ML20105A894

Forwards Rev 2 to PRA North Anna Power Station,Svc Water Preservation Project,Part One, Final Rept & PRA for North Anna Power Station,Svc Water Preservation Project,Part 1, Supplemental Rept,Per Environ Qualification of CR Chillers

ML20105A894
Person / Time
Site:	North Anna
Issue date:	09/11/1992
From:	Stewart W VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
Shared Package
ML20105A897	List: ML20105A894 ML20105A900 ML20105A904
References
92-450A, NUDOCS 9209180036
Download: ML20105A894 (12)

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YlltGINI A ELI:CTItIC AND POWi-:lt CO)fl'ANY N I C11.St O N D, Yl](O INI A O H L* 6 )

September 11, 1992 U.S. Nuclear Regulatory Commission Serial No. 92-450A Attention: Document Control Desk NL&P/JBL: R3 Washington, D.C. 20555 Docket Nos. 50-338 50-339 License Nos. N PF-4 NPF-7 Gentlemen:

VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION UNITS 1 AND 2 EXEMPTION REQUEST FROM THE REQUIREMENTS OF 10 CFR 50.49 ENVIRONMENTAL QUAllFICATION OF CONTROL ROOM CHILdBS DURING SERVICE WATER SYSTEM RESTOR ATION PROJECT Virginia Electric and Power Company has informed the NRC of plans to perform extensive refurbishment activities for restoration of cortain portions of the service water system common to North Anna Units 1 and 2. By letter dated July 16,1992 (Serial No.92-450), a temporary exemption pursuant to 10 CFR 50.12 was requested from the requirements of 10 CFR Part 50, Appendix A, Criterion 2 (GDC-2), " Design basis for protection against natural phenomena," for North Anna Power Station Units 1 and 2 for implementation of Phase 1 of the service water system restoration project. The basis for that request and supporting justification were provided in Attachment 2 to our July 16,1992 letter.

We have recently reached the 70% review milestone in developing the detailed design change package that supports the activities described in our July 16,1992 letter. During that review, an additional issue was identified for which a temporary exemption from the governing regulation appears to be the most appropriate resolution. This issue is further discussed below.

During the first stage of the service water restoration project, it will be necessary to isolate service water from the Unit 1 control room chillers for the majority of the North Anna Unit 1 steam generator replacement outage. It is planned to provide a temporary water supply and return path to the Unit 1 control room chillers from the common bearing coo!ing water system to provide normal control room temperatures and provide a reliable backup cooling system to the Unit 2 air conditioning design basis.

However, it was recently determined that this will affect the normal environmental qualification design basis for the Unit 2 control room chillers. Therefore, pursuant to 10 CFR 50.12(a), Virginia Electric and Power Company supplements its previous request and further requests a temporary exemption from the requirements of 10 CFR 50.49, Environmental Qualfication of Electric Equipment important to Safety for 920918o036 92o911 PDR ADOCK 05000338 0 P PDR 1 ,

Nuclear Power Plants, for environmental qualification of the North Anna Unit 2 control room chillers for approximately the duration of the North Anna Unit 1 steam generator replacement outage.

The provisions of 10 CFR 50.12 provide that specific exemptions from the requirements of 10 CFR Part 50 may be granted provided the exemptions are authorized by law, are consistent with the common defense and security, are accompanied by special circumstances, and do not present an undue risk to the public health and safety. Virginia Electric and Power Company concludes that the activities sought to be conducted under this exemption request are clearly authorized by law 1 and are consistent with the common defense and security.

As described in 10 CFR 50.12(a)(2), special circumstances must be present for the NRC to consider granting an exemption. Three of the examples of special circumstances stated in the regulation apply in this case. The first special circumstance is that compliance would result in undue hardship or other costs that are significantly in excess of those contemplated when the regulation was adopted. The second special circumstance is that the completion of the project as proposed would result in an overall benefit to the public health and safety that compensates for any decrease in safety that may result from the granting of the exemption. The third special circumstance is that the exemption provides only temporary relief from the applicable regulation and that the licensee has made good faith efforts to comply with the regulation. Further description of these special circumstances is provided in the attachment to this letter. In addition, the attachment provides information on the environmental qualification requirements for the Unit 2 control room chillers and an evaluation that ensures the effects resulting from the implementation of this temporary exemption will not adversely affect the health and safety of the public.

