Reactor noise analysis of the experimental fast reactor JOYO

Abstract

Experimental fast reactor JOYO achieved its first criticality in 1977 spring. Since then a series of reactor noise analysis has been carried out. During zero power testing, the reactor kinetic parameters, i.e. β/l and subcriticality, were obtained, applying band-pass filter and polarity correlation methods. As the high power operation started from 1978, the reactor noise analysis based on neutron flux spectra has been proceeded. This paper presents study of noise analysis for rather lower frequency region at high power operation. At 50 Mwt power level operation, noises of neutron flux signals, reactor inlet temperatures and subassembly outlet temperatures were measured, and then auto power spectral densities and coherence functions were calculated. The frequency range for the noise analysis is for 10 −3 − 10 −1 Hz. In the APSD of neutron flux, a low peak was observed at 2.5×10 −2 Hz. For neutron noise, it was estimated that the noises in the frequency region lower than 1.5 × 10 −2 Hz is a core inherent temperature noises, while that in the frequency region higher than 1.5 × 10 −2 Hz is a core inherent neutron noise. The coherence function of neutron fluxes, which are signals of ex-core detectors located at opposite positions against the core center, indicated that some space dependent phenomena of neutron noise might exist in 5×10 −3 − 6×10 −2 Hz. The space dependency in 5×10 −3 − 1.5×10 −2 Hz were estimated to be produced by the sodium temperature noise due to the insufficient mixing of two primary coolant loops. In 1.5×10 −2 − 6×10 −2 Hz, the peak existed at 2.5×10 −2 Hz and reverse phases were observed in the coherence of two neutron signals at opposite side of the core. With the investigation of phase relations between neutron flux and subassembly outlet temperature in this frequency region, the phenomenon was estimated to be due to mechanical vibration of some reactor core components. When the static gain of reactivity-to-power transfer function increased, the changes of reactor noise characteristics were observed. Since the change of the gain indicates a change of the reactor core characteristics, the observation of reactor noise characteristics has a potential for the surveillance of the reactor core.