ML20024J205
| ML20024J205 | |
| Person / Time | |
|---|---|
| Site: | Seabrook  |
| Issue date: | 10/05/1994 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20024J204 | List: |
| References | |
| NUDOCS 9410120145 | |
| Download: ML20024J205 (8) | |
Text
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I t UNITED STATES j
fl, j NUCLEAR REGULATORY COMMISSION WASHINGTON. D.C. 20555-0001 k.....
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 32 TO FACILITY OPERATING LICENSE NO. NPF-86 NORTH ATLANTIC ENERGY SERVICE CORPORATION SEABROOK STATION. UNIT NO. 1 DOCKET NO. 50-443
1.0 INTRODUCTION
By application dated February 26, 1993, as supplemented by letter of March 11, 1994, and application dated April 7,1993, as supplemented by letter dated February 24, 1994, North Atlantic Energy Service Corporation (North Atlantic) proposed changes to the Appendix A Technical Specifications (TS) for the Seabrook Station, Unit 1 (Seabrook).
The proposed changes would revise the Appendix A TS relating to the operability requirements for the primary component cooling water (PCCW) system, the service water (SW) system, and the ultimate heat sink (VHS).
The proposed changes would redefine the requirements for operable PCCW and SW systems and would combine the TS requirements for the SW system and the UHS.
The changes would affect TS sections 3/4.7.3, 3/4.7.4, and 3/4.7.5.
North Atlantic's letters dated March 11, 1994 and February 24, 1994, provide additional clarifying information related to risk calculations but neither letter changes the initial proposed no significant hazards consideration determinations.
2.0 DISCUSSION AND EVALUATION 2.1 Service Water System and Ultimate Heat Sink 2.1.1 Discussion The SW system at Seabrook employs two independent and redundant cooling loops.
Each loop can be supplied by either of two full-capacity SW pumps (4 pumps total) drawing water from the Atlantic Ocean via a pumphouse, or alternatively each loop can be supplied by a full-capacity cooling tower SW pump (2 pumps i
total) drawing water from a mechanical draft cooling tower.
Each of the six pumps is a 100% capacity pump capable of handling all of the necessary heat loads for all normal and design basis events.
1 Because the tunnels between the Atlantic Ocean and the pumphouse are not designed to seismic Category I requirements, a seismic Category I cooling tower is provided to protect against their failure due to a seismic event.
Therefore, to meet the design basis for the SW system, each loop must have an operable SW pump and an operable cooling tower SW pump.
9410120145 941005 PDR ADOCK 05000443 P
4 By letters dated April 7,1993, and February 24, 1994, North Atlantic proposed changes to the Seabrook TS to redefine the requirements for an operable SW system and to consolidate the SW system requirements with the requirements for the UHS. TS 3/4.7.4 for the SW system requires two operable SW loops with each loop having three operable pumps (two SW pumps and one cooling tower SW pump) when the plant is in Modes 1 through 4.
North Atlantic asserts that this requirement is unnecessarily restrictive since the second SW pump in each loop is not required for normal or any design basis event, and the associated cooling tower SW pump provides redundancy to cover all design basis events.
Operation of any one of the pumps will satisfy the SW cooling requirements during normal operation or any design basis event.
North Atlantic has proposed to amend the plant TS to:
(1) Redefine SW system operability in terms of one operable SW pumphouse, two SW loops each with one operable SW pump, an operable cooling tower, two cooling tower SW loops each with one operable cooling tower SW pump, and a portable cooling tower makeup system in operational readiness, (2) Extend the allowed outage time (A0T) for one inoperable SW loop from 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> and for one inoperable cooling tower SW loop from 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to seven days, (3) Add an A0T of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> for two inoperable SW pumps and 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> for two inoperable cooling tower SW pumps, (4) Revise the surveillance requirements for SW pumps, and (5) Add two new action statements based on the proposed definition of SW system operability.
In conjunction with the changes related to the number of pumps required to operable, North Atlantic has proposed to delete TS 3/4.7.5 for the UHS and consolidate the UHS requirements into TS 3/4.7.4 for the SW system. North Atlantic asserts that this consolidation would reduce the potential for confusion between the specifications and to control station operation in a manner consistent with station design. The current specification for the UHS specifies the requirement for the SW pumphouse and the mechanical draft cooling tower. The Atlantic Ocean is the UHS when the SW pumps are usec, and the atmosphere, via the cooling tower, is the UHS when the cooling tower SW pumps are used.
