Evolving Defect Chemistry of (Pu,Am)O_2±x

Abstract

The β decay of 241 Pu to 241 Am results in a significant ingrowth of Am during the interim storage of PuO 2 . Consequently, the safe storage of the large stockpiles of separated Pu requires an understanding of how this ingrowth affects the chemistry of PuO 2 . 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 PuO 2 evolves due to the incorporation of Am. The model predicts that Am occupies Pu sites in (Pu,Am)O 2± 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)O 2± x as the negatively charged (Am Pu 1– ) defect, requiring charge compensation from holes in the valence band, thereby increasing the conductivity of the material compared to Am-free PuO 2 . Oxygen vacancies take over as the charge compensation mechanism at low O/M ratios. In (Pu,Am)O 2± x , hypo- and (negligible) hyperstoichiometry is found to be provided by the doubly charged oxygen vacancy (V O 2+ ) and singly charged oxygen interstitial (O i 1– ), respectively.

Accommodation of helium in PuO2±x and the role of americium

“…In a previous study, Am( iii ) was found to be accommodated in PuO 2 as substitutions on Pu sites, with a net charge of −1 (Am Pu 1− ). 37 The ratio of Am( iv ): Am( iii ) in (Pu,Am)O 2± x was found to vary with external conditions and total Am concentrations. However, in conditions where Am( iv ) is the dominant oxidation state, Am( iii ) concentrations remain high, and Am( iii ) is dominant in hypo-stoichiometric PuO 2− x , due to reduction of Am( iv ).…”

“…). 37 The ratio of Am(IV): Am(III) in (Pu,Am)O 2AEx was found to vary with external conditions and total Am concentrations. However, in conditions where Am(IV) is the dominant oxidation state, Am(III) concentrations remain high, and Am(III) is dominant in hypo-stoichiometric PuO 2Àx , due to reduction of Am(IV).…”

Synthesis and multi-scale properties of PuO2 nanoparticles: recent advances and open questions

“…Investigations dealing with the local environment and valences of the cations in (Pu,Am)O 2Àx indicated that Am occupies the Pu sites in the oxide and will reduce to Am(III) prior to any reduction of Pu(IV). 154,186 In comparison to the theoretical a 0 values of 5.396 Å and 5.373 Å for PuO 2 and AmO 2 , respectively, it was demonstrated that the presence of Am in (Pu 1Ày , Am y )O 2Àx may induce an increase in the lattice parameter for the oxide studied (a 0 ¼ 5.400 and 5.411 Å for Am/(Pu+Am) ¼ 0.008 and 0.2, respectively). 187 At high temperature, low Am concentration and low oxygen-to-metal ratio, Am is mainly found in the +III state.…”

The accommodation of lithium in bulk ZrO2

“…The Brouwer diagrams presented in this investigation that used Fermi-Dirac statistics gave subtle differences, with enhanced accuracy but do not fundamentally alter the picture previously reported in literature when considering no lithium [23]. The ability to stipulate a defect species concentration is novel to the Brouwer diagram script created by Murphy et al [30]. This is particularly advantageous as it provides a basis for comparison in real world applications where lithium concentrations are added to coolant water within a nuclear reactor.…”

“…DFT can also provide electronic information concerning the perfect supercell structures including band gap, valence and conduction bands, which are used in the creation of the Brouwer diagram, using Fermi-Dirac statistics to provide electron and hole concentrations upon which charge defect equilibria could be attributed. A novel aspect of the Defect Analysis Package V2.7 script created by Murphy and Neilson [30] is that a particular defect concentration may be stipulated where all other defect equilibria, including those of electrons and holes, are calculated. In the case of this investigation, lithium concentrations are stipulated from 10 -3 to 10 -11 per ZrO2 formula unit and a temperature of 635K was used for all calculations as a normal (water) operating temperature of a PWR [31].…”