ML20136G354

TR WCAP-14857, AP600 Long-Term Core Cooling Summary Rept

ML20136G354
Person / Time
Site:	05200003
Issue date:	03/31/1997
From:	Doug Garner, Hochreiter L, Kemper R WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:
Shared Package
ML20136G344	List: ML20136G354 NSD-NRC-97-5016, Forwards WCAP-14857, AP600 Long Term Cooling Summary Rept, Which Provides Overview of Matl Presented in Wcobra/Trac Long-Term Cooling Validation Rept (WCAP-14776)
References
WCAP-14857, NUDOCS 9703180015
Download: ML20136G354 (30)

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T 4

i Westinghouse Non-Proprietary Class 3 l WCAP-14857

+ + + + + + + + .

/

LAP 600 Long-Term Core Cooling. -

Summary Report I

Westinghouse Energy Systems l l

" U I - ,

m:\3539w.wpf:Ib/ua42n

AP600 DOCUMENT COVER SHEET TDC: IDS: 1 S Form 58202G(5/94)(rnA3539w.frm] AP600 CENTRAL FILE USE oNLY:

0058.FRM RFSs; RFS ITEM #:

AP600 DOCUMENT NO. REVISION NO. ASSIGNED TO LTCT-GSR-005 0 Page i of ALTERNATE DOCUMENT NUMBER: WORK BREAKDOWN #: 3.1.1.19.10 l

. DESIGN AGENT ORGANIZATION: l PROJECT: AP600  !

TITLE: AP600 Long-Term Core Cooling Summary Report l l

ATTACHMENTS: DCP #/REV, INCORPORATED IN THIS DOCUMENT REVISION:

None

. CALCULATION / ANALYSIS

REFERENCE:

ELECTRONIC FILENAME ELECTRONIC FILE FORMAT ELECTRONIC FILE DESCRIPTION m:\3539w.wpf Wordperfect 5.2 m:\3539w.frm (C) WESTINGHOUSE ELECTRIC CORPORATION 1997 0 WESTINGHOUSE TNs document containsPROPRIETARY CLASS informahon proprietary 2 to Weshnghouse Electric Corporation: it is submitted in confidence and is to be used solely for the purpose for which it is fumsshed and retumed upon request. This document and such information is not to be reproduced, transmitted, disclosed or used otherwise in whole or in part without pnor wntten authorization of Washnghouse Electnc Corporation, Energy Systems Business Urwt, subt ect to the legends contained hereof.

O WESTINGHOUSE PROPRIETARY This document is the property CLASS of and contains 2Cinformation owned by Westinghouse Electnc Corporation and/or its subcontractors and Proprietary  !

suppleets it is transmitted to you in confidence and trust, and you agree to treat tNs document in strict accordance with the terms and condebons i of the agreement under wNch it was provided to you.

@ WESTINGHOUSE CLASS 3 (NON PROPRIETARY)

COMPLETE 1 IF WORK PERFORMED UNDER DESIGN CERTIFICATION OR - COMPLETE 2 IF WORK PERFORMED

- UNDER FOAKE, 1 E DOE DESIGN CERTIFICATION PROGRAM - GOVERNMENT LIMITED RIGHTS STATEMENT [See page 2)

Copyright statement A license is reserved to the U.S. Government under contract DE AC03-90SF18495.

O DOE Sutg'ectCONTRACT DELIVERABLES to specified excephons, disclosure of tNs(DELIVERED data is restrictedDATA)l unb September 30,1995 or Design Cerbfication under DOE co 90SF18495, whichever is later.

EPRI CONFIDENTIAL: NOTICE: 1 2 334 5 CATEGORY: A 3 B C D E F 2 O ARC FOAKE PROGRAM - ARC LIMITED RIGHTS STATEMENT (See page 21 Copyright statement A license is reserved to the U.S. Govemment under contract DE-FC02-NE34267 and subcontract ARC-93-3-SC-001.

ARC CONTRACT DELIVERABLES (CONTRACT DATA) .

Sa_W to eW excaaha'is disclosure of this data is restricted under ARC Subcontract ARC-93-3-SC-001.

l ORIGINATOR SIGNATUR T f l D. C. Garner

- AP600 RESPONSIBLE MANAGER SIG w 5/#/f7 APPROVAL DATE E. H. Novendstern f,, _ C 3 f/- f y

• Approval of tie responsible manager signifies that docufhont as complete, all required reviews are complete, electroruc fil(es attached and document is i

. . released for use.

5 l cosajew om 1

AP600 DOCUMENT COVER CHEET Page 2 l

} Form ss202G(s/94) LIM!TED RIGHTS STATEMENTS '

DOE GOVERNMENT UMITED RIGHTS STATEMENT I (A) These data are submitted with limited rights under govemment contract No. DE-AC03-90SF18495. These data rriay be rep'oduced r and used by the govemment with the express hmitation that they will not, without wntten permission of the contractor, be used for purposes of rnanufacturer nor disclosed outside the govemment; except that the govemment may disclose these data outside the govemment for the following purposes, if any, provided that the govemment makes such disclosure subject to prohibition against further use and disclosure:

(1) This " Proprietary Data" may be disclosed for evaluation purposes under the restrictons above.

(11) The " Proprietary Data' may be disclosed to the Electnc Power Research institute (EPRI), electric utility representatives and their direct consultants, excluding direct commercial competitors, and the DOE National Laboratories under the prohibitions and restrictons above.

(B) This notice shall be marked on any reproduction of these data, in whole or in part.

ARC UMITED RIGHTS STATEMENT:

This proprietary data fumished under Subcontract Number ARC-93-3-SC-001 with ARC may be duplicated and used by the govemment and )

ARC, subject to the limitabons of Article H-17.F. of that subcontract, with the express limitations that the proprietary data may not be disclosed '

outside the govemment or ARC, or ARC's Class 1 & 3 members or EPRI or be used for purposes of manufacture without prior permission of the Subcontractor, except that further disclosure or use rnay be made solely for the following purposes:

This propnetary data rnay be disclosed to other than commercial competitors of Subcontractor for evaluation purposes of this subcontract under 1 the restnction that the propnetary data be retained ir. onfidence and not be further disclosed, and subject to the terms of a non-disclosure

, agreement between the Subcontractor and that organitation, excluding DOE and its contractors.

i DEFINITIONS CONTRACT / DELIVERED DATA - Consists of documents (e.g. specifications, drawings, reports) which are i generated under the DOE or ARC contracts which contain no background proprietary data. j

~

i EPRI CONFIDENTIALITY / OBLIGATION NOTICES NOTICE 1: The data in this document is subject to no confidentiality obligabons.

