Transient thermohydraulic processes in a fast-reactor core

Abstract

An important aspect of the thermophysical substantiation of fast-reactor fuel assemblies is the determination of the maximum temperature of the fuel-element cladding and the probability of local boiling of the liquid-metal coolant during transient operating regimes, for example, an abrupt change in the energy release. The factors leading to overheating, including lattice deformation, nonuniform energy release, stochastic temperature nonuniformities, and others, must be/aken into account in order to calculate correctly at the current level the most stressed reactor fuel assemblies, determining the maximum temperature level.In the present paper we develop a channelwise method [ 1] based on a local approach for transient processes with a time constant of not less than several tenths of a second. Data are presented on transient processes accompanying an abrupt change in the energy release.Method of ehtnnelwise caleulatioll. The distribution of the velocity and heating of the cooling in the fuel assemblies is found by solving a system of macroscopic-transfer (balance) equations [2] written for the elementary cells of the fuel assembly ( Fig. 1). The closing relations are found from the relations presented in [3]. The boundary conditions are the values of the velocity, pressure, and enthalpy at the entrance and exit of a fuel assembly (or group of fuel assemblies) under nonadiabatic conditions on the cladding of the fuel assembly.The temperature of the ~an'face of fuel elements in a distorted lattice is determined by interpolating the values at the characteristic points of the perimeter (narrow and wide parts of the cells). The temperature at these points is found as a superposition of the coolant temperature in the cells and the local overheating of the surface.The computational method is implemented in the MID program. Many verification tests of the program were performed using the data obtained at the State Science Center -Physics and Power Engineering Institute and other organizations [4, 5] (Figs. 2 and 3). In Figs. 2 and 3 the temperature of the fuel elements was calculated using Duhamel's integral based on our investigations.Nonstationary temperature fields in the central zones of the fuel assemblies. The method for solving numerically the heat-transfer equations and the characteristic nonstationary temperature distributions in the fuel and the coolant are presented in [6]. The calculations were performed for the turbulent coolant flow with different Reynolds numbers. Convective heat transfer and molecular and turbulent diffusion in the radial and vertical directions were taken into account.Fuel assemblies with different coolants (sodium, water, lead) with uranium and uranium dioxide fuel as well as the temperature distributions in model fuel assemblies and ring:shaped channels with the fuel elements arranged in a circular tube with adiabatic boundary condition on the outer wall of the tube were investigated. The numerical results were conftrmed by experimental data obtained with the heater in the model switch...