United States Nuclear Regulatory Commission - Protecting People and the Environment

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 
50.63]). 

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, 

9104290113 
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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. 

                                        Sincerely,


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


Technical Contacts:  Chu-Yu Liang, NRR
                     (301) 492-0878

                     Paul Gill, NRR
                     (301) 492-3265

Page Last Reviewed/Updated Friday, June 28, 2013