Effect of thermal gradient on in reactor oxidation of MTR fuel plates

Aluminium oxide is a poor thermal conductor. lt interferes with the cooling process as it grows during Research Reactors operation. This factor has to be taken into account in the design process of high flux reactors, because oxidation excess may affect fuel performance by causing the plate's surface to spall, generating blisters, dimensional changes and even intergranular perforation. The oxidation mechanism is not well understood, being strongly influenced by water chemistry, mainly its pH, water flow, heat flux and thermal gradient, among other variables. In oxidation tests in static autoclaves at water reactor temperature the oxide layers produced are much thinner than those generated during reactor operation. It is possible that the process controlling temperature be the aluminium plate temperature instead that of the bulk water. Or, a temperature gradient - associated with heat transfer- is needed to accelerate the reaction. This paper describes the efforts carried out to study the effect of heat transfer on oxidation, through experiments performed in similar conditions, some in isothermal systems, using an autoclave, and some others in a device capable to produce thermal transfer through the plates. In both approaches the hydrodynamic conditions and other parameters have been set up in such a way that the results can be compared. For instance, the exposure temperature in the autoclave is not the reactor bulk water temperature, but the value measured with a thermocouple inserted inside the aluminium plate during the heat transfer test, a much more demanding condition. The water flow in the fuel channel was simulated in the autoclave by means of a rotating sample, where the tangential velocity is similar to the one of the coolant. If the oxidation would progress similarly in both conditions, then it could be possible to develop a full oxide growth correlation with simple autoclave tests, a much more accessible tool than the one needed to simulate a reactor fuel channel with heat transfer. Tests at different times were carried out, ranging from one week to two months, in both systems. In all cases, the oxide obtained in thermal transfer conditions is thicker than the one produced in autoclave, up to an order of magnitude for longer times, indicating a strong influence of the thermal gradient on oxidation mechanism. These results are discussed considering the migration of reacting species under temperature gradients, the effect of hydrodynamics in the formation of the diffusion layer and other factors. (author)