anp-10288np, revision 2, 'u.s. epr post-loca boron ... · revision 2 u.s. epr post-loca boron...

AAREVA

U.S. EPR Post-LOCA BoronPrecipitation and Boron Dilution

ANP-10288NPRevision 2

Technical Report

June 2013

AREVA NP Inc.

(c) 2013 AREVA NP Inc.

AREVA NP Inc. ANP-10288NPRevision 2

U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Renort Paaei

Nature of Changes

Revi.Num

sion Section(s) orber Page(s)0 All1 Abstract

NomenclatureSection 1.0

Section 2.0and allsubsectionsFigures 2-7through 2-10

Figure 2-19

Section 3.0and allsubsections

Section 3.1

Section 3.2Section 3.3,Table 3-1,Table 3-2, &Table 3-3.

Section 4.02 All

Description and JustificationOriginal issueEditorial clarifications

Added acronyms needed from new text

Editorial clarifications

Revised to incorporate the updated SBLOCA and LBLOCAboron precipitation analysis

Replaced for updated SBLOCA analyses

Figure 2-19 replaced with new boron precipitation curves

Editorial clarifications

Modified to focus on the key conclusions of the PKL tests.Modified to include PKL III F1.1 test results.

Added section for the CFD analyses

Added editorial clarification and added results from anadditional neutronics analysis

Added reference 6

Update consistent with RAI 403 responses


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Page ii

ContentsPaqe

1.0 INTRO DUCTIO N ............................................................................................... 1-1

2.0 BO RO N PRECIPITATIO N ................................................................................. 2-1

2.1 Introduction ............................................................................................. 2-1

2.2 LO CA Categories .................................................................................... 2-1

2.3 Transient Evolution ................................................................................. 2-2

2.3.1 Large Break LO CA ....................................................................... 2-2

2.3.2 Sm all Break LO CA ....................................................................... 2-4

2.4 Boron Concentration Analysis ................................................................. 2-5.

2.4.1 Boron Solubility Lim it .................................................................... 2-62.4.2 Boron Concentration Equation ..................................................... 2-72.4.3 Injected Boron Concentration ....................................................... 2-72.4.4 Core M ixing Volum e ..................................................................... 2-82.4.5 Steam ing Rate ............................................................................. 2-82.4.6 Time-Dependent Calculation of Boron Concentration .................. 2-92.4.7 Boron Precipitation Conclusions ................................................ 2-10

3.0 BO RO N DILUTIO N ............................................................................................ 3-1

3.1 Introduction ............................................................................................. 3-1

3.2 Analytical Approach ................................................................................ 3-1

3.3 Physical Phenomena Relative to the Boron Dilution Event ..................... 3-23.3.1 Transition from Forced Circulation to Two-phase

Natural Circulation ........................................................................ 3-23.3.2 Interruption of Natural Circulation ................................................. 3-33.3.3 Refill and Resum ption of Natural Circulation ................................ 3-33.3.4 Transport of Deborate towards the Core Inlet .............................. 3-4

3.4 PKL Tests ............................................................................................... 3-43.4.1 Refill and Restart Dynam ics ......................................................... 3-7

3.5 System Code Analyses ........................................................................... 3-9

3.6 JULIETTE Tests for CFD M odel Q ualification ....................................... 3-13

3.7 CFD Analyses ....................................................................................... 3-14

3.8 Critical Concentration ............................................................................ 3-16


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Page iii

Contents(continued)

Pa3e

3.9 Recriticality Evaluation ...................................................... 3-16

3.10 C onservatism s ...................................................................................... 3-17

3.11 Boron D ilution C onclusion ..................................................................... 3-17

4 .0 R E F E R E N C E S .................................................................................................. 4-1


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Page iv

List of Tables

Paqe

Table 2-1: Solubility Lim it Table for Boric Acid .......................................................... 2-10

Table 2-2: Core Mixing Volume for LBLOCA ............................................................. 2-11

Table 3-1: U.S. EPR CFD Input Comparison to Selected S-RELAP5 Case .............. 3-18

Table 3-2: U.S. EPR Minimum Core Inlet Concentration-Critical ConcentrationC o m pa riso n .......................................................................................... 3-19

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U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report


Paae v

List of Figures

Page

Figure 2-1: Integrated Flow from the Upper Plenum to the Hot Legs - LBLOCA ...... 2-12

Figure 2-2: Integrated Flow from the Core Regions to the Upper Head -L B LO C A ................................................................................................ 2-13

Figure 2-3: Integrated Flow through the Core Bypass Regions - LBLOCA ............... 2-14

Figure 2-4: Integrated Flow from the Lower Plenum Flow to the Core Regions -L B L O C A ................................................................................................ 2 -15

Figure 2-5: Integrated Flow from the Lower Head to Lower Plenum - LBLOCA ....... 2-16

Figure 2-6: Integrated Break Flow- LBLOCA ........................................................... 2-17

Figure 2-7: Water Entrainment into the Loops after Hot-Leg Injection Initiation -L B LO C A ................................................................................................ 2 -18

Figure 2-8: Pressurizer Pressure and Steam Generator 1 Secondary Pressure -

6 .5-Inch B rea k ...................................................................................... 2-19

Figure 2-9: Integrated Flow from Upper Plenum to the Hot Legs - 6.5-Inch Break... 2-20

Figure 2-10: Integrated Flow from the Core Regions to the Upper Plenum -6 .5-Inch B re a k ...................................................................................... 2-2 1

Figure 2-11: Integrated Flow from the Lower Head to the Lower Plenum -6 .5-Inch B rea k ...................................................................................... 2-22

Figure 2-12: Pressurizer Pressure and Steam Generator Pressure - 2-Inch

B re a k .................................................................................................... 2 -2 3

Figure 2-13: Core Exit Node Void Fraction - 2-Inch Break ....................................... 2-24

Figure 2-14: Hot Fuel Assembly Collapsed Liquid Level - 2-Inch Break .................. 2-25

Figure 2-15: Inlet Flow Rate at the Reactor Coolant Pumps - 2-Inch Break ............. 2-26

Figure 2-16: S olubility Lim it ....................................................................................... 2-27

Figure 2-17: Time Dependent Boron Concentration - LBLOCA ................................ 2-28

Figure 2-18: Margin to Solubility Limit - SBLOCA ..................................................... 2-29

Figure 3-1: Inherent Boron Dilution Analytical Methodology ...................................... 3-20

Figure 3-2: P K L III Test Facility ................................................................................. 3-21

Figure 3-3: PKL Test E2.2 Loop Flows Prior to RNC ................................................ 3-22


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Page vi

List of Figures(continued)

Page

Figure 3-4: PKL Test F1.1 Loop Flow Rates Prior to RNC ........................................ 3-23

Figure 3-5: 1.60-Inch Break: Primary and Secondary Pressures .............................. 3-24

Figure 3-6: 1.60-Inch Break: RCS Inventory ............................................................. 3-25

