A FAILURE ANALYSIS FOR THE LOW-TEMPERATURE PERFORMANCE OF DISPERSION FUEL ELEMENTS

An analytical approach is proposed which allows the bunnup (by fission) of uranium required to cause failure in a uranium dioxide-stainicss steel dispersion fuel element to be calculated. The analysis is developed by assuming the matrix of the fuel eicment to be made up of a uniform, close-packed array of spherical UO/sub 2/ particles, each surrounded by and associated with a hollow stainless steel sphere. Equations are then written for the amount of fission gas released into the stainless steel cavity in terms of the UO/sub 2/ particle size and density and the burnup. The release mechanism is by recoil only, since diffusion is unimportant for the particle sizes and temperatures (<1000 tained F) of interest. The gas atoms recoiled from the UO/sub 2/ particle are assumed to diffuse from the stuinless steel shell into the caviiy. The pressure thus exerted in-side the stuinless steel sphere is computed by the application of a real gas law. A suitable failure criterion for an internally pressunized, heavy-walled metal sphere appears to be when the sphere becomes entirely plastic. An equation for the pressure at failure and displacements of the sphere is written in terms of the UO/sub 2/ loading and the yield strength of the steel. By combination with the previous expressions, the burnup required to cause failure is calculated. Thus, the effects of UO/sub 2/ density and particle size, temperature, strength of the matrix material, and UO/sub 2/ loading on the burnup at failure are predicted by the theory. (auth