Spin-Lattice Model of Plutonium Hydride Nucleation

Plutonium hydride forms when plutonium is exposed to hydrogen gas or water vapor. The mechanism of plutonium hydride nucleation is a key material property in this process. Conducting experiments with plutonium is difficult and expensive due to its toxicity and radioactivity. Computer models allow us to probe the phase transition to plutonium hydride without physically handling plutonium.

We develop a spin-lattice model of plutonium hydride, PuH₂, wherein the presence of a hydrogen atom in an interstitial site is a binary variable. We parameterize the site energy of each interstitial as a function of the number of nearest-neighbor occupied sites from the DFT energies of a small system (N_Pu = 32). We then compute the phase equilibrium between Pu and PuH₂ from the spin-lattice model using a combination of Wang-Landau sampling and histogram reweighting. The size of the critical PuH₂ nucleus is then computed using a much larger system than would be accessible with DFT (N_Pu = 500,000).