Physical interpretation of neutron-noise specimens of the VVÉR-1000 reactor

Abstract

Following the success of the phenomenological theory of globality-locality (G-L theory) of neutron noise of a boiling-water reactor [1] numerous attempts have been made in the past few years to develop a similar theory for pressurizedwater reactors. For this purpose sink structures, the periodic minima of the spectral characteristics and the linear phases between different pairs of signals from neutron detectors, have been used as the characteristics of the spectral features.Acting by analogy with the G--L theory, some researchers link sink structures with the coolant velocity, proving that are located at points k/~-o, where ~r 0 is the time taken by the coolant to pass through the core, k = 1, 2, 3 .... [2][3][4][5][6]; other researchers link those structures with the velocity of the vapor phase that arises in the coolant because of subcooled boiling [7][8][9]. However, the fact that sink structures cannot be reproduced from one core channel to another, their enormous number, and the experimentally observed different phase shifts in different frequency ranges between signals from in-reactor monitoring detector indicate that simple models do not work here. This was first noted in [10], after which space-dependent models began to be developed [11][12][13][14][15]. Unfortunately, the models were too complex for a final result to be obtained in closed analytical form and numerical solutions of those models was not introduced into the practice of noise measurements in operating reactors. In this study a physical interpretation is made, using approximate analytical methods developed on the basis of a space-dependent neutron-noise model [16].In neutron-noise measurements on the first unit of the Kalinin Nuclear Power Plant with a VVI~R-1000 reactor signals from three ionization chambers and 14 self-powered detectors (SPDs) were recorded simultaneously. The arrangement of the ionization chambers (ICs) and the numbers of the sets of SPDs, the signals from which are interpreted here, is shown schematically in Fig. 1.The following findings were made:--phases of the mutual spectral power density of SPD--SPD signals are observed and are found to be nearly rectilinear in two frequency ranges, 0.2-1.2 and 2-5 Hz (Fig. 2); and --rectilinear phase relations are observed between the signals from SPDs put in different fuel assemblies (Fig. 3) and between SPD and IC signals (Fig. 4).If in the range 0.2-1.2 Hz the phases of the mutual spectral power density of SPD-SPD signals are approximated by a straight line, the delay time is extracted, and a transition is made from it to velocity (Fig. 5), then clearly the measured velocity is always higher than the design velocity and the error of velocity determination is larger when the chosen pair of SPDs are closer together.As shown in [16], in the horizon of the first SPD the G-L relation is most unfavorable for the flow-rate contribution (Fig. 6). The largest error here, that of the estimate of the coolant velocity, incases with the coordinate z of the L component and the SPDI -SPD7 pair...