ML20236D445

SER Re Licensee 870814 & 21 Responses Re Single Failure Potential in Nuclear Svc Water Sys.Nuclear Svc Water Sys Meets Requirements of GDC 5 & 44 Re Sharing & Provisions for Suitable Redundancy in Cooling Water Sys

ML20236D445
Person / Time
Site:	Catawba
Issue date:	09/30/1987
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20236D436	List: ML20236D434 ML20236D445
References
NUDOCS 8710280212
Download: ML20236D445 (4)

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I j PLANT SYSTEMS BRANCH l SAFETY EVALUATION REPORT RESPONSE TO 50.54(f) REGARDING SINGLE FAILURE POTENTIAL IN NUCLEAR SERVICE WATER SYSTEM CATAWBA NUCLEAR STATION, UNITS 1 & 2 DOCKET NOS. 50-413/414 INTRODUCTION-

-During a Quality Validation Inspection Audit, the staff reviewed the Catawba 1 nuclear service water (RN) system design and identified concerns regarding whether the shared system satisfied the requirements of General Design Criteria (GDCs)

5. " Sharing of Structures, Systems, and Components", and 44, " Cooling Water."

Specifically, the concern is related to at least one scenario in which one unit is shut down (i.e., in Modes 5 or 6) with one of two diesel generators in

! that unit out of service, and the remaining unit has a postulated loss-of-coolant accident (LOCA). Because there is no Technical Specification (TS) l limiting condition for operation (LCO) with one diesel generator out of service l in Modes 5 or 6, this condition could continue indefinitely. In this condition. l a single failure to open of a particular opposite train motor operated RN valve in the suction suppl ultimate heat sink (y transfer from the normal heat sink (Lake Wylie) to theUHS),thesta leave one RM pump to serve both units. This results in a condition which exceeds the design basis for Catawba. The licensing basis as described in the Final Safety Analysis Report (FSAR) states that two out of four RN pumps are necessary to remove LOCA heat loads in one unit and shutdown and cooldown heat loads in the other unit (i.e., the requirements of GDC 5). i As a result of this concern, the staff transmitted a letter to the 'iicensee, Duke Power Company, dated August 7,1987, which requested the licensee, pursuanttotheprovisionsof10CFR50.54(f),toprovide,(1)ananalysis justifying the adequacy of the single RN pump to serve both units, or (2) a description of actions implemented or planned to be implemented that justify continued plant operation. In the interim, the staff notified the licensee by phone, that it should notify the NRC within one hour whenever a diesel generator was taken out of service, and that consideration should be given to switchover to the SNSWP in a time frame less than the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> action statement currently

% being used by the licensee's interpretation of the existing Technical Specifications. The licensee consnitted to comply with these actions and proposed to switchover to the NSWP if the diesel generator is unavailable for 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> s in lieu of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.

By letter dated August 14, 1987, the licensee responded to the staff's 10 CFR 50.54(f) request. In addition, by letter dated August 21, 1987, the licensee

• ' provided minor corrections to the figures provided in the August.14.' 1987 letter.

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.4 BRIEF DESCRIPTION OF THE CATAWBA NUCLEAR SERVICE WATER SYSTEM (RN)

.The RN system consists of two redundant trains (A and B) of essential equip-ment per unit. TheAtrainissuppliedbytwoARNpumps(oneperunit)and

l. the B. train by the two B RN pumps. Two pumps-(A and B) are designated for l

each unit and are. powered from separate emergency power sources associated ,

with that unit. The A and B pumps are interconnected by a comon discharge '

header such that operation of any two pumps is sufficient to supply all cooling l water requirements for the two units for all plant conditions including a LOCA in one unit end shutdown and cooldown of the other unit. During normal power operation, only one pump is needed to remove operating plant heat loads from both units. The crossover connections between train A and train B are automatically isolated on a high-high containment pressure or low RN pump pit level signal to ensure train integrity. Refer to the FSAR Section 9.2.1 for a more complete description. Thus, with all trains operable including power supplies, no single failure will reduce the capability of the RN system to less than two pumps.

DISCUSSION AND EVALUATION l

The licensee's response to the 50.54(f) letter discussed conformance to GDCs 5 and-44, single pump operation, actions justifying continued operation and L planned actions to improve safety. Further clarification regarding the system design and the licensee's interpretation of the current Technical Specifications

were provided by the licensee at a meeting with the staff on August 27, 1987.

Further, the licensee stated in its response that as previously concluded in Section 9.2.1 of the Catawba Safety Evaluation Report (NUREG-0954), the RN system design meets GDCs 5 and 44 and that the staff's previous conclusions remain valid. While the staff's previous conclusions regarding the design of the RN system and GDCs 5 and 44 remain valid as discussed below, it should be noted that the staff's basis for meeting GDC 5 as identified in NUREG-0954 is incorrect. 'In NUREG-0954 Section 9.2.1, the staff stated that its basis for concluding the design meets GDC 5 was the automatic separation capability of the RN system between Units 1 and 2. However, such isolation capability only exists between trains not between units. As a result of this reevaluation the staff concludes that the design as described above meets the requirements of GDC 5. Specifically the design of the RN system is such that no single failure can keep the system from performing its safety function, namely two pumps will be available to handle a LOCA in one unit and bring the other unit to a safe

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a cold shutdown. The RN system is continuously shared by each unit. .Therefore, l the Technical Specifications (TS) should be revised to adequately address sharing the RN system between units in order to ensure adequate heat removal capability. The current TS for the emergency diesel generators require that two separate and independent diesel generators be operable in Modes.1, 2, 3 and 4. With one diesel generator out of service, operation may continue for up to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. The nuclear service water system TS requires at Isast two in-dependent service water " loops" to be operable in Modes 1, 2, 3, and 4. With l

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only one loop operable, operation may continue for up to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. Neither of these specifications indicate that the action statement is applicable to both

' units despite the shared nature of the RN system. TS 3.0.5.a specifies that "whenever the Limiting Condition for Operation refers to systems or components which are shared by both units, the ACTION requirements will apply to both

. units simultaneoung. This will be indicated in the ACTION statement". As ar example,.the SNSWP specification, TS 3.7.5, identifies that the ACTION statement applies.to Units 1 and 2.

