Bulletin 89-03: Potential Loss of Required Shutdown Margin During Refueling Operations
OMB No.: 3150-0011 NRCB 89-03 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 November 21, 1989 NRC BULLETIN NO. 89-03: POTENTIAL LOSS OF REQUIRED SHUTDOWN MARGIN DURING REFUELING OPERATIONS Addressees: All holders of operating licenses or construction permits for pressurized water reactors. Purpose: This bulletin is being provided to alert addressees to the potential loss of required shutdown margin during the movement and placement of highly reactive fuel during refueling operations. It is expected that recipients will take the actions in this bulletin to prevent potential violations of required shutdown margin and, in extreme cases, inadvertent criticality during refueling. Description of Circumstances: Baltimore Gas and Electric Company (BG&E) submitted a 10 CFR Part 21 report to the NRC on March 15, 1989, regarding the potential loss of shutdown margin during refueling operations at its Calvert Cliffs Nuclear Power Plant, Units 1 and 2. BG&E reported that it had increased the fuel enrichment during the last several fuel cycles. In the current cycle, 4.3 weight percent U-235 fuel was loaded into the core. During a review of NRC Information Notice No. 88-21, "Inadvertent Criticality Events at Oskarshamn and U.S. Nuclear Power Plants," BG&E discovered that the Calvert Cliffs refueling procedures allowed the placement of fuel assemblies in intermediate positions during core alterations. In addition, BG&E determined that the potential existed for losing some of the shutdown margin required by the plant technical specifications, even if several fresh reload assemblies with enrichments as low as 4.1 weight percent were grouped together. Calculations also showed that under extreme conditions an inadvertent criticality could occur if a number of highly reactive assemblies were grouped together. In response to the BG&E notification, the NRC issued Information Notice No. 89-51, "Potential Loss of Required Shutdown Margin During Refueling Operations," dated May 31, 1989. BG&E has revised the Calvert Cliffs refueling procedures to ensure that fuel assemblies will not be placed in intermediate positions during core alterations without first verifying their potential reactivity. The revised procedures 8911200248 . NRCB 89-03 November 21, 1989 Page 2 of 4 allow fuel to be positioned only in intermediate core locations that will contain fuel of equal or greater reactivity in the final core configuration. Discussion: As a result of longer fuel operating cycles, utilities have been increasing the enrichment of reload fuel. Some of these fresh reload assemblies may be highly reactive under certain refueling conditions. Although analyses are performed for PWRs to confirm that the refueling boron concentration is sufficient to maintain the required shutdown margin (subcriticality) for the final core configuration, these analyses may not be sufficient to assure that the shutdown margin will be maintained for all intermediate fuel assembly positions. In addition, explicit procedural controls may not exist to control the location and movement of highly reactive fuel assemblies during refueling. Because a significant amount of reactivity can be added to subcritical configurations by the addition of a single highly reactive assembly, it is possible that an inadvertent criticality could occur if a number of such assemblies are grouped together. With this highly reactive fuel, subcritical multiplication (inverse count rate) may not provide adequate warning of an approach to criticality. An inadvertent criticality could result in fuel failures, system damage, and potentially high radiological doses to onsite workers. Combustion Engineering, Inc. (CE), the nuclear fuel design consultant for Calvert Cliffs, recognized this potential problem and issued an information bulletin regarding shutdown margin during refueling to all utilities with CE-designed plants. These utilities have been advised to review their fuel shuffle procedures that control the location of highly reactive fuel and to be aware that the refueling boron concentration necessary to maintain the required shutdown margin, which is based on the final core configuration, may not be sufficient to assure that their required shutdown margin will be maintained for all intermediate fuel assembly positions. In circumstances in which explicit analyses are not available for each intermediate fuel assembly position, CE recommends positioning fuel only in intermediate core locations that will contain fuel of equal or greater reactivity in the final core configuration. Requested Actions: Operating Reactors: All PWR licensees are requested to assure that adequate shutdown margin is maintained during all refueling operations. This should be accomplished through the following actions: 1. Assure that any intermediate fuel assembly configuration (including control rods) intended to be used during refueling is identified and evaluated to maintain sufficient refueling boron concentration to result in a minimum shutdown margin of approximately 5 percent. 2. Assure that fuel loading procedures only allow those intermediate fuel assembly configurations that do not violate the allowable shutdown margin and that these procedures are strictly adhered to. . NRCB 89-03 November 21, 1989 Page 3 of 4 3. Assure that the staff responsible for refueling operations is trained in the procedures recommended in Item 2 above and understand the potential consequences of violating these procedures. This training should include the fundamental aspects of criticality control with higher enriched fuel assemblies. Construction Permit Holders: All PWR construction permit holders are requested to complete the above-listed actions before the date scheduled for fuel loading. Reporting Requirements: Within 60 days of receipt of this bulletin, pursuant to 10 CFR 50.54(f), all holders of OLs for PWRs shall notify the NRC by letter whether they have taken, or will take, the actions requested above. Prior to initial fuel loading, CP holders shall advise the NRC by letter whether the requested actions have been implemented. The written reports required above shall be addressed to the U.S. Nuclear Regulatory Commission, ATTN: Document Control Desk, Washington, D.C. 20555, and shall be submitted under oath or affirmation pursuant to the provisions of Section 182a, Atomic Energy Act of 1954, as amended and 10 CFR 50.54(f). In addition, a copy shall be submitted to the appropriate Regional Administrator. Backfit Discussion The objective of the actions requested in this bulletin is to ensure that an adequate shutdown margin is maintained during the movement and placement of fuel during refueling operations. The issuance of the bulletin is justified on the basis of the need to provide adequate protection to the health and safety of the public and is consistent with the provisions of 10 CFR 50.109a(4)(ii). The actions proposed by the bulletin will ensure that an adequate shutdown margin is maintained. Failure to take these actions may result in an inadequate shutdown margin during refueling operations. In extreme cases this could result in inadvertent criticality with potential for fuel failures, reactor damage, and relatively high radiological doses to onsite workers. This request is covered by Office of Management and Budget Clearance Number 3150-0011 which expires December 31, 1989. The estimated average burden hours are 100 person-hours per licensee response, including assessment of the new recommendations, searching data sources, gathering and analyzing the data, and preparing the required letters. These estimated average burden hours pertain only to these identified response-related matters and do not include the time for actual implementation of the requested actions. Send comments regarding this burden estimate or any other aspect of this collection of information, . NRCB 89-03 November 21, 1989 Page 4 of 4 including suggestions for reducing this burden, to the Information and Records Management Branch, Division of Information Support Services, Office of Information Resources Management, U.S. Nuclear Regulatory Commission, Washington, D.C. 20555; and to the Paperwork Reduction Project (3150-0011), Office of Management and Budget, Washington D.C. 20503. If you have any questions about this matter, please contact one of the technical contacts listed below or the appropriate NRR project manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical Contacts: Laurence I. Kopp, NRR (301) 492-0879 N. Prasad Kadambi, NRR (301) 492-1153 Attachment: List of Recently Issued NRC Bulletins .
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