Dataset Information

DFT + U Study of Uranium Dioxide and Plutonium Dioxide with Occupation Matrix Control.

ABSTRACT: DFT + U with occupation matrix control (OMC) is applied to study computationally bulk UO₂ and PuO₂, the latter for the first time. Using the PBESol functional in conjunction with OMC locates AFM and NM ground states for UO₂ and PuO₂, respectively, in agreement with experimental findings. By simulating the lattice parameter, magnetic moment, band gap, and densities of states, U = 4.0 eV is recommended for AFM UO₂, yielding data close to experiments for all considered properties. U = 4.5 and 4.0 eV are recommended for NM and AFM PuO₂, respectively, though much larger U values (c. 10 eV) are required to yield the most recently reported PuO₂ band gap. For both oxides, several excited states have similar properties to the ground state, reinforcing the need to employ OMC wherever possible.

SUBMITTER: Chen JL

PROVIDER: S-EPMC9289946 | biostudies-literature | 2022 Jul

REPOSITORIES: biostudies-literature

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DFT + U Study of Uranium Dioxide and Plutonium Dioxide with Occupation Matrix Control.

Chen Jia-Li JL Kaltsoyannis Nikolas N

The journal of physical chemistry. C, Nanomaterials and interfaces 20220701 27

DFT + U with occupation matrix control (OMC) is applied to study computationally bulk UO2 and PuO2, the latter for the first time. Using the PBESol functional in conjunction with OMC locates AFM and NM ground states for UO2 and PuO2, respectively, in agreement with experimental findings. By simulating the lattice parameter, magnetic moment, band gap, and densities of states, U = 4.0 eV is recommended for AFM UO2, yielding data ...[more]

PMID: 35865792

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Project description:Over 80 heat treatment experiments have been made on samples of chloride-contaminated plutonium dioxide retrieved from two packages in storage at Sellafield. These packages dated from 1974 and 1980 and were produced in a batch process by conversion of plutonium oxalate in a furnace at around 550 °C. The storage package contained a poly(vinyl chloride) (PVC) bag between the screw top inner and outer metal cans. Degradation of the PVC has led to adsorption of hydrogen chloride together with other atmospheric gases onto the PuO2 surface. Analysis by caustic leaching and ion chromatography gave chloride contents of ∼2000 to >5000 ppm Cl (i.e., μgCl g-1 of the original sample). Although there are some subtle differences, in general, there is surprisingly good agreement in results from heat treatment experiments for all the samples from both cans. Mass loss on heating (LOH) plateaus at nearly 3 wt % above 700 °C, although samples that were long stored under an air atmosphere or preexposed to 95% relative humidity atmospheres, gave higher LOH up to ∼4 wt %. The majority of the mass loss is due to adsorbed water and other atmospheric gases rather than chloride. Heating volatilizes chloride only above ∼400 °C implying that simple physisorption of HCl is not the main cause of contamination. Interestingly, above 700 °C, >100% of the initial leachable chloride can be volatilized. Surface (leachable) chloride decreases quickly with heat treatment temperatures up to ∼600 °C but only slowly above this temperature. Storage in air atmosphere post-heat treatment apparently leads to a reequilibration as leachable chloride increases. The presence of a "nonleachable" form of chloride was thus inferred and subsequently confirmed in PuO2 samples (pre- and post-heat treatment) that were fully dissolved and analyzed for the total chloride inventory. Reheating samples in either air or argon at temperatures up to the first heat treatment temperature did not volatilize significant amounts of additional chloride. With regard to a thermal stabilization process, heat treatment in flowing air at 800 °C with cooling and packaging under dry argon appears optimal, particularly, if thinner powder beds can be maintained. From electron microscopy, heat treatment appeared to have the most effect on degrading the square platelet particles compared to those with the trapezoidal morphology.

Dataset Information

DFT + U Study of Uranium Dioxide and Plutonium Dioxide with Occupation Matrix Control.

Publications

DFT + <i>U</i> Study of Uranium Dioxide and Plutonium Dioxide with Occupation Matrix Control.

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