North Atlantic has proposed these changes to take advantage of the extra redundancy in the SW system and UHS designs to provide enhanced flexibility during station operation.
Presently, TS 3/4.7.4 and 3/4.7.5 are unduly restrictive in that no credit is given for the extra redundancy. These current TS are more restrictive than the Westinghouse Standard TS (STS),
NUREG-0452, and the improved Westinghouse STS, NUREG-1431, both of which are based upon a station design with minimum redundancy.
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. l The proposed changes to TS 3/4.7.4 reflect the design basis of the SW system in that with two operable loops, each having one operable SW pump and one operable cooling tower pump (given each pump's UHS is operable), the system is capable of performing its safety function for all design basis events given the worst case single active failure, including the failure of either diesel generator.
The additional backup SW pumps are not required by NRC regulation and are not relied upon for any design basis event, therefore, an action statement is not necessary if one of the backup pumps becomes inoperable.
The proposed change also added an A0T (72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />) if the portable cooling tower makeup pump is not stored in its proper position. After 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, plant operation may continue provided a report is made to the NRC within I hour in accordance with the requiremente of 10 CFR 50.72. The current TS does not specify an A0T for the portab'. C up pump. Under the proposed TS, if the makeup pump is inoperable for re other than improper storage, a one hour
.3 repor't is required with no A0T, wnich is consistent with the current TS. The staff concludes that this A0T is acceptable since it allows North Atlantic to perform periodic testing and maintenance of the pump without an hourly report describing the actions taken to ensure an adequate supply of makeup water for the cooling tower for a minimum of 30 days. The 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> is conservative given the low probability of an event requiring a makeup system during this time frame.
The staff also concludes that the consolidation of the SW system and UHS specifications to one TS is acceptable and necessary to achieve and maintain clarity, within the specifications, of the overall requirements for system operability.
North Atlantic asserts that the proposed changes would enhance plant operations and likely improve component reliability by providing more flexibility to perform maintenance on the SW system when the system is required to be operable.
As a result of the removal of the backup SW pumps from the TS, North Atlantic has also proposed to eliminate the related surveillance requirement that d
verifies each of the four SW pumps starts automatically upon loss of or failure to start of its redundant SW pump within the loop. This automatic start feature is not relied upon in any accident analysis or any other design basis event and is, therefore not required to be part of the TS even without the proposed changes to remove the backup pumps from the specifications.
The proposed changes would impact risk by increasing the likelihood that a SW pump would be unavailable due to planned or unplanned maintenance. North' Atlantic has evaluated the impact of the proposed changes on system unavailability and on the potential increase in the total reactor core damage frequency.
North Atlantic supported the proposed changes with a probabilistic risk assessment.
North Atlantic used the results published in the "Seabrook Station Probabilistic Safety Study", July 1993 and the generic pump
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b maintenance data from " Data Base for Probabilistic Risk Assessment of Light Water Nuclear Power Plants - Maintenance Data", PLG-0500, Volume 3, Revision 1, August 1989.
In the analysis of SW system unavailability (current TS), North Atlantic made the following assumptions:
No planned maintenance is done on the SW system during power
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operation that would cause a pump to be inoperable; i
No contribution is given to two SW pumps in unplanned maintenance at the same time; No explicit maintenance contribution is modeled for valves and instrumentation that would make a SW loop inoperable; No maintenance contribution is included from failures of SW or cooling tower ventilation; and Maintenance is unrecoverable.
For the two redundant standby SW
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pumps, it was assumed that (a) each pump is unavailable due to planned maintenance once every four years for 14 days; (b) planned r
maintenance is done one pump at a time; and (c) the pumps are repaired in unplanned maintenance with no special priority.
The SW system configuration is quantified for a number of different boundary conditions, including the number of support systems available, loss of offsite power, safety injection signals and whether the cooling tower is included. A sensitivity study was evaluated for the case assuming that the two standby SW pumps are permanently removM. The results of the analysis show that the change in system unavailability due to the proposed amendment is less than two percent for all the boundary cases and up to seven percent for the sensitivity case.
