ML20136G193

Forwards Comments on NRC Rept on Probabilistic Safety Study, Per 850826 Request.Two Suggestions in Encl Warrant Implementation.Third Suggestion Re Mod of diesel-driven Containment Spray Pumps Not Cost Justifiable

ML20136G193
Person / Time
Site:	Zion  File:ZionSolutions icon.png
Issue date:	11/19/1985
From:	Reed C COMMONWEALTH EDISON CO.
To:	Harold Denton Office of Nuclear Reactor Regulation
References
0899K, 899K, NUDOCS 8511220278
Download: ML20136G193 (7)

Text

N) Commonwealth Edison

One hrst N; hon

I Plan, Chicigo libnojs t 7 Address Reply to. Post Office Box 767

/ Chicago, Ilknois 60690 November 19, 1985 Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, DC 20555

Subject:

Zion Nuclear Power Station Units 1 and 2 Comments on ZPSS NRC Docket Nos. 50-295 and 50-304 References (a): August 26, 1985 letter from S. A. Varga to D. L. Farrar.

(b): September 8,1981 letter from L. O.

DelGeorge to H. R. Denton.

(c): November 18, 1985 letter from Cordell Reed to H. R. Denton.

(d): January 27, 1984 letter from Cordell Reed to H. R. Denton.

Dear Mr. Denton:

The purpose of this letter is to transmit Commonwealth Edison Company's coments on the NRC Staff's report on the Zion Probabilistic Safety Study as requested by reference (a). These comments also address the recommendations for altering Zion's design and operations.

The Zion Probabilistic Safety Study (ZPSS) was originally supplied to the NRC Staff by reference (b). The Staff's review of the ZPSS, along with a request for comments, was transmitted to Commonwealth Edison Company by reference (a). The Staff's report also contained a number of suggestions for increasing the level of safety at Zion Station.

Commonwealth Edison's general comments on the Staff's review of the ZPSS are contained in Attachment 1. In general, Commonwealth Edison Company is impressed with the quality of the Staff's review and in agreement with the report's conclusion that Zion Station poses no undue risk to public health and safety. The report also states that its conclusion does not depend upon the continued enforcunent of the Confirmatory Order of February, 1980. Commonwealth Edison has prepared an amendment request to runave the Confirmatory Order and has submitted it to the NRC in reference (c).

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H. R. Denton November 19, 1985 A discussion of the Staff's specific suggestions is supplied in Attachment 2. Connonwealth Edison has determined that two of these suggestions warrant implementation. The third suggestion, modification of the diesel driven containment spray pumps, is not cost justifiable and will not be implemented.

If there are any further questions regarding this matter, please contact Connonwealth Edison Company's Nuclear Licensing Department.

Very truly yours, Cndt Re.m,}s c Cordell Reed Vice-President im i

Attachments cc: NRC Resident Inspector - Zion J. A. Norris - NRC 0899K

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ATTACHW NT 1 CEERAL COMMENTS ON THE NRC STAFF'S REVIEW OF THE ZPSS l

l Commonwealth Edison Company has reviewed the NRC Staff's report on  ;

the ZPSS. In general, Commonwealth Edison is impressed with the quality of the Staff's review and in agreement with the Staff's conclusion that Zion l does not pose an undue risk to public health and safety. Further, we agree l that the conclusion does not depend upon the continued enforcement of the Zion Confirmatory Order of February,1980. As a result, a request to delete the Order has been prepared and is being submitted separately.

The Staff's specific suggestions for further risk reductions are addressed in detail in Attachment 2. The specific comments that have resulted from our review are listed below. In general, the comments involve issues which are the subject of ongoing study. Commonwealth Edison recognizes that the NRC Staff is also considering these issues. However, we  ;

believe the eventual resolution of these items will result in the reduction of Zion's Core Damage Frequency (COF). j f

1. The Staff's report indicates that the loss of component cooling water  !

(CCW) event is a dominant contributor to the core damage frequency '

(CDF). Table 3.2 assigns a CDF of 1 x 10-4 to this event, which represents over 60% of the report's total CDF estimate. Use of this COF for the loss of CC# event is thought to be overly conservative, for the following reasons.

a. The Westin0 house Owners Group has been performing tests to determine the likelihood and magnitude of reactor coolant pump (RCP) seal leakage upon loss of seal cooling. Preliminary results that indicate the earlier assumption that the RCP seals will leak at a rate of 300 gpm within 30 minutes after a loss of seal cooling ,

is overly conservative. In fact, the RCP scals may be able to l tolerate on extended loss of cooling without appreciable leakage.

Without the assumption of an RCP seal LOCA, the report's total C0F  ;

would decrease by a factor of 4 to a value of 4 x 10-5 (Table  ;

3.10).

a. The loss of CCW event assumes the failure of the high pressure injection pumps (safety injection and charging pumps). However, Zion has implemented procedures and training for the loss of the CC# event, which instruct the operator to secure the high pressuru '

injection pumps before pump dama00 can occur. The procedure also provides instruction for systematically identifying and isoluting the leak, for refilling and restarting the CC# system, and for re-establishing h10h pressure injection. Therefore, it would appear appropriate to take some credit for recovery from the loss of CCW t. vent, which would further reduce the CEF.

