium

horizontal view

verti

cal v

iew



MPBR-80

MCNP4B Model of Shutdown Site

1

2

3

1) empty channel2) channel filled with

absorber balls3) 0.25 in. absorber ball

with graphite coating



MPBR-81

MCNP4B/VSOP Model Output

Top ofcore

top

0

161

242

402

483

644

725

0 108 13

4 175

0

1

2

3

4

5

6

7

8

MW

/m3

Axial Position (cm)

Radial Position

(cm)

Power Density in PBMR Equilibrium CoreControl Rods 1/4 Inserted (z = 201.25 cm)

7.00E+00-8.00E+006.00E+00-7.00E+005.00E+00-6.00E+004.00E+00-5.00E+003.00E+00-4.00E+002.00E+00-3.00E+001.00E+00-2.00E+000.00E+00-1.00E+00

Power density in annular core regions



MPBR-82

MCNP4B/VSOP Model Output .. 2

bottom

top

Average fluxin annularcore regions

Bottomof core

016124

240248

364472

5

0

73

108

134

156

0.E+00

5.E+13

1.E+14

2.E+14

2.E+14

3.E+14

3.E+14

4.E+14

Neu

tron

s.cm

-2. s

-1

Axial Position (cm)Radial

Position (cm)

Thermal Neutron Flux in PBMR Equilibrium CoreControl Rods 1/4 Inserted (z = 201.25 cm)

3E+14-3.5E+142.5E+14-3E+142E+14-2.5E+141.5E+14-2E+141E+14-1.5E+145E+13-1E+140-5E+13


bottom



MPBR-83

MCNP4B/VSOP Model Output .. 3

bottom

Bottomof core

08116124

232240

348356

464472

5805

073

108

134

156

0.E+00

2.E+13

4.E+13

6.E+13

8.E+13

1.E+14

1.E+14

1.E+14

2.E+14

2.E+14

2.E+14

Neu

tron

s.cm

-2.s

-1

Axial Position (cm)

RadialPosition

(cm)

Fast Neutron Flux in PBMR Equilibrium CoreControl Rods 1/4 Inserted (z = 201.25 cm)

1.8E+14-2E+14

1.6E+14-1.8E+14

1.4E+14-1.6E+14

1.2E+14-1.4E+14

1E+14-1.2E+14

8E+13-1E+14

6E+13-8E+13

4E+13-6E+13

2E+13-4E+13

0-2E+13


bottomof core



MPBR-84

NonproliferationPebble-bed reactors are highly proliferation resistant:• small amount of uranium (9 g/ball)• high discharge burnup (100 MWd/kg)• TRISO fuel is difficult to reprocess• small amount of excess reactivity limits

number of special production balls

Diversion of 8 kg Pu requires:

• 260,000 spent fuel balls – 2.6 yrs• 790,000 first-pass fuel balls – 7.5• ~50,000 ‘special’ balls – 3

Spent Fuel

Pu238 5.5%Pu239 24.1Pu240 25.8Pu241 12.6Pu242 32.0

First Pass

Pu238 ~0 %Pu239 64.3Pu240 29.3Pu241 5.6Pu242 0.8



MPBR-85

MCNP4B Analysis of Heavy Balls

1 Ball with depleted U2 Surrounded by BCC

lattice of driver balls3 Spherical driver core

with fuel compositionfrom VSOP model

• Critical core (keff = 1)• 238U(n,γ)239U

2

R = 1.75 m

UO2

driverfuel

R = 2.1 cm

1

3

Supercell Model



MPBR-86

Path ForwardPath Forward•• Complete implementation of MCNP/VSOP linkComplete implementation of MCNP/VSOP link•• Improve model of PBMR Improve model of PBMR ⇒⇒ ESKOM inputESKOM input•• Validate MCNP4B using PROTEUS criticals &Validate MCNP4B using PROTEUS criticals &

VSOP predictions of PBMR startup coreVSOP predictions of PBMR startup core

•• Investigate proliferation resistance of PBR fuelInvestigate proliferation resistance of PBR fuelcycles using VSOP (Th/LEU, OTTOcycles using VSOP (Th/LEU, OTTO--PAP2)PAP2)

•• Assess applicability of Monteburns to modelingAssess applicability of Monteburns to modelingburnup in PBRs using MCNP4B and ORIGEN2burnup in PBRs using MCNP4B and ORIGEN2



