Prediction of cross sectional void distribution in a BWR fuel bundle configuration

Conventionally, the nuclear calculations for a BWR fuel bundle assumes certain cross sectional void fraction profile and calculates the axial void fraction variation based on the bundle axial power shape. This process can be evaluated using the detailed void distribution in the channel. Furthermore, the distribution of the liquid in the different subchannels as well as the split between film and entrained droplets in the annular flow regime has a direct impact on the rod critical power. GE's subchannel code COBRAG has the capability to calculate the three-dimensional void distribution in a BWR fuel bundle. The models in COBRAG include two-phase flow mixing and void drift as they are important for predicting the cross sectional distribution of liquid in the fuel bundle. The NUPEC facility has measured detailed distributions of the liquid and vapor phases in a 8 x 8 bundle. In this study the cross sectional void distributions calculated by COBRAG are compared to NUPEC data. The comparison between the calculation and the test results serves as a good guide to gain insight on how different phenomena compete and contribute to the actual local void variation. This understanding will help future space design and modeling. (author)