GI-23, "Reactor Coolant Pump Seal Failures" and its Possible Effect on Station Blackout (Generic Letter 91-07)
TO: ALL POWER REACTOR LICENSEES AND HOLDERS OF CONSTRUCTION PERMITS
SUBJECT: GI-23, "REACTOR COOLANT PUMP SEAL FAILURES" AND ITS POSSIBLE
EFFECT ON STATION BLACKOUT (GENERIC LETTER 91-07)
The staff published a notice in the Federal Register (56 FR 16130) on April
19, 1991, soliciting public comments on Generic Issue (GI)-23, "Reactor
Coolant Pump Seal Failures." This letter informs licensees of the possible
effect of GI-23 on their responses to the station blackout (SBO) rule
(Section 50.63 of Title 10 of the Code of Federal Regulations [10 CFR
Reactor coolant pump (RCP) seals and their supporting systems have
experienced degradations and failures, some of which have been of sufficient
severity to be classified as small break loss of coolant accidents
(SBLOCAs). Preliminary analyses by the NRC indicated that the RCP seal
failures could dominate the overall probability of a core melt caused by
SBLOCA. As a result, the staff established GI-23 to provide for the
evaluation of the adequacy of current licensing requirements relating to RCP
seal integrity and to determine if the U. S. Nuclear Regulatory Commission
(NRC) should take any further action.
In resolving the SBO issue (USI A-44), the NRC staff considered the
relationship of this issue with other NRC generic issues, including GI-23.
In determining estimates of core damage frequency for SBO events in
NUREG/CR-3226, "Station Blackout Accident Analyses," the staff assumed that
the RCP seals would leak at a rate of 20 gallons per minute (gpm) per pump.
During the final resolution of the station blackout issue and the
development of the Nuclear Management and Resources Council (NUMARC) Report
87-00, "Guidelines and Technical Bases for NUMARC Initiatives Addressing
Station Blackout at Light Water Reactors," NUMARC and the staff agreed to
assume that RCP seal leakage during an SBO would be no greater than 25 gpm
per pump. Further, NUMARC and the staff agreed that if the final resolution
of GI-23 results in higher RCP leakage rates, then the SBO analyses would
have to be reevaluated.
The SBO rule became effective on July 21, 1988, and the NRC received
responses from all licensees addressing the SBO rule by April 21, 1989.
Licensees may have analyzed their reactor coolant inventories for the SBO
conditions using the specific guidance provided in NUMARC Report 87-00 of 25
gpm for RCP seal leakage for pressurized water reactors (PWRs) and 18 gpm
for boiling water reactors (BWRs). These leak rates could be greater if the
seals failed during the SBO event.
The preliminary results of the staff's studies for GI-23 indicate that the
pump seal leak rates could be substantially higher than those assumed for
the resolution of the SBO issue. The staff determined that RCP seal leakage
could exceed 25 gpm and lead to core uncovery during an SBO in any of the
PWRs and in any of the four BWRs (Millstone Unit 1, Oyster Creek, Nine Mile
Point Unit 1,
Generic Letter 91-07 - 2 - May 2, 1991
and Big Rock Point) that do not have an AC-independent makeup capability.
Having made these findings, the staff is soliciting public comments on its
current understanding of GI-23. One possible outcome may be that seal
cooling be provided by an independent cooling system during off-normal plant
conditions involving the loss of all seal cooling, such as could occur
during an SBO. This recommendation, if adopted, would apply to all PWRs and
might apply to the above-mentioned BWRs, which do not have AC-independent
makeup capabilities for their reactor coolant systems. Therefore, utilities
should recognize that such a recommendation could affect their analyses and
actions addressing conformance to the SBO rule.
This generic letter consists of information only and does not require
specific action or written response. Therefore, an Office of Management and
Budget (OMB) clearance number is not necessary. If you have any questions
about this matter, please contact one of the technical contacts listed below
or the Regional Administrator of the appropriate regional office.
James G. Partlow
Associate Director for Projects
Office of Nuclear Reactor Regulation
Technical Contacts: Chu-Yu Liang, NRR
Paul Gill, NRR
Page Last Reviewed/Updated Thursday, March 25, 2021