Figure 3-7: 1.60-Inch Break: SG Apex Liquid Flow ................................................... 3-26

Figure 3-8: Mesh Used in the U.S. EPR CFD Analyses ............................................ 3-27

Figure 3-9: U.S. EPR CFD, 11 m3 - Minimum and Average Core InletC o ncentratio ns ...................................................................................... 3-28

Figure 3-10: U.S. EPR CFD - Minimum Core Inlet Concentration Slug SizeC o m pa riso n .......................................................................................... 3-28

Figure 3-11: U.S. EPR CFD, 11 m 3 - Core Inlet Concentrations at Minimum InletC oncentration T im e .............................................................................. 3-29

Figure 3-12: U.S. EPR CFD, [ ]- Core Inlet Concentrations at MinimumInlet C oncentration Tim e ....................................................................... 3-29

AREVA NP Inc. ANP-i 0288NPRevision 2

U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTec~hnir~l Renoot Pane viiTechnical Report Pane vii

Nomenclature

AcronymCFDEBSECCECCSGSIHLIRWSTLBLOCALHSILOCALHLPMHSIMSRTNCPCTPKL

PMPWRPZRRCPRCSRHRSRNCRPVRVSBLOCASFSISGTSPUP

DefinitionComputational Fluid DynamicsExtra Borating SystemEmergency Core CoolantEmergency Core Cooling SystemGeneric Safety IssueHot LegIn-Containment Refueling Water Storage TankLarge Break Loss of Coolant AccidentLow Head Safety InjectionLoss of Coolant AccidentLower HeadLower PlenumMedium Head Safety InjectionMain Steam Relief TrainNatural CirculationPeak Cladding TemperaturePrimArkreislauf-Versuchsanlage (primary coolant loop testfacility)Preventative MaintenancePressure Water ReactorPressurizerReactor Coolant PumpReactor Coolant SystemResidual Heat Removal SystemRestart of Natural CirculationReactor Pressure VesselReactor VesselSmall Break Loss of Coolant AccidentSingle FailureSafety InjectionSteam GeneratorTrisodium PhosphateUpper Plenum


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Paqe viii

ABSTRACT

This technical report provides an evaluation of two phenomena that potentially occur

following loss-of-coolant accidents (LOCAs) in the U.S. EPR plant:

* Boron precipitation

" Boron dilution during a small break LOCA (SBLOCA)

Boron Precipitation

During the pool boiling phase that follows the initial mitigation of all but the smallest

LOCAs, boron could concentrate in the reactor core region if countermeasures are not

taken. If the concentration of boron reaches the solubility limit, boron precipitates out of

solution and potentially causes coolant channel blockage. The design of the U.S. EPR

plant provides the operator with two actions that prevent the concentration of boron from

reaching the solubility limit:

" Continue the initial automatic cooldown of the steam generators to cool and

depressurize the reactor coolant system (RCS) for small breaks. This allows the

low head safety injection (LHSI) system to refill the RCS and establish natural

circulation with a small enough break.

* Redirect part of the LHSI to the RCS hot leg. The simultaneous injection into

both hot and cold legs limits the boron concentration in the core, regardless of

break location, and prevents the concentration of boron from reaching the

solubility limit when the breaks are too large to establish natural circulation.

This report demonstrates that these measures are adequate to prevent boron

precipitation.


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Page ix

Boron Dilution

Generic Safety Issue 185 (GSI 185) identified a concern that, during an SBLOCA event,

deborated water could accumulate due to the condensation of steam in the SGs and, be

transported to the reactor vessel and core when circulation is restored, potentially

causing a recriticality and fuel damage. This report presents test data from the PKL

integral test facility and U.S. EPR analyses to demonstrate that sufficient reboration

occurs such that recriticality will not occur.


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Page 1-1

1.0 INTRODUCTION

The reactor coolant system (RCS) contains boric acid at a concentration that maintains

the core subcritical with all but the most reactive rod inserted in the core following a

loss-of-coolant accident (LOCA). The required boron concentration varies with time

during an operating cycle. The boric acid may concentrate in the core region during the

long-term pool boiling phase of a LOCA, potentially reaching the point where it

precipitates causing coolant channel blockage. Because the boric acid concentrates

slowly, the operator has time to act to prevent precipitation.

For the small break sizes, the operator prevents excessive boron concentration in the

core region by establishing natural circulation (NC). Natural circulation is achieved by

cooling the RCS with the steam generators (SGs) to reduce pressure so that the

medium head safety injection (MHSI) and low head safety injection (LHSI) pumps can

refill the primary loops. For the larger break sizes, the operator prevents excessive

boron concentration in the core region by diverting a portion of the LHSI to the hot leg.

Delivery of emergency core cooling system (ECCS) water to the core from both the hot

legs and cold legs eliminates the potential for boric acid precipitation during a large

break LOCA (LBLOCA), regardless of break location.

Generic Safety Issue 185 (GSI 185) identified a concern that, during an SBLOCA event,

deborated water could accumulate due to the condensation of steam in the SGs and be

transported to the reactor vessel and core when circulation is restored, potentially

causing a recriticality and fuel damage. Test data and analyses demonstrate that

sufficient reboration occurs such that recriticality will not occur.



2.0 BORON PRECIPITATION

2.1 Introduction

Boron concentrates in the core region during the continued pool boiling that follows the

initial recovery phase of a LOCA. Boron precipitates when its concentration exceeds

the solubility limit. The U.S. EPR plant incorporates two design features that limit boric

acid concentration in the core to levels which preclude precipitation:

" Main steam relief trains (MSRTs). The MSRTs depressurize the SGs during a

small break LOCA (SBLOCA), which cools and depressurizes the RCS allowing

for the safety injection (SI) shutoff head to be reached. The SI flow increases

with the continued depressurization of the RCS. The refill of the RCS by the SI

allows natural circulation to be reestablished. A partial cooldown is initiated

automatically on a safety injection signal. This action depressurizes the SGs to

870 psia at a rate corresponding to 180°F/h. Operator action continues the

partial cooldown of the SGs.

* LHSI realignment allows the operator to redirect a portion of the flow to the hot

legs. This action prevents the concentration of boron from reaching the solubility

limit, regardless of break location.

S-RELAP5 analyses are performed to evaluate the plant thermal-hydraulic behavior

during the post-LOCA period that is potentially susceptible to boron precipitation.

Bounding calculations of the boron concentration demonstrate that there is sufficient

margin to the solubility limit.

