ML19327A653

Forwards Safety Evaluation of Topical Rept WCAP-11736, RHR Sys Autoclosure Interlock (ACI) Removal Rept, Denying Permission to Remove ACI for Any Plant

ML19327A653
Person / Time
Site:	Summer
Issue date:	08/08/1989
From:	Thadani A Office of Nuclear Reactor Regulation
To:	Newton R WESTINGHOUSE OPERATING PLANTS OWNERS GROUP
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ML19327A654	List: ML19327A653
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UNITED STATES

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. NUCLEAR REGULATORY COMMISE ON' I(

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August 8, 1989

.Nr. Roger A.- Newton, Chairman a

Westinghouse Owners Group l.

2310 W. Michigan Avenue r

Milwaukee, Wisconsin 53201 L

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Dear Mr. Newton:

W SUEJECT: ACCEPTANCE FOR REFERENCING WCAP-11736 REY. 0.0, RESIDUAL HEAT REMOVAL SYSTEM AUTOCLOSURE INTERLOCK (ACl) REMOVAL REPORT" IN PLAKr SPECIFIC SUBNITTALS

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'We have completed our review of the subject topical report submitted with your.

letter of April 22, 1988.

We have concluded that the information in this report can be used to supplement plant specific requests to remove the ACI

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from the Westinghouse plants covered by this' report.

HowcVer, the attached SER does not grant permission to remove the ACI for any plant; such permission trust be requested on a plant-specific basis.

We do not intend to repeat our review of the matters described in the reports s

L and found acceptable when the. reports appear as references in license applications except to assure that the material presented is applicable to the specified plant involvert. Our acceptance applies only to the matters i

described in the reports.

In accordance with procedures established in NUREG-0390, it is reqdested that Westinghouse publis1 accepted proprietary and non-proprietary versions of this topical report within 3 months of receipt of this letter. The acceptec versions should incorporate this letter and the encitsed evaluation between the title page and the abstract. The accepted versions shall include an -A (designated accepted) following the report identification symbol.

Should our criteria or regulations change, such that our conclusions as to the acceptability of the reports are invalidated, Westinghouse and/or the licensees referencing the topical reports will be expected to revite and resubmit their respective documentation, or submit justification for the continued effective applicability of the topical reports witk ;t revision of their respective documentation.

Sinc ely.

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AA db A2 ok Thadani,Wrector or Systems 011ision of Engineering & Systems Technology Office of Nuclear Reactor Regulation

Enclosure:

Safety Evaluation WCAP-11736 cc w/ enclosure:

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S/FETYEVALUATIONBYTHEOFFICEOFNUCLEARREACTOP.REGULATig RELATING TO REMOVAL OF AUTO CLOSURE INTERLOCK FUNCTION WESTINGHOUSE OWNERS GROUP WCAP.11736. VOLUME I AND'il 4

1.0 INTRODUCTION

i-ll By letters dated April 22, 1988 and January 3,1989 (References 1, 2) the -

736

-' Westinghouse Owners Group (WOG)' requested that.the staff review WCAP-11 Volume I and II, " Residual Heat Removal System Auto Closure Interlock Removal The staff review of this issue has focused on assuring that this-l Report."

gener.ic report is consistent with the staff position on the removat of the autoclosure interlock, as set forth in the staff's safety evaluation for DiabloCanyon(Reference 3).

2.0 EVALUATION 2.1 WCAP-11736 Summary WCAP-11736 was written with the support and funding of WCG.

It provides an evaluation of the removal of the autoclosure interlock (ACI) from suctio isolation valves in the residual heat removal system (RHRS) at four reference l

Salem Unit 1, Callaway Unit 1, North Anna Unit 1, and Stiearon Harris plants:

The WOG plants participating in the program were categorized into one Unit 1.

of four groups based on RHR$ configuration and design characteristics that were i

I similar to ors of the four reference plants. The plants listed by group cre:

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Group l'- Salem Unit 1 Group 2 - Callaway Unit 1 D

Salem Unit 2-Braidwood Units.1 & 2-D.C. Cook Units 1- & 2 -

Byron Units 2: 8 2 Indian Point Unit 3.

Catawba Units 1 & 2 1

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.McGuire Units 1 & 2 Comanche Peak Units 1412 1

Sequoyah Units 1 & 2 Trojan Unit'l a

Watts Bar Unit's l'& 2 seabrook Unit 1 l

Zion Units 1 & 2 Vogtle Units 1 6 2 Wolf Creek Unit 1 M111 stone' Unit 3 4

South Texas Units 1..& 2 f

Group 3 Nt;rth Anna' Unit 1 Group 4 - Shearon Harris Unit 1.

