RELAP5 code improvement for horizontal passive containment cooling system (PCCS) thermal hydraulic performance evaluation

A passive containment cooling system (PCCS) is under planning to use in a next-generation-type BWR for long-term cooling by condensing steam in horizontal u-tube heat exchangers during a severe accident when a containment spray system is in failure. Steam and non-condensable gas released into the containment flow into the primary side of the tubes by pressure difference between the drywell and the wetwell, and the steam is condensed in the tubes. So far, a single u-tube test for studying fundamental heat exchanger performance and a prototypical-scale test were completed. This paper describes modification of the RELAP5/MOD3.2.1.2 code to evaluate the PCCS system responses including condensation heat transfer behavior in the PCCS horizontal heat exchanger tubes. The original RELAP5 code underpredicted condensation heat transfer in the tubes. So the original RELAP5 code was improved employing optimum existing condensation heat transfer correlations (1. Nakamura et al., 2000). After employment of the optimum existing heat transfer correlations, the RELAP5 code still underpredicted condensation heat transfer. Therefore the RELAP5 code was further improved. This paper describes improvement after employment of the optimum existing correlations which is a) Kondo et al. correlation (the new condensation heat transfer correlation for annular flow taking into account the condensation enhancement by roll waves), b) modified flow regime transition criterion which uses the maximum of the annular and the wavy flow correlations, c) an improvement of the heat transfer correlation after transition to wavy flow and d) Arai et al. condensation degradation correlation for better prediction of condensation degradation by non-condensable gas. Consequently the RELAP5 code predicted closer primary side equilibrium quality and temperature distribution to that of the test results. (author)