COBRAG is a subchannel code for the two-phase flow in a BWR fuel bundle. COBRAG has been extensively qualified against bundle critical power and pressure drop data. Recently its capability has been extended to include transient process analysis for the fuel bundles. New models for the heat conduction in the fuel and heat transfer to the two-phase mixture have been incorporated. The new models were compared against the transient test data collected at the ATLAS facility at GE Nuclear Energy. Results of comparison show good agreement on the characteristics of transient dryout. The code is then used to analyze a typical pressurization transient event. It is found that for a typical pressurization event the transient DELTACPR calculated with COBRAG is very similar to the system code analyses with the hot bundle model.
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