H.B. Robinson Unit 2 Farley Units 1 & 2 Turkey. Point Units 3 & 4 Beaver Valley Unit 2 Beaver Valley Unit 1 V.C. Sumer Unit I t

'N Prairie Island Units 1 & 2 North Anne Unit E

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Tne choice of the four particular reference plants was intended to provide the.

maximum number of the other WOG members with the best possible fit should they It.is choose to delete the ACI in the future and reference WCAP-11736.

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expected that, should a plant desire to delete the ACI, a plant-specirge analysis would still be required, but that substantially less resources would need to be expended to produr' and review this analysis if refertnce were made to WCAP-11736.

The staff agrees with this approach and gives some guidance by sumarizing the

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WOG position.ar what is expected in the plant-specific response.

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WCAP.11736 proposes to remove the ACI function from the RHR suction valves.

-The open permissive interlock will remain intact. An alarm will be added to

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each valve which ' ill actuate if the valve is open.

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is a plant-specific concern. A general rule for establishing the alarm setpoint, is that the setpoint prest.ure ba greater than the open permissive setpoint and less than the RHR system design pressure minus the RHR pump heaa pressure.

In a6dition, the status lights on the operator's panel, which indicate that these valves are open or closed, will remain functional after power has been removed from these valves.

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2.3 Procedura) Changes I V.e 4 d d &

wj WCAP-11736 proposes the following for generic procedural requirements:

Each plant will be expected to review its operating procedures to deterrr.ine the continued applicability of the procedures and to make any changes necessary, to ensure continued safe operation without the ACI. Plant operating Drocedures should be reviewed to determine the effect of removing the ACI and installing a control room alarm. Listed below are a few of the general procedures that'may require modification, p

- RHRS operating procedure

- Plant startup from cold shutdown operating procedure (s)

- P1snt shutdown from hot standby operating procedure (s)

- Alarrn surveillance procedures (to include the new alarm)

- Leak rate testing procedures (caution regarding alarms and power removal from RHR suction valves) h L

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~ In addition, the alarm response procedure used during plant startup should be modified to reflect the appropriate (new) alarm recognition and responses for 2

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the added alarm. The procedure should be revised to direct the operator to' L

take the necessary actions to close the open RHR suction' valve (s), if they are l

not closed following alarm actuation during normal startup operations.

If this is not possible, the operator should be instructed to not pressurize further i

.and return to the safe-shutdown mode of operation.

7 The staff agrees with this generic guidance assuming a surveillance procedure L

for the RHR suction valve alarms is added to ensure these alarms remain l

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operable, but in addition, the staff believes that further protection can be

. achieved by removing. power from the RHR suction valves before they are leak-checked in order to ensure that they remain-in the tested configuration.

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this.is not feasible for a particular plant, the reasons for not performing I

this procedure should be justified in the plant-specific submittal.

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2.4 Reasons for Removing Autoclosure Interlock The main reasons for removing ACI have been previously elucidated by the staff in tne AEOD report, " Decay Heat Removal Problems at U.S. Pressurized Water Reactor.s"-(Referen:e 4). This rcport points out that of the 130 loss-of-RHR events that were documented at U.S. pressurized water reactors (PWRs) between 1976 and 1983, 37 were caused by the automatic closure of the suction / isolation valves. The AE00 report also quotes a Sandia Laboratory study (Reference 5) that evaluated the competing risks associated with RHR suction / isolation valve closure and Event V.

Sandia concluded that:

"The lowest core melt frequency due to the combination of loss of RHR suction during cold shutdown and V-LOCAs is obtained when there are no autoclosure interlocks on the RHR suction valves... removing the overpressure interlocks from the RHR suction valves gives the best RHR suction arrangement for PWRs based upon this analysis.

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...when interlocks are present, loss of RHR suction is the largest contributor to core melt frequency for all assumed values of P(CM-LRHRs). However, when the interlocks are not present, the core I

melt frequency due to loss of RHR suction is comparable to or less than the V-LOCA core melt frequency for the "best estimate" cases."

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The AE00 report concluded that even though it was most likely a good idea to remove Atl, the effects of ACI removal upon plant safety must be evaluated on a plant-by-plant basis because of numerous plant-specific differences. The WOG submittal (Reference 1) contains such a plant-specific analyses for four different groups of plants.

An additional benefit associated with removal of ACI is that the isolation valves'will remain open during low-temperature overpressurization (LTOP) events which allows the RHR relief valves to relieve pressure and aid in the LT0p protection of the RCS.

2.5 Safety Function of the Autoclosure Interlock The WOG has shown in Reference 1 that the RHR relief valves have adequate capacity-to mitigate pressure transients that occur during RHR operation.

l Therefore, the purpose of ACI is to ensure that there is a double barrier between the RCS and the RHRS when the plant is at normal operating conditions (hot and pressurized) and not in the RHRS cooling mode. The ACI function is to preclude conditions that could lead to a LOCA outside of containment, Event V, due to operator error. The sequence that concerns the staff in particular is that case in which the operator closes one of the isolation valves and not the other, since if both valves were left open, the operator would not be able to pressurize the plant.

