Effectiveness of External Reactor Vessel Cooling (ERVC) Strategy for - - PowerPoint PPT Presentation

• Nov. 8th, 2005

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

S.J. OH and H.T. KIM

se_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) Strategy

Evolved since mid. 80’s One of high level candidate strategy to mitigate severe accident for

• perating plants (EPRI SAMG Technical Basis Report)

With the application to AP600, systematic evaluation has been performed Submerged 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 APR1400 In-vessel retention (IVR) strategy

• Submerging the reactor vessel exterior using SCP and BAMP
• Inject into the vessel to arrest core melt if possible
• APR1400 –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 APR1400

Risk-oriented accident analysis methodology (ROAAM) by Theofanous is

adopted for the systematic evaluation

Supporting material for level 2 PSA quantification Do not try to claim ‘vessel breach is physically unreasonable’

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Damper Steam Venting Reflective Insulation Molten Oxides

Nucleate Boiling

• f External Water

Nucleate Boiling of Water Injected

Conduction through Wall In-Core Instrument Nozzles Solid Crust Natural Convection 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 used Inherent difficulty: rare event with incomplete evidence (diverging expert

• pinion)
• 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 scenarios

Examine the critical issues based on the physically-based decomposition with

bounding assumption

Similar 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 ROAAM

P= 0.1: Behavior is within known trends but obtainable only at the edge of

spectrum parameters

P= 0.01: Behavior cannot be positively excluded, but is outside the spectrum

• f reason

P= 0.001: Behavior is physically unreasonable and violates well-known reality Question: Are these reasonable value?

• Application of ROAAM to AP600 IVR study by Theofanous

Identify the key issues of vessel integrity Thermal failure criterion is the limiting one

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

Sensitivity study to find out the thermal margin with the known CHF limit

and thermal load

Peer 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 Level Melt Quantity Melt Composition Geometry, Properties Geometry Flow Paths Weights (Net) Thermal Stresses

) (θ

w

q

) (θ

CHF

q

) (θ δ w

F

δ

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• ULPU-III: extension of CHF tests performed for AP600
• ULPU-V series: full-height 1/84 slice geometry representation of AP1000

36 tests Series M: streamlined geometry Series C and P: effect of surface condition and power shape

• Key findings

With streamlined insulation design, CHF limit would be increased to 1.8 -2.0

MW/m2

Microlayer scale phenomena are important for CHF Surface 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

determined

Based on the method developed for AP600 study, thermal margin is examined A 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-17 GTRN-17 LOFW-6 LOFW-17 SLOCA-23 MLOCA-4 LLOCA-4 SLOCA-23 WGTR-28 SLOCA-22 MLOCA-3 MLOCA-4 MLOCA-2 LLOCA-4 Percentage

• f Total

CDF (% ) Steel mass molten (Msteel), (tons) Zirconium

• xidation

fraction (fox) Core melt fraction (fU02) Time to Full Core Melt (hr) MHFR LOFW 35.2 32 0.38 0.85 10.14 0.50 SLOCA 26.7 28.4 0.42 0.78 9.5 0.51 MLOCA 9.6 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

Reactor Dome Upper Plenum

2 1 14

Hot Leg Hot Leg Tube Cold Leg Press- rizer Cold Leg Tube Intermediate Leg

3 4 6 7 5

Hot Leg Hot Leg Tube Cold Leg Intermediate Leg

9 10 12

Core Downcomer Cold Leg Tube

11 13 1 8

Broken Loop Unbroken Loop

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U pper C om par t m ent , 11 C ont ai nm ent D om e 12 R eact

• r

C avi t y 1 Annul ar C om par t m ent 9 Annul ar C om par t m ent 9 I R W ST (N o Spar ger s) 14 I R W ST (Spar ger s) 15 2 5 8 S/G # 1 C om pt . 7 H VT 13 N ot e 2

• I

n-C or e I nst r u. C hase 3

• C or

i um C ham ber R oom 4

• C avi

t y Access A r ea 5

• R eact
• r

Vessel Annul us 8

• PZR

C om pt . 10

• R ef

uel i ng Pool B ol d : com par t m ent no. I t al i c : j unct i

• n

no. 254. 5 f t 156. f t 100 f t 69 f t

1

27 5 7

3

6 9 8 28 26 29 16, 17, 18 19 10, 11, 12, 36 15 14 37, 38 33

32

23 25 25 24

4 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

10 20 30 40 50 60 70 80 90 200 400 600 800 1000 1200 1400 1600 1800 2000 2200

YANG & CHEUNG LOFW MLOCA LLOCA SLOCA ULPU-2000 Correlation (ULPU-III) ULPU-V

Heat Flux (kw/m

2)

Angle (degrees)

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Summary

• IVR is an effective SAM strategy for APR1400

Except LLOCA limiting case, there are enough thermal margin with ULPU-III

CHF data

With 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-vessel

With 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 importance

However, 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 SNU

SNU provides an input to the streamlined thermal insulation design using CFX

• Base Technology R&D with Center for Risk Studies and Safety, UCSB

Robust quantification and enhancement of IVR margins

• APR1400 melt progression information exchange with INEEL (MAAP, SCDAP)
• Close coordination with PSU on insulation design

PSU will generate CHF data from 3-D scaled geometry of APR1400