Loss of the SW system would have the following consequences:
For transients and loss of coolant accidents, loss of SW would fail primary component cooling leading to loss of cooling to the reactor coolant pump seals and to the emergency core cooling system pumps; and For a loss-of-offsite-power event, loss of SW would fail the diesel generators leading to station blackout.
In the estimate of the potential increase in the reactor core damage frequency, North Atlantic compared the dominant sequences in the current TS with those resulting from the proposed amendment. North Atlantic found that the change in the core damage frequency is dominated by the initiating event frequency for loss of one SW train. The resulting increase in the total core 4
damage frequency from the increase in SW system unavailability is 1.9x10 per year, or 2.4 percent of the current value.
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2.1.2 Evaluation and Finding The staff has determined that the methodology used in the North Atlantic's I
analysis is appropriate, the supporting data are pertinent and current, the assumptions are sound and conservative, and the results of the analysis are reasonable.
The staff has further determined that the change in the SW system unavailability due to the proposed TS amendment and the resulting increase in the total reactor core damage frequency are insignificantly small.
The staff concludes that the existing TS for the SW system is unnecessarily restrictive and exceeds applicable regulatory requirements by requiring more pumps to be operable than is necessary to meet the system design basis.
The staff further concludes that the proposed changes to TS 3/4.7.4 accurately reflect the SW system and UHS design bases and provide an adequate level of safety while providing considerable flexibility. The proposed changes are also consistent with the Westinghouse STS, both NUREG-0452 and NUREG-1431.
On the basis of the above, the staff finds that the proposed TS changes for i
the SW system and UHS at the Seabrook Nuclear Station, Unit No.1, are acceptable.
2.2 Primary Component Cooling Water System
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2.2.1 Discussion The PCCW system at Seabrook employs two independent and redundant cooling l
loops, each with two full capacity pumps.
Each PCCW loop supplies cooling water to safety related heat exchangers that are required for safe shutdown j
during normal operation and following design basis events.
Each loop also i
supplies cooling water to the reactor coolant pump thermal barrier loop and to other nonsafety-related heat exchangers. The PCCW system is designed such that a single pump in either loop can handle the heat loads for all design t
basis events, including a loss of coolant accident (LOCA) coincident with a loss of offsite power.
TS 3/4.7.3 currently requires two operable PCCW loops with each loop having two operable PCCW pumps. North Atlantic asserts that this requirement is.
4 unnecessarily restrictive since the second pump in each loop is not required to mitigate any design basis event. With any one of the four pumps inoperable the present TS provide a 7 day A0T before the plant must be brought to cold shutdown. With two pumps (one in each loop) inoperable the A0T is 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, i
and with two pumps in the same loop inoperable the A0T is 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
By application dated February 26, 1993, as supplemented by letter dated March 11, 1994, North Atlantic proposed to amend the plant TS to:
(1) Redefine a PCCW loop as having one operable PCCW Pump, t
- (2) Change TS 3.7.3 Action Statement a to refer to PCCW loops vice PCCW pumps and establish the A0T for an inoperable PCCW loop as 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, and delete TS 3.7.3 Action Statements b and c, and (3) Delete Surveillance Requirement 4.7.3 b.2.
The proposed changes reflect the design basis of the PCCW system in that with two operable loops, each having one operable pump, the system is capable of performing its safety function for all design basis events given the worst case single active failure, including the failure of either diesel generator.
The additional backup pumps are not required by any NRC regulations and are not relied upon for any design basis event, therefore, an action statement is not necessary if one of the backup pumps becomes inoperable.
The proposed A0T for one loop (72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />) is consistent with the Seabrook A0Ts for other engineered safety feature systems for a loss of redundancy.
The proposed changes are consistent with the Westinghouse STS, NUREG-0452, and are also consistent with the improved Westinghouse STS (NUREG-1431).
North Atlantic supported the proposed changes with a probabilistic risk assessment. North Atlantic asserted that the proposed changes would enhance plant operations and outage flexibility by allowing planned and corrective maintcnance to be performed on line without affecting adversely the ability of the system to mitigate a postulated design basis event.
