Investigation of Spatial Effects Accompanying Local Boiling of Coolant in VVER Core on the Basis of Neutron Flux Noise Analysis

Situations with local boiling of the coolant become more likely when VVER-type power units are transferred to a heightened power level. In a VVER core, neutron flux detectors are not present in all FA. An important problem for evaluating the possibility of detecting local boiling in FA which are not equipped with detectors is the study of the spatial distribution of the change brought about in the neutron flux noise by local boiling of the coolant. The results of an investigation of the spatial distribution of the neutron flux noise initiated by fluctuations of the coolant parameters at the ingress into the core with non-equilibrium local surface boiling are reported. The sensitivity of the neutron flux noise in FA with no boiling to local boiling in neighboring FA is analyzed. The results could be helpful in analyzing the data recorded by the in-reactor noise diagnostics system in commercial operation in operating power-generating units.