ML20205F632

Safety Evaluation Supporting Amends 70 & 56 to Licenses NPF-4 & NPF-7,respectively

ML20205F632
Person / Time
Site:	North Anna
Issue date:	10/25/1985
From:	Office of Nuclear Reactor Regulation
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ML20205F628	List: ML20205F623 ML20205F632
References
NUDOCS 8511120344
Download: ML20205F632 (4)

Text

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UNITED STATES NUCLEAR REGULATORY COMMISSION

j WASHINGTON, D. C. 20555

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NOS. 70 AND 56 TO FACILITY OPERATING LICENSE NOS. NPF-4 AND NPF-7 VIRGINIA ELECTRIC AND POWER COMPANY OLD DOMINION ELECTRIC COOPERATIVE NORTH ANNA POWER STATION, UNITS NO. 1 AND NO. 2 DOCKET NOS. 50-338 AND 50-339 l

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Introduction:==

By letters dated March 29 and July 1,1985, the Virginia Electric and Power Company (the licensee) requested a temporary change to the Technical Specifications (TS) for the North Anna Power Station, Units No. I and No. 2 (NA-1&2). Specifically, the Condition for Operation (LCO) proposed change would revise the Limiting from 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to 168 hours0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br /> for which one out of two Service Water System (SWS) headers could be out of service. The licensee is requesting this temporary change in order to expedite and perform a mechanical cleaning and refurbishing program for removal of corrosion j

products on the inner surfaces of the SWS piping and valves. A detailed description of the proposed mechanical cleaning program was submitted to the NRC by the licensee in a letter dated February 27, 1985. The licensee's safety evaluation supporting the temporary TS change was based on a probabilistic safety assessment of the NA-182 SWS. Our discussion and evaluation of the licensee's submittal is provided below.

Discussion:

l The licensee's probabilistic safety assessment concluded that increasing the

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number of SWS pumps and auxiliary SWS pumps required to be operable during the 168 hour0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br /> LCO provided an increased reliability of the SWS even though the allowable outage time for one SWS header was increased from 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to 168 hours0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br />.

The probabilistic safety assessment of the NA-182 SWS consisted of a system description and definition of success criteria, and qualitative and quantitative analysis. The quantitative analysis consisted of system fault trees, a data base for the possible hardware failure rates and human errors, comon cause failures, and the resultant minimal cut sets for the dominant failure modes of the SWS. The system unavailability for the SWS and those safety-related systems that require SW flow (recirculation spray heat exchangers and the charging pumps) were calculated for four different SWS operating conditions. The four configurations analyzed were:

1) both kD p[

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. headers in operation; 2) one header out of service for a 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> period;

3) one header out of service for a 168 hour0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br /> period; and 4) one header out of l

service for a 168 hour0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br /> period with an increase in the number of SWS pumps required to be operational. Condition 2 above represents the current Technical Specification LCO on SWS header operability and Final Safety Analysis which requires that 2 out of 4 main service water pumps and 1 of 2 auxiliary SWS pumps be operational. The fourth condition mentioned above is based on 3 of 4 main SWS pumps and 1 of 2 auxiliary SWS pumps operational during the proposed mechanical cleaning program. The probabilistic study performed by the licensee showed that the reliability of the SWS and SW supply through the recirculation spray heat exchangers and charging pumps for condition 4 was higher, by approximately 10-50 percent, than for condition 2, which is the current LCO for the NA-182 SWS.

The licensee also analyzed the change in reliability of the component cooling water (CCW) heat exchangers which are cooled by the SWS. Although the CCW heat exchangers are not needed to mitigate the consequences of a Design Basis Accident (DBA), they are important to the temperature reduction of the reactor coolant from a hot shutdown state to a cold shutdown state. The reliability decreased somewhat (by approximately 70%) for the SW flow through the CCW heat exchangers from condition 4 described above. When one header is out of service, the cooling flow to the CCW heat exchanger is vulnerable to a blockage failure in either of two valves in the working header. The CCW heat exchanger system reliability decreased because the benefit of the extra SW pump was not enough to overcome the incremental vulnerability to the failure of the two particular valves. The risk contribution of maintaining the plant in a hot shutdown state for the brief period of time until SW can be restored to the CCW heat exchangers is judged to be negligible.

Based on these results, the liceasee concluded that the temporary TS request would not result in an overall decrease in plant safety during the mechanical cleaning program.

The staff's review of the information contained in the March 29, 1985 submittal resulted in a number of questions regarding the methodology used in developing the fault trees and data base and the results of the minimal cut sets. These questions were transmitted to the licensee in a letter dated August 9, 1985. By letter dated August 26, 1985 the licensee responded to the staff's request for additional information.