NOTICE 2: The data in this document is proprietary and confidential to Westinghouse Electne Corporation and/or its Contractors. It is forwarded to recipient under an obligabon of Confidence and Trust for hmited purposes only. Any use, disclosure to unauthorized persons, or copying of this document or parts thereof is prohibited except as agreed to in advance by the Electnc Power Research Institute (EPRI) and Westinghouse Electnc Corporabon. Recipient of this data has a duty to inquire of EPRI and/or Westinghouse as to the uses of the informttion contained herein that are permitted.

NOTICE 3: The data in this document is proprietary and confidential to Westinghouse Electric Corporabon and/or its Contractors. It is forwarded to recipient under an obligation of Confidence and Trust for use only in evaluabon tasks specifically authorized by the Electnc Power Research Institute (EPRI). Any use, disclosure to unauthorized persons, or copying this document or parts thereof is prohibited except as agreed to in advance by EPRI and Westinghouse Electnc Corporabon. Recipient of this data has a duty to inquire of EPRI and/or Westinghouse as to the i uses of the informabon contained herein that are permitted. This document and any copies or excerpts thereof that may have been generated

are to be returned to Westinghouse, directly or through EPRI, when requested to do so.

NOTICE 4: The data in this document is proprietary ur.d confidential to Westnghouse Electric Corporabon and/or its Contractors. It is being revealed in confidence and trust only to Employees of EPRI and to certain contractors of EPRI for limited evaluation tasks authorized by EPRf.

4 Any use, disclosure to unauthonzed persons, or copying of this document or parts thereof is prohibited. This Document and any copies or excerpts thereof that may have been generated are to be returned to Westinghouse, directly or through EPRI, when requested to do so.

) NOTICE 5: The data in this document is proprietary and confidental to Westinghouse Electric Corporation and/or its Contractors. Access to this data is given in Confidence and Trust only at Westinghouse facilibes for limited evaluation tasks assigned by EPRI. Any use, disclosure to unauthonzed persons, or copying of this document or parts thereof is prohibited. Neither this document nor any excerpts therefrom are to be removed from Westnghouse facilities.

EPRI CONFIDENTIALITY / OBLIGATION CATEGORIES

CATEGORY *A* - (See Delivered Data) Consists of CONTRACTOR Foreground Data that is contained in an issued reported.

CATEGORY *B' -(See Delivered Data) Consists of CONTRACTOR Foreground Data that is not contained in an issued report, except for

, computer programs.

CATEGORY *C'- Consists of CONTRACTOR Background Data except for computer programs.

CATEGORY "D" - Consists of computer programs developed in the course of performing the Work.

CATEGORY *E'- Consists of computer programs developed prior to the Effective Date or after the Effective Date but outside the scope of the Work CATEGORY *F"- Consists of administrative plans and administrative reports.

cosa_new w

. . 1 WESTINGHOUSE NON-PROPRIETARY CLASS 3 )

l WCAP-14857

\

l l

l AP600 Long-Term Core l Cooling Summary Report I l

1 l

D. C. Garner L. E. Hochreiter R. M. Kemper March 1997 l

l I

I Westinghouse Electric Corporation Energy System Business Unit P.O. Box 355 Pittsburgh, PA 15230-0355 C 1997 Westinghouse Electric Corporation All Rights Reserved m:\3539w.wptib431197

. . iii

' . TABLE OF CONTENTS LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v 1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 -1 1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 -1 1.2 Long-Term Cooling Thermal-Hydraulic Phenomena of Interest . . . . . . . . . 1-2 1.3 AP600 Phenomena Identification am1 Ranking Table . . . . . . . . . . . . . . . . 1-2 2 DESCRIPTION OF THE WINDOW MODE METHODOLOGY USING ECOBRA / TRAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1 3 ECOBRA/ TRAC VALIDATION FOR LONG-TERM COOLING . . . . . . . . . . . . . 3-1 3.1 ECOBRA/ TRAC Initial Condition Convergence . . . . . . . . . . . . . . . . . . . 3-2 3.2 ECOBRA/ TRAC Extended Time Calculation Convergence . . . . . . . . . . . 3-3 3.3 ECOBRA/ TRAC Boundary Condition Convergence . . . . . . . . ....... 3-3 3.4 ECOBRA/ TRAC Comparisons with OSU Test Data . . . . . . . . . . . . . . . . 3-3 4 AP600 PLANT LONG-TERM COOLING WINDOW MODE CALCULATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............. 4-1

4.1 Computation of AP600 Emergency Core Cooling Syster.

c Performance in the Short Term . . . . . . . . . . . . . . . . . .............. 4-1 L

4.2 IRWST Draindown and Core Boiloff Calculations to Establish Mass / Energy Releases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 4.3 EGOTHIC Containment Pressure Computation . . . . . . . . . . . . . . . . . . . . 4-2 l

4.4 WCOBRA/ TRAC Long-Term Cooling Emergency Core Cooling System Performance Calculations . . . . . . . . . . . . . . . . . . . . . . . . 4-4 5

SUMMARY

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1 6 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1 i

l m:\3s39w.wpf:1b-031197 March 1997 l l

L t

l l

iv '

LIST OF TABLES ,

Table 1-1 Phenomena Identification and Ranking Table for AP600 LOCA LTC Transient (Rev.1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4 i

i l

l 1

l l

March 1997 m:\3539w.wpt:1b-031197

• y

- LIST OF FIGURES Figure 1-1 AP600 Small-Break LOCA and LTC Scenario . . . . . . . . . . . . . . . . . . . . . . 1-6 Figure 4-1 Simplified AP600 Internal Containment Flow Network . . . . . . . . . . . . . . 4-7 Figure 4-2 Small-Break LOCA LTC Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8 1

i

)

i l

l l

l l

m g 3997 m:\3539w.wpf:1b.031197

4 1-1 1 ' INTRODUCTION 1 l

1.1 BACKGROUND

l One of the requirements of 10 CFR 50.46 is to show how the engineered safeguards systems ]

can maintain long-term cooling (LTC) of the reactor core after a loss-of-coolant accident

-(LOCA). In current operat ng i p ants, l t eh eng neered i safety systems consist of safety class pumps and heat exchangers that maintain a continuous flow of subcooled borated water into the reactor vessel to maintain the core in a cooled, subcritical state indefinitely. Current plant safety systems consist of active components that are powered by an emergency diesel. These systems are designed to be single-failure-proof and can be tested during outages.

