A method for channel-bow indication by neutron flux measurements - 025

Channel bow in boiling water reactors (BWR) can be detected by simultaneous measurement of the thermal and fast neutron fluxes. There is a robust linear correlation between the ratio of the thermal and fast neutron fluxes and the void fraction close to the detector through the core. The moderation will be modified by channel bow and this change in moderation will affect the thermal neutron flux strongly and the fast flux weakly, i.e., the thermal flux will act as an indicator that the moderation or power has changed but the fast flux will reveal if the change is due to difference in moderation or power. The thermal and fast fluxes are furthermore affected in opposite directions, thus enhancing the deviation from linearity of the ratio between the fluxes, caused by channel bow. By comparing the void fraction calculated by a core simulator with the void fraction predicted by the ratio of the thermal and fast neutron fluxes, channel bow can be detected as a deviation from the linear void correlation of 4% per mm bow. This implies that it is reasonable to detect channel bow 2 mm or larger, assuming that the uncertainty of the detectors does not exceed 5%. (authors)