2.2 LOCA Categories

The LBLOCA is characterized by a rapid inventory depletion followed by lower plenum

refill and core reflood from the accumulators and pumped SI. The RCS pressure falls to

the containment pressure. The break flow matches the injected flow and the pool

boiling period ensues. In the SBLOCA, there is not a sudden emptying, refill, and



transition to pool boiling period. The size of the break dictates the inventory loss and

the initial depressurization. SBLOCA breaks on the larger side of the spectrum behave

similarly to a LBLOCA. With these breaks, the RCS depressurizes rapidly, the core

level drops and refills from the pumped injection and accumulators. Smaller SBLOCAs

retain more inventory, have less pressure drop, and have longer periods of natural

circulation. With continued natural circulation, the decay heat and stored energy can be

removed by the coolant flow through the core. Furthermore, during this time period, the

concentrating volume is comprised of the entire RCS; therefore, any steaming which

does occur has a negligible impact on the RCS boron concentration. Once natural

circulation ceases, there is insufficient coolant flow through the core and the heat

cannot be removed except by boiling. The core continues to be fed by the water from

the downcomer, extra borating system (EBS) injection (once manually initiated), and

ECCS injection (once it reaches the shutoff head) and the concentration increases.

Also, as opposed to the LBLOCA, the SBLOCA event goes through a range of

concentrating volumes for several hundred seconds due to the break and the delayed

injection. There are three categories of LOCA relative to the boron precipitation

long-term cooling scenario:

* LBLOCA.

" SBLOCA that rely on hot leg injection.

" SBLOCA that re-establishes natural circulation.

2.3 Transient Evolution

2.3.1 Large Break LOCA

2.3.1.1 Cold Leg Breaks

LBLOCA events are evaluated using the approved realistic LBLOCA methodology

described in the U.S. EPR Realistic Large Break Loss of Coolant Accident Topical

Report (Reference 1).



The post-LOCA phenomena associated with boron concentration in the core is

independent of break size once the break is too large for the LHSI to refill the loops. A

representative LBLOCA case is analyzed to demonstrate the effectiveness of hot leg

injection for break sizes too large for the MHSI and LHSI to refill the loops.

In the analysis depicted in the following figures, the operator is assumed to switch to hot

leg injection at one hour. As seen in Figure 2-1, the switch to hot leg injection causes

flow to reverse from the hot legs that receive hot leg injection back into the upper

plenum. Flow reversal is indicated when the slope becomes negative. The flow

proceeds down the peripheral region, the guide tubes, and the heavy reflector into the

lower plenum as seen in Figure 2-2 and Figure 2-3. Forward flow continues into and

out of the hot assembly, the central core region, and the average core and into the two

loops without hot leg injection. Figure 2-4 shows the reverse flow into the lower plenum

from the peripheral region and the forward flow to the central regions. This upper

plenum and core flow behavior is similar to that observed in test facilities with hot leg

injection, such as the Slab Core Test Facility, the Cylindrical Core Test Facility, and the

Upper Plenum Test Facility. Approximately 75 percent of the peripheral region flow

mixes with the other core regions, with the remainder penetrating into the lower plenum.

Downflow into the lower plenum penetrates further into the lower head, as seen in

Figure 2-5. The flow reverses to the downcomer, increasing the flow out of the vessel

side of the break (Figure 2-6). This removes the concentrated boron that accumulated

prior to the initiation of the hot leg injection.

The steam flows to the SGs have the potential to entrain a portion of the LHSI injected

into the hot legs. The amount of water entrained as predicted by the Wallis correlation

is shown in Figure 2-7. The maximum percentage of LHSI water entrainment into the

loops after the initiation of hot leg LHSI injection at one hour is lower than 15 percent,

except for a brief peak value slightly below 45 percent. The prediction does not take

credit for de-entrainment in the hot leg bend or the critical velocity required for droplet

suspension in the steam generator inlet plenum. The brief periods and small



magnitudes of entrainment do not degrade the effectiveness of the core penetration.

Furthermore, analyses assuming degraded flow rates with bounding entrainment

percentages for the entire hot leg injection period show the same flow patterns which

remove the concentrated boron.

2.3.1.2 Hot Leg Breaks

The large hot leg break scenario results in high circulation of flow through the reactor

vessel and liquid entrainment from the reactor vessel out the break which mitigates the

accumulation of boron in the core regions. The ECCS is initially all injected via the cold

legs. All the ECCS flows through the core towards the break providing continuous

dilution. After the redirection of a majority of the LHSI to the hot leg, a portion of the

ECCS continues to flow into the cold legs. The remaining cold leg injection and the hot

leg flow in excess of the break flow prevent the concentration of boron in the core.

2.3.2 Small Break LOCA

SBLOCA events are evaluated using the approved S-RELAP5-based methodology

described in the Codes and Methods Applicability Report for the U.S. EPR

(Reference 2).

2.3.2.1 Larger Small Break LOCAs that Rely on Hot Leg Injection

The limiting peak cladding temperature (PCT) SBLOCA break size, 6.5 inches, was

analyzed assuming redirection of a majority of the LHSI to the hot leg and continued

cooldown of the steam generators at 90 0F/h at 1800 seconds. This break size is

representative of this category of SBLOCAs. The large break size results in a pressure

reduction of the RCS significantly below that of the secondary side (Figure 2-8). As a

result of the LHSI redirection, a hot leg injection flow pattern develops that returns the

LHSI flow from the loops receiving hot leg injection to the upper plenum and down the

peripheral regions of the core. Figure 2-9 shows the reversal of the redirected flow in

Hot Legs 1 and 4 into the upper plenum. Figure 2-10 shows that the hot leg injected



water continues to flow from the upper plenum, further reversing the fuel assembly flow

in the peripheral region of the core. Some of the downflow continues down through the

lower plenum to the lower head region (Figure 2-11). Thus, the hot leg injection is

effective at penetrating the core, thereby reducing the boron concentration in the core

region and preventing precipitation of boron in the core.

2.3.2.2 Small Breaks that Re-Establish Natural Circulation

For the smaller end of the SBLOCA spectrum, two trains of ECCS can refill the RCS

and re-establish natural circulation. The transient evolution of a 2-inch diameter break

size case is presented as representative of this range of break sizes. Figure 2-12

compares RCS and SG secondary side pressures. This figure shows the automatic

partial cooldown at 180°F/h, followed by the operator initiated cooldown at 90*F/h

starting at 1800 seconds. Primary system pressure increases at 9500 seconds, when

the LHSI refills and pressurizes the RCS.

The core outlet void fraction for this case (Figure 2-12) increases to 40 percent in

1500 seconds, then slowly increases to 50 percent at 7500 seconds, at which time the

LHSI starts to refill the RCS. As natural circulation restarts, the core outlet void fraction

drops to zero. The collapsed liquid level in the hot assembly (Figure 2-14) is

approximately 9 feet at 1500 seconds. It fluctuates near this level until 7500 seconds,

when two-phase natural circulation begins. The liquid level then increases to cover the

core at approximately 9000 seconds.

Figure 2-15 shows the flow through the RCP. Natural circulation stops at 1500

seconds. Two-phase circulation restarts at 7500 seconds. Single-phase flow is

established by 10,000 seconds.

2.4 Boron Concentration Analysis

This section establishes the allowable solubility limit for boron and determines the time

available to terminate the concentrating process.