MPBR-87



MPBR-88

The Safety Analysis of PBMR

Student: Tieliang ZhaiAdvisors: Prof. A. Kadak

Dr. W.Y. Kato



MPBR-89

Objectives• Identification of the Safety Issues for MHTGR

• Analysis of a Major Accident

• The Temperature Distribution Calculation After the Depressurization With the Failure of RCCS



MPBR-90

The Identification of the Safety Issues

• Review of Safety Literature on HTGR• Based on a Specific Design Concept, Identified the

Critical Safety Issues:A. Fuel PerformanceB. Completely Passive System for Ultimate Heat SinkC. Air IngressD. Lack of ContainmentE. Calculation of Source Term



MPBR-91

Air Ingress Accident• Important parameters governing these reactions:

Gas flow ratesTemperaturePressure

• 3 steps• Mainly Depends on Possibility of Enough and

continuous air supply



MPBR-92



MPBR-93

Specific areas for additional research:

• Chimney effect• Airflow rate• The extent that a below-grade confinement building

could limit the supply of air



MPBR-94

The Temperature Distribution Calculation After the Depressurization With the Failure of RCCS

• The Code “Heating-7”Heating-7 is a general-purpose 3-D

conduction heat transfer program written in Fortran 77. It can solve steady-state and/or transient heat conduction problems



MPBR-95

The Model Established• 20-Region 3-Dimensional model. The core void is filled with

stagnant helium at atm.• The air in the confinement is steady and heat transfer only through

conductivity and radiation.• The outer radius of this model is 28.4m, i.e. all the heat transfer

happen only in this huge volume.• The Materials and their thermal properties (Conductivity, Density

and Specific Heat): In the case that the thermal properties could not been fully determined, the conservative values would be selected



MPBR-96



MPBR-97

The Conclusion• The core peak temperature is 15570C. This is below the 1600 C SiC fuel

temperature target and No Core Melt is predicted assuming only conduction.

• 192 hours (8 days) later, the peak temperatures of the pressure vessel and the Concrete Wall will be 1348 °C and 1322°C , respectively. The temperatures of the vessel and the concrete wall are out of the design capability range

.• The sensitivity analyses of the initial temperature of the core, the conductivity

of the air, the initial temperature of the air gap, the conductivity of the concrete wall and the soil indicate that: The key factors that determine the temperatures of the pressure vessel and concrete wall are the conductivity and heat capacity of the concrete and soil.

• Some convection cooling will be required to keep the temperature of the vessel and concrete within acceptable ranges - conduction alone is not sufficient.



MPBR-98

The Temperature Curve of the Hot-Points

0

200

400

600

800

1000

1200

1400

1600

0 50 100 150 200 250 300 350 400 450 500

Time (hr)

Tem

pera

ture

(C)

the Hot-Point ofthe Core

The Hot-Point oftheVessel

The Hot-Point oftheConcreteWall



MPBR-99

0

0.4

0.8

1.2

1.6

2

2.4

2.8

3.2

4.4

6.4

8.3 7.

9 7.5 7.

1 5.5 3.

5 1.5 -0.5 -2

.5 -4.5 -6

.5 -7.3 -7

.7 -8.1

0

200

400

600

800

1000

1200

1400

1600

Tem

pera

ture

(C)

Distance to the Central Line of the Core r (m)

The Depth Z (m)

Fig-3: Temperature Distribution on the 8th Day

1400-16001200-14001000-1200800-1000600-800400-600200-4000-200



MPBR-100

200

400

600

800

1000

1200

1400

0 50 100 150 200 250 300 350

Vessel Temperature (v=0.0)Vessel Temperature (v=0.5)Vessel Temperature (v=1.0)Vessel Temperature (v=2.0)Concrete Temperature (v=0.0)Concrete Temperature (v=0.5)Concrete Temperature (v=1.0)Concrete Temperature (v=2.0)

Tem

pera

ture

(K)

Time(Hrs)

Temperatures with Convection



MPBR-101

Future Work

• The perfection of the model, especially how to

model the core of the reactor and the heat transfer

through convection and how to obtain the accurate

initial condition of the accident.

• Air ingress accident simulation.

modular high temperature pebble bed reactor

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