i The AP600 plant utilizes passive safety systems to perform the same functional requirements j as the active engineered safeguards systems in current plants. In place of the active pumps l l

and heat exchangers of the active plant, the AP600 uses gravity injection from large water supplies within the containment, which can provide indefinite coolant flow into the reactor vessel. The core's decay heat is removed from the containment through the steel ,

containment shell, which then retums subcooled condensate to the reactor sump and/or to l the in-containment refueling water storage tank (IRWST). The gravity injection from both the I

IRWST and the reactor sump replaces the active pumps found in current plants. The passive heat removal through the passive shell replaces the sump heat exchangers also found in  ;

current plants. Therefore, the methods to provide safety injection flows and heat removal are different for the AP600 as compared to current operating plants; however, the functional requirements for the passive safety systems remain the same for the AP600.

l t

Since the AP600 passive safety systems for LTC are different from current operating plants, the performance of these systems for postulated accidents must be confirmed. The performance of the AP600 passive safety systems is confirmed using validated computer 3 codes that model the thermal-hydraulic phenomena associated with this period of the postulated accident. The performance of the passive safety systems is judged to be adequate l if the core remains in a coolable state using a conservative model of the reactor coolant system (RCS), with the imposed accident initial and boundary conditions, and consideration of the worst single failure. To perform these calculations with confidence, the selected computer code must be validated against experimental data that capture the LTC thermal-hydraulic phenomena.

This report supports the application for AP600 design certification as specified in 10 CFR 52.47 for passive safety system plants by demonstrating that the AP600 LTC safety features have been evaluated in tests and by analysis with a suitable code, ECOBRA/ TRAC.

Test data from the Oregon Statr University (OSU) facility have been compared to March 1997 WCAP 14857 m:\3539w.wpf:1b-031197

7

=

1-2 *

}V_ COBRA / TRAC calculations for a wide range of break sizes and locations, and at a series of times during the LTC transient.

1.2 LONG-TERM COOLING THERMAL-HYDRAULIC PHENOMENA OF INTEREST The small-break LOCA and LTC periods are shown in Figure 1-1. LTC is defined as the time after which injection from the IRWST has become stable, until the plant is fully recovered with active systems. The same definition applies for the LTC period after a large-break LOCA. LTC occurs after the initial periods of the transient are complete and the RCS is fully depressurized. The higher-pressure passive safety components, such as the core makeup tanks (CMT) and the accumulators, have all fully injected. The automatic depressurization system (ADS) has activated, the reactor vessel is filled to approximately the hot-leg elevation, and the volumes above the reactor vessel cold legs, such as the steam generators, pressurizer, and pressurizer surge line, are steam-filled. The passive residual heat removal system (PRHR) is inactive and does not provide a significant heat removal path for the core decay heat. The ADS stage 4 valves are open to the containment and provide the primary flow path for mass and energy from the reactor vessel to the containment. The core is in a weak boiling mode since the decay power is low and safety injection is initially from the IRWST, which provides a high flow of subcooled water into the reactor vessel. Once the IRWST is  ;

depleted (assuming no credit for the condensate gutters, which direct most of the condensate j flow to the IRWST), injection is initiated from the reactor sump. )

1.3 AP600 PHENOMENA IDENTIFICATION AND RANKING TABLE The thermal-hydraulic phenomena that are important when the AP600 i<, in tb lag-term passive cooling mode have been assessed. Table 1-1 gives a relevant y;uon of the phenomena identification and ranking table (PIRT), which was in%11y developed for the Oregon State University (OSU) long-term cooling tests, and has been refined over time through NRC reviews and as testing and analysis of the tests and AP600 plant have progressed.

Since the entire primary system is near containment pressure, the resulting core flow is determined by the gravity driving head in the downcomer, the head in the core, and the two-phase pressure drop in the core, hot legs, and ADS-4. The LTC PIRT contains the phenomena ranking for the IRWST injection phase as well as for the sump injection phase.

Most of the highly ranked items are the same for both IRWST and sump injection. The levels in the core, upper plenum, and downcomer are all ranked high since the levels determine if the core remains covered and coolable. Most of the RCS components above the hot legs and cold legs are empty and full of stagnant steam and do not contribute to the LTC phenomena.

These components are either ranked very low in the PIRT or are not applicable for this WCAP-14857 March 1997 m:\3539w.wpf:1b-031197

4 1-3 peridd of the transient. The decay power is ranked high since this is the source of steam generation within the vessel. Using the Appendix K assumptions for the decay heat in the AP600 plant calculations is clearly conservative for the LTC period.

The hot-leg flow pattern and the effects of the "T" connection at the top of the hot leg are ranked high since these components determine the void fraction and quality of the flow that is vented out the ADS-4 valves. The venting of ADS-4 valves is important since reduced ADS vent area or increased pressure drop adversely affects the flow through the core ai 3 the core steam generation rate. Higher ADS-4 pressure drop reduces the core flow and increases the steam generation rate, and hence, the volume of steam that must be vented at low pressure.

The IRWST flow and the sump flow are ranked high since these flows are needed to maintain core cooling. The temperature of the sump flow is also ranked high since a reduced subcooling of the sump flow results in additional steam generation in the core, which must be vented.

The direct vessel injection (DVI) line resistance is ranked high. This is an important quality, because for a given head difference between the IRWST or the ' amp and the RCS, it is the DVI line resistance that determines the flow into the reactor sessel.

All the parameters ranked high in Table 1-1 will be evaluated in the analysis of the OSU LTC tests.