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l 2. The report indicates that loss of off-site power events are a dominant l

contributor to the overall core damage frequency. These events comprise seven of the fourteen dominant sequences identified in Table 3.2, and .

i are assigned a C0F of 1.2 x 10-4 in Table 1.1. Use of this C0F for '

these events is thought to be overly conservative because it is based  !

i solely on generic estimates for the frequency of loss of off-site power from Sandia studies, with no credit given for Zion-specific performance

, data and design features. This is not consistent with the treatment of I

other events in the ZPSS, for which both generic and plant-specific data -

are used whenever possible in the derivation of event frequencies. The appropriate use of plant specific data has been essential to the performance of all other plant-specific PRA's and is endorsed by the (

NRC's PHA Procedure Guide (f0 REC /CR-2300). It would appear inappropri-atu to arbitrarily impose, for certain specific accident secuences, the use of a diffurent methodology not consistent with accepted inoustry and NRC practice. j

3. The report indicates that fires are a significant contributor to the i core damago frequency (5 x 10-b). Use of this COF is overly conservative, for the following reasons:

a. The Sandia analysis assuned that a fire in the cable spreading room (

would lead to a loss of all component cooling water, auxiliary l reedwater, h10 h pressuru injection, and containment cooling.

Subsequent evaluation perfarined as part of the Appendix R review I process revealed that, even before the implenentation of any Appendix R modifications, such an event could not occur; 1.0., the necessary power and control cables for those components were not all routed through the nrune fitu area. Followin0 the completion of committed Appendix R modifications, at least one train of euch of the above systems will remain operable following a fire in the i cable spreading room or any other fire area,

b. The Sandia analysis is ulso based on thu assumption of an RCP seal I l.0CA leading to core darmgo upon loss of seul cooling.

4. The report indicates that the suismically induced loss of all AC power ,

event externalrepresents events (Table virtually)all 1.1 . This the result early fatality risk to is thought associated be overly with i conservative, for the following reasons:

a. This event assunen un RCP seal LOCA leading to coro danage upon loss of sual cooling,

b. Work performed urdur thu Industry Degraded Core Rulemaking Program (10COR), which included a re-evaluation of fission product releases from coro darnage events, has demonstrated that thu early fatuilty risk from coro damago events is troch loss than previously bolleved.

l This work is currently being reviewud by the NkC.

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5. The report expresses a concern about the ability of the containment I cool,ing systems to function for a long time after a core melt accident.

A specific concern is raised about the potential for plugging the filters associated with the containment air coolers. These fliters have  !

been removed by a modification. Therefore, there is no possibility of '

them being plugged by particulates from a core melt accident.

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I ATTACF#{NT 2 f ASSESSNENT OF THE STAFF'S SPECIFIC RECOMMENDATIONS I

The NRC Staff's review of the ZPSS was transmitted to Commonwealth Ecison by Heference (j). This report concluded in Section 1.3 that Zion posed no undue risk to public health and safety. However, the Statf i l identified three actions that, if performed, would reduce the level of risk I

( at Zion Station. These proposed actions are as follows; l t

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1. Continue to test the Residual Heat Removal (RHR) System check I

valves. '

2. Implement testing of the Rm suction valves. ,

3. Modify the diesel containment spray pumps to be totally indepencent of AC power.

Commonwealth Edison's response to each of thesu throu suggestions is provided below:

TESTING OF HHH CHECK VALVES Commonwealth Edison Company has previously acknowledged thu value of this testing in References (b) and (d). This testing has been ,

incorporated into station procedures. Zion Station will continuu to perform F these tests subsequent to the formal removal of Ittm A.5.

TESTING OF HHH StX TION VALVE 5 Corunonwealth Edison has revluwed the desih of the two valves in I question and has compared them to the class of valves used to generate the assumed failure rates. It appears likely that if a reas9essment were to be porformed, the valvo specific fulluto rate would be substantially lower than the rates used in thu ZPS$. This would result in u lower core melt frequency for the affected sequencus. >

However, Coneonwealth Edison agrees that the integrity of these two valvos is important to reactor safety. In order to provido a periodic check of these valvo's integrity, Zion Sthtlon's procucures will be altured to includo a leak test each timo the RHR system is pinced in service durin0 1 normal cooldown.

Thu test will involvu ob u rving RHR nystem pressure with one suction valvo open and onu closed just prior to placing the RHH systeai in service, typically, the differential pressure across the valvu will be about 350 psi ut this time. Lihu opening sequence of the valves will be varied to unsure l that both suction valves are tested ovot a purlod of tino. This proceduro l chango is expected to be in plocu by February 1,1986.

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,o MODIFICATION OF DE DIESEL CS PUW

( This modification would involve the diversion of approximately 50 i gpm of the discharge from the containment spray pump to cool the pump's oil cooler. This oil cooler is currently cooled by a portion of the discharge from the electrically driven Service Water Pumps. It also would involve the addition of battery power to control containment spray system valve operators. This modification is intended to increase the probability that containment integrity will remain intact after a core melt accident und an l

l extended loss of AC power.

l l Commonwealth Edison believes that the potential exposure savings of

1530 person-rem / unit is overstated. That estimate was based on the overly I

conservative estimates for RCP seal LOCAs and the frequency of loss of off-site power. Both of these issues are discussed in Attachment 1.

In addition, the IOCOR prcgram has demonstrated that, as a result of the re-avtsluation of the fission product releases from core damage events, thu expected population do;,e is mudi less than previously believed. Based on the 10COR work, it is estinnted that the modification to the diesel contuirvnent spray system would result in a reduction in the expected population dose of less than 72 person-rem / unit over the remainin0 life of the plant. A detulled cost estimate for the modification has also been performed which indicates that tne cost #0r altering both Zion units will be approximately *,,435,000 in 1984 dollars. Utilizing the same cost / benefit analysis method as thu NRC Staff results in a maxinum justifiable modification cost of $144,000 for both units, based upon these results, the inodification cannot be justified.

Thus, Comconwealth Edison does not intend to proceed with this change.

However, it is recognized that the NFC is completing its source term and severo accident work. It in our understanding that, this work will be publirhed in 1986. Commonwealth Ldison believes that this work will support i.he above conclusion.

0099K

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