Improved methodologies in ECCS large break LOCA analysis

Currently, Loss-of-Coolant Accident (LOCA) analyses performed to evaluate Light Water Reactor (LWR) Emergency Core Cooling System (ECCS) designs must comply with the rules layed down by Appendix K of U.S. Nuclear Regulatory Commission (NRC) regulation 10 CFR 50. The regulations set forth in Appendix K place severe restrictions on the methods which can be utilized for large break LOCA analysis. Several conservative assumptions and models are explicitly required in Appendix K. As the early ECCS computer codes were reviewed by the NRC, additional conservatisms were added in those areas where knowledge or phenomenological understanding was limited. Westinghouse Electric Corporation has developed an improved thermal hydraulic transient model, called the BART code. BART is a reflood safety analysis code which can be used to address ECCS performance during reflood. The formulation used in BART utilizes mechanistic models to describe the two-phase flow and heat transfer, axial conduction, clad rewetting, fuel clad swelling and radiation heat transfer to drops and vapor in the flow. This paper will show that mechanistic models can produce significant LOCA peak clad temperature (PCT) licensing margin while maintaining the overall calculation conservatism required by Appendix K