Localized void feedback effects under single rod drop transient in BWR

A postulated single control rod drop transient was analyzed for a typical BWR plant. In order to include feedback effects of the infra-bundle void distribution in those bundles neighboring the withdrawn control blade, transient pin power distributions were reconstructed by the plant simulator TRAC/BF1-ENTREE and were exported to the subchannel code, NASCA The cross section libraries with flat and distorted void distributions were combined in the two-way coupling calculation. Exposure trends of neutronic properties were compared between two void distributions. Although the infinite multiplication factor wasn't notably influenced, the radial peaking factor changed significantly due to void distortions. A pin-by-pin concentration change of fissile materials also contributes to this result. At the initial state, the void fraction in peripheral subchannels facing the withdrawn blade became prominently low. The power level in the surrounding bundles decreased due to larger neutron leakage caused by distorted void distributions. During transient, the linear power density at the critical fuel rod increased faster than that of the flat void model. Change in the fuel surface heat flux was attenuated due to the heat conduction time delay. The peak cladding temperature was slightly lower than that of the flat void model due to cancellation among these factors. (authors)