Evaluation of coupled codes RELAP5/PARCS capability for BWR global stability prediction

The present study is concerned with capability of a coupled neutron-kinetic/thermal-hydraulic code system RELAP5/PARCS for the numerical prediction of global core stability condition and instability transients. The work is motivated by the need to assess safety significance of a number of stability transients which trigger core instability and challenge reactor protection systems. The technical approach adopted is both to learn from real stability events and to perform analysis of idealized well-defined transients in a real plant and core configuration. In this paper, we show that the code system can serve as a unique and powerful tool to provide a consistent and reasonably reliable prediction of stability boundary even in complex plant transients. However, the prediction quality of the instability transients, i.e. core behavior without scram, namely parameters of the limit cycle remains questionable. We identify two main factors for future studies (two-phase flow regimes in oscillatory flow and algorithm for effective grouping of thermal-hydraulic channels) as key to enhancing the predictive capability of the existing coupled code system for BWR stability.