Evolving Defect Chemistry of (Pu,Am)O2±x.

William D Neilson,Helen Steele,Samuel T Murphy

doi:10.1021/acs.jpcc.1c03274

William D Neilson, Helen Steele + Show 1 more

Open Access

https://doi.org/10.1021/acs.jpcc.1c03274

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Abstract

The β decay of 241Pu to 241Am results in a significant ingrowth of Am during the interim storage of PuO2. Consequently, the safe storage of the large stockpiles of separated Pu requires an understanding of how this ingrowth affects the chemistry of PuO2. This work combines density functional theory (DFT) defect energies and empirical potential calculations of vibrational entropies to create a point defect model to predict how the defect chemistry of PuO2 evolves due to the incorporation of Am. The model predicts that Am occupies Pu sites in (Pu,Am)O2±x in either the +III or +IV oxidation state. High temperatures, low oxygen-to-metal (O/M) ratios, or low Am concentrations favor Am in the +III oxidation state. Am (+III) exists in (Pu,Am)O2±x as the negatively charged (AmPu1–) defect, requiring charge compensation from holes in the valence band, thereby increasing the conductivity of the material compared to Am-free PuO2. Oxygen vacancies take over as the charge compensation mechanism at low O/M ratios. In (Pu,Am)O2±x, hypo- and (negligible) hyperstoichiometry is found to be provided by the doubly charged oxygen vacancy (VO2+) and singly charged oxygen interstitial (Oi1–), respectively.

Highlights

The management of the large stockpiles of Pu, separated from spent nuclear fuel or nuclear weapons programs, pose a series of technical challenges associated with its potential storage, disposal, and reuse
Incorporation of Am is predicted to alter the defect chemistry of PuO2; in a density functional theory (DFT) investigation on Pu−Am mixed oxide surfaces, Chen et al.[4] report that the presence of Am promotes the formation of O vacancies that increase the favorability of molecular adsorption of water on PuO2 surfaces while reducing the favorability of dissociative water adsorption
DFT simulations were performed using the Vienna Ab initio Simulation Package (VASP)[22−25] employing the projector augmented wave (PAW)[26,27] method implemented with the frozen-core approximation

Summary

Introduction

The management of the large stockpiles of Pu, separated from spent nuclear fuel or nuclear weapons programs, pose a series of technical challenges associated with its potential storage, disposal, and reuse. Oxidation of the material during interim storage and the formation of hyperstoichiometric PuO2+x may initiate chemical reactions that cause potential pressurization of PuO2 storage canisters.[1] Previous theoretical investigations of the defect chemistry of PuO2 suggest that pure PuO2 is very reluctant to form hyperstoichiometric PuO2+x.2. Am builds up relatively quickly due to the short half-life of 241Pu (14.4 years), with Am concentrations peaking after approximately 70 years, at which point, it too begins to decay faster than it is produced.[3] Incorporation of Am is predicted to alter the defect chemistry of PuO2; in a density functional theory (DFT) investigation on Pu−Am mixed oxide surfaces, Chen et al.[4] report that the presence of Am promotes the formation of O vacancies that increase the favorability of molecular adsorption of water on PuO2 surfaces while reducing the favorability of dissociative water adsorption The consequence of this could be an increased likelihood of chemical reactions including the aforementioned pressurization.[4]

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