United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 92-21, Supplement 1: Spent Fuel Pool Reactivity Calculations

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                               April 22, 1992


NRC INFORMATION NOTICE 92-21, SUPPLEMENT 1:  SPENT FUEL POOL REACTIVITY
                                             CALCULATIONS


Addressees

All holders of operating licenses or construction permits for nuclear power 
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this supplemental 
information notice to update information initially supplied by ABB 
Combustion Engineering (CE) and incorporated in Information Notice (IN) 
92-21.  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

IN 92-21 described errors which were discovered in reactivity calculations 
for spent fuel pools.  Initially, CE attributed one of the errors in their 
calculations for the Millstone, Unit 2, spent fuel pool to using the 
transport cross section as an approximation for the total cross section in a 
region containing a strong neutron absorber (Boraflex).  CE has recently 
informed the NRC that, upon further investigation, they believe that this 
approximation had little effect on calculated reactivity.  CE now attributes 
the error to incorrect treatment of the self-shielding effect in Boraflex 
for the epithermal energy group.  Revised calculations have shown that the 
absorption cross section in Boraflex for the epithermal energy group is 
significantly self-shielded; however, this was not accounted for in the 
original calculations.  This oversight resulted in overestimating neutron 
absorption and a corresponding lower calculated keff in that region.  The 
remainder of the discrepancy is still attributed to the inaccurate geometric 
buckling term used, as discussed in IN 92-21.

Discussion 

Criticality calculational methodologies are benchmarked against criticality 
experiments which include neutron absorbers; however, the negative 
reactivity worth of the neutron absorber is usually low compared to that 
utilized in an 


9204160193 
.

                                                  IN 92-21, Supplement 1
                                                  April 22, 1992
                                                  Page 2 of 2


actual spent fuel pool storage rack (approximately 2 percent versus 20 
percent).  Modest errors in calculated absorber worth in these criticality 
experiments would result in relatively small errors in total reactivity.  
However, similar errors when applied to the much higher absorber worth in 
actual spent fuel pool storage racks may result in significantly larger 
errors in total reactivity.

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 (NRR) project manager. 




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

Technical contacts:  Jack Ramsey, NRR
                     (301) 504-1167

                     Larry Kopp, NRR
                     (301) 504-2879

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