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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 Attachment: List of Recently Issued NRC Information Notices .
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