APPARATUS FOR MEASURING SUBCHANNEL VOID RATE IN FUEL-BUNDLE TESTING APPARATUS FOR LIGHT WATER REACTOR

PURPOSE: To measure the sub-channel voidity in the twophase region of a simulated fuel bundle in a fuel bundle tester for simulatedly reproducing the performance of the fuel bundle. CONSTITUTION: A simulated fuel bundle has a plurality of simulated fuel rods R, which are individually heated not by nuclear reaction but by a current. A radiation source for radiating γ-ray is mounted on a probe, and arranged in the specific rod R capable of moving vertically. On the other hand, a detector is arranged in another rod R capable of moving vertically oppositely to the radiation source. The γ-ray radiated from the source and incident to the detector via the tubular wall of the rod R is measured. When the attenuation factor of the γ-ray between the source and the detector is used, the voidity in the two-phase region of the bundle is obtained.