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For conservatism, the analysis does not credit the following:

.[


Page 2-6

]

2.4.1

[Boron Solubility Limit

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Page 2-7

I

]

2.4.2 Boron Concentration Equation

The fundamental equation for the concentration of boron in the core region as a function

of time is:

]

The bases for the terms are described in the following sections.

2.4.3 Injected Boron Concentration

The U.S. EPR plant uses a minimum of 37 percent enriched B-10 boron for all sources

of borated water. In an LBLOCA, the SI flows are at their maximum due to the rapid

RCS pressure reduction. The boron concentration of the injected flow is maximized

using a flow rate weighted injection concentration with the limiting single failure (SF) and

preventative maintenance (PM) combination. It conservatively assumes that the EBS,

an operator-initiated system, is started at reactor trip. The maximum flow rate weighted



concentration of the injected flow is a constant, 2051 ppm. In an SBLOCA, since the

flow MHSI and LHSI are pressure dependent and the pressure reduces more slowly,

the injection concentration is a function of time.

2.4.4 Core Mixing Volume

In an LBLOCA, the RCS inventory rapidly decreases due to the break. The PCT

transient portion of event is terminated by refill from the combination of accumulators

and pumped SI. The RCS pressure is quickly reduced and the pumped SI is at its

maximum. A balance is reached between the break and the injected flow. During the

pool boiling period, the water in the regions in and around the core increases as the

decay heat decreases. The volume of water in the concentrating region is

conservatively set to a constant value based on the volume at the end of the PCT

transient. The five sub-regions comprising the liquid volume of the concentrating region

(core region, lower support plate to heated core, upper plenum to the bottom of the hot

legs, heavy reflector, and guide tubes) are included because of the recirculation flow

pattern which develops in the core following a LOCA. The volumes, based on a

representative LBLOCA case at the end of the PCT transient, are shown in Table 2-2.

Additional extended S-RELAP5 transient analyses and long-term cooling static balance

calculations have demonstrated the conservatism of this assumption.

As opposed to the LBLOCA event, the SBLOCA event goes through a range of

concentrating volumes for several hundred seconds due to the slower inventory

depletion from the smaller break and delays in the pressure-dependent SI. As such,

the SBLOCA volumes are a function of time. During the pool boiling period, the

concentrating volumes for the SBLOCA are larger and therefore less limiting than the

LBLOCA.

2.4.5 Steaming Rate

For the LBLOCA and SBLOCA boron concentration analyses, the decay heat is treated

as a time-dependent function based on the ANS 1971 draft decay heat standard



ANS 5.1-1971 (Reference 3) with a 20 percent uncertainty added. The steaming rate

can, therefore, be calculated as a function of time. The LBLOCA model is based on the

evaporation of saturated water at 212'F, a density of 59.81 Ibm/ft3, and a latent heat of

evaporation of 970.30 BTU/lbm. The model assumes that the boiling and concentrating

phase starts at the decay heat power level at reactor trip. The SBLOCA model includes

the fuel and passive metal stored heat and the water properties are a function of time.

Neither model includes sensible energy even though the water makeup from the IRWST

is subcooled.

2.4.6 Time-Dependent Calculation of Boron Concentration

2.4.6.1 Large Break LOCA

For the LBLOCA, the core region boron concentration is calculated to be approximately

27,000 ppm at one hour (Figure 2-17) when the operator would switch to hot leg

injection, thereby redirecting the majority of the LHSI flow to the hot legs. This reverses

net flow through the reactor vessel. With the large capacity of the LHSI, this conclusion

is not impacted by the entrainment predicted with the Wallis correlation. Without

crediting hot leg injection at one hour, the 14.7 psia/212°F solubility limit of 38,500 ppm

would be reached at 1.74 hours. When hot leg injection is initiated at one hour, the

concentration of boron in the core quickly decreases to approximately 2500 ppm.

2.4.6.2 Small Break LOCA

Because the solubility limit is a function of the pressure, the results are presented as a

margin to the limit. The margin to the limit is displayed in Figure 2-18. As can be seen,

the smaller the break, the greater the margin throughout the transient. Without hot leg

injection or a restart of natural circulation and assuming the RCS depressurizes to

atmospheric pressure, the limit is reached at approximately 3.75 hours for the 2.8-inch

break and 3.5 hours for the 6.5-inch break. For the 2.8-inch break, the results with only

two ECCS trains available for refill show a time to restart of natural circulation around

3.1 hours. In both cases, the hot leg injection flow is sufficient to provide excess ECCS

to penetrate and dilute the core well before the limits are reached.

AREVA NP Inc.

U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTec~hnical1 R*ennrt


Pane 2-10............ R pr t .... . 2-10. .

Since the LBLOCA boron precipitation time is 1.74 hours, the SBLOCA is bounded by

the LBLOCA.

2.4.7 Boron Precipitation Conclusions

The design of the U.S. EPR plant is adequate to prevent the precipitation of boron for

the entire spectrum of LOCAs. For LBLOCAs, the operator has approximately

1.74 hours to initiate hot leg injection in order to terminate the concentrating process.

For SBLOCA breaks small enough to refill and establish natural circulation, the

manually initiated depressurization of the SGs following the automatic partial cooldown

is adequate to establish this circulation before the boron solubility limit is reached. For

the larger SBLOCAs, hot leg injection terminates the concentrating process. For both

categories of small breaks, the time to precipitation is bounded by the LBLOCA.

Table 2-1: Solubility Limit Table for Boric Acid

- Notes:

1. [

]


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Renort Paae 2-11

Table 2-2: Core Mixing Volume for LBLOCA

Region Volume LBLOCA LiquidVoid Volume(f3)Fraction[I] (ft')

975.8 0.635 356.2

127.5 0 127.5

363.5 0.992 2.9

37.8 0.633 13.9

93.4 0.552 41.8

Totals 1598 542

Notes:

1. The void fractions are representative void fractions for each of the five sub-regions. Liquid volumes

within the detailed nodal model of the sub-region are determined and then void fractions

representative of that liquid volume within a sub-region are calculated.

AREVA NP Inc. U.S.EPR ost-OCA oronANP- 10288NP

U-S. EPR POst-LOCA Boron Precipitation and Boron Dilution Revision 2

Technical Re oil

Figure 2-1: Integrated Flow from the Upper Plenum to the Hot Legs -

LBLOCA

400

200 . . . -. ..---

-o A

It

07 -2000

4'

S-400

-600

-800

-10000 3000 4000

Time (sec)5000 6000


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Reoort DarM MI1,1

if

Figure 2-2: Integrated Flow from the Core Regions to the UpperHead - LBLOCA

10000

5000

/,/

E0O

LL0

CDO'D

- I=

0 - --- - 8 8 fl-ý- ~- -

-

N

-5000

E) Integrated Hot Assy to UP FlowIntegrated Central Core to UP Flow

- -- Integrated Average Core to UP Flow- ? Integrated Peripheral Core to UP Flow

~1

-100000 1000 2000 3000

Time (sec)4000 5000 6000

AREVA NP Inc. ANP- 0288NPRevision 2

U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Page2-14

Figure 2-3: Integrated Flow through the Core Bypass Regions -LBLOCA

0

100

0

-100

-200

-300

-400

-500

-600

-700

-800v-80 0 1000 2000 3000 4000 5000 6000

Time (sec)


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionT~e.hnie.~l Renort Paae 2-15

Figure 2-4: Integrated Flow from the Lower Plenum Flow to the CoreRegions - LBLOCA

E

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4000

3000

2000

1000

0

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-3000

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- -- - -- - -

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- - Integrated LP to Average Core Flow-- i,. Integrated LP to Peripheral Core Flow

0 1000 2000 3000 4000 5000 6000Time (sec)

AREVA NP Inc.

