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

9/16/2020 2020 - #03 Thermal physics and thermal hydraulics

https://doi.org/10.26583/npe.2020.3.08

### UDC: 536.2:519.6

The paper presents the results of a study by numerical simulation methods of the thermal regime of an underground facility for long-term storage of spent nuclear fuel in the version of an built-in reinforced concrete structure. A multiphysical computer model in a two-dimensional setting was built in the COMSOL program. The mathematical model is based on the continuity equation, Navier-Stokes equations and the general heat transfer equation. The conditions of mixed convection are taken into account in the «incompressible ideal gas» approximation, in which the thermophysical parameters of air are a function of temperature only. The choice of thermophysical parameters of different parts of the model and a set of boundary conditions for the solved ones are substantiated. For two parameters of the model, the following values are taken: the air flow rate which provides the velocity at the input boundary of 0.01, 0.03 and 0.05 m/s, and the effective thermal conductivity of the material of the integrated design is 1.0 and 2.0 W/(m⋅K). Numerical experiments were performed for a period of up to 5 years of fuel storage. The formation features of the velocity fields in the facility are noted when the values of these parameters are changed. Special emphasis is given to the fundamental difference between the non-stationary structure of the velocity fields predicted in the model of an «incompressible ideal gas» and the «frozen» picture of aerodynamic parameters in the model of an incompressible fluid. An analysis is made of the dynamics of spatial distributions of temperature fields in different areas of the model. The distribution features of maximum temperature values in the different areas of the model are also noted. It is shown that the requirements for not exceeding the criterion temperature values are met when the facility is operated under conservative ventilation conditions in terms of the air flow rate and the thermal conductivity coefficient of the material of the built-in structure. 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 is analyzed. The heat flow dominance from the structure surface is noted. Finally, the influence of the effective coefficient of thermal conductivity of the material of the built-in structure and the air flow rate on the value of heat fluxes into the air and rock mass is demonstrated.

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heat-producing materials spent fuel storage facility numerical simulation mixed convection thermal safety