High temperature chemomechanical interactions between alkali metals and urania-base oxide reactor fuels

The chemical reactions that occur when alkali metals such as Cs and Rb fission products or Na and K coolants react with urania or mixed urania-plutonia solid solutions at 550 to 1150/sup 0/C, are accompanied by volume increases that can contribute to swelling in irradiated oxide nuclear fuel and breeder materials. These reactions, and the associated phase equilibria, are somewhat complex, and the actual complex oxide reaction products that form are determined by the relative quantities of alkali metal (A), oxide fuel and available/reactive oxygen, the chemical potentials of A and O/sub 2/, and temperature. The conditions necessary for formation of the various ternary oxide reaction products are reviewed, then both theoretical estimations and experimental determinations of swelling in several important alkali metal-polycrystalline oxide reaction systems are described. It is shown that reaction swelling in intact or breached LMFBR fuel pins can be modelled with reasonable precision using swelling factors derived from crystallographic densities of the Cs-fuel or Na-fuel reaction products. Potassium reactions, however, are an exception, the experimental reaction swelling being approx. 77% lower than theoretical. It is also shown, principally from FCMI simulation experiments performed on sintered UO/sub 2 + x/ pellets reacting with Cs, that stainless steelmore » cladding generally cannot restrain alkali metal-oxide fuel reaction swelling, however, at initial pellet densities less than or equal to 80% theoretical this limited restraint may be sufficient to fracture (and hence densify) reacted/embrittled regions of the polycrystalline oxide. The critical load to cause this type of brittle fracture in reacted 80% TD UO/sub 2/ /sub 035/ was estimated to be approx. = 15 MPa.« less