A nondiffusive solution method for RETRAN-03 BWR stability analysis

Boiling water reactors (BWRs) are susceptible to thermal-hydraulic instabilities which must be considered in their design and operation. Early BWRs were designed to be very stable while operating under natural circulation conditions. As reactor designs have been modified, stability margins have been reduced and the potential for stability events, such as occurred at the La Salle and Vermont Yankee plants, has increased. These events and other considerations point to the need for a reliable analysis tool for predicting the dynamic behavior of these events. This paper presents the development and validation of a conservation equation transport model for the RETRAN-03 time domain thermal-hydraulic analysis code that extends the range of application to simulating the dynamic behavior of stability events. The model has been developed for use with the RETRAN-03 mixture energy and vapor continuity equations that eliminates the numerical diffusion resulting from current models in RETRAN. The transport model has been implemented in RETRAN-03 and results using the five-equation model are presented. Since most time domain thermal-hydraulics systems analysis codes use a donoring approach which makes them susceptible to numerical diffusion, comparisons of the RETRAN-03 solution with the MOC transport model will be made to demonstrate the impact that numerical diffusion can have on stability analyses. Additional RETRAN-03 analyses are presented in the paper that compare simulations of hydrodynamic instability events with data