A number of factors determine the length of time that nuclear fuel rods can safely be left in pressurised water reactors, although it is generally accepted that the most important of these is waterside corrosion of the zirconium alloy cladding that contains the fuel. Over the past twenty years standard Zircaloy-4 tubes have performed satisfactorily in reactors throughout the world, but the economic and environmental constraints that determine the present day reactor operating parameters have changed somewhat, and now the fuel has to withstand more arduous working conditions for considerably longer times.

To ensure that fuel cladding materials are capable of meeting the requirements of reactor operators over the next decade a number of new zirconium alloys have already been developed. Now researchers in South Africa have reported the results of an investigation to establish if the oxidation resistance of Zircaloy-4 can be improved by alloying palladium into the surface layer (“Improvement in Oxidation Resistance of Zircaloy-4 by Surface Alloying with a Thin Layer of Palladium”, G. A. Eloff, C. J. Greyling and P. E. Viljoen, J. Nucl Mater., 1993, 202, (3), 239–244).

A 2 µm thick layer of palladium was electrodeposited onto the surface of test samples cut from standard fuel cladding tube material, which were then vacuum annealed at 950°C for various times to diffuse the palladium into the Zircaloy-4 before they were vacuum quenched to room temperature. Selected samples were then oxidised at 450°C, the rates of oxidation being monitored at intervals by determining the increases in weight of the samples.

The study showed that a substantial improvement in the short term oxidation resistance of Zircaloy-4 was achieved when the electrode-posited palladium layer was diffused into the surface, by annealing for an optimum period of time. This resulted in a high concentration of intermetallic particles at the sub-grain boundaries of the α-Widmannstätten structured outer layer. It is suggested that this increases the plasticity of the oxide layer and permits a greater degree of deformation to take place before the onset of cracking occurs, from the outer surface of the oxide layer.