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UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555-0001 October 14, 1994 NRC INFORMATION NOTICE 94-75: MINIMUM TEMPERATURE FOR CRITICALITY Addressees All holders of operating licenses or construction permits for pressurized- water reactors (PWRs). Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to potentially non-conservative initial conditions that were used in the analysis of some design-basis transients. As a result, some plant technical specifications for minimum temperature for criticality may not be adequately conservative. 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 Commonwealth Edison Company (Zion Nuclear Station Units 1 and 2) and New York Power Authority (Indian Point Nuclear Power Plant Unit 3) have informed the NRC that their technical specifications for minimum temperature for criticality were not supported by the safety analyses for their plants. The licensing analysis performed by Westinghouse for Zion assumed a nominal hot- zero-power (no-load) operating temperature of 286 oC [547 oF], but Zion's technical specifications allow criticality if the average reactor coolant system temperature is greater then 260 oC [500 oF]. The safety analysis for Indian Point 3 was also performed at 286 oC [547 oF], but its technical specifications allow criticality at 232 oC [450 oF], a limit which was set by reactor vessel material considerations. A review of Indian Point records indicated that the reactor was brought critical below 286 oC [547 oF] several times in the early life of plant operations (before 1988). The lowest temperature during these instances was 272 oC [521 oF]. After discovering a potential to operate the plant in a region outside that analyzed, both licensees instituted administrative controls to ensure that the minimum temperatures for criticality are bound by the safety analyses performed for their plants. In addition, both licensees submitted license amendments to NRC to revise the minimum temperature for criticality. Further details concerning these events are in a 10 CFR Part 21 report to NRC prepared for the Zion plant, dated March 18, 1993, and in Indian Point 3 Licensee Event Report 93-046-00, dated December 1, 1993. 9410070143. IN 94-75 October 14, 1994 Page 2 of 2 Discussion For transient analysis performed for hot-zero-power cases, small changes (e.g., 3 oC [6 oF]) in initial conditions such as allowed by standard technical specifications would have a negligible impact on analysis results. However, if PWRs are allowed to achieve criticality significantly below the temperature that was previously analyzed at hot-zero-power, the following safety concerns would be raised: (1) The transient analyses, such as "rod withdrawal from subcritical," "rod ejection," "zero power feedwater malfunction," and "boron dilution event" documented in the Final Safety Analysis Report, might not have been analyzed at temperatures below hot-zero-power and could be non- conservative. This could cause the analyses results with small margins to violate specified fuel design limits (i.e., centerline fuel melt or departure from nucleate boiling) for one or more of these postulated transients. (2) The response of the power range ex-core nuclear instrumentation may be adversely effected by the increased density of the reactor coolant at lower temperatures. This could result in a higher power being reached before a power range reactor trip occurs which might violate specified fuel design limits for transients that rely on this trip. (3) The moderator temperature coefficient will become more positive, perhaps causing a violation of existing technical specifications. Higher values of moderator temperature coefficient could exceed those used in some of the transient safety analyses. A more positive moderator temperature coefficient at power would result in reactivity insertion that could increase the consequences of an anticipated-transient-without-scram event. New analyses may justify criticality at somewhat lower temperatures. For example, Commonwealth Edison Company performed the necessary safety analyses for the Zion station and the staff approved a technical specification amendment to change the minimum temperature for criticality to 277 oC [530 oF]. This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact the technical contact listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager. original signed by Brian K. Grimes, Director Division of Project Support Office of Nuclear Reactor Regulation Technical contact: George A. Schwenk, NRR (301) 504-2814 Attachment: List of Recently Issued NRC Information Notices
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