Analytic initialization method for xenon oscillation in power reactor simulation

Abstract

The purpose of simulating nuclear power reactor core transients is to predict variations in core behavior due to changes in operating conditions. In the simulation, the initial condition of the core transient should be consistent with the real core state. Especially, the initial conditions of iodine and xenon distributions, which cannot be measured in the core, have dominant effects on the accuracy of the simulation for the transients related with xenon dynamics. If the transient starts from non-equilibrium xenon condition, the accurate initialization of the non-equilibrium iodine and xenon distributions is essential for the prediction of the core transient behavior. In this study, a simple initialization method of non-equilibrium iodine and xenon distributions for the core transient simulation was developed using axial power difference, which is the only measurable parameter related with the non-equilibrium state of the core. For this purpose, the spatial distribution of two-group one-dimensional neutron diffusion equation combined with xenon dynamics equation was expanded with Fourier sine series. It was found that the nonlinearly combined terms of flux and xenon distributions in the equations were canceled by the orthogonal characteristics of the Fourier series using adequate approximation. Therefore, the dynamics equations for axial difference parameters of power, iodine and xenon can be completely linearized and can be converted into an equation system which can be solved analytically. Because the equation system provides with an analytic relationship between the axial differences of power, iodine and xenon, the non-equilibrium iodine and xenon can be represented as functions of the axial power difference. It implies that the initial condition of the transient starting from a non-equilibrium state can be determined from the initial condition of the axial power difference which is a measurable parameter of the core. Using the analytic solution obtained in...