Finite element analysis of local overheating within plutonium enriched UO2 fuel rods caused by PuO2 islands

Within natural UO2 fuel elements enriched with plutonium, this last material should form PuO2 solid solutions inside the UO2 pellets, in a wide range of concentrations. If the solutions are obtained by mechanical mixing of the oxides, PuO2 islands are formed in the UO2 matrix. These islands may be the source of several problems in the fuel behaviour, the most important being the overheating of the matrix in the neighbourhood of the particles. It is caused by the large fission cross section of plutonium compared with that of uranium. A detailed study of the thermal effects produced by PuO2 particles in the UO2 matrix and the cladding is then important for the specification of their permissible size. A portion of the fuel rods with spherical particles in the most significant places was studied. In order to obtain the dimensionless overheating of the fuel and cladding produced by the presence of those particles, the spatial distribution of temperature was calculated, solving the stationary and linear bidimensional equation of heat conducting using a finite element code. Several geometrical variables and material properties have been taken as dimensionless parameters. A satisfactory convergence of the numerical results to an asymptotic limit with a well-known exact solution, has been obtained. (orig.)