In the August 14,'1987 response, the licensee stated that plant procedures were revised in 1986 to require both units to enter a 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> action statement anytime.a diesel . generator _ on either unit' was removed from service. It was also stated that prior to an emergency diesel generator being or expected to be out of service for 72 haurs or more, plant procedures require that the un-laced in a non-degraded condition by (1)

.affected unaffected unit train be p(A or B) for SNSWP recirculation, isolating andor(2) pre-aligning other- th wise restricting flow to the degraded train of the degraded unit in a manner which insures adequate post-accident flows to components of this (affected) train of the other (operating) unit. Pre-aligning the unaffected train trans-fers the suction and discharge of the RN system train from Lake Wylie to the SNSWP. Following this realignment, the licensee would remove the unaffected unit from the action statement, because the licensee concluded that following the realignment and the shutdown of the affected unit, adequate flow would be-available to supply post-accident heat loads in the operating unit given any single active failure i.e., two complete OPERABLE loops were restored to the operating unit as required by TS. At the August 27, 1987 meeting, the licensee stated that since both units have become operational, the RN Technical Specification has been interpreted to apply to both units.

The staff concludes that the Technical Specifications should be revised to reflect the fact that with a diesel generator or a shared RN component out of service, the action statement should address both units, and should also ad-dress.the alternate actions associated with pre-alignment of the unaffected train from Lake Wylie to the ultimate heat sink including isolation and/or restricted flow to the degraded train of the degraded unit to ensure two pumps are operable and the requirements of GDCs 5 and 44 are met. The bases section of the specification should also identify or define what is to be considered a

" loop" and/or " train". Based upon discussion with the licensee, the staff con-

'cludes that the definition of an RN loo) is not clear. Specifically, the RN system description in the FSAR and it s1ared operation are in terms of " units" and " trains" while the TS discusses " loops".

In the August 14, 1987 letter, the licensee also stated that the transfer from Lake Wylie to the SNSWP as a source of water for the RN system.in)the absense of a loss of-suction from Lake Wylie as originally designed (inipated whenever a high containment pressure was sensed) represents an unnecessary ghallenge to the associated RN suction valves from the SNSWP. To improve the evere11 reliability of the RN system, the licensee is implementing two modifications to reduce the potential for unnecessary challenges. The first modification would remove the containment pressure signal that realigns the RN pumps suction from Lake Wylie

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to the SNSWP. This signal .is not necessary since realignment to the SNSWP will. .

occur on loss of suction from Lake Wylie as sensed by low RN pump pit level.  !

The second modification will reduce the possibility of spurious transfer signals l by installing additional transmitters to upgrade the logic of the low RN pump l pit level . signal'from a 1 out of 2, to a 2 out of 3 system. The staff finds j the proposed modifications acceptable as they do not degrade the original design basis for' provision of an assured post-accident water source and improve the overall reliability of the RN system.

The licensee also performed an analysis to demonstrate the adequacy of a single.

RN pump to remove LOCA heat loads in one unit and the shutdown heat loads of the other unit. Review of this. analysis is beyond the scope of this SER. The original I and, current licensing design basis requires two pumps as described above and i in the FSAR. It should also be noted that this evaluation did not include a  !

review of the affects of the pre-alignment of the RN system for SNSWP recircu-  !

1ation on the ultimate heat sink. The staff assumed that the initial FSAR review .I consisidered such effects as overflow capability and temperature changes (in-cluding instrumentation capability) while in the SNSWP recirculation mode.

CONCLUSION Based on'its review as described above, the staff has reached the following conclusions:

1. 'he T nuclear service water system design meets the requirements  !

of General Design Criteria 5 and 44 as they relate to sharing T and provisions for suitable redundancy in cooling water systems.

The procedures implemented by the licensee and revisions of the Technical _ Specifications which the licensee has committed to submit will ensure that at least two RN pumps will be available for removal of design basis accident (LOCA) heat loads in one

. unit and shutdown and cooldown heat loads of the other unit in the event of a sinole active failure.

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2. Prompt notification (within one hour as previously committed to by the licensee) of the NRC anytime a diesel generator is taken out of service is no longer necessary, similarily switchover to the pond within a 12 hour1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> period with a diesel generator out of service is no longer necessary.

3. The modifications to elim uate the high containment pressure sional from the SNSWP transfer logic and the upgrade of the RN pump pit level logic are acceptable. l

4. The licensee shall propose revised Technical Specifications and associated bases to specifically address the fact that the j

' nuclear serv :e water system action statement applies to both j units anytime a diesel generator or shared RN component is taken i out of service, and is applicable during all Modes of operation.

The licensee has comitted to provide the above TS changes. The  ;

staff considers a 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> LCO on the RN pumps to be appropriate.  :

The staff, therefore, concludes that continued operation of Catawba is acceptable, i

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