Compatibility studies of irradiated carbide fuel pins

When the free energies of formation of mixed (U, Pu) monocarbide and sesquicarbide, the phases present in as-manufactured carbide fuel, are plotted on an Ellingham type diagram along with the free energies of formation of the carbides of the three main constituent elements of austenitic stainless steel, that is nickel, iron and chromium, it can readily be seen that chromium forms the most stable carbide. Mixed (U,Pu) carbides, therefore, are thermodynamically unstable in contact with stainless steel. In consequence, when mixed (U,Pu) carbides are heated in contact with stainless steel, carbon transfer from the fuel to the clad should occur at a rate dependent upon the kinetics of the actual mechanism of carbon transfer. The rate of carbon transfer increases in the presence of a medium such as a sodium bond, which can act as a transfer agent. In this instance the wetting of the steel surface provides a greater area of contact compared to the condition prevailing in the absence of such a bond and may explain the effectiveness of the transfer agent. The mixed (U,Pu) carbide as currently manufactured is less pure, is less well characterised and much less stable than oxide fuel. It is possible, therefore, to control stoichiometry only within broad limits and the method of manufacture, by carbon reduction of oxide, leaves an appreciable amount of residual oxygen in the lattice or as discrete particles of oxide. During fuel preparation, localised reaction between oxide (or oxygen) and carbide may lead to the appearance of free (U,Pu) metal which could in a fuel pin react with the clad. It has further been postulated that similar reactions occur in mixed (U, Pu) carbide pins of high centre temperature in the reactor and that although the free (U,Pu) metal so formed is mainly concentrated at the 1200 deg. C fuel isotherm, some of the free metal together with the carbon monoxide produced in the reaction migrates to the clad surface and reacts. The occurrence of these two possible types of reaction, carburization and fuel-clad reaction, in mixed (U, Pu) carbide fuel pins in-pile, are discussed in this paper with particular consideration being given to the effect these two reactions could have on the evolution of a successful (U,Pu) carbide fuel pin design