Nov. 8th, 20051

Effectiveness of External Reactor Vessel Cooling (ERVC) Strategy for APR1400 and Issues of

Phenomenological Uncertainties

S.J. OH and H.T. KIMse_oh@khnp.co.kr and hyeong@khnp.co.kr

Nuclear Environmental Technology Institute, Korea Hydro & Nuclear Power Co., Ltd

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Contents

Introduction

External Reactor Vessel Cooling (ERVC) Strategy

Effectiveness Consideration :Risk-Oriented Accident Analysis Methodology (ROAAM)

Application to APR1400 (Advanced Power Reactor 1400 MWe)

Summary

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External Reactor Vessel Cooling (ERVC) StrategyEvolved since mid. 80’sOne of high level candidate strategy to mitigate severe accident for operating plants (EPRI SAMG Technical Basis Report)With the application to AP600, systematic evaluation has been performedSubmerged water will help remove the decay heat and maintain vessel’s integrity

In-Vessel Melt Retention (IVR) as Severe Accident Management

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Loviisa VVER-440

Westinghouse AP600

In-Vessel Melt Retention (IVR) as Severe Accident Management

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IVR History

AP600 Implementation and Base Technology R&D (UCSB, W)

AP1000 Implementation of IVR (W, UCSB)

APR1400 Implementation of IVR (KHNP/KAIST/PSU)

IVR was developed for AP600. The AP600 technology offers too narrow a thermal margin for IVR in high-power reactors.

In-Vessel Retention as SAM Strategy

IVR implementation to AP600. Develop a basic understanding of governing phenomena (NC inside and CHF outside). Effectiveness examined using ROAAM process

APR1400 plant-specific natural circulation tests for the insulation design. CHF tests to support the effectiveness evaluation. Systematic evaluation to support the IVR implementation to APR1400

Streamlined insulation design, lower support structure optimization to enhance the thermal margin

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IVR Technology and Implementation in APR1400

External Reactor Vessel Cooling (ERVC) Primary severe accident management (SAM) strategy for APR1400In-vessel retention (IVR) strategy

Submerging the reactor vessel exterior using SCP and BAMPInject into the vessel to arrest core melt if possibleAPR1400 –specific insulation design to promote heat removal and natural circulation

Implementation as a part of Severe Accident Management Guidance (SAMG) in Korea

Examine the effectiveness of ERVC and its implementation in APR1400Risk-oriented accident analysis methodology (ROAAM) by Theofanous is adopted for the systematic evaluationSupporting material for level 2 PSA quantificationDo not try to claim ‘vessel breach is physically unreasonable’

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Damper

SteamVentingReflective Insulation

Molten Oxides

Nucleate Boilingof External Water

Nucleate Boiling ofWater Injected

Conductionthrough Wall

In-Core InstrumentNozzles

Solid Crust

NaturalConvection

Water Ingress

Liquid Metal Layer

Reactor Cavity

Reactor Pressure Vessel

IVR Technology and Implementation in APR1400

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Basic Design of ERVCS for IVR and Cavity Flooding System (CFS) of APR1400

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In general, the effectiveness of severe accident mitigation features have been examined as a part of Level 2 PSA

As a part of NUREG1150, expert elicitation process was usedInherent difficulty: rare event with incomplete evidence (diverging expert opinion)

Risk-oriented accident analysis methodology (ROAAM)To overcome the difficulty of quantifying under uncertainty, Prof. Theofanous proposed to ‘resolve’ uncertainty using a structured evaluation with bounding scenariosExamine the critical issues based on the physically-based decomposition with bounding assumptionSimilar to expert elicitation, independent reviews by experts will be conducted.Prof. Theofanous proposed that the issue is closed once experts agree on the result

ROAAM Approach

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Proposed numerical value as a part of ROAAMP=0.1: Behavior is within known trends but obtainable only at the edge of spectrum parametersP=0.01: Behavior cannot be positively excluded, but is outside the spectrum of reasonP=0.001: Behavior is physically unreasonable and violates well-known realityQuestion: Are these reasonable value?

Application of ROAAM to AP600 IVR study by TheofanousIdentify the key issues of vessel integrityThermal failure criterion is the limiting one

Wall heat flux vs. CHF heat fluxWall thickness vs. Min thickness required for structural integrity

Sensitivity study to find out the thermal margin with the known CHF limit and thermal loadPeer Review with documented response

Key issue seems to be the melt configuration in view of complex physico-chemistry effect

ROAAM Approach

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Overall view of in-vessel retention issues

Thermal Loading

(Long Term)

Thermal Loading

(Long Term)

Thermal

Criteria

Thermal

Criteria

Structural

Criteria

Structural

Criteria

Power LevelMelt Quantity

Melt CompositionGeometry, Properties

GeometryFlow Paths

Weights (Net)Thermal Stresses

)(θwq )(θCHFq

)(θδ w Fδ

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ULPU-III: extension of CHF tests performed for AP600

ULPU-V series: full-height 1/84 slice geometry representation of AP100036 testsSeries M: streamlined geometrySeries C and P: effect of surface condition and power shape

