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. 2021 Jul 22;125(28):15560-15568.
doi: 10.1021/acs.jpcc.1c03274. Epub 2021 Jul 7.

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

William D Neilson  1 , Helen Steele  2 , Samuel T Murphy  1
Affiliations

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

William D Neilson et al. J Phys Chem C Nanomater Interfaces. .

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 (AmPu 1-) 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 (VO 2+) and singly charged oxygen interstitial (Oi 1-), respectively.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Defect formation energies for AmPu, AmO, and Ami defects in (Pu1–yAmy)O2±x (y = 0.001) as a function of Fermi energy. Calculated at 1000 K and an oxygen partial pressure of 0.10 atm. Only the charge state with the lowest formation energy for a given Fermi level is shown for each defect, represented with numeric label.
Figure 2
Figure 2
Final relaxed structure for the AmPu1– defect in PuO2. Plutonium, americium, and oxygen are represented with gray, blue, and red spheres, respectively.
Figure 3
Figure 3
Brouwer diagrams showing the defect concentrations and value of x in (Pu1–yAmy)O2±x as a function of oxygen partial pressure at a temperature of 1000 K and y value of 0.0 (left) and 0.001 (right). At partial pressures to the left of the vertical dashed line, (Pu,Am)O2±x is predicted to be thermodynamically unstable with respect to Am2O3.
Figure 4
Figure 4
Values of x in (Pu1–yAmy)O2–x as a function of oxygen partial pressure at y values of (a) 0.09 and (b) 0.072, with comparison to the experimental results of Osaka et al. and Matsumoto et al.
Figure 5
Figure 5
Defect concentrations in (Pu1–yAmy)O2±x as a function of temperature at an oxygen partial pressure of 0.1 atm and y value of 0.0 (left) and 0.001 (right). At temperatures to the left of the vertical dashed lines (a) and (b), (Pu,Am)O2±x is predicted to be thermodynamically unstable with respect to Am2O3 and AmO2, respectively.
Figure 6
Figure 6
Defect concentrations in (Pu1–yAmy)O2±x as a function of the concentration of Am at an oxygen partial pressure of 0.1 atm and temperature of 1000 K. At Am concentrations to the right of the vertical dashed line, (Pu,Am)O2±x is predicted to be thermodynamically unstable with respect to AmO2.
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