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February 11, 2000 Dr. William D. Travers Dear Dr. Travers:
During the 469th meeting of the Advisory Committee on Reactor Safeguards, February 3-5, 2000, we reviewed the proposed final revision of Appendix K to 10 CFR Part 50. During this review, we had the benefit of discussions with representatives of the NRC staff and the Caldon Corporation. We also had the benefit of the documents referenced. We had previously commented on the proposed revision to Appendix K in a letter dated July 22, 1999. The proposed final rule will permit a reduction in the conservatism of the reactor power level assumed for loss-of-coolant accident analysis by relaxing the requirement that a licensee assume 1.02 times licensed power for the Appendix K emergency core cooling system analysis. This rulemaking is in response to requests from licensees seeking credit in safety analyses for reduction in uncertainty of reactor power resulting from the use of highly accurate flow measurement systems. This rule change will allow licensees to credit use of such measurement systems and will avoid an expected large number of exemption requests, thereby reducing regulatory burden. Licensees are expected to pursue small power increases or other cost-saving changes to plant operating parameters through license amendment requests. Recommendations
Discussion This rule is an example of allowing an appropriate reduction of conservatism in the regulations when the uncertainties that led to this conservatism can be shown to have been reduced. In principle, this is a straightforward matter. Implementation of the rule will require specific guidance about the definition of uncertainties. For example, does "x% uncertainty" imply that there is some confidence level, such as 95%, that the deviations between actual and measured values are less than x% of the measured values? How are uncertainties in several values contributing to power calculation, such as temperatures and flowrate, to be combined? Answers to these questions as well as a suitable reference should be provided in the guidance to the licensees. In our July 22, 1999 letter on this matter, we recommended that the staff evaluate the possible impact of the proposed rule on parts of the regulations other than Appendix K. Some changes to guidance documents may be necessary, as mentioned in the Statement of Considerations accompanying the rule revision.
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