WCAP-14857 March 1997 m:\3539w.wpf:1b-031197

1-4 ,. ,

i 1

1 Table 1-1 Phenomena Identification and Ranking Table for AP600 LOCA LTC Transient (Rev.1)  !

1 Sump Component Phenomenon IRWST Injection

• Injection

• Break Critical flow M N/A Subsonic flow M L ADS Stages 1 to 3 Critical flow M N/A Subsonic flow M L Two-phase pressure drop L L Valve loss coefficients M/L L Single-phase pressure drop L L Vessel Core Decay heat H H Flow resistance L L j Flashing N/A N/A Wall-stored energy M M l Natural circulation flow and heat transfer M M Mixtute level mass inventory H H l'ressurizer Pressurizer fluid level L N/A Wall-stored heat L N/A Pressurizer Surge Line Pressure drop / flow regime L L Downcomer/ Lower Plenum Pressure H H Liquid level H H Condensation M M Upper Head Liquid level N/A N/A Flow through downcomer top nozzles M M Upper Plenum Liquid level H H Entrainment/deentrainment M M Cold Legs Condensation L L Separation at balance line tee L L 1

Steam Generator 2$ - natural circulation N/A N/A l Steam generator heat transfer L/NAc2) N/A Secondary conditions L/NA(2) N/A l WCAP-14857 March 1997 m:\3539w.wpf:1b-031197

+

35 Table 1 1 Phenomena Identification and Ranking Table for AP600 LOCA LTC Transient [

(cont) (Rev.1)  !

f Sump Component Phenomenon IRWST Injection"' Injection"'

Hot Leg Flow pattern transition H/M H/M Separation at ADS-4 tee H/M H/M ADS-4 Critical flow H N/A l Subsonic flow H H CMT l Recirculation injection N/A N/A Gravity draining injection L L

]

Vapor condensation rate L L 1 I

CMT Balance Lines i Pressure drop N/A N/A l Flow composition L L Accumulators Noncondensable gas entrainment N/A N/A l l

IRWST I Gravity draining injection H M I Vapor condensation rate L L  ;

Temperature distribution M M i DVI Line Pressure drop H H PRHR Liquid natural circulation flow and heat transfer N/A N/A Sump Gravity draining injection N/A H Level N/A H Temperature N/A H 1

Notes:

1. H = Hic' M = Me om l L = bw N/A = Not Applicable

2. The rankings for steam generator heat transfer and secondary conditions are Low for IRWST injection after a large break and Not Applicable for IRWST injection after a small break.

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1-6 ,. ,

d

)

J 3

1 3

i l AP600 SBLOCA Scenario i

5 . . . .

l  :

SBLOCA ': : LTC  :

I I I I i

Blowdown l ADS l IRWST l Sump

! 't i Blowdown iI Injection i Injection i I i

1. I 1 l 1 1 1 i e- l l l l a l Natural l l l

$u l Circulation l l ,

o. - l l 1 1 I I I I I I I i i 1 1 I I 1 I I i i i i i i l I I I I I I i i i l i I ,

I I I I I I I I l 51897B.1 O

Figure 1-1 AP600 Small-Break LOCA and LTC Scenario WCAP-14857 March 1997 m:\3539w.wpf:1b431197

1

. '. 2-1 l l

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DESCRIPTION OF THE WINDOW MODE METHODOLOGY 2

USING ECOBRAfrRAC The LTC mode is characterized by a quasi-steady gravity injection into the reactor vessel downcomer through the DVI line and a venting process from the reactor through the ADS stage 4 valves on the hot legs. The pressure is low and slightly above the containment pressure, typically 25 psia. As the PIRT indicates, the key phenomena of interest are the single- and two-phase levels in the reactor vessel, downcomer, IRWST and the sump, as well as the resistances between the injection sources and the vessel and resistances for venting from the reactor vessel.

It was desired to use a safety analysis code that had been qualified for low-pressure applications to perform the AP600 LTC safety analysis calculations. At the time such a code was needed, the NOTRUMP code had not been modified and validated for low-pressure applications. The only approved low-pressure safety analysis computer code was ECOBRA/ TRAC (EC/T). ECOBRA/ TRAC had also been validated against a large number l of low-pressure separate effects tests as well as low-pressure integral systems effects tests such as those at the Cylindrical Core Test Facility (CCTF) and the Slab Core Test Facility (SCTF) Both of these facilities simulated gravity reflood phenomena at low pressure that are similar to the AP600 LTC phenomena, as given in Table 1-1. Therefore, it was decided to use the ECOBRA/ TRAC code for the AP600 LTC safety analysis.

The LTC period of a postulated accident continues in a quasi-steady-state fashion indefinitely until the plant recovers from the accident in a post-accident situation with active systems.

Therefore, the LTC transients to be evaluated are tens of thousands of seconds long as compared to the much shorter small-break LOCA transients, which may be only several thousand seconds in duration.

The ECOBRA/ TRAC code has been designed for the much shorter large-break LOCA analysis and runnmg times for longer transients are prohibitively long. To perform calculations that assess the performance of the passive safety systems over this long time duration, a " window mode" calculational approach has been used in which a time slice of the quasi-steady-state transient is analyzed using ECOBRA/ TRAC. The initial conditions are obtained from either earlier window mode calculations, or from the small-break conditions when IRWST injection is established, or similar conditions from the large-break LOCA calculation. The boundary conditions for the window mode calculation are obtained from the calculated containment conditions, which give the sump level and fluid temperature, as well as the containment pressure.

The strategy for the window mode is that the RCS is in a quasi-steady condition, the assumed initial conditions for the calculation will be damped and dimmished over time if the WCAP-14857 March 1997 m:\3539w.wpf:1b-031197

._ . _ _ . . _ _. __ ~ . _ _ _ .. . _. .. - . _ _ _ _ _ _ _ _ . _ . -

2-2 .

l l selected window time is longer than the transient flow time through the RCS. Th,e estimated transient time for the injection flow to pass through the AP600 reactor vessel downcomer and lower plenum is approximately 500 to 700 seconds. Therefore, if the window mode calculation is carried out over multiple RCS transient times, the response of the RCS becomes dependent on the imposed boundary condition, not the selected initial conditions. Since the containment and the IRWST (or sump) boundary conditions of pressure and temperature are calculated throughout the long-term transient, they will determine the resulting injection flow into the reactor vessel,'and therefore, the coolability of the core for a given decay heat power level.

The window mode approach is a valid method of assessing the passive safety injection l system performance since the transient times for draining of the IRWST are much longer than l the fluid transient time through the RCS. Therefore, calculations that have been performed both in modeling the OSU experiments as well as the AP600 plant need only be sufficiently I

long to ensure that sufficient transient times through the RCS have occurred. Typically,

[ calculations have been performed for window times of 1000 seconds or longer to ensure that l the calculation has reached a quasi-steady state and that the boundary conditions are determining the RCS behavior. In this fashion, the AP600 LTC transient can be examined in l time segments to ensure core coolability.

l l The AP600 plant calculations are also performed in a conservative manner, such as by using .

l American Nuclear Society (ANS) ANS-1971 plus 20 percent decay power, assuming a failure

' of one of the fourth-stage ADS valves and bounding the injection line resistance. In addition to these assumptions, the most limiting periods for LTC are examined where the injection temperature is highest, and the driving heads for injection are minimized. In this fashion, a conservative estimate of the AP600 plant passive safeguards system performance for LTC is i obtained.