The changes to the facility will be in the form of temporary piping to the Unit 1 control room chillers from the bearing cooling system to provide normal temperature controlin the control room and provide a reliable backup cooling system to the Unit 2 air

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conditioning chillers. These proposed temporary changes have been evaluated in accordance with the requirements of 10 CFR 50.59. Conditional on the acceptance of this exemption request,it has been determined that the changes to the control room air conditioning system as described in the attachment do not involve an unreviewed safety question. This exemption request has been reviewed and approved by the Station Nuclear Safety and Operating Committee and has been reviewed by the Management Safety Review Committee.

In as much as the criteria established by 10 CFR 51.21 may require the NRC to perform an environmental assessment for the regulatory action of granting this temporary exemption request, we have reviewed the proposed temporary plant modifications and determined that they will have no significant effect on the quality of the human environment. A discussion of our evaluation is provided in the attachment.

North Anna Unit 1 is currently scheduled to conclude Cycle 10 operation and begin the steam generator replacement outaga on January 2,1993. In our July 16,1992 letter, we requested your approval of the exemption request from GDC-2 by November 13,1992 to support implementation of Phase I, Stage 1 of the service water restoration

project. We request NRC approval of this supplemental exemption request from the requirements of 10 CFR 50.49 by that date.

If you have any questions or require additionalinformation, please contact us.

Very truly yours,

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W. L. Stewart Senior Vice President - Nuclear Attachment cc: U.S. Nuclear Regulatory Commission Region 11 101 Marietta Street, N.W.

Suite 2900 Atlanta, Georgia 30323 Mr. M. S. Lesser NRC Senior Resident inspector North Anna Power Station w

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l ATTACHMENT i

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i TIMPOR ARY EXEMPTION FROM 10 CFR' 50.49 ' REQUIREMENTS l FOR THE UNIT 2 CONTROL ROOM AIR CONDITIONING CHILLERS

! SERVICE WATER SYSTEM RESTORATION PROJECT l

NORTH ANNA POWER STATION l

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VIRGINIA ELECTRIC - AND POWER COMPANY i

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TEMPOR ARY EXEMPTION FROM 10 CFR 50.49 REQUIREMENTS FOR THE UNIT 2 CONTROL ROOM AIR CONDITIONING CHILLERS SERVICE WATER SYSTEM RESTORATION PROJE.Q.I

NORTH ANNA POWER STATION 1.0 B ACKGROUND 1 Virginia Electric and Power Company is planning an extensive refurbishment project for the existing uncoated, buried, and concrete encased 24-inch service ,

water pipe sections. As discussed in our letter dated July 16,1992 (Serial No. l 92-450), the overall objective of the project is to clean and restore internal pipe j surfaces as required to assure continued structural integrity and to apply a protective coating to minimize or eliminate further corrosion. in general, the refurbishment process will only be used on concrete encased pipe sections.

4 Sections of 24-inch pipe that are direct buried will be replaced with new piping similarly coateJ internally and protected externally from corrosion. Attachment 2 j to the July 16,1992 letter described the sequence of work activities mquired to perform the repair and replacement activities on these pipe sections varing the upcoming Unit 1 steam generator replacement outage.

As part of Phase I, Stage 1 of the service water restoration project, it is proposed to isolate the service wa.er headers to the Unit 1 recirculation spray heat exchangers during the Unit 1 1993 steam generator replacement outage. As discussed m our August 25,1992 meeting, isolating-service water from the recirculation spray heat exchangers will also temporarily isolate the service

water supply and return to ine Unit 1 control room chillers. To maintain normal control room temperatures and provide a reliable backup to the Unit 2 air conditioning syuem, it is proposed to supply bearing cooling water to the Unit 1 control room chillers during thi )utage period. The portion of the bearing cooling system to be used to supp v the Unit 1 chillers can be fed from either the Unit 1 nr Unit 2 bearing cooling wawr pumps.

However, during a review of the engineering package for Phase I, Stage 1 of the service water restoration project, a concern was identified in that a prelirninary

engineering evaluation had previously identified an environmental qualification 1

concern for the control room chillers. In general, following a certain main steam

. line break accident scenario, the chiller room of the affected unit may become a harsh environment. Therefore, to meet the 10 CFR 50.49 environmental qualification requirements, at least one of the opposite unit's chillers must remain operable to provide air conditioning to the control room.

2.0 SUPPLEMENTAL EXEMPTION REQUEST An exemption from 10 CFR 50.-49 for the North Anna Unit 2 chillers is requested for the period the service water system is isolated from the recirculation spray heat exchangers and the control room chillers. This supplemental exemption request was discussed with the NRC staff in our meeting on August 25,1992.