U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical ReDort


Paae 2-16

Figure 2-5: Integrated

1000

800 L

Flow from the Lower Head to Lower Plenum -LBLOCA

C).000

.0_0

()

0CU)

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400

200

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U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionThchnioal Renort Paae 2-17

Techncal epor Po 2-17

Figure 2-6: Integrated Break Flow - LBLOCA

4000

3000

E.00

4-2000

1000

00 1000 2000 3000

Time (sec)4000 5000 6000

AREVA NP Inc.

U.S. EPR Post-LOCA Boron Precipitation and Boron Dilution


Paae 2-18

Figure 2-7: Water Entrainment into the Loops after Hot-Leg InjectionInitiation - LBLOCA

45

40

35

30

250

15

10

% of Water Entrainment in Hot Leg 3 Nodes

I

hot leg

ode 310-2

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AREVA NP Inc. ANP-1 0288NPRevision 2

U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTp~r.h nirjAI Renort PAne 2-19Technical. ReotPn,-1 9

Figure 2-8: Pressurizer Pressure and Steam Generator I SecondaryPressure - 6.5-Inch Break

2000

1500

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CD 1000

500

00 1000 2000 3000 4000 5000 6000 7000

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U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTprhnir(Al Renort Paoe 2-20

-c-I

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-8000 1000 2000 3000 4000

Time (s)5000 6000 7000


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTpec.hnir.•l Renort Pane 2-21

Figure 2-10: Integrated Flow from the Core Regions to the Upper Plenum -6.5-Inch Break

20000

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2000

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AREVA NP Inc. AN P- 10288NPRevision 2

U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Reoort P nA 7-9:V

Pane 2-23

Figure 2-12: Pressurizer Pressure and Steam Generator Pressure -

2-Inch Break

2 inch Break2400

2000

1600

2 1200U)

800

400

00 2000 4000 6000 8000 10000 12000 14000

Time (sec)


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTp~nhnir.•l Re~nort Don m:)-A

Technical Report

Figure 2-13: Core Exit Node Void Fraction - 2-Inch Break

2 inch Break1.00

0.90

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U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTe~ch n ir•1 Renort Pann 9--r

Figuca Rp ret -4 o ulAsml ClasdLqi ee

Figure 2-14: Hot Fuel Assembly Collapsed Liquid Level -2-Inch Break

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AREVA NP Inc.

U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Renort

AN P-10288NPRevision 2

Paoe 2-26............ R prt ... .oe 2-26

Figure 2-15: Inlet Flow Rate at the Reactor Coolant Pumps -2-Inch Break

2 inch Break

(D

E.0

LLC.00-J

5000

4500

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3500

3000

2500

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U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical ReDort Paae 2-27

Figure 2-16: Solubility Limit

AREVA NP Inc.

U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTc.chnir.il R~nort


Paoe 2-28Technical Renort Pane 2-28

Figure 2-17: Time Dependent Boron Concentration - LBLOCA

70000

60000

50000

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40000

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20000

10000

0

-LBLOCA, No Hot Leg Inj.

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i ! i

0.0 1.0 2.0 3.0

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Figure 2-18: Margin to Solubility Limit - SBLOCA

140000

120000

100000

E80000

a60000

40000

20000

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000

Time (seconds)



3.0 BORON DILUTION

3.1 Introduction

GSI 185 identified a concern that, during an SBLOCA event, deborated water could

accumulate due to the condensation of steam in the SGs and be transported to the RV

and core when natural circulation is restored, potentially causing recriticality and fuel

damage. This event is typically referred to as inherent boron dilution. The transport

could also occur due to the restart of an RCP, but this is precluded by procedure for the

U.S. EPR plant.

There is a narrow range of break sizes in the SBLOCA spectrum which are susceptible

to this event. Breaks smaller than this range do not interrupt natural circulation and,

therefore, do not accumulate deborated water. Those larger than this range

depressurize quickly such that the secondary sides of the SGs become a heat source to

the primary system. Even if heat transfer is re-established to the SGs after they are

depressuized, the break is too large for the LHSI to refill the loops and restart natural

circulation.

The evaluation of the SBLOCA boron dilution event is described in this report.

3.2 Analytical Approach

While the true criteria for this event is the maintenance of long-term core coolability and

the prevention of fuel damage (10 CFR 50.46, Reference 5; GDC 28, Reference 6 and

Reference 7), a conservative decoupling criterion of no return to criticality is used. In

order to assess the potential for recriticality during the inherent boron dilution event a

combination of experimental test results, system code analyses, and computation fluid

dynamics (CFD) analyses are used. A schematic of the approach is shown in

Figure 3-1. The PKL test facility investigated the thermal hydraulic phenomena

associated with the event (Reference 8). System code analyses were performed with

S-RELAP5 to evaluate the particular performance of the U.S. EPR plant and the effects



of break size and systems availabilities on the transient evolution. However, neither the

PKL tests nor system code analyses can evaluate the mixing process during transport

following the restart of natural circulation (RNC). For this, mixing tests and CFD

analysis are needed. CFD analyses, when validated by tests, are capable of accurately

simulating the mixing processes in the cold legs, downcomer, and lower plenum, and

determining the boron concentration at the core inlet. The minimum boron

concentration at any location at the core inlet is then compared to critical concentration

for the U.S. EPR plant to ensure that there is no return to criticality. The use of this

criterion represents a large conservatism in preventing recriticality. It neglects the large

variations in the boron concentration across the core inlet, the increased concentration

in the core, and the mixing in the core. Additional conservatisms are discussed in

Section 3.10.

3.3 Physical Phenomena Relative to the Boron Dilution Event

Contrary to the classical evaluation of SBLOCA transients which are particularly

interested in the core uncovery phase, the SBLOCA transients for the boron dilution

event have a longer duration, lasting from one to six hours, depending on the break size

and cooldown rate. There are four characteristic phases of the accident relative to the

evolution of the boron dilution event:

3.3.1 Transition from Forced Circulation to Two-phase Natural Circulation

The phase of forced circulation comes to an end after the shutdown of the reactor

coolant pumps. The draining of the RCS continues because the break flow exceeds

any external compensation. This progressive draining causes greater evaporation of

the water due to a reduced flow rate to the core. The steam flow increases to the SGs,

but water is still entrained with the high steam flows. This is the two-phase natural

circulation period in which there is continuous flow of liquid over the SG apex and

through the system. Any condensate generated in the tubes of SGs is mixed with the

borated water in the RCS. The natural circulation flow maintains a rather homogeneous

boron concentration in the RCS.


