Information Notice No. 91-54: Foreign Experience Regarding Boron Dilution

                               UNITED STATES 
                           WASHINGTON, D.C.  20555

                              September 6, 1991



All holders of operating licenses or construction permits for pressurized 
water reactors (PWRs).  


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information 
notice to alert addressees to a potential sequence of events that may result 
in a rapid injection of unborated reactor coolant water into the reactor 
core.  It is expected that recipients will review the information for 
applicability to their facilities and consider actions, as appropriate, to 
avoid similar problems.  However, suggestions contained in this information 
notice are not NRC requirements; therefore, no specific action or written 
response is required.  

Description of Circumstances

The NRC has learned of analyses, performed by a foreign program, of certain 
events and event sequences which indicate that there is a potential for a 
rapid injection into a PWR reactor core of unborated reactor coolant whose 
effects would be more severe than similar events considered in the safety 
analyses of most U.S. plants.  

The adverse events involve the accumulation of a significant volume of 
unborated water in a part of the primary coolant system during plant 
shutdown.  Various actions performed during plant shutdown could result in 
such an accumulation.  Once the unborated water is accumulated, the startup 
of an idle reactor coolant pump could send the unborated water into the 
reactor core causing a significant and unplanned reactivity insertion.  

The following sequence of events is evaluated in the foreign study.  

The sequence is initiated by the loss of one of two offsite electrical power 
sources.  This loss of power occurs while the reactor coolant water is being 
diluted for reactor startup.  At this time, decay heat is insufficient to 
cause natural circulation of the reactor coolant.  It is assumed that the 
operating reactor coolant pump(s) are powered from the failed offsite power 
source and cease running.  However, the charging pumps, powered from the 
alternate power source, continue to operate and a significant volume of 
unborated water is accumulated in the reactor coolant system.  If, at this 


                                                       IN 91-54 
                                                       September 6, 1991 
                                                       Page 2 of 2

point, an idle reactor coolant pump is started, the accumulated volume of 
weakly borated or unborated water would be injected into the reactor core. 

Discussion of Safety Significance

The safety analyses for most U.S. nuclear plants do not address this 
explicit sequence of events.  However, the staff is aware of one 
representative analysis of a similar sequence for a U.S. PWR.  In that 
analysis, the results indicate that such an accident could cause some core 
damage but that breach of the reactor coolant system would be unlikely.  The 
NRC is continuing to study other possible sequences that could cause a 
similar unplanned injection of unborated water.  The above postulated 
sequence of events would occur as a result of actions performed during plant 
restart from a shutdown condition and as such is being considered as part of 
the NRC's review of shutdown risk.

This accident sequence involves several assumptions regarding plant 
conditions and equipment configuration and, therefore, may not apply to a 
particular U.S. plant.  Training and procedures that emphasize the need to 
ensure uniform boron concentration in the reactor coolant system at all 
times and the implementation of appropriate caution in starting an idle 
reactor coolant loop can reduce the probability of occurrence of such an 

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 Office of 
Nuclear Reactor Regulation project manager.  

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

Technical contacts:  N. Prasad Kadambi, NRR 
                     (301) 492-0841

                     M. Caruso, NRR 
                     (301) 492-3235

Attachment:  List of Recently Issued NRC Information Notices


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