NRC Generic Letter 91-09, Modification of Surveillance Interval for the Electrical Protective Assemblies in Power Supplies for the Reactor Protection System

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June 27, 1991

TO: All HOLDERS OF OPERATING LICENSES FOR BOILING-WATER REACTORS

SUBJECT: MODIFICATION OF SURVEILLANCE INTERVAL FOR THE ELECTRICAL PROTECTIVE ASSEMBLIES IN POWER SUPPLIES FOR THE REACTOR PROTECTION SYSTEM (Generic Letter 91-09)

This generic letter provides guidance for requesting a license amendment to modify the surveillance interval for electrical protective assemblies (EPAs)

used in power supplies for the reactor protection system (RPS). The current standard technical specifications (STS) for boiling-water reactors (BWRs)

require the licensee to perform channel functional tests of EPAs at a 6-month interval. The modification of EPA test interval provided by this generic letter is to change the TS to state that the test shall be performed each time the plant is in cold shutdown for more than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, unless the test was performed in the previous 6 months.

In a proposal on December 15, 1988, for Nine Mile Point Nuclear Station (NMP), Unit 2, the Niagara Mohawk Power Corporation provided a justification that this TS change benefits plant safety. During the recent review of this TS change for Dresden Nuclear Power Station, Units 2 and 3, the U.S. Nuclear Regulatory Commission (NRC) staff noted that this change from the guidance of the BWR STS had been implemented in the TS issued with 9 of the last 14 BWR operating licenses. The staff concludes that the TS change applies generically for BWRs as a line-item TS improvement. The enclosure provides guidance for preparing a license amendment request to implement this line-item TS improvement.

Licensees are encouraged to propose TS changes for BWR facilities that are consistent with the guidance provided in the enclosure. The NRC project manager for the facility will review amendment requests conforming to this guidance. Please contact the NRC project manager or the contact identified below if you have questions on this matter.

This letter does not require any licensee to propose changes to its plant TS. Therefore, any action taken in response to the guidance provided in this generic letter is voluntary and is not a backfit under 10 CFR 50.109.

Sincerely,

James G. Partlow Associate Director for Projects Office of Nuclear Reactor Regulation

Enclosure: As stated

Contact: Tom Dunning, NRR/OTSB (301) 492-1189Generic Letter 91-09 Enclosure

MODIFICATION OF THE SURVEILLANCE INTERVAL REQUIREMENTS FOR ELECTRICAL PROTECTION ASSEMBLIES IN REACTOR PROTECTION POWER SUPPLIES

Introduction

This enclosure provides guidance for preparing a request for a license amendment to modify the technical specifications (TS) surveillance interval requirements for the electrical protection assemblies (EPAs) used in power supplies for the reactor protection system (RPS) for boiling-water reactors (BWRs). This change reduces the possibility for inadvertent reactor trips caused by testing of EPAs during power operation.

Discussion

To protect RPS equipment from abnormal operating voltage or frequency produced by RPS motor generator (MG) sets or an alternate power supply, EPAs will trip a breaker between the MG sets and the RPS. TS 4.8.4.4 in the standard technical specifications (STS) for BWRs addresses the surveillance requirements for EPAs. This TS specifies that licensees perform a channel functional test every 6 months.

To functionally test an EPA channel, the licensee transfers the power for the RPS from the associated MG set to the alternate power supply. Because the transfer of RPS power involves a dead-bus transfer, power is momentarily interrupted which causes a half scram or group isolation. Alternatively, the licensee could perform tests without a bus transfer, but this procedure also results in a momentary interruption of power to the RPS when each EPA channel is tripped during the channel functional test. At many BWR plants, licensees have encountered problems with the reset of the half-trip conditions, following testing of EPAs during power operation, resulting in inadvertent scrams and group isolations that challenge safety systems.

An alternative to testing the EPAs every 6 months during power operation has been to test them each time the plant is in cold shutdown for more than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> if this test has not been performed within the previous 6 months. This alternative eliminates the need to test the EPAs during power operation and, thereby, reduces the possibility of inadvertent challenges to the protection systems. However, this alternative retains testing within the existing 6-month interval when the unit is shut down for more than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> during an operating cycle.

If the licensee does not encounter a cold shutdown of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> or more during a fuel cycle, the effect of not testing EPAs during this interval is a small risk to safety. This alternative benefits safety by reducing the possibility for inadvertent trips and challenges to safety systems. The staff concludes that the benefit to safety of reducing the frequency of testing during power operation more than offsets the risk to safety from relaxing the surveillance requirement to test EPAs during power operation.

The following guidance provides an acceptable alternative in the format of the current BWR STS for these surveillance requirements:

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Generic Letter 91-09Enclosure

4.8.4.4 The above specified RPS electrical power monitoring assemblies shall be determined OPERABLE:

a. By performance of a CHANNEL FUNCTIONAL TEST each time the plant is in COLD SHUTDOWN for a period of more than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, unless performed in the previous 6 months.

Summary

The modification of the surveillance interval for performing channel functional tests for EPAs in accordance with this guidance will eliminate the requirement to test EPAs during power operation. The elimination of this testing during power operation will reduce the possibility for inadvertent trips and challenges to safety systems. The implementation of this line-item TS improvement will produce a net benefit for safety.