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6 l-2.6 Probabilistic Risk Assessment of the Event V Sequence and Safety Analyses of Transients o

WCAP-11736 provides a plant-specific probabilistic risk assessment (PRA) for thefourleadplants(Salem,Callaway,NorthAnna,ShearonHarris);these l

analyses follow the PRA format previously reviewed by the staff for Diablo Canyon (Reference 3). The results ofa these analyses agree with the Diablo p

Canyon finding that the proposed configuration with the ACI removal and an I

alarm added will result in a lower probability for the Event Y sequence than-the present plant configuration. The staff has no requirements based on the absolute values in the PRA analyses and will not require a plant-specific PRA l

from each. licensee proposing to remove the ACI. However, the licensee should i

do sufficient PRA and safety analyses to ensure that its plant will not show results that will invalidate the conclusions of WCAP-11736.

l 3.0 STAFF POSITION The staff finds that the removal of the ACI for Westinghouse plants covered by t

WCAP-11736 can produce a net safety benefit provided that the following five key improvements are in place. Furthermore, the staff finds that WCAP-11736 may be referenced in the licensee's plant-specific submittals to show compliance L

with those items that are not plant specific. However, this SER does not grant permission to remove the ACI for any plant.

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c (1) An alarm will be added to each RHR suction valve which will actuate if the l

valve is open and the pressure is greater than the open permissive I

setpoint and less than the RHR system design pressure minus the RHR pump head pressure [ justified by WCAP-11736].

(2) Valve position indication to the alarm must be provided from the stem-mounted limit switches (SMLSs) and power to the SMLSs must not be affected by power lockout of the valve [ justified by WCAP-11736].

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(3) The procedural improvements described in WCAP-11736 should be implemented.' -

Procedures themselves are plant specific.

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(4) Where feasible, power should be removed from the RHR suction valves prior j

to their being leak-checked [ plant specific).

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c (5) The RHR suction valve operators should be sized so that the valves cannot be opened aga, inst full system pressure [ plant specific).

4.0 REFERENCES

(1)- N. L. Burns (et al.), " Residual Heat Removal System Autoclosure j

interlock Removal Report for the Westinghouse Owners Group " February

-1988, WCAP-11736, Revision 0.0.

(2) Letter, Roger A. Newton (WOG) to M. W. Hodges (f;RC), January 3,1989.

(3) Letter, Harry Rood. (NRC) to J. D. Shiffer (PGE), " Safety Evaluation of Removal of RHR Autoclosure Interlock Function and Installation of an Alarm at Diablo Canyon Units 1 & 2," February 17, 1988.

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(4)

H. O. Ornstein, " Decay Heat Removal Problems at U.S. Pressurized Water Reactors," December 1985, AEOD/C503.

1 (5)

D. R. Gallup, D. M. Kunsman, M. P. Bohr; Sandia National Laboratories, j

" Potential Benefits Obtained by Requiring Safety-Grade Cold Shutdown j

Systems," NUREG/CR-4335, July 1985.

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The proposed interlock change for V. C. Summer removes the autoclosure l'

interlock feature from the RHRS suction / isolation valves (8701A/B, 8702A/B).

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All other valve interlock features described in Section 2.3 of this report l

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remain in place.

With removal of the autoclosure interlock feature, valves 8701A&8 and 8702A&B will not close automatically on increasing RCS pressure greater than the valve closing setpoint. Alarms will be added (for each RHRS I

suction / isolation valve) that actuate in the main control room given a " VALVE dp NOT FULL CLOSED" signal in conjunction with a "RCS PRESSURE-HIGH" signal.

The intent of the alarms is to alert the operator that a RCS-RHRS, series, suction / isolation valve (s) is not fully closed, and that double valve

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isolation from the RCS to the RHRS is not being maintained.

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ndication to the alarm must be provided from the valve limit switches and power to the limit switches must not be affected by power lockout to the valve.

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L The proposed interlocks for valves 8701A&B and 8702A&B are shown functionally on Figure 3-1.

In addition, the proposed valve interlock changes are shown on the elementary wiring diagrams in Figures 3-2, 3-3, 3-4 and 3-5.

The only change to the valves interlock and circuitry is to remove the autoclosure portion of the interlock and add a control room alarm; the valves open l

permissive circuit will not be altered.

In summary, the proposed V. C. Summer interlock changes provide deletion of the autoclosure interlock feature from the RHRS suction / isolation valves, while still meeting the regule *equirements to retain the open permissive portion of the interlock.

t ;;e.on, the change provides a control room alarm to alert the operator if e i suction / isolation valve is not fully closed, and provides justification for elimination of power lockout of the suction / isolation valves during shutdown.

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