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. . 3-1 3

W COBRA / TRAC VALIDATION FOR LONG-TERM COOLING To validate EC/T for the long-term core cooling analysis of AP600, a series of calculations was made to demonstrate the behavior of the code. A second series of calculations was also performed to demonstrate applicability of the code by comparison to OSU LTC test data.

The LTC analysis of AP600 is essentially a boundary value problem dependent on the boundary conditions, e.g., liquid source levels / temperatures and core decay heat levels. A quasi-equilibrium solution is obtained that is independent of the initial conditions:

• Vessel initial fluid levels, downcomer, core, and upper plenum

- Vessel initial fluid temperatures, downcomer, core, and upper plenum

. Initial vessel and internals metal temperatures

• Fuel rod (or heater rod) temperatures

. Initial system pressure The solution is determined by the boundary conditions, which, for the LTC problem, include:

. IRWST level and temperature

. Sump level and temperature

- Steam generator secondary-side conditions

• CMT level and temperature

• Break separator level and temperature (OSU test)

. Core decay heat level The insensitivity of the quasi-equilibrium solution to initial vessel conditions, vessel liquid level, and downcomer temperature, was first shown by a set of comoarative calculations as l

detailed in Section 3.10 of Reference 1. In each case, the same quasi-equilibrium solution was obtained independent of the initial conditions.

Subsequent to the issuance of Reference 1, the effect of boundary condition variations was also shown in a set of calculations (Reference 2) and the extended time convergence of EC/T l

was evaluated with comparative long and short calculations for OSU test simulations. In the latter, it was shown that the same quasi-equilibrium solution was obtained at a calculation time of 3000 seconds, as obtained for calculations started 2000 seconds later and run for a 1000-second period. In these calculations, windows were selected that covered the 2-inch cold-leg break transient (SB01) from the initiation of IRWST flow through switchover to sump injection.

For the calculational comparisons with OSU test data (see Section 5 of Reference 1), the window periods were selected at conditions that challenge the AP600 LTC design, i.e., at WCAP-14857 March 1997 m:\3539w.wpf:Ib-031197

i 3-2 .- .

i  !

l times when the IRWST and sump levels are low, providing the minimum driving. heads to  !

{ the DVI flows and the minimum vessel liquid levels. This occurs prior to and following l l switchover from IRWST injection to sump injection. To cover the range of break sizes and i

locations, a set of four OSU tests was selected.

I 4 In response to NRC concerns regarding solution divergence at extended calculation times,

three calculations were performed for periods of 3000 seconds and were compared to i OSU test data in Sections 2.3,2.4, and 2.5 of Reference 2. Three different window l periods were evaluated during IRWST draindown and sump operation, covering the time period of 1260 seconds to 16,500 seconds.

} Modeling of some early transient phenomena that occur only with high vessel liquid levels j l was not performed since they tend to be self-limiting, e.g., the CMT refill phenomenon is 4

terminated when the vessel liquid level falls below the top of the cold leg and steam is i

drawn into the CMT balance line. Similarly, the vessel /DVI flow oscillation period was not l observed once the upper plenum level dropped below the centerline of the hot legs as occurs )

l' late in the transient prior to switchover to the sump injection. l

\

l 3.1 ECOBRAMRAC INITIAL CONDITION CONVERGENCE I i

To demonstrate the lack of sensitivity of the calculations to the initial vessel conditions selected, the initial vessel liquid level was set alternately at 75 percent of core height, the top of the core, and the hot-leg centerline, in three comparative calculations. Identical boundary conditions (initial IRWST level, core power, steam generator secondary conditions, etc.) were used for each calculation. Within 300 to 400 seconds, the varying vessel conditions had converged to the quasi-equilibrium solution and the same values were maintained for the remainder of the 1000-second window. Inlet and outlet vessel flows were also identical after 400 seconds. These calculations were performed for two OSU tests, the 2-inch cold-leg break (SB01) and the CMT balance line break (SB10). These results are contained in Section 3.10 of WCAP-14776 (Reference 1).

Two similar sets of calculations were performed in which the vessel levels were held constant and the vessel downcomer initial temperatures were set at different values. Within 500 seconds, the quasi-equilibrium solutions were essentially identical and remained so for the remainder of the 1000-second window. These calculations were performed for the 2-inch cold-leg break and the CMT balance line break. These results are also contained in Section 3.10 of Reference 1.

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. a. 3-3 3.2 ' ECOBRA/ TRAC EXTENDED TIME CALCULATION CONVERGENCE Subsegur.nt to issuance of WCAP-14776 and in response to NRC concerns and questions, one transient window was extended from the normal 1000-second length to a length of more than 3000 seconds, to demonstrate solution convergence for calculations at extended times. The results of this calculation are provided in Section 2.1 of Reference 2. This was performed for the 2-inch cold-leg break test (SB01), starting at initial IRWST injection (1260 seconds) and running to a test time of 4600 seconds. Initial vessel conditions were taken from the test data at 1260 seconds. The comparison calculation was performed using identical initial vessel conditions but was started at a transient time of 3600 seconds and run with the test boundary ,

conditions occurring from 3600 to 4600 seconds. Within 500 seconds, the quasi-equilibrium {

solution was established and the vessel conditions were essentially identical. The transient followed the same track for the remaining period from 4100 seconds to 4600 seconds. )

i l

3.3 .W_ COBRA / TRAC BOUNDARY CONDITION CONVERGENCE l i

Also following issuance of WCAP-14776 and in response to NRC concerns and questions,it l was decided to demonstrate that the solution would converge to the quasi-equilibrium values  ;

if the boundary conditions were perturbed and then retumed to the correct values. For the 2-inch cold-leg break transient (SB01), the IRWST level was arbitrarily set at 2.5 feet above the test data level for 200 seconds and then returned to the correct level for the remainder of the window. While the early portion of the solutions reflected the differences in initial I IRWST levels, identical results were obtained for the baseline and the variational calculations well before the end of the 1000-second window. These results are documented in Section 2.2 of Reference 2.

