United States Nuclear Regulatory Commission - Protecting People and the Environment

Transmittal of NUREG-1190 Regarding the San Onofre Unit 1 Loss of Power and Water Hammer Event (Generic Letter No. 86-07)


                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                          WASHINGTON, D. C.  20555 

                               March 20, 1986 

TO ALL REACTOR LICENSEES AND APPLICANTS 

Gentlemen: 

SUBJECT:  TRANSMITTAL OF NUREG-1190 REGARDING THE SAN ONOFRE UNIT 1 LOSS OF 
          POWER AND WATER HAMMER EVENT (GENERIC LETTER 86-07) 

On November 21, 1985, while operating at 60% power, Southern California 
Edison Company's San Onofre Unit 1 Nuclear Power Plant experienced a loss of
ac electrical power followed by a severe water hammer in the secondary 
system which caused a steam leak and damaged plant equipment. Shortly after 
the event, the NRC Executive Director for Operations directed that an NRC 
Team be sent to San Onofre, in conformance with the recently established 
Incident Investigation Program, to investigate the circumstances of this 
event. The NRC Team has now completed its investigation and has documented 
the factual information and their findings and conclusions associated with 
the event (see enclosed NUREG-1190, entitled "Loss of Power and Water Hammer 
Event at San Onofre Unit 1, on, November 21, 1985"). 

In this report, the team has concluded that the event was significant 
because (a) all inplant ac power was lost for 4 minutes; (b) all steam 
generator feedwater was lost for 3 minutes; (c) a severe water hammer caused 
by check valve failures was experienced in the feedwater system which caused 
a leak, damaged plant equipment and challenged the integrity of the 
auxiliary feedwater system; (d) all indicated steam generator water levels 
dropped below scale; and (e) the reactor coolant system experienced an 
acceptable but unnecessary cooldown transient. In the team's view the most 
significant aspect of the event was that five safety-related feedwater 
system check valves degraded to the point of inoperability during a period 
of less than a year, without detection, and that their failure jeopardized 
the integrity of safety-related feedwater piping. The cause of the feedwater 
system check valve failures has been preliminarily identified by SCE as 
partial or complete separation of the check valve disc assemblies due to 
fluid flow conditions. Information submitted to the staff on this subject is 
currently under review. 

You should review the information in the enclosed report for applicability 
to your facility. In addition, you should ensure that the information in 
NUREG-1190 is made available to your plant staff as part of your training 
program in connection with the Feedback of Operating Experience to Plant 
Staff (TMI Action Plan Item I.C.5). 



8603210334 

                                                            March 20, 1986 

                                     - 2 -

On February 4, 1986, the Executive Director for Operations (EDO) identified 
and assigned responsibility or generic and plant-specific actions resulting 
from the investigation of the San Onofre event. Some of the generic actions 
may be applicable to your facility. A copy of the EDO memorandum is included
for your information.  

This generic letter is provided for information only, and does not involve 
any reporting requirements. Therefore, no clearance from the Office of 
Management and Budget is required. The enclosed report is currently under 
NRC review. 

Any generic requirements stemming from the report will be transmitted at a 
later date following completion of the appropriate procedural steps.   

                                   Sincerely, 


                                   Harold R. Denton, Director 
                                   Office of Nuclear Reactor Regulation 

Enclosures:
1. NUREG-1190 
2. EDO Memorandum of February 4, 1986 
3. List of Generic Letters
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