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Information Notice No. 89-69: Loss of Thermal Margin Caused by Channel Box Bow
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 September 29, 1989 Information Notice No. 89-69: LOSS OF THERMAL MARGIN CAUSED BY CHANNEL BOX BOW Addressees: All holders of operating licenses or construction permits for boiling-water reactors (BWRs). Purpose: This information notice is intended to alert addressees to potential problems involving loss of thermal margin caused by excessive bowing of BWR fuel channel boxes. 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 do not constitute NRC requirements; therefore, no specific action or written response is required. Description of Circumstances: During a refueling outage in August 1988, four failed fuel rods in separate assemblies were identified at a foreign BWR facility. Subsequent evaluation of sipping, visual inspection, gamma scan, and hot-cell data led to the con- clusion that these rods failed because the rods were operated under dryout conditions during steady-state operation for an extended period of time (between 2 and 7 days). The failed fuel rods were located symmetrically in the core. The fuel assem- blies containing the rods that had failed were located adjacent to once-burned fuel assemblies with highly exposed fuel channels (see Figure 1). These fuel channels were in their second bundle lifetime and had excessive channel bowing. In each assembly with failed fuel, the corner rod facing the adjacent control rod was heavily oxidized and the cladding was penetrated just below the top spacer grid. In addition, each of the four failed rods had typical secondary internal hydriding damage near the bottom of the fuel rods, resulting in loss of fuel material. Discussion: Dryout of the fuel rods in this foreign facility occurred because of modeling errors in the plant process computer, which resulted in nonconservative calcu- lated values of the minimum critical power ratio (MCPR) of the core. These 8909250252 . IN 89-69 September 29, 1989 Page 2 of 2 modeling errors were caused by neglecting the effects of channel bowing and the geometric variation between the reloaded and once-burned fuel assemblies. These effects substantially increased the widths of the control rod water gaps for the assemblies that contained these four fuel rods beyond that assumed in the plant process computer calculations. The increased neutron moderation as- sociated with the increased water gap widths led to very high localized power peaking at these four fuel rods. However, these effects were not properly ac- counted for in the MCPR calculations. For some time, the plant operators were misled by these erroneous MCPR calculations and were operating the plant in steady-state beyond the MCPR safety limit. The modeling error of generic concern to all BWRs, regardless of the fuel supplier, relates only to the greater-than-expected bowing of fuel channel boxes, which contributed about 15 percent error in the calculated MCPR value for this foreign facility. Channel bowing is a manifestation of differences in the channel growth of opposite sides of the channel box and is proportional to channel growth. The information obtained by the NRC indicates that the channel growth shows an accelerated trend at higher burnup exposure, especially when the fuel channels are being reused in their second bundle lifetime. The effect on core operating MCPR is magnified when fresh fuel is located adjacent to the bowed fuel channels. Core operating limits imposed by technical specifications may be exceeded if the reduction in margin caused by fuel channel bowing is not properly accounted for in the plant process computer for thermal limits monitoring. Based on a preliminary evaluation by BWR fuel vendors of U.S. reactors, the impact of the new data on actual versus calculated MCPR values is expected to range from 0.0 to 0.03 CPR units. However, the impact could be much greater (about 15 percent) for any reactors operating with fuel channels being reused in their second bundle lifetime. 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 manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical Contacts: Peter C. Wen, NRR (301) 492-1172 Daniel B. Fieno, NRR (301) 492-3236 Attachments: 1. Figure 1, "Channel Bow" 2. List of Recently Issued NRC Information Notices .
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