3.4 ECOBRA/ TRAC COMPARISONS WITH OSU TEST DATA To demonstrate the adequacy of WC/T to accurately perform LTC calculations, a group of calculations has been performed and compared to OSU test data. The calculations are documented in Section 5 of Reference 1. The window mode approach was used with the time periods selected just prior to, during, or immediately following the switchover from the IRWST to the sump. Calculations were performed for 1000 seconds in each case, with the first 300 seconds defined as the initialization period that was required to transition from the assumed vessel initial conditions to a quasi-equilibrium condition that would compare to the test data. Calculations for the remaining 700 seconds showed a quasi-steady-state condition that could be compared to the measured data. To show a representative selection of small l

l l

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WCAP-14857 March 1997 m:\3539w.wpf:1b-031197 l

l 1

a i

3-4 -

breaks, calculations and test data comparisons have been performed for the following set of four tests:

SB01 inch cold-leg break SB10 - Balance line break

• SB12 - DVI line break

• SB23 - 1/2-inch cold-leg break Initial conditions within the vessel were selected from conditions observed during the test at the start of IRWST flow or in a conservative fashion for the particular parameter.

Specifically, the downcomer fluid temperature was set at the test value observed at the start of IRWST flow. In all cases, this value was above the values observed subsequently during the tests and was considered an upper-bound temperature. The core liquid and heater rod temperatures were set slightly below saturation temperature at the test pressure, primarily to l minimize liquid level perturbations at the start of the calculations. The vessel collapsed liquid level was set at the top of the core in all calculations. This level is below the levels observed in the test data and results in high DVI line flows until the proper vessel level is achieved. Consistent with the vessel collapsed liquid level, no liquid was assumed in the loops, pressurizer, surge line, steam generator (primary side), or reactor coolant pumps. As shown in Section 3 of Reference 2, these conditions can be determined arbitrarily since their effects typically persist for less than 300 seconds into the calculation.

Break location and geometry were adjusted as appropriate for each test while other facility  !

parameters such as geometry and hydraulic line resistances were held constant for all i comparisons. j 1

In all calculations, the vessel levels showed reasonably good comparisons with the OSU test  ;

data. From the quasi-equilibrium portion of the solutions, the collapsed liquid levels in the upper plenum were within 10 percent of the measured values. The collapsed liquid levels calculated in the downcomer were slightly conservative, being within +0 to -20 percent of the measured values. Of the four test calculations performed, three showed reasonable agreement with the measured total DVI and ADS-4 flows, with the deviations being within 215 percent. Test SB01 indicated somewhat larger deviations. Calculated draindown of the IRWST was in good agreement with the data in SB10 and SB23. The SB01 calculation predicted a higher DVI flow rate and a more rapid draindown of the IRWST than indicated by the data. The SB12 calculation predicted a lower total DVI flow rate and a slower draindown than indicated by the data. In all cases, however, the core remained covered as indicated by the test data and upper plenum liquid was well above the top of the core.

In response to NRC concerns regarding solution divergence at extended calculation times, an additional set of data comparison calculations was performed subsequent to the issuance of WCAP-14776. Two additional windows were selected for the 2-inch cold-leg break (SB01)

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3-5 and the sump switchover window at 13,500 seconds for the CMT balance line break (SB10) was extended to 16,500 seconds. The three calculations were performed for periods of at least 3000 seconds and were compared to OSU test data in Sections 2.3,2.4, and 2.5 of Reference 2. In all cases, no solution divergence was detected and the quasi-equilibrium solutions were as accurate at 3000 seconds as when they were first reached at 300 to 400 seconds.

1

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1 41 l

1 AP600 PLANT LONG-TERM COOLING WINDOW MODE 4 i' CALCULATIONS To confirm the successful emergency core cooling system (ECCS) performance of the AP600 plant during the LTC phase of postulated LOCA events, window mode calculations are performed as previously described. The windows selected for the AP600 are at limiting i times during LTC as judged by the prevailing core decay power, sump level, water )

temperature, and other conditions. A spectrum of LOCA break cases is provided, and I stipulation of conservative assumptions provides confidence in the capability of the plant. In I particular, the long-term core cooling analyses in the AP600 Standardized Safety Analysis i Report (SSAR) Subsection 15.6.5.4c are performed in compliance with the conservatisms l outlined in 10 CFR 50 Appendix K. I l

i 4.1 COMPUTATION OF AP600 EMERGENCY CORE COOLING SYSTEM PERFORMANCE IN THE SHORT TERM Large- and small-break LOCA events are analyzed in the short term using computer codes specifically designed for each break category. Large-break LOCA events (SSAR Subsection 15.6.5.4a) are analyzed by applying the ECOBRA/ TRAC best-estimate methodology to the AP600 (Reference 3) to analyze the core response until total fuel rod quench is preciicted. The small-break LOCA events of s 1.0 ft.2 in area (SSAR Subsection 15.6.5.4b) are analyzed using a NOTRUMP version specifically created and validated for AP600 (Reference 4) to perform an Appendix K analysis until the steady, continuous injection of water from the IRWST is established. The time of completion of the short-term ECCS analyses provides information needed to establish the boundary conditions for the LTC window mode analyses.

4.2 IRWST DRAINDOWN AND CORE BOILOFF CALCULATIONS TO ESTABLISH MASS / ENERGY RELEASES As shown in Figure 4-1, the mass and energy releases for the pre-LTC portion of a SSAR LOCA transient are supplied to WGOTHIC (Reference 5) from the AP600 large- or small-break LOCA ECCS analysis results. In addition, the mass and energy releases during the IRWST injection period also must be determined. To accomplish this, first the drain rate of the IRWST is computed based on the minimum initial inventory and the tank condition at the initiation of IRWST injection. An initial WGOTHIC run provides an estimate of the containment pressure during LTC.

The computed IRWST drain rate then provides the basis for the mass and energy release determination. A specific conservatism applied in computing the mass / energy releases during the IRWST delivery period is that all the IRWST flow injected into the reactor vessel WCAP-14857 March 1997 m:\3539w.wpf:1b-031197

- -- --.--- .-. - -- .- .---- . . - . . . ~

4-2 .- ,

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as the tank drains to the low-3 level setpoint is assumed to pass through the core,,and in the  ?

[ process is heated to saturation temperature. Any core decay heat present that is beyond that necessary to heat all the injected liquid to saturation is then allocated to vaporize liquid. In this way, the saturated liquid flow through ADS stage 4 flow paths is maximized, while the steam flow out through ADS stage 4 is minimized, consistent with the decay heat model employed. As a result, the temperature of the containment sump liquid is maximized, while the steam release, which causes an increase in containment atmospheric pressure, is  ;

minimized and the time at which the IRWST is empty is shortened. Consistent with the OSU Facility test data, no fluid is 'assumed to flow through ADS stages 1,2, and 3 and no flow i enters the PRHR heat exchanger during IRWST injection (Reference 6). This conservative l approach maximizes the likelihood that the containment sump, which receives no subcooled l ADS stage 4 flow, will contain saturated liquid at the inception of sump injection. l Conditions at or near saturation challenge the ability of sump injection to continue removal of core decay heat and provide the core liquid throughput that precludes any concentration of boric acid in the core during the long term post-LOCA.

