Modification of Surveillance Interval for the Electrical Protective Assemblies in Power Supplies for the Reactor Protection (Generic Letter 91-09)
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 hours, 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.
James G. Partlow
Associate Director for Projects
Office of Nuclear Reactor Regulation
Enclosure: As stated
Contact: Tom Dunning, NRR/OTSB
Generic Letter 91-09 Enclosure
MODIFICATION OF THE SURVEILLANCE INTERVAL REQUIREMENTS FOR
ELECTRICAL PROTECTION ASSEMBLIES IN REACTOR PROTECTION POWER SUPPLIES
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.
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 220.127.116.11 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
hours 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 hours
during an operating cycle.
If the licensee does not encounter a cold shutdown of 24 hours 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:
Generic Letter 91-09 - 2 - Enclosure
18.104.22.168 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 hours,
unless performed in the previous 6 months.
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.
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