Analysis of neutron flux noise due to coolant parameter fluctuations in a VVÉR core

Abstract

The results of investigations of noise appearing in the signals from direct-charge sensors of the in-reactor monitoring system of VVÉR reactors as a result of coolant parameter fluctuations are presented. The calculations and experimental data are used to analyze the dependence of the amplitude of the neutron flux oscillations (local intensity) at the locations of sensors as a function of the magnitude and frequency of the fluctuations of a specific coolant parameter. The NOSTRA program was used to perform the calculations. The results of the analysis were used in the in-reactor noise diagnostics system in the No. 3 unit of the Kalinin and the No. 1 unit of the Tianwan (China) nuclear power plants.In-reactor noise diagnostics has been developed on the basis of an analysis of the noise in various parameters of the core of a nuclear reactor. Analysis of neutron flux noise is one direction of noise diagnostics whose main goals are detection and localization of coolant boiling sites inside a reactor core. Monitoring the moment when coolant begins to boil makes it possible to refine the real conditions for removing heat in the core and to determine the design limits more accurately. The computational and experimental works described in the present article were performed to solve the problem of instrumentation monitoring of coolant boiling.The standard in-reactor monitoring sensors distributed over a core are direct-charge sensors with a rhodium emitter [1]. Calculations and experiments have confirmed that such sensors make it possible to detect neutron flux fluctuations in a wide frequency range due to the instantaneous component of the current [2] that is due to the Compton effect. The instantaneous component of the current is proportional to the main (activation) component of the sensor current. For the same coolant density fluctuations, the amplitude of the sensor current fluctuations is proportional to the neutron flux or the main component of the sensor current. The standard deviation of the variable component of the sensor current reduced to its average main current -the dimensionless parameter ∆ -was chosen as the parameter characterizing the state of the coolant.The neutron flux fluctuations depend on the coolant density fluctuations, which influence the change in the moderating properties of the coolant and arise as a result of fluctuations of the temperature, flow rate, and pressure of the coolant in the reactor. When steam starts to form, the nonlinearity of the dependence of the coolant density on the amplitude of the fluctuations of the coolant parameters at the entrance into a fuel assembly becomes stronger. As a result, the amplitude of the coolant density fluctuations increases, which increases the amplitude of the neutron flux noise in the region of boiling.Experimental Results. The experiments confirmed that rhodium sensors make it possible to detect the moment local boiling of the coolant appears and starts to develop. The possibility of detecting boiling onset was shown in experi-

Compton-emissive hafnium detector of neutrons for in-core monitoring

“…The critical thermal load q critical predetermines the energy release level of the FE, at which the crisis of heat transfer begins. In this case, the most dangerous stage is when the bubble boiling of the coolant on the surface of the FE transforms to the film boiling (bubble boiling crisis) [5,6,27]. Above the starting temperature of the surface boiling, the volume density of the coolant decreases due to the formation of vapor bubbles in it.…”