Measurement and analysis of noise spectra – an effective diagnostics method for nuclear reactors

Abstract

The basic principles of measuring and analyzing nuclear-reactor noise are described. The results obtained for the IBR-2 reactor by the noise method are presented. It is shown that analysis of the noise spectra of the power and the main reactor parameters makes it possible to find deviations from normal reactor operation at the level 10 -6 ∆k/k of the change in reactivity.Diagnostics is one of the important components of reliable and safe operation of any system, including one with a nuclear reactor. The power noise observed during the operation of different types of reactors in the stationary and pulse operating regimes is due to the stochastic nature of fission and to reactivity fluctuations associated with the operation of the basic systems.In the present article, we present the principles for constructing a system for measuring and analyzing reactor noise spectra. The main results of measurements in IBR-2 are presented.IBR-2 is a periodic-pulsed reactor in which the power pulses are generated with a reactivity modulator -a mobile reflector with heterogeneous structure (Fig. 1). The nominal average power of the reactor is 2 MW. Liquid sodium with flow rate 80-100 m 3 /h is used as the coolant in the first loop. The duration of a power pulse is 245 µsec and the repetition rate is 5 sec -1 . The power of the reactor at the peak of the pulse reaches 1.4 GW [1].The method studied in the present article is applicable for any nuclear reactor. Power Fluctuations. The relative dispersion of the fluctuations ∆ 2 Q = (σ Q /Q) 2 can be represented in the formwhere is the standard deviation of the measurement results Q n from the average value Q, n = 1, 2, ...; N is the number of measurements; δ 0 denotes the relative power fluctuations due to the effect of the external devices on the reactivity; ∆ 2 st = νD ν /2Sτ denotes the relative stochastic fluctuations of the energy of the pulses; ν is the average number of neutrons which arise in a single fission event; D ν = 0.815 is the relative variance of the number of neutrons which arise with the fission of a nuclide; S is the total intensity of the emission of spontaneous and delayed neutrons determined in the time interval between neigh-σ Q n n N Q Q N = − − = ∑ ( ) / ( ) 2 1 1