Detection and monitoring of boiling in PWRs by incore neutron noise analysis

Abstract

Incore neutron noise analysis can be an effective method of detecting and monitoring boiling rate in operating PWRs. This paper presents recent developments on this topic. This research aims to develope a method of measuring the boiling rate in the area around an incore neutron detector and to determine the sensitivity level of the method. Theoretical developments and experimental investigations on operating PWRs and the IRENE loop set in the OSIRIS reactor (70 MWth) at SACLAY, have been carried out. - Theoretical developments Statistical characteristics of fluctuations of an incore detector signal depends on the thermohydraulic parameters of the flow (void fraction α, bubble mean radius, flow rate) and on the effect of a given local change of water density on space-energy neutron transport. The expression of the Power Spectral Density of an incore detector signal fluctuations is presented and discussed. - IRENE loop experiments Special experiments concerning boiling phenomena have been achieved on the IRENE SAT 1 loop (2 x 2 fuel rods, outlet in saturated two-phase conditions) fitted with neutron detectors in appropriate locations. This program of experiments was defined and carried out in collaboration by CEA, EDF, FRAMATOME and WESTINGHOUSE. Corresponding neutron noise fluctuations were measured and analysed. Boiling effets were clearly observed and allowed to check the theoretical developments and to adjust physical parameters (bubble mean radius) - Operating PWR experiments The sensitivity of the method has been determined by comparing the existing noise level on the incore detectors in operating PWRs (due to noise sources other than boiling) to the level of fluctuations due to boiling.