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Paqe 3-3

3.3.2 Interruption of Natural Circulation

For large enough break sizes, the loss of system inventory is sufficient to cause the

interruption of natural circulation. First a period of intermittent flow arises in which liquid

continues to be carried over the SG apex with the steam flow. As the system inventory

continues to decline, a complete interruption of natural circulation occurs. It is indicated

by the loss of liquid flow at the SG tube apex. According to the PKL test results

(Reference 8), the accumulation of deborate during this phase begins when the RCS

liquid level falls below that of the SG tube bundle at the SG outlet. The very weakly

borated steam produced in the core passes through the hot legs into the SG tubes

where it is condensed. This mode of heat transfer by evaporation/condensation is

characteristic of a low system inventory and requires the cooldown by the secondary

side. The liquid resulting from the condensation on the upsides of SG tubes falls back,

countercurrent to the steam flow, towards the upper plenum mixing again with the

borated water in the upper plenum and core. The PKL test results demonstrated that

there was no accumulation of highly deborated liquid in the SG inlet plenum. On the

other hand, the accumulation of deborate in the SG outlet plenum and loop seal could

lead to the formation of deborated slugs.

3.3.3 Refill and Resumption of Natural Circulation

The filling of the RCS is achieved, for the smaller breaks, by MHSI alone; or for the

larger breaks, by the addition of the accumulators and/or the LHSI. Once the level

reaches the bottom of the SG tubes at the SG outlet, the carryover of liquid, as

demonstrated in the PKL test results, ends the phase of deborate accumulation. The

refill of the RCS results in a phase of intermittent flow followed by a resumption of

continuous natural circulation. The restart of natural circulation is necessary for the

transport of the deborate towards the reactor and the challenge to recriticality. The

phenomena in the refill phase leading up to the restart and the restart dynamics are

discussed in more detail in Section 3.4.1.



3.3.4 Transport of Deborate towards the Core Inlet

The final phase of the event is the transport of the deborated slug to the core inlet.

During this movement, the slug mixes in the cold legs with borated water injected from

the SI and accumulators and EBS, if present, and finally with the borated water in the

downcomer and lower plenum. This mixing raises the boron concentration before it

reaches the core.

3.4 PKL Tests

The PKL test facility is based on a typical four-loop PWR of German design

(Reference 8). The entire primary side, the most significant components of the

secondary side (excluding the turbine and condenser), and the appropriate system

technology, are represented (Figure 3-2). The major components heights on the

primary and secondary side are scaled with a 1:1 ratio to the reference plant. The

volumes, power, and mass flows are scaled with a ratio of 1:145. For some

components, the exact volume scaling is not applied in order to simulate certain thermal

hydraulic phenomena; for example, counter current flow limitation in the hot legs. This

allowed dimensionless numbers (e.g., the Froude number) to be maintained in the

correct parameter range. Due to its full-scale height and symmetric layout of the four

loops, the PKL test facility is well suited for the study of reflux condensation and natural

circulation phenomena.

Four tests relevant to the inherent boron dilution event were conducted with the PKL III

E and F test series. Test E2.2 and Test F1.1 were transient simulations. Test E2.2

simulated a cold leg break with asymmetric cold leg injection from only two safety

injection pumps. This is representative of the U.S. EPR design with two out of four SI

trains unavailable due to single failure and preventative maintenance assumptions. The

test had a very long reflux condenser period and excessive condensate production in

order to obtain the maximum possible volume of accumulated condensate. Test F1.1

also simulated a cold leg break, but with symmetric cold-side injection into all four loops.

A major focus of this test was to investigate the potential for simultaneous restart of


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Paqe 3-5

natural circulation. Test F1.2 and Test F4.1 were parametric studies that investigated

the relationship between the primary inventory drain and refill and the start and end of

reflux condensation and condensate accumulation. The matrix of tests allowed for

conclusive findings to be made relative to the boron dilution processes and to the refill

and restart of natural circulation.

While Test E2.2 had a long condensation period, the refill and restart of natural

circulation processes limited the amount of condensate which could accumulate as a

slug. [

J As soon as the refill allows for a level

above the SG outlet side tube sheet, transport phenomena over the SG apex enable

further disintegration and reboration of the slug. These transport phenomena were

investigated in detail and confirmed with the parametric study tests, Test F1.2 and

Test F4.1. Therefore, the size of the slug is limited to the loop seal and SG outlet

plenum.

The tests also concluded that natural circulation arises in different loops at different

times due to the inherent asymmetries between the tubes of a single SG and between

the refill processes of the individual loops. Test E2.2 demonstrated that the loop which

restarts first is a loop without injection. The colder water in the loop seal of the loops

with injected ECC creates a temperature and density difference which delays the restart

of natural circulation. The restart of natural circulation in one loop causes a decrease in

the steam production, which reduces the swell levels in the SGs in the other loops and

delays the restart of natural circulation in these loops. [

I



[ J Test

F1.1, which was set up for the most symmetric boundary conditions, confirmed the

single loop restart and delay to restart of natural circulation in the other loops.

The main conclusions from the test matrix are:

" In order for the accumulation of condensate, the water level must be below the

SG outlet tube sheet.

* The size of the deborated slug is limited to the volume of the loop seal and the

SG outlet plenum.

" The exchanges between the SG inlet plenum, hot leg, upper plenum, and core

region prevent the formation of a deborated slug on the SG upside.

* Natural circulation does not restart simultaneously, regardless of whether ECC

injection is symmetric or asymmetric.

" Natural circulation restarts first in loops without ECC.

" The first restart of natural circulation impedes the restart of natural circulation in

subsequent loops.

" During the refill phase, there are intermittent flows of borated water from the cold

leg back into the loop seal and from the inlet SG plenum, over the SG tubes

apex. These flows mix, reborate, and partly disintegrate the deborated slug prior

to its transport towards the core.

" Weakly borated slugs were observed only in loops without ECC.

A more detailed description of the evolution of the system refill, restart dynamics, and

the thermal-hydraulic phenomena that limit the slug size and preclude multiple restarts,

as evidenced in the PKL tests, is provided in the Section 3.4.1.



3.4.1 Refill and Restart Dynamics

The tests in the PKL boron dilution matrix allowed for an in-depth understanding of

characteristics and thermal-hydraulic processes which occur during the system refill and

lead up to the restart of natural circulation.