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Environmental Qualification Reouirement Each unit has three control room air conditioner chillers located in a missile protected room of the service building off the respective unit's turbine building basement. Ventilation for each unit's chiller room is taken from and exhausted to the respective unit's turbine building basement. Hence, the chillers for each unit are located in the same environmental zone which is also common to the unit's turbine building' basement. Therefore, as the result of an environmental qualification evaluation of the control room air conditioning systems, a station standing order was issued to require at least one of the opposite unit's chillers to rernain operable while shutdown. Specifically, the station standing order requires that at least one control room chiller on the unit in Mode 5 or 6 be maintained operable while the other unit is in Mode 4 or above. This measure assures that the air conditioning system serving the control room and emergency switchgear room would be available during a certain postulated main steam line break accident in the turbine building.

However, with bearing cooling water supplied to the Unit 1 chillers instead of service water, the reliability of the Unit 1 chillers is called into question because bearing cooling is not safety-related. Bearing cooling would not be available in <

the event of a loss of offsite power event or design basis earthquake coincident with the main steam line break accident in the turbine building. Therefore, an exemption from 10 CFR 50.49 for the North Anna Unit 2 chillers is requested for

, the period the service water system is isolated from the recirculation spray heat exchangers and the control room chillerc.

While the shutdown unit's Technimi Specifications do not require the air conditioning systems to remain operuoie in Modes 5 and 6, the environmental qualification design basis for the operating unit's air conditioning systems requires at least one of the shutdown unit's chillers to be operable as a backup to operating unit.

Detailed Discussion of Exemotion Period During the Unit 1 steam generator replacement outage, a portion of the service water system servicing Unit 1 is scheduled to be isolated from the main service water system. As a compensatory measure, bearing cooling water will supply cooling to the Unit 1 control room chillers instead of service water to provide normal cooling to the common control room.

The required exemption period is technically from entry into the second 168-hout action statement through the c' earing of the fifth 168-hour action statement for the work activities associated with Phase I, Stage 1 service water restoration project to be performed during the outage. A detailed description of the use of the 168-hour action statements is provided ir. Attachment 2 of our July 16,1992 letter.

Tha period for the Unit 1 chillers to oe operating on bearing cooling water is projected to be between 90 and 120 days.

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j Accident Scenario of Concern i

j The situation of concern is an environmental condition in the Unit 2 chiller room j for which the Unit 2 control room chillers are not qualified and may cease to function properly. The only postulateo accident event that could cause this 4

condition is the failure of a main steam line in the turbine building basement in proximity of the Unit 2 chiller room. However, in order to have sufficient steam

concentration in the area to disable the Unit 2 chillers, the main steam trip valve

on the line would also have to fail to close.

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This is unlikely because the trip valves are essentially check valves reversed to the flow of steam with the check disk physically held out of the steam flow path.

.' Failure to hold the disk out of the steam flow path would cause the trip valve to i slam shut. Failure of the valve where the disk is stuck open is unlikely.

l Because the Unit 1 chillers provide backup to the Unit 2 chillers, the Unit 1 i

chillers would have to also fail to provide cooling. Because bearing cooling

' water is supplied to the Unit 1 chillers for the exemption period, the failure of the bearing cooling system would also have to occur. Bearing cooling is a non- i f

safety related system. Therefore, bearing cooling is assumed to be lost in the

event of either a loss of offsite power or a design basis earthquake event.

Enoineerino Evaluation of System Peliability l A Probabilistic Risk Assessment (PRA) of the postulated accident scenario and j the affected systems was performed to support this exemptbn request. A number of combinations and sequences were considered in this evaluation. The specific sequences are described in the attached supplement to the PRA. The sequences can be grouped into two categories es described below

1) Main steam line rupture in the Unit 2 turbine building with concurrent l loss of the bearing cooling backup on Unit 1. The loss of bearing

cooling backup could be a result of loss of offsite power or othei loss of bearing cooling system components.

2) Design basis earthquake (DBE) resulting in main steam line rupture i

and loss of bearing cooling.

Quantification of the various event probabilities was performed using fault tree

models and results fram the North Anna Individual Plant Examination (IPE). The

worst case probability for the sequences considered in item 1 above.was

2.5 x 10-9 Based on these probabilities, we conclude that the events associated with item 1 are not credible and do not warrant further analysis.

Both the Electric Power Research Institute (EPRI) and the Lawrence Livermore National Laboratory (LLNL) seismic curves were used to evaluate item 2 above.

