Development of an Appendix K Version of RELAP5-3D and Associated Deterministic-Realistic Hybrid Methodology for LOCA Licensing Analysis

Abstract

The Loss of Coolant Accident (LOCA) is one of the most important design basis accidents (DBA). In light water reactors, particularly the pressurized water reactor (PWR), the severity of a LOCA will limit how high the reactor power can operate. In the regulatory analysis (USNRC, 1987), it was estimated that if the peak cladding temperature (PCT) during a LOCA decreases by 100°F, it would be possible to raise the plant power by 10%. The revision of 10 CFR50.46 in 1988 stated that two kinds of LOCA licensing approaches can be accepted, namely the realistic and Appendix K methodologies. The realistic licensing technique describes the behavior of the reactor system during a LOCA with best estimate (BE) codes. However, the uncertainties of BELOCA analysis must be identified and assessed so that the uncertainties in the calculated results can be estimated to a high confidence level. Alternatively, the Appendix K approach will guarantee the conservatism of the calculation results, instead of answering the analytical uncertainty. It is widely believed that the realistic approach can more precisely calculate the sequences of a LOCA accident, and therefore provides a greater margin for the PCT evaluation. The associated margin can be more than 200K (Westinghouse, 2009). However, the development of a realistic LOCA methodology is long and costly, and the safety authority is highly demanding in their approach to evaluate uncertainties. Instead, implementation of evaluation models required by Appendix K of 10 CFR 50 (USNRC, 1988) upon an advanced thermal–hydraulic platform, such as RELAP5-3D (RELAP5-3D Code development Team, 1998), TRAC (Liles et al., 1981), CATHARE (Bestion, 1990) et al., also can gain significant margin in the PCT calculation. For instance, the PCT of Taiwan’s Maanshan Nuclear Power Plant calculated by the latest Westinghouse Appendix K Evaluation Model BASH (Westinghouse, 1987) is 445°F (2170°F→1725°F) lower than that of 1981 s calculation (Taipower Company, 1982). To develop a new Appendix K LOCA licensing tool using the most advanced version of RELAP5, namely RELAP5-3D, the compliance of the advanced RELAP5-3D code with Appendix K of 10 CFR 50 has been evaluated, and it was found that there are nine areas where code assessment and/or further modifications were required to satisfy the requirements set forth in Appendix K of 10 CFR 50. All of the ten areas have been evaluated