Transient subchannel analysis of BWR fuel thermal hydraulic performance

Traditionally, the complexity of the phenomena inside the fuel has been analyzed with one-dimensional thermal hydraulic models coupled with empirical correlations based on full-scale or sub-scale tests to determine the appropriate design margins. The disadvantage of this approach is that physical phenomena often are not modeled from first principles and therefore a large database covering all expected conditions is needed. A more basic approach is to calculate the detailed bundle flow is multi-field subchannel analysis coupled with phenomenological first principle models in 3-dimensional detail. This is done with Global Nuclear Fuel’s subchannel code COBRAG. COBRAG solves the two-phase multi-field time dependent governing equations including continuous vapor, entrained droplets and liquid films for the annular flow regime along with models for shear and heat transfer, turbulent mixing and void drift to calculate the quality and void distributions across the bundle subchannels. Critical power is determined from mechanistic film dryout model derived based on selected non-dimensional local conditions parameters that capture the dynamic at which the liquid film breaks down. The purpose of this paper is to present the critical film model and the validation of this model for GNF2 and GNF3 fuel type steady-state and transient data collected at the Stern facility. Comparisons for steady-state critical power which includes GNF2 and GNF3 data show excellent agreement. Trends with mass flux, pressure, inlet subcooling, rod power distribution and axial power shape are also well predicted. Results of comparisons to transient dryout also show very good agreement. The capabilities of the COBRAG code allows it to be used for analyses of typical plant transient event including phenomena such as direct moderator heating and time varying axial power shapes that cannot be simulated in experimental test facilities. The results are used to benchmark design codes like TRACG using the GE critical quality versus boiling length (GEXL) correlation. The transient ΔCPR calculated with COBRAG is very similar to the design code analysis.