Modification of the Hydriding of Uranium using Ion Implantation
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MODIFICATION OF THE HYDRIDING OF URANIUM USING ION IMPLANTATION R. G. MUSKET, G. ROBINSON-WEIS,
AND R. G.PATTERSON
Lawrence Livermore National Laboratory Livermore, California 94550 ABSTRACT The hydriding of depleted uranium at 76 Torr hydrogen and 130 0C has been significantly reduced by implantation of oxygen ions. The high-dose implanted specimens had incubation times for the initiation of the reaction after exposure to hydrogen that exceeded those of the non-implanted specimens by more than a factor of eight. Furthermore, the nonimplanted specimens consumed enough hydrogen to cause macroscopic flaking of essentially the entire surface in times much less than the incubation time for the high-dose implanted specimens. In contrast, the ion-implanted specimens reacted only at isolated spots with the major fraction of the surface area unaffected by the hydrogen exposure. INTRODUCTION The results presented here represent the first known use of ion implantation for the modification of the hydriding properties of a material. In particular, oxygen ions were implanted in uranium up to concentrations calbe stable under the hydriding culated to form surface oxides that should conditions (76 Torr hydrogen and 130 0C). The inhibiting effect of surface oxides on the hydrogenation of uranium has been recognized for many years. In addition, uranium hydride ignites in oxygen to form U3 08 or, when insufficient oxygen is present, U02 and reacts at elevated temperature with water to form U02 [1,2]. Since these oxides are thermodynamically more stable than the hydride and inhibit the hydrogenation reaction, it is quite logical to consider using oxide films for protection of uranium from hydrogen. The oxide formed under ambient atmospheric conditions does not form a useful, protective barrier, presumably because of its porosity, defects, impurities, and tendency to spall off the surface. Oxidation at higher temperatures with high-purity oxidizing gas would probably result in an improved barrier. However, heating the uranium may be inconsistent with other constraints for the bulk material (eg, grain size, precipation of undesirable phases, dimensional tolerances). Implantation of oxygen into uranium surfaces under high energy, low current conditions (ie, low power conditions) insures that the bulk properties are retained while the oxygen concentration in the layer increases. EXPERIMENTAL CONSIDERATIONS The specimens used all came from one piece of high-purity depleted uranium. Spectrochemical analysis revealed the following impurities or detection limits (in ppm by weight): Tm
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