Two Interesting Phenomena Observed in the Axial Power Distribution of a BWR

Two interesting phenomena were observed in the axial power distribution in the core of the Japan Power Demonstration Reactor. One is the control-rod and void coupled bump on the axial power shape; this is a kind of local power peaking produced by the interacted effect between the partially inserted control rod and the axial void changes in the moderator. The other may be termed a “burnup oscillation of the axial power distribution” this is a long-term, periodic change of the axial power distribution due to core exposure and to the withdrawal of the control rods. It is found that the upper and lower half-channel integrations of the local leakage of thermal neutrons are essential factors for the occurrence of those phenomena.