[I

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Paqe 3-9

3.5 System Code Analyses

Many thermal-hydraulic aspects of boron dilution, except the mixing effects, can be

analyzed by using system codes such as CATHARE and S-RELAP5. The mixing

processes from the loop seal to the core involve multidimensional flow effects which

typically are not modeled in system codes. Furthermore, these codes exhibit far too

much numerical diffusion to be useful for tracking a relatively sharp concentration

gradient around the system (Reference 9). Such system code analyses are used to


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Paaqe 3-10

look at the overall system evolution as a result of break sizes in the range of concern.

The depletion and refill characteristics of the transient are well captured by system

codes. Since the timing of the different phases in the boron dilution event evolution are

dependent on the system inventory, system codes can be used to provide an indication

of the critical times in the event. The system characteristics: pressure, temperature,

flow rates, etc. at these times are relevant. The condensation in SGs during the phases

can also be captured by the system code analyses.

Additionally, system code analyses can provide a means to compare the effects of

different break sizes, equipment availabilities, and operator actions. A spectrum of

cases was analyzed with S-RELAP5 to determine the effect of these parameters on the

transient evolution. The spectrum included a range of break sizes and different SF and

PM combinations. The system analyses, in combination with tests, can be used to

identify limiting characteristics of the inherent boron dilution event. The following

conclusions are drawn from those studies and the PKL tests:

" The PKL tests showed that the loop most probable to restart NC first will be a

loop without SI.

* The PKL tests showed that the size of the slug is limited to the volume of the loop

seal and the SG outlet plenum. For the EPR RCS, this is 11 M3 .

1 The break sizes in these ranges are large

enough that intermittent flow over the SG apex stops, but not so large as to

depressurize the system well below that of the secondary which allows for the

condensation and potential accumulation of deborated condensate. The break

size is also not so large as to require LHSI for the system refill and restart of

natural circulation. For the U.S. EPR plant, the LHSI flow rates greatly exceed



those of the MHSI which enhances the mixing and reboration. The LHSI trains

are also cross-connected, which in some SF/PM scenarios alleviates the concern

for a loop without direct boration.

" With respect to EFW, since the primary side cooldown rate for the U.S. EPR

plant is fixed, the total condensate generated in the primary side is approximately

the same regardless of the number of fed SGs. Once an un-fed SG dries out, no

more condensate will be produced in that loop.

" A slower operator-initiated cooldown results in longer times until the restart of

natural circulation and larger amounts of condensate. Physically, this is

counterbalanced by the extended period of SI injection and increased boron

concentration in the core. [

]

" The slower cooldown is tied to the operation of only one EBS pump. While the

time to restart is extended, the total amount of EBS injected at the time of restart

of continuous natural circulation is less.

A scenario that demonstrates these limiting characteristics was selected for additional

evaluation. The scenario selected was:

.[I



[

I The break size is large enough to

result in an extended period of time between the end of and restart of intermittent flows;

thereby maximizing the condensate, but small enough for the refill to occur without

LHSI.

In this case, the RCS pressure reaches the low primary pressure setpoint at

101 seconds and the reactor is tripped. With an assumed loss of offsite power, the

pumps trip and forced circulation stops. Steam is produced in the core and a period of

two-phase natural circulation begins. Any condensate generated during this period is

borated by the continued circulation of liquid flow through the system. The RCS

pressure continues to fall and reaches the low-low primary pressure setpoint at

710 seconds which actuates the SI system and initiates the automatic partial cooldown

at a rate of 180°F/hr (Figure 3-5). The MHSI flow increases as pressure decreases, but

it is not enough to compensate yet for the flow out the break. The RCS inventory

continues to decline (Figure 3-6) and continuous natural circulation is lost at

-1300 seconds (Figure 3-7). A period of intermittent flow begins in which the high

steam flow rates continue to carry borated liquid over the SG apex and mix with any

condensate. At -2000 seconds, intermittent flow stops and the period of single-phase

steam flow begins (Figure 3-7). This begins the reflux condensation period during

which deborated condensate could accumulate as a slug below the SG outlet tube

sheet. The RCS continues to cooldown and depressurize due to the operator-initiated

cooldown following the completion of the automatic partial cooldown. The ECC flow

eventually exceeds the break flow rate due the increase in MHSI flow and the

contribution from the accumulators. The minimum RCS mass occurs at -4300 seconds



and the system refill begins (Figure 3-6). Intermittent flow, in which borated liquid is

carried over the SG apex, starts at -9,300 seconds (Figure 3-7). Further increases in

the RCS level lead to the conclusion of the intermittent flow period and the first restart of

continuous natural circulation at -11,600 seconds Figure 3-8). System code analyses

cannot accurately evaluate the mixing process during transport following the restart of

natural circulation. As such, CFD analyses are used. Table 3-1 summarizes the

comparison of the parameters from this case to those used in the CFD analyses

described in Section 3.7.

3.6 JULIETTE Tests for CFD Model Qualification

The transport phase and the mixing that occurs in the cold leg, downcomer, and lower

plenum during the transport are evaluated with CFD analyses. CFD analyses have

been proven to be an effective tool for calculating three-dimensional mixing flows, but

the applied numerical methods and turbulence models require experimental validation

with detailed local resolution of flow and temperature fields. The JULIETTE boron

dilution tests provide a database for the qualifying of the CFD model at mock-up scale.

The JULIETTE test facility is fully representative of the U.S. EPR reactor vessel

internals at a 1:5 scale. The EPR CFD model for evaluating the transport of a

deborated slug and determining the minimum boron concentration at the core inlet is

based on the qualified mock-up scale CFD model.

I

AREVA NP Inc.

U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnicali Renort


Paae 3-14............ R pr t .... . 3 14.

3.7 CFD Analyses

The CFD analyses simulate the transport of the deborated slug and the mixing between

the slug and the surrounding borated water in the cold leg, downcomer, and lower

plenum in order to determine the local distribution of boron concentrations at the core

inlet as a function of time. The concentration results of the CFD analyses are compared

to the critical concentration to assess the potential for a core recriticality.

The CFD model, shown in Figure 3-8, is qualified on the basis of the benchmarks to the

inherent boron dilution tests as described in Section 3.6. [

]

The boundary conditions for the CFD analyses are based on the combination of system

analyses and the PKL conclusions. [

The conditions that are supported by system analyses include:

AREVA NP Inc.

U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Renort


Paae 3-15............ Re or .... . 3 15.

. [

I

To justify the applicability of these conditions in the CFD analysis, the results of the

S-RELAP5 analysis of the case demonstrating limiting conditions are compared to the

inputs to the CFD analysis in Table 3-1. [

Figure 3-9 displays the minimum core inlet concentration and average core inlet

concentration as a function of time for the 11 m 3 slug. The minimum value throughout

the transient of the minimum core inlet concentration was []

Figure 3-10 compares the minimum core inlet concentration as a function of time for the

three slug sizes. Figure 3-11 and Figure 3-12 show the core-wide distributions of boron

I



concentrations at the inlet at the moment the minimum inlet concentration occurs for the

11 m3 [ 1, respectively. These figures clearly demonstrate that the

minimum concentration is both localized to the periphery and short lived. The lowest

concentration water will rapidly mix with the more borated water following it and with the

surrounding water in the highly borated core.

