MECHANISTIC PREDICTION OF VOID FRACTION DISTRIBUTION IN BWR FUEL ASSEMBLY USING THE SUBCHANNEL CODE CAPE

In order to predict the critical power or void fraction in BWR fuel bundles and the DNB heat flux of PWR fuel assemblies, the boiling transition analysis code called "CAPE" with mechanistic models has been developed in the IMPACT project by NUPEC. The objective of the CAPE code development is to perform with good accuracy the safety evaluation for a new type or improved fuel bundle design of BWR and PWR without full-scale experiments or any tuning parameters in the analysis code. In the present study void fraction distribution of BWR fuel assembly were analyzed by the CAPE code and compared with experimental data of BFBT benchmark test carried out by NUPEC. The analyses were carried out by changing the operational parameter such as the inlet subcooling, mass flow rate and the power output of the fuel bundles. Resultantly, the thermal equilibrium quality at the outlet ranges 2% similar to 25%. Averaged and local void fractions were compared between experiment and analysis. The results indicated that the CAPE code satisfactorily predicted void fraction distribution of fuel assembly for wide range of pressure, mass flux, subcooling and bundle geometries obtained in BFBT benchmark test. However, the detailed analysis showed that in some subchannels, which was surrounded by the heated fuel rods and partially unheated wall such as an unheated rod, a water rod and a separation wall of the channel box, certain difference between experiment and prediction appeared. In the CAPE code, the drift flux model is used for predicting void behavior in fuel assembly. In drift flux model, correlations of drift velocity and distribution parameters are quite important in predictive accuracy of void fraction. In particular, it is considered that distribution parameter plays important role in void distributions in subchannel surrounded by unheated rods. In addition to the original correlations in the CAPE, some typical correlations of drift velocity and distribution parameter were tested for the prediction of void fraction distribution. The evaluations of correlations of drift velocity and distribution parameter were carried out, and some recommendations for improvement of predictive capability of the CAPE code were made.