A STUDY ON BOILING WATER-REACTOR REGIONAL STABILITY FROM THE VIEWPOINT OF HIGHER HARMONICS

A quantitative study on a mechanism for boiling water reactor regional stability has been carried out from the viewpoint of higher harmonics . In the mechanism, the gain decrease in the void-to-power transfer fu nction can be explained by the higher harmonics mode subcriticality. I t is shown that the thermal-hydraulic feedback effect can compensate f or the gain decrease, and regional oscillation can be sustained that w ay. For quantitative evaluations, a three-dimensional higher harmonics analysis model has been developed. The results show that the first az imuthal harmonics subcriticality has a relatively small value under a regionally unstable condition. Comparing the subcriticality and the st eady-state power distribution, it is shown that the distribution exist s whose first azimuthal harmonics subcriticality takes a small value. A method of decomposition for the oscillated power responses into the harmonics modes is presented. The results show that the corewide oscil lation power response consists almost entirely of the fundamental mode , and the regional oscillation power response consists almost entirely of the first azimuthal harmonics mode. This indicates that regional o scillation is a phenomenon in which the first azimuthal harmonics mode oscillates on the basis of the fundamental mode.