THERMAL ANALYSIS OF THE STEEL CONCRETE CASK CONSTOR

In this article the methods used for thermal analysis of the CONSTOR steel concrete cask for spent nuclear fuel transport and storage under normal and accident fire conditions are presented. The methods are valid to calculate the heat transfer in the inner cavity of the gas filled cask when the main part of the residual heat is transfered by means of radiation and conductivity. The calculative model covers the heat transfer in the assembly both under symmetrical and non-symmetrical boundary conditions, the latter are typical for multi-places casks or canisters.One of the main features of the method is the real multi-rod assembly substitution by homogenous heat releasing bar with effective heat conductivity enclosed into the heat conducting casing which models assembly cover or basket tube. Two-zone model is used with separate consideration o( heat transfer in the rod bundle inner zone and in the edge zone disposed between the edge row of rods and casing. The method have been verified by experimental data obtained by means of electricaly heated bundle model consisting of 127 rods and a good correspondens has been shown. For the casks with a metal-concrete body a method and code for thermal analysis during the accident f1re conditions has been elaborated. The method includes elements of the joint thermal-strength calculation of the cask body and allows to carry out calculations taking into account disappearance and arising of new gaps and cavities and altering their widths in time. The joint thermal and thermal-strength analysis has made clear some peculiarities of the multilayer cask body response in different regimes and allowed to determine its design. In an adopted cask construction the outer liner is not directly bound with the inner one and with the system of reinforcing rod heat conductors which are welded with the inner liner. The relatively free outer liner provides reliable heat contact in the cask body under normal stationary conditions and increases essentially a cask fire resistance under accidents. Resulting cask construction is unloaded from thermal stresses in this way. The fire-protecting effect of the free outer shell is shown most clearly under conditions when the environment temperature rises gradually, that is typically for ship-hold fire.