3.8 Critical Concentration

The minimum concentration of boron in the core to maintain subcriticality depends on

the time in the fuel cycle because PWR cores contain excess reactivity to achieve

criticality over the entire fuel cycle. Boron compensates for this excess reactivity. PWR

core designs use boric acid, dissolved in the coolant, and burnable poison, in some fuel

rods, to offset the excess reactivity. Therefore, boron concentrations are highest early

in the fuel cycle.

Xenon-1 35 is a strong neutron absorber that helps to maintain the core subcriticality.

It's negative worth peaks at about eight hours after reactor trip, after which it steadily

decreases and reaches its initial equilibrium value at 24 hours. Because the SBLOCA

event is mitigated within this period, the assessment of the boron dilution event

conservatively assumes that the xenon-135 remains at its equilibrium value throughout

the event.

For additional conservatism, it is assumed that the most reactive control rod assembly is

stuck out of the core. The resulting minimum boron concentration for the U.S. EPR

reactor is calculated to be the equivalent of 905 ppm of natural boron assuming

equilibrium xenon in the core. Applying a 100 ppm uncertainty raises the critical boron

concentration to the equivalent of 1005 ppm of natural boron.

3.9 Recriticality Evaluation

The minimum core inlet concentrations from the U.S. EPR CFD analyses (Section 3.7)

are compared to the critical concentration, 1005 ppm, natural boron (Section 3.8) in


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Renort Paae 3-17

Table 3-2. [

] Therefore, the conservative criterion is met and there is no return to

criticality.

3.10 Conservatisms

The U.S. EPR approach for evaluating the inherent boron dilution event contains a

number of large conservatisms in addition to the conservatisms which are included in

the analysis inputs. The conservatisms of the approach and analyses include:

3.11 Boron Dilution Conclusion

To demonstrate the ability of the U.S. EPR plant to maintain long-term cooling and

prevent fuel damage in the SBLOCA inherent boron dilution event, it is conservatively

shown that there is no threat of recriticality due to the transport of deborate towards the

AREVA NP Inc.

U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionT•.chnic.il R~nnrt


Paae 3-18............ Re or .... . 3 18.

core. [

] Therefore,

there is no recriticality and no safety concern for the U.S. EPR plant in an SBLOCA

inherent boron dilution event.

Table 3-1: U.S. EPR CFD Input Comparison to Selected S-RELAP5Case



Table 3-2: U.S. EPR Minimum Core Inlet Concentration-CriticalConcentration Comparison

Notes:

1.[


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Pa-ne 3-20

Figure 3-1: Inherent Boron Dilution Analytical Methodology


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Pagje 3-21

Figure 3-2: PKL III Test Facility

Volume :1:145 M IElevations :1 1Max. pressure: 45 barMax. power :2.5 MW (10%)


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Paae 3-22

Figure 3-3: PKL Test E2.2 Loop Flows Prior to RNC


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Renort DPhn a q-'Y. ..... ...... R p r t. .. 1 . .

Figure 3-4: PKL Test F1.1 Loop Flow Rates Prior to RNC


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Pacie 3-24

Figure 3-5: 1.60-Inch Break: Primary and Secondary Pressures

PZR Press ure-- SG-1 Pressure- - SG-2 Pressure

- SG-3 Pressure-- SG-4 Pressure

2400 . . . .. .

2000

1600

1200U)i,-

800

400

0 2000 4000 6000 8000 10000 12000 14000Time (sec)

AREVA NP Inc.



Page 3-25

Figure 3-6: 1.60-Inch Break: RCS Inventory

I RCS Mass

850000

800000

750000

700000

650000

600000

550000

500000

450000

400000

350000

E

C/)CO2)

300000

250000

2000000 2000 4000 6000 8000 10000 12000 14000

Time (s)


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTechnical Report Paae 3-26

Figure 3-7: 1.60-Inch Break: SG Apex Liquid Flow

ESG 1 Apex-- SG 2 Apex--. 2SG 3 Apex

-e, SG 4 Apex

5000

4000

3000

E__o

tL

-g

2000

1000

I0

-1000

-2000-2000 0 2000 4000 6000 8000 10000 12000 14000

Time (sec)


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionThchnicrl Renort Paae 3-27

Figure 3-8: Mesh Used in the U.S. EPR CFD Analyses



Figure 3-9: U.S. EPR CFD, 11 m3 - Minimum and Average Core InletConcentrations

Figure 3-10: U.S. EPR CFD - Minimum Core Inlet Concentration SlugSize Comparison


U.S. EPR Post-LOCA Boron Precipitation and Boron DilutionTh~chnir.al R~nnrt On:) n, 'It-gQ............ R pr t .. 1. .

Figure 3-11: U.S. EPR CFD, 11 m3 - Core Inlet Concentrations atMinimum Inlet Concentration Time

Figure 3-12: U.S. EPR CFD, [ ]- Core Inlet Concentrations atMinimum Inlet Concentration Time



4.0 REFERENCES

1. ANP-10278PA, Revision 1, "U.S. EPR Realistic Large Break Loss of Coolant

Accident Topical Report," AREVA NP Inc., November 2011.

2. ANP-1 0263PA, Revision 0, "Codes and Methods Applicability Report for the

U.S. EPR," AREVA NP Inc., August 2007

3. ANS 5.1-1971, Proposed ANS Standard, "Decay Energy Release Rates Following

Shutdown of Uranium Fueled Thermal Reactors," American Nuclear Society,

October 1971.

4. Letter from USNRC Advisory Committee on Reactor Safeguards Chairman,

M. Bonaca, to USNRC Chairman, N. Diaz, "Proposed Resolution of Generic Safety

Issue 185," October 2004.

5. "Acceptance Criteria for Emergency Core Cooling Systems for Light-water Nuclear

Power Reactors." Code of Federal Regulations Title 10, Pt. 50.46, 2007 ed.

6. "Appendix A to Part 50-General Design Criteria for Nuclear Power Plants." Code of

Federal Regulations Title 10, Pt. 50, Appendix A, 2007 ed.

7. Memorandum to L. Reyes from C. Paperiello, "Closure of Generic Safety Issue 185,

'Control of Recriticality Following Small-Break LOCAs in PWRs,"' September 23,

2005.

8. Umminger, K., Thomas Mull, and Bernhard Brand. "Integral Effect Tests in the PKL

Facility with International Participation" Nuclear Engineering and Technology Vol. 41,

No. 6 (2009): 765-774.

9. Nourbakhsh, H.P. and Z. Cheng "Mixing Phenomena of Interest to Boron Dilution

During Small Break LOCAs in PWRs" 7th International Meeting on Nuclear Reactor

Thermal-Hydraulics, Saratoga Springs, NY, September 10-15, 1995.

anp-10288np, revision 2, 'u.s. epr post-loca boron ... · revision 2 u.s. epr post-loca boron...

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