Hydrogen-Vacancy Effects in Pu-2 at. % Ga Alloys
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Hydrogen-Vacancy Effects in Pu-2 at. % Ga Alloys
Daniel S. Schwartz, Scott Richmond, Alice I. Smith, Alison Costello, and Christopher D. Taylor Los Alamos National Laboratory, Los Alamos, NM 87545, USA
ABSTRACT
Plutonium and Pu-Ga alloys have been observed to have anomalous hydrogen solubility behavior, including a significant concentration dependence of hydrogen diffusivity in the dilute regime, a sharp drop off in the hydrogen solubility constant in the dilute regime, and a near complete absence of change in the Sieverts’ constant as the alloys are heated across phase transformation boundaries. We are investigating the possibility that a vacancy mechanism is responsible for this behavior. X-ray diffraction measurements show a 0.14% lattice contraction in Pu-2 at. % Ga alloys when they are charged with ~2 at. % hydrogen. The lattice re-expands when the hydrogen is removed. Density functional calculations show that increasing the number of hydrogen atoms associated with a vacant lattice site in Pu lowers the energy of the hydrogenvacancy complex. These observations support the idea that vacancies are stabilized by hydrogen in the Pu lattice well beyond their thermal equilibrium concentration and could be responsible for the anomalous hydrogen response of Pu.
INTRODUCTION: ANOMALOUS HYDROGEN BEHAVIOR IN PLUTONIUM
During our ongoing work to investigate the influence of hydrogen and deuterium gas on Pu alloys, we have observed several anomalous behaviors [1]. The Sievert’s solubility constant, Ks, was seen to sharply decrease (Figure 1) in the dilute regime where the hydrogen concentration was less than 0.1 at. %. In this regime, where solute atoms are clearly non-interacting, we expect the solubility to be approximately constant, independent of hydrogen concentration. The hydrogen diffusivity was also observed to anomalously drop with increasing hydrogen concentration in this regime, as seen in Figure 1. Again, at these low hydrogen levels the diffusion constant should not show any significant dependence on hydrogen concentration. This type of behavior points to a hydrogen trapping mechanism that operates at low hydrogen concentration.
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Figure 1. Solubility constant and diffusion coefficient at 400°C, as a function of deuterium concentration.
Figure 2 shows the hydrogen solubility constant behavior for both pure Pu and Pu2 at. % Ga. Indicated on the plots are the phase boundaries that are crossed for these materials: į ĺ į´ĺ İ for Pu, and į ĺ į+İ ĺ İ for Pu-2Ga. There is an anomalous lack of discontinuity across these boundaries. The large change in crystal structure that results from crossing these phase boundaries should result in a large and discontinuous change in Ks due to entropic changes, as is observed in other metals that undergo phase transformations (e.g. for a fcc ĺ bcc transition, there is typically a 3x increase in Ks). Vacancy formation may explain this unusual behavior. Hydrogen is known to stabilize large numbers of vacancies in some metals [2] at high hydrogen concentrations. Hydrogen-vac
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