Key findingsWith streamlined insulation design, CHF limit would be increased to 1.8 -2.0 MW/m2

Microlayer scale phenomena are important for CHFSurface effect and water chemistry are important

UCSB ULPU Test Series

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Experimental Setup of ULPU-2400 Configuration V

1152 heaters (power control)

Magnetic Flowmeter

72 thermocouples

7 pressure transducers

Flow visualization

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Experimental Setup of ULPU-2400 Configuration V

Three Baffle Configurations

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Results of ULPU-2400 Configuration V

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Effectiveness of IVR Strategy for APR1400

Effectiveness Examination Procedures

Choose representative scenarios from Level 1 PSA results

Examine the BC for ERVC strategy using MAAP4 code

Structured examination using ROAAM framework

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APR1400 IVR Performance

Effectiveness of IVR strategy is evaluated using the structured approach developed for AP600

Four representative scenarios are selected from Level 1 PSA Using MAAP4 code, the boundary condition at the time of vessel failure is determinedBased on the method developed for AP600 study, thermal margin is examinedA limiting scenario is developed from LLOCA scenario at the recommendation of the peer review

full core melt in 3.72 hrs from shutdown. Steel mass is estimated to be 30 tons.

The two layer melt pool configuration (metallic layer above oxidic layer) is assumed in the study

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APR1400 Dominant Scenarios

Category Sequences Bounding Sequence

LOFW-17GTRN-17LOFW-6

LOFW-17

SLOCA-23

MLOCA-4

LLOCA-4

SLOCA-23WGTR-28SLOCA-22

MLOCA-3MLOCA-4MLOCA-2

LLOCA-4

Percentage of Total CDF (%)

Steel mass molten (Msteel), (tons)

Zirconium oxidation fraction(fox)

Coremelt fraction(fU02)

Time toFull CoreMelt(hr)

MHFR

LOFW35.2 32 0.38 0.85 10.14 0.50

SLOCA26.7 28.4 0.42 0.78 9.5 0.51

MLOCA9.6 32.7 0.44 0.88 5.6 0.62

LLOCA 2.3 25.2 0.34 0.82 3.72 0.74

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RCS Nodalization of MAAP4 in APR1400

ReactorDome

UpperPlenum

2

1

14

Hot Leg

Hot Leg Tube

Cold Leg

Press-rizer

Cold Leg Tube

Intermediate Leg

3

4

67

5

Hot Leg

Hot Leg Tube

Cold Leg

Intermediate Leg

9

10

12

Core

Downcomer

Cold Leg Tube

11

13

1

8

Broken LoopUnbroken Loop

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U pper C om partm ent, 11

C ontainm ent D om e 12

R eactor C avity 1

AnnularC om partm ent

9

AnnularC om partm ent

9

IR W ST(N o Spargers)

14

IR W ST(Spargers)

15

2

5

8

S/G # 1 C om pt.

7

H VT13

N ote2 - In-C ore Instru. C hase3 - C orium C ham ber R oom4 - C avity Access A rea5 - R eactor Vessel Annulus8 - PZR C om pt.10 - R efueling Pool

B old : com partm ent no.Italic : junction no.

254.5 ft

156.0 ft

100 ft

69 ft

1

27

5

7

3

6

98

28

26

29

16,17,18

19

10,11,12,36

15

14

37,38

33

32

23

25 25

244

3

2

4

21

S/G # 2 C om pt.

6 10

39

40

13

20

22

30,31

34

35

41,42

Containment Nodalization of MAAP4 in APR1400

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APR1400 IVR Assessment

0 10 20 30 40 50 60 70 80 900

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

YANG & CHEUNG

LOFW

MLOCALLOCA

SLOCA

ULPU-2000 Correlation (ULPU-III)

ULPU-V

H

eat F

lux

(kw

/m2 )

Angle (degrees)

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Summary

IVR is an effective SAM strategy for APR1400Except LLOCA limiting case, there are enough thermal margin with ULPU-III CHF dataWith the adoption of streamlined insulation, there are enough thermal margin even for the limiting LLOCA case

There are still ongoing discussion about plausible melt configuration in-vesselWith proper SAM action (inject into vessel with IVR), severe accident will be arrested in-vessel before full core melt and the melt configuration issue would be less of importanceHowever, study similar to that by Seiler et al would be useful

The structured approach of ROAAM is useful in evaluating effectiveness. If there is no consensus, one needs to resort back to expert elicitation process

Experience shows that, in this case, there is a strong bias toward failure by participating experts due to the bounding assumptions and the worst case scenario focused during the ROAAM process

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Collaboration Works

Close coordination with SNUSNU provides an input to the streamlined thermal insulation design using CFX

Base Technology R&D with Center for Risk Studies and Safety, UCSBRobust quantification and enhancement of IVR margins

APR1400 melt progression information exchange with INEEL (MAAP, SCDAP)

Close coordination with PSU on insulation designPSU will generate CHF data from 3-D scaled geometry of APR1400