Information Notice No. 90-74: Information on Precursors to Severe Accidents

                               UNITED STATES
                          WASHINGTON, D.C.  20555

                             December 4, 1990

Information Notice No. 90-74:  INFORMATION ON PRECURSORS TO 
                                   SEVERE ACCIDENTS 


All holders of operating licenses or construction permits for nuclear power 


This information notice is intended to alert addressees to the existence of 
reports that examine events that have occurred at commercial nuclear power 
plants and that quantify how close these events were to causing severe core 
damage.  This information notice does not contain NRC requirements; 
therefore, no specific action or written response is required.


The Office for Analysis and Evaluation of Operational Data and its 
consultants at the Oak Ridge National Laboratory have published a series of 
accident sequence precursor study reports ("Precursors to Potential Severe 
Core Damage Accidents:  [year] A Status Report").  The studies reviewed 
licensee event reports (LERs), for the period from January 1969 through 
December 1989, to identify and categorize precursor events to potential 
severe core damage accidents.  Accident sequences considered in these 
studies were those associated with the failure of one or more safety 
systems, the degradation of two or more safety systems, or the occurrence of 
an initiator; such as loss-of-offsite power, loss of coolant accident, steam 
line break, or a transient with complications.  When coupled with one or 
more other postulated events, precursor events may result in a plant 
condition in which core cooling is inadequate.  Study of accident precursors 
has indicated that the causes of the most safety significant events are 
essentially the same as those of minor events and can reveal areas of a 
plant's design, procedures, or operation that potentially could contribute 
to a severe core damage accident. The accident sequence precursor study 
reports estimate the conditional probability of core damage, given the 
occurrence of precursor events.  


                                                         IN 90-74 
                                                         December 4, 1990 
                                                         Page 2 of 2 

The accident sequence precursor study reports may be obtained from the 
Superintendent of Documents, U.S. Government Printing Office, P.O. Box 
37082, Washington, D.C.  20013-7082.  The following reports are available:

                   Year(s)              Document Number

                 1969 to 1979   (NUREG/CR-2497, vol. 1 and 2)
                 1980 to 1981   (NUREG/CR-3591, vol. 1 and 2)
                 1984           (NUREG/CR-4674, vol. 3 and 4)
                 1985           (NUREG/CR-4674, vol. 1 and 2)
                 1986           (NUREG/CR-4674, vol. 5 and 6)
                 1987           (NUREG/CR-4674, vol. 7 and 8)
                 1988           (NUREG/CR-4674, vol. 9 and 10)
                 1989           (NUREG/CR-4674, vol. 11 and 12)

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
one of the technical contacts listed below or the appropriate NRR project 

                                Charles E. Rossi, Director
                                Division of Operational Events Assessment
                                Office of Nuclear Reactor Regulation

Technical Contacts:  Adel El-Bassioni, NRR
                     (301) 492-1094

                     Fred Manning, AEOD
                     (301) 492-4426

Attachment:  List of Recently Issued NRC Information Notices

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