Forecast of the thermal regime of an underground storage facility for heat-generating materials under mixed convection conditions

Abstract

The paper presents the results of a study based on numerical simulation methods of the thermal regime of an underground facility for long-term spent nuclear fuel storage in the version of a built-in reinforced concrete structure. A multiphysical computer model was constructed in a two-dimensional setting by means of the COMSOL software. The mathematical model was based on the continuity equations, Navier-Stokes equations and the general heat transfer equation. The conditions of mixed convection were taken into account in the ‘incompressible ideal gas’ approximation, in which the thermophysical properties of air were a function of temperature only. For two parameters of the model, the following values were taken: the air flow rates providing the velocity at the inflow boundary = 0.01, 0.03 and 0.05 m/s, and the effective heat conductivity coefficients of the material of the built-in structure = 1.0 and 2.0 W/(m×K). Numerical experiments were performed for a period of up to 5 years of fuel storage. Special emphasis was given to the fundamental difference between the non-stationary structure of the velocity fields forecasted in the model of an ‘incompressible ideal gas’ and the ‘frozen’ picture of aerodynamic parameters in the model of an incompressible fluid. An analysis was made of the dynamics of spatial temperature field distributions in different areas of the model. It was shown that the criterion temperature control requirements were met when the facility was operated under conservative ventilation conditions in terms of the air flow rate and the heat conductivity coefficient of the built-in structure material. The dynamics of heat flows directed into the rock mass through the base and from the surface of the built-in structure into the air was analyzed. The heat flow dominance from the structure surface was also noted. Finally, the influence of the effective heat conductivity coefficient of the built-in structure material and the air flow rate on the values of heat flows directed into the air and rock mass was demonstrated.