VOID FRACTION DISTRIBUTION IN BWR FUEL ASSEMBLY AND EVALUATION OF SUBCHANNEL CODE

Void fraction measurement tests for BWR fuel assemblies have been cond ucted as part of a Japanese national project. The aim was to verify th e current BWR void fraction prediction method. Void fraction was measu red using an X-ray CT scanner. This paper describes typical results of void fraction distribution measurements and compares subchannel-avera ged void fraction data with current subchannel analysis codes. The sub channel analysis codes COBRA/BWR and THERMIT-2 were used in this compa rison. The agreement between data for an actual BWR fuel assembly with two unheated rods was good, but in the case of many unheated rods, th e codes were unable to predict well large void fraction gradient in th e radial direction observed in the measured data over the unheated rod region. The prediction errors of COBRA/BWR and THERMIT-2 for the subc hannel-averaged void fraction were [A] (average of difference between measurement and calculation)=-1.1%, sigma(standard deviation)=5.3% and [A]=-2.2%, sigma= 6.3%, respectively.