With the exception of including in the fault trees the conditional events that alternate SW pumps and emergency diesel generators are not in maintenance, the staff concludes that the methodology utilized in the study is correct. The staff disagrees with the inclusion _ of this type of condition in fault trees because it leads to minimal cut sets that have events that do

. not contribute to system failure. However, the staff agrees that this inclusion, calculated by the licensee to be approximately 9%, does.not exceed the difference between unavailabilities for conditions 2 and 4 and will not significantly change the final results. Therefore, the staff finds the system fault trees, minimal cut sets, and the data base as used to be acceptable.

, For completeness, the stsff also examined the potential for other adverse effects that the mechanical cleaning program could have on plant operations beyond those examined by the licensee. The staff examined the potential for adverse effects from the flooding of essential areas and equipment through the open SW train (due to an accidental connection with the intact train).

The procedures and steps to be taken by the licensee to prevent and, if necessary, mitigate the effects of flooding during the mechanical cleaning program include the following:

1)

The licensee will utilize administrative controls such as " tag out" procedures, locking closed those valves (in lines greater than 18" in diameter) that could connect the open SW train with the operating train.

Also, constant on-location surveillance by plant personnel will minimize the possibility of c flood occurring.

2)

'The charging pump cubicles will be sealed to the 44 inch level to provide the operators with ample time to isolate the open line before the charging pumps are flooded. The flow through the open train can be terminated by using a blind flange that can be slid over the open end, by re-closing the valve interconnecting the two SWS trains (if inadvertently opened), or by stopping the SW pumps.

3)

Certain MOVs and pumps in systems used to bring the plant to a cold shutdown state could be damaged by the postulated flood. Analyses and procedures provided by the licensee show that the necessary cold shutdown equipment could be restored to service within approximately two days.

Based on the above information provided by the licensee, the staff concluded that the licensee could successfully maintain the plant safely in a hot shutdown condition, but that the possibility exists of losing certain cold shutdown equipment if a flood should occur.

Evaluation:

The staff concludes that temporarily extending the LCO for one header of the NA-1&2 SWS from 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to 168 hours0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br /> is acceptable from a probabilistic risk viewpoint. The basis for this conclusion is as follows:

1.

The end result of the mechanical cleaning program will be a more reliable and efficient SWS.

2.

The temporary TS change request will include a LC0 on the SW pumps requiring 3 out of 4 main SW pumps and 1 out of 2 auxiliary SW pumps to be operable while the 168 hour0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br /> LC0 is in effect. This restriction on removing SW pumps from service will more than negate the increase in the allowable header outage time. The above statement is verified by the licensee's probabilistic safety assessment which the staff has reviewed and finds sufficiently complete and reasonably representative to assess the differential system unavailability between the present and proposed SWS arrangements.

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.. l 3.

Although not assessed numerically, the staff considers the incremental probability of plant damage due to flooding through an open service water line during tile outage time from 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to 168 hours0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br /> to be of low order if proper administrative controls on " tag out" of components in conjunction with increased surveillance by plant personnel is implemented. Finally, the risk contribution from maintaining the plant in a hot shutdown state for several days while repairs are made to cold shutdown equipment is judged to be negligible.

Therefore, based on the above, we find acceptable on a temporary basis, the allowable time that one of the two NA-182 SWS loops can be inoperable to hereby be revised from 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to 168 hours0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br /> provided 3 out of 4 service water pumps and 1 out of 2 auxiliary service water ;, umps are operable during the 168 hour0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br /> LCO. The period of time permissibb for an LCO of 168 hours0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br /> is applicable for only such time as may be requ hed to complete the NA-182 SWS i

upgrade. Finally, the NRC will monitor the licensee's administrative process and surveillance activities for mitigating the possible occurrence of flooding during the time as may be required to upgrade the NA-182 SWS.

Environmental Consideration:

These amendments involve a change in the installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20.

The staff has determined that the amendments involve no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Comission has previously published a proposed finding that these amendments involve no significant hazards consideration and there has been no public coment on such finding. Accordingly, these amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR 651.22(c)(9).

Pursuant to 10 CFR 951.22(b), no environmental impact statement or environmental assessment need be prepared in connection with the issuance of these amendments.

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Conclusion:==

We have concluded, based on the considerations discussed above, that (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (2) such activities will be conducted in compliance with the Comission's regulations, and the issuance of the amendments will not be inimical to the common defense and security or to the health and safety of the public.

Date: October 25, 1985 Principal Contributors:

M. Fields, RRA8 P. Hearn, ASB s

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