According to recent investigations, the neutron noise field in a boiling water reactor can be separated into a local and a global component. In the present paper, these two components are discussed further via two-group diffusion theory. The expediency of the local-global concept is compared to another concept based on separating components corresponding to the two roots of the dynamic eigenvalue problem. The mathematical discussion of the neutron response to a propagating perturbation of the moderator density is given. Point reactor behavior and “linear-phase behavior” appear as two extremes of the neutron response. The mathematical results are illustrated numerically for the cases of a large power reactor core and a small highly enriched core.

Measurements, taken in the Ringhals-1 boiling water reactor after revision in 1990, showed that instability occurred at high power and low core flow. Measurements in several points of the power-flow map showed that the decay ratio (DR), obtained by conventional methods, jumps from a moderate value directly to unity. This was valid for DR values calculated from both average power range monitor (APRM) and local power range monitor (LPRM) signals. Thus, the conventional DR cannot be used as a measure of the margin to instability. It was found that both global (in-phase) and regional (out-of-phase) oscillations occur, the global with low DR but large signal amplitude, and the regional with high DR but low signal amplitude. The former dominates the DR calculated from both APRMs and LPRMs, except when the instability is fully developed and impedes detection of the actual margin to instability. Methods for obtaining the stability characteristics of both modes separately from neutron noise signals were de...