The proposed TS changes would impact risk by increasing the likelihood that a PCCW pump would be unavailable due to maintenance, either planned or unplanned.
The loss of either train of the PCCW system would affect the plant power generation through loss of cooling to two reactor coolant pump motors.
North Atlantic evaluated the impact of the proposed changes on the unavailability of the PCCW system and on the potential increase in the total reactor core damage frequency.
Two sensitivity cases were evaluated:
(1) the two standby PCCW pumps were assumed to be permanently removed; and (2) the impact of the proposed TS changes for SW systems discussed in Section 2.1 was combined with the impact of the proposed TS changes for the PCCW system.
North Atlantic used the results published in the "Seabrook Station Probabilistic Safety Study", July 1993, and the generic pump maintenance data from " Data Base for Probabilistic Risk Assessment of Light Water Nuclear Power Plants - Maintenance Data", PLG-0500, Volume 3, Revision 1, August 1989.
In the analysis of PCCW system unavailability (current TS), North Atlantic made the following assumptions:
No planned maintenance is done on the PCCW system during power operation that would cause a pump to be inoperable;
No contribution is given to two PCCW pumps in unplanned maintenance a
at the same time; No explicit maintenance contribution is modeled for valves and instrumentation that would make a loop inoperable; No maintenance contribution is included from failures of the PCCW
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pump area ventilation; and Maintenance is unrecoverable.
For the two redundant standby PCCW pumps, it was assumed that (a) each pump is unavailable due to planned maintenance once every four years for 14 days; (b) planned maintenance is done one pump at a time; and (c) the pumps are repaired in unplanned maintenance with no special priority.
The PCCW system configuration is quantified for a number of different boundary conditions, including the number of support systems available, loss of offsite power, and containment isolation. The results of the analysis show that the increase in system unavailability due to the proposed changes is less than six percent for all the boundary cases and up to a factor of ten for the sensitivity study case where the two standby PCCW pumps were assumed to be permanently removed, in the estimate of the potential increase in the reactor core damage i
frequency, North Atlantic compared the dominant sequences in the current TS with those resulting from the proposed amendments. The increase in the total core damag,e frequency from the increase in unavailability of the PCCW system
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is 8.0x10^ per year, or 14 percent of the current value. When combined with the previously proposed changes discussed in Section 2.1, the increase in the total core damage frequency is 20 percent of the current value.
2.2.2 Evaluation and Finding The staff has determined that the methodology used in the North Atlantic's analysis is appropriate, the supporting data are pertinent and current, the assumptions are sound and conservative, and the results of the analysis are reasonable.
The staff has determined that the change in the PCCW system unavailability due to the proposed TS amendment is insignificantly small.
The resulting increase in the total reactor core damage frequency is also small.
The staff concludes that the proposed changes to TS 3/4.7.3 c.ccurately reflect the PCCW design basis and provide an adequate level of safety while providing considerable flexibility.
The proposed changes are also consistent with the Westinghouse STS, both NUREG-0452 and NUREG-1431.
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i On the basis of the above, the staff finds that the proposed TS changes for the PCCW system at Seabrook, are acceptable.
In future proposed amendments affecting the SW or PCCW systems, North Atlantic should demonstrate that the resulting potential increases in the total core damage frequency would not cumulate to an unacceptable level.
3.0 STATE CONSULTATION
In accordance with the Commission's regulations, the New Hampshire and Massachusetts State officials were notified of the proposed issuance of the amendment. The State officials had no comments.
4.0 ENVIRONMENTAL CONSIDERATION
The amendment changes requirements with respect to installation or use of facility components located within the restricted area as defined in 10 CFR Part 20 and changes surveillance requirements. The NRC staff has determined that the amendment involves no significant increase in the amounts, and no significant change in the types of any effluents that may be released offsite, i
and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has previously issued proposed findings that the amendment involves no significant hazards consideration, and there has been no public comment on such findings j
(58 FR 25860 and 58 FR 34082). Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR 51.22(b), no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendment.
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5.0 CONCLUSION
The Commission has 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, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributors:
J. Wing
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W. LeFave A. De Agazio Date: October 5, 1994
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