1 For LTC computations, the DVI piping break location is conservatively modeled at the outlet of the IRWST isolation valve to maximize the tank spill rate. The IRWST tank draindown l calculation is performed modeling vessel injection via one DVI line of the parallel IRWST i outlet lines, while at the same time modeling the second IRWST outlet line as broken and l spilling with a minimum line resistance. A simple NOTRUMP model of the IRWST and pertinent piping predicts draining of the IRWST. ]

For pipe breaks at reactor coolant loop locations, the IRWST tank draindown calculation is performed modeling injection into the reactor vessel through both of the parallel IRWST outlet lines. A simple NGOTHIC model computes this draining while at the same time modeling the other RCS flow resistances.

4.3 .W. GOTHIC CONTAINMENT PRESSURE COMPUTATION Removal of decay heat from the AP600 in the long term occurs via condensation of steam on the inside of the containment shell. The AP600 is equipped with gutters to return condensate formed on the containment shell as a result of heat transfer to the env.i onment, i The gutters can be used to direct condensate back into the IRWST. These gutters are not safety-related devices, so they cannot be credited in the SSAR design basis analyses if a more limiting condition exists when they are presumed inoperable.

I The AP600 SSAR Section 15.6 analysis of post-LOCA LTC does not credit condensate retum into the IRWST, except for one sensitivity case. When the gutter return is presumed, condensate retuming to the IRWST maintains the water level therein to some extent and extends the TRWST drain period. The increase in the hydrostatic head from the retumed condensate increases the rate of IRWST injection into the reactor vessel through the DVI lines WCAP-14857 March 1997 m:\3539w.wpf:1b-031197

. s 4-3 f l

i at any given time during the IRWST drain period. Furthermore, when the gutters are assumed to be effective in retuming condensate to the IRWST, injection from the containment  ;

sump with its lower liquid head is delayed by several hours during a LOCA LTC transient.

The " gutters unavailable" scenario is the more limiting case, and it was the scenario simulated in the OSU LTC tests.

For the SSAR LTC analysis, the ECOBRA/ TRAC calculation of RCS performance is interfaced with the EGOTHIC prediction of AP600 containment response during postulated LOCA events. EGOTHIC is a well benchmarked, state-of-the-art containment analysis computer code that is suited to the AP600 passive systems application. In the SSAR LTC analysis methodology, a EGOTHIC analysis is performed using the mass / energy releases defined as indicated above to provide the following information for use as boundary condition input to the ECOBRA/ TRAC long-term ECCS performance analysis window mode calculations: containment pressure, sump levels in the various containment compartments, and the liquid temperatures within those compartments. EGOTHIC is executed from time  ;

zero of the LOCA event using mass / energy releases as indicated in Figure 4-2 to generate the  ;

subject information for use in ECOBRA/ TRAC. The two computer codes are interfaced as j shown in Figure 4-2 to accomplish the analysis of breaks postulated to occur in AP600 ,

piping.

l EGOTHIC is applied in such a manner that it provides a conservative boundary condition for the ECOBRA/ TRAC computation. The noding of the lumped-parameter AP600  ;

EGOTHIC containment evaluation model is applied to compute not only the containment pressure transient but also the filling of the sump with liquid. The SSAR Subsection 15.6.5c '

LTC ECCS performance analysis use of EGOTHIC involves only containment phenomena I

for which EGOTHIC is already validated; the code version employed is the one used for the AP600 SSAR Section 6.2 analyses. However,in contrast with the AP600 containment  ;

integrity analysis, no penalties in heat transfer or in mixing / stratification modeling are ]

1 included for this application. The initial and bcundary conditions for EGOTHIC are conservatively established as follows to minimize the computed pressure in a manner consistent with the spirit of 10 CFR 50 Appendix K:

• Best-estimate heat sink heat transfer areas x 1.05

• Maximum passive containment cooling system (PCS) water flow external to containment

. Initial atmosphere values of 14.7 psia,99-percent humidity,120 F

- No single failure of any containment system device

. Best-estimate net free volume x 1.05 I

• Containment wall gutters do not return flow to the IRWST

= Communication between compartments as per the design

. PCS water temperature at the minimum value of 40*F

= Maximum PCS tank water volume WCAP-14857 March 1997 m:\3539w.wpf:1b-031197

44 / . .

l l ,

- Maximum wetting of the external containment surface .

No heat and/or mass transfer correlation penalties Modeling of the containment compartments using the EGOTHIC computer code is slightly different for the double-ended direct vessel injection (DEDVI) line break, which is the limiting break in terms of the earliest onset of sump injection. The EGOTHIC containment calculation considers the IRWST outlet spill flow as an additional source of liquid, which, together with the break flow from the reactor vessel, directly fills the appropriate passive core cooling system (PXS) compartment. For this case, EGOTHIC models a drain path that provides flow communication between the break compartment and the RCS loop compartment (see Figure 4-2). As the PXS compartment liquid level increases, flow is calculated to pass into the RCS compartment at the proper elevation. For cases in which the break is postulated to occur in the RCS loop compartment, check valves in the drain lines prevent backflow into PXS compartments.

The containment parameters from EGOTHIC are supplied to ECOBRA/ TRAC as boundary conditions at the time of the window being analyzed. An ECCS performance calculation of a sump injection window can employ constant values of the sump level and enthalpy as well as containment pressure because these parameters vary little during the 3000- to 4000-second l duration of the calculational windows. Any significant changes in the rate of condensation in j containment occur long before sump injection begins. The quantity of steam present and the l condensation behavior in the containment are essentially constant for the stable, quasi-steady-state condition that exists within the AP600 containment at the sump injection intervals of l interest for EC/T window-mode LTC ECCS performance calculations.

4.4 WCOBRAfrRAC LONG-TERM COOLING EMERGENCY CORE COOLING SYSTEM PERFORMANCE CALCULATIONS The EC/T nodalization of the AP600 is consistent with the modeling of the OSU Test Facility presented in the LTC validation report (Reference 1). In particular, the RCS hot legs and cold legs are modeled using the COBRA VESSEL component channels to make use of their two-fluid, three-field capability. A simplified reactor vessel noding that is consistent with that of Reference 1 is employed. The PXS and RCS loops are also modeled consistent with the Reference 1 test simulations.