The change in failure probability of the Unit 2 air conditioning syctem for the event described in item 2 was calculated to be 1.4 x 10-6 to 6.8 x 10-6 using the EPRI and LLNL seismic hazard curves, respectively. The change in core Page 3 of 8

damage probability (CDP) could be substantially less than this value. However, without performance of a seismic PRA, the change in CDP cannot be specifically calculated. Thus, the range calculated above is used as an upper bound change in CDP. This is judged to be an acceptable level of risk and implementation of this temporary bearing cooling supply is justified.

The results of the PRA reported Wre for items 1 and 2 above are based on a conservative 120-day period of providing bearing cooling water to the control room chillers instead of service water. The attached supplement to the PRA includes evaluations for both a 90-day period and a 120-day period.

In addition, it should be notcd that at least one Unit 1 bearing cooling water pump and flow path will remain operable and will supply the Unit 1 chiller piping while the normal service water source is unavailable. The Unit 2 bearing cooling system will also be available to suprly this system if required.

3.0 BASIS FOR EXEMPTION REQUEST As discussed in our July 16,1992 letter, the exemption from GDC-2 is necessary ,

to permit restoration activities to the service water system piping without t extension of the North Anna Unit 1 steam generator replacement outage scheoule. When completed, these modifications will result in increased reliability of the service water system. Completing the necessary excavation work during this non-outage exemption period will allow the restoration work to be performed more efficiently. For Phase I, Stage 1, the exemption is necessary primarily for the 30-day period from early-December,1992 until the beginning of '

the 1993 steam generator replacement outage and for a 30-day period following completion of the outage.

At the time of the July 16,1992 submittal, it was determined that Unit 2 was not affected by the partialisolation of the service water systern during Phase I, Stage 1 of the project. Hosvever, it will be necessary to isolate service water from the Unit 1 control room chillers for the majority of the North Anna Unit 1 steam generator replacement outage. It is planned to provide a temporary water supply and return path to the Unit 1 cc irol room chillers from the bearing cooling water system to provide normal control room temperatures and provide a reliable backup cooling system to the Unit 2 air conditioning design basis. However,-it was recently determined that this will affect the normal environmental qualification design oasis for the Unit 2 control room chillers. Therefore, pursuant to 10 CFR 50.12(a), Virginia Electric and Power Company also requests an exemption from the requirements of 10 CFR 50.49, Environmental Qualification of Electric Equipment important to Safety for Nuclear Power Plants, for environmental qualification of the North Anna Unit 2 control room chillers for ,

approximately the duration of the North Anna Unit 1 steam generator replacement outage. This exemption will permit the temporary use of bearing cooling water to the Unit 1 control room chillers instead of the normal cooling water supply from the service water system during the service water restoration project.

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The procedures set forth in 10 CFR 50.12 provide that specific exemptions from

, the requirements of 10 CFR Part 50 may be granted which:

are authorized by law,

+ are consistent with the common defense and security,-

will not present an undue risk to the public health and safety, and ,

a are accompanied by special circumstances.

Virginia Electric and Power Company submits that the activities sought to be conducted unoer this exemption request are clearly authorized by law and are consistent with the common defense and security. As detailed below, the remaining standards for the exemption are also satisfied.

No Undue Risk to Public Health and Safety The likelihood of the accident scenario discussed above removing all control room chillers from service is small et the North Anna site during the periods for which the requested exemption would apply. The principal risk is a main steam line break inside the Unit 2 turbine building basement. In addition, the risk associated with failures due to seismic events or combinations of main steam line break accidents coincident with a loss of offsite power is unlikely to cause a total loss of control room chilling. An engineering evaluation and probabilistic risk assessment have been performed for this accident scenario. We have assessed the likelihood of such an event- usino regional meteorological information and concluded that the risk is acceptably low.

Moreover, the proposed exemption will not otherwise affect radiological plant effluents, nor result in any significant occupational exposure. Thus, there are no significant radiological or non-radiological environmental impacts associated with the proposed exemption.

Special Circumstances Exist Special circumstances are present to warrant granting the requested exemetion.

Three of Me examples of special circumstancer.,, as provideo in 10 CFR 50.12(e)p)(iii), (iv), and (v), apply in this case. The first special circumstance, paragraph (iii), is that compliance would resuit in ondue _ hardship or other costs that are significantly in excess of those contemplated when the regulation was adopted. The second special circumstance, paragraph (iv), is that the completion of the project as proposed would result in an overall benefit to the public health and safety that compensates for any decrease in safety that may result from the granting of the exemption. The third special circumstance, paragraph (v), is that the exemption provides only temporary relief from the applicable regulation and that the licensee has made good faith efforts to comply with the regulation. It has been determined by PRA methods that the temporary relief does not affect the safe operation of either unit.

