Potential thermal margin available from changes in the appendix K rule

Over a decade of research has been completed on light water reactor safety. This research was the result of the 1972 core cooling hearing in the United States which resulted in the Appendix K rule for evaluating safety system performance. The objectives of the research were to confirm that the Appendix K rule was conservative, determine and quantify the amount of conservatism such that the excess conservatism could be reviewed by future rule changes, and provide a basis for future rule changes. The lightwater reactor research for the large break LOCA has, in the main, been completed and the first two objectives of the research program have been achieved. Presently there are ongoing efforts in the United States to find methods of factoring into the licensing process the results of this research. This paper discusses the impact on the W safety analysis if some of the Appendix K requirements could be relaxed. Calculations are presented with revised models which have been shown, by the LWR program, to be more accurate and which have the identified excessive conservatisms removed. LOCA calculations are presented to show the increased power capability, as well as power peaking margin which can result from such changes. These calculations are also compared with best estimate calculations using more mechanistic computer codes