Initial conditions are specified consistent with the input boundary conditions that were established in the EGOTHIC analysis. Among the conservatisms imposed to ensure compliance with 10 CFR 50 Appendix K are the following:

- ANS-1971 standard decay heat with +20 percent uncertainty

- Use of the locked-rotor reactor coolant pump K-factor

- Computation and use of a low containment pressure WCAP-14857 March 1997 m:\3539w.wpf:1b-031197

r . -- - - . . - . . . - __- . -_. _- - -

' 5 45 The 'AP600 SSAR LTC cases analyzed are outlined below. They represent a suitable spectrum of cases to demonstrate that ECCS performance of the AP600 complies with the requirement l of 10 CFR 50.46 to provide adequate core cooling in the long term post LOCA. The eight

.W. COBRA / TRAC analyses to be included in SSAR Subsection 15.6.5.4c are outlined below:

a. 2-inch cold-leg break encompassing the end of IRWST and start of sump injection

1. Base case-single failure of one ADS-4 valve path

2. Alternate single failure-a valve in the injection path

b. Double-ended cold-leg guillotine (DECLG) break

1. Continuation of the short-term transient to the 20-minute point, when the CMTs have drained beyond the ADS-1 to -3 actuation level and approach the ADS stage 4 actuation / IRWST injection actuation level

2. Based on conditions from (1) above, examine two condensate retum scenarios with an ADS-4 path failure:

a. Behavior once the sump level reaches the broken cold-leg pipe elevation, i modeling liquid delivery between the sump and downcomer through the break

b. Behavior assuming the gutters return containment condensate to the IRWST perfectly at the established temperature when the sump level nears, yet remains below the break elevation l

l c. DEDVI break - ADS stage 4 valve failure l

1. A PXS-only event, at the start of sump injection, during which the PXS compartment in which the break is located fills preferentially.

2. The corresponding case in which the normal residual heat removal system (RNS) pump activates to initially drain the IRWST rapidly but later fails at the sump

. injection inception. In this case, reliance on sump injection occurs even earlier than in C(1) above.

3. Later on during sump injection, the reduced sump head associated with complete intercompartmental leakage is modeled at an appropriate time. This is the so-called " wall-to-wall flooding" case.

l l

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4-6 t ,

The cases outlined above address the spectrum of possible LOCA break sizes and.the issues i of adverse nonsafety-related system operation interactions, potential passive failure (s) in the long-term that degrade the available sump liquid head, and identified Draft Safety Evaluation Report (DSER) open items.-

The 2-inch break represents a typical small-break LOCA event, in which the break size and location are insignificant with regard to long-term ECCS performance; the cooling mode is injection from the IRWST/ sump through both DVI nozzles, with energy removal through the ADS-4 flow paths. The initial window chosen is at the end of IRWST injection on into sump injection. Selection of this point in time bounds the injection phase of LTC because the IRWST level is at its minimum value. Upon receipt of the low-3 level signal from the IRWST, the sump isolation valves open. In the ECOBRA/ TRAC window, the IRWST injection is modeled for more than 1000 seconds until the plant steady-state condition for the specified boundary conditions is established. Conservatively low liquid levels are assumed in both the IRWST and sump and the case continues to be executed until the steady-state injection of saturated sump liquid in established. The window selected is bounding for long-term sump injection because the decay power level is at its maximum value for the prevailing sump level and temperature values. As required by Appendix K, the limiting single failure is established.

The initial DECLG break window is analyzed to establish that the CMTs provide the injection necessary to maintain the core in a quenched state once the accumulators are empty. The analysis is carried out beyond the actuation of ADS stages 1 to 3 using the detailed large-break LOCA WCOBRA/ TRAC model to establish the initial conditions for the later windows.

Once the CMTs drain to the low-low level and the IRWST isolation valves open, safety injection water at a greater driving head becomes available. The first of the later DECLG break windows identifies the impact of a large cold-leg break on calculated ECCS performance. The second of these windows addresses the possible effect of thermal stratification within the IRWST, in response to a request for additional information (RAI).

The double-ended rupture of a DVI line is the limiting-case break in terms of initiating sump injection at a maximum core decay heat level. By postulating a DVI break location immediately downstream of the IRWST/ sump isolation valves, the IRWST drain rate through the broken pipe is maximized. The overall drain rate of the IRWST is also maximized, and the time before the initiation of sump injection is minimized. The time to initiation of sump injection can be shortened even further by presuming that the nonsafety-related RNS pump operates. If this pump operated during the sump injection phase, abundant core cooling via pumped injection would be ensured; however, if the RNS pump is assumed to become unavailable at the start of sump injection, continued safety injection would occur solely by gravity. Because the DVI break location drains the IRWST most rapidly, it is the location for which intercompartmental leakage produces " wall-to-wall" flooding earlier than for other cases. A " wall-to-wall flooding" case in which the water level in the sump has equalized among all of the containment subcompartments will be presented in the SSAR.

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SUMMARY

i The preceding discussion provides an overview of the approach selected by Westinghouse to address the long-term cooling of the AP600 for LOCA transients. The complete process is addressed, from the development of the PIRT for AP600, through validation of the EC/T j code for the LTC calculations with OSU test data, and with a discussion of how EC/T will be applied to plant calculations to be documented in the SSAR. This report also provides a "roadmap" to appropriate references that contain additional details.

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l WCAP-14857 March 1997 m:\3539w.wpf:1b-031197

. i 6-1 6 REFERENCES

1. Garner, D. C., et al., "WCOBRA/ TRAC Long-Term Cooling Final Validation Report,"

WCAP-14776, November 1996.

2. Letter, B. McIntyre to T. Quay, "WC/T Long Term Cooling Letter Report,"

NSD/NRC-97-5014, March 1997.

3. Hochreiter, L. E., et al., WCAP-14171, Revision 1, "WCOBRA/ TRAC Code Applicability to AP600 Large Break Loss-of-Coolant Accident," October 1996. ,

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4. Fittante, R. L., et al., "NOTRUMP Final Validation Report for AP600," WCAP-14807, Revision 1, January 1997.

5. Forgie, A., et al., "WGOTHIC Application to AP600," WCAP-14407, September 1996.

6. Andreychek, T. S., et al., "AP600 Low-Pressure Integral Systems Test at Oregon State University Test Analysis Report," WCAP-14292, Revision 1, September 1995.

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