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Providing a safety-related water supply to the Unit 1 chillers would result in an unnecessary additional expense without a significant increase in system reliability or safety. Any other options would result in an extended unplanned outage or a dual unit outage situation. As the NRC is well aware, replacement power costs result in substantial undue hardship to the Company. In addition, the exemption will indirectly result in benefits to the public from increased unit availability by combining planned outage activities and not requiring a dual unit outage. Finally, the exemption would provide only temporary relief from the applicable regulations. The exemption is requested only for a specified period time, i.e., from the end of the second 168-hour action statement through the end of the fifth 168 hour0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br /> action statement (between approximately 90 and 120 days) during the steam generator replacement outage. We are committed to making good faith efforts to provide control room cooling during tho exemption periods.

Special circumstances exist in that we have made.a good faith effort in considering alternatives to an exemption request and have concluded that the project could only be conducted without an exemption by an extensive temporary safety related sewice water supply or during a period when both units are shutdown. As thera are no dual unit outages planned or scheduled, we believe that this alternative represents an undue hardship. The impact of scheduling such a dual unit outage would have potentially significant consequences in terms of power supply and replacement power costs. We believe that it was never the intent of the regulation to require such actions to ensure compliance with the design enterion. Also, the schedule that we are proposing for this portion of the service water prohct will accelerate the timetable >

for restoring the portions of the scrvice water system that have become degraded. Therefore, we conclude that several of the criteria described in the special circumstances portion of the regulations are met.

4.0 DISCUSSION OF SERVICE WATER SYSTEM INTEGRITY The service water restoration project work activities to be performed in conjunction with the North Anna Unit 1 steam generator replacement outage have been evaluated to ensure safe operation of the plant. As part of our engineering evaluation of these efforts, a Probabilistic Risk Assessment (PRA) was performed for both units by Halliburton NUS Environmental Corporation.

Per your request at our August 25,1992 meeting, the following is a discussion of the conservatism built into the PRA with respect to the current material cor dition of the service water system. In addition, a copy of the PRA report is attached for yaur review.

A number of methods are available to quantify the pipe rupture frequency for use i n this assessment. The three methods evaluated for applicability to the service water restoration project are listed below:

1) Application of a generic failure probability model as presented in the WASH-1400 Reactor Safety Study.

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2) Empirical correlations based on actual service failure statistics and piping characteristics (known as Thomas correlation model). '

3) Calculation using a log-linear model which considers system specific failures based on actual failure events reported for the U.S. nuclear power industry.

Evaluation of specific North Anna service wator system piping was performed using the three approaches described above. The log-linear model yielded the most conservative values and was used to support the PRA.

5.0 ENVIRONMENTAL ASSESSMENT Pursuant to 10 CFR 50.12(a), Virginia Electric and Power Company is requesting an exemption from the requirements of 10 CFR 50.49, Environmental Qualification of Electric Equipment important to Safety for Nuclear Power Plants, for the environmental qualification of the North Anna Unit 2 control room chillers for approximately the duration of the North Anna Unit 1 steam generator replacement outage. This exemption will permit temporary cooling of the Unit 1 control room chillers from the common bearing cooling water system to provide normal control room temperatures and provide a reliable backup cooling system to the Unit 2 air conditioning design basis. The proposed exemption is needed in crder to permit the completion of repair and replacement activities on the service water system without unduly extending the next several scheduled refueling outages.

The, .osed exemption does not involve any measurable environmental impa'. ' auring normal operation since the plant configuration is changed only miniihatly and operation of Unit 2 is not changed. The likelihood of the above accident scenario during the time the exemption would be in effect is low. Thus, the proposed exemption would not significantly affect the probability or consequences of potential reactor accidents and would not otherwise affect radiological plant _ effluents. Consequently, there are no significant radiological impacts associated with the proposed exemption.

With regard to potential non-radiological impacts, the proposed exemption involves features located entirely within the restricted area as defined in 10 CFR Part 20. It does not affect non-radiological plant effluents and has no other environmental impact. Therefore, we conclude that there are no significant non-radiological environmental impacts associated with the proposed exemption.

The principal alternative to requesting the temporary exemption for the Unit 2 chillers would be to c.omply with the requirements of 10 CFR 50.49. However, this alternative would not significantly enhance the protection of the environment, and would result in a significant loss of power generation as the next several refueling outages for North Anna Units 1 and 2 would have to be extended considerably.

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Based on the above assessment, we conclude that the NRC granting of the requested exemption discussed above would not have a significant effect on the quality of the human environment.

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