The plutonium/hydrogen reaction: The pressure dependence of reaction initiation time and nucleation rate controlled by a plutonium dioxide over-layer

As part of an ongoing programme to quantify those parameters which influence the early stages of the plutonium hydriding reaction, the hydrogen pressure dependence of both plutonium hydriding initiation time (It) and hydriding nucleation rate (Nr) have been determined for plutonium covered in a reproducible dioxide over-layer. The data show that initiation time is inversely proportional to hydrogen pressure, while nucleation rate is proportional to hydrogen pressure. Both observations are consistent with a model of hydriding attack in which the dioxide over-layer acts as a diffusion barrier, controlling the flow of hydrogen from the gas phase to the oxide/metal interface. The low scatter and reproducibility of the experimental data set demonstrate the importance of synthesising well controlled and characterised oxide layers prior to determining these experimental parameters.