About the event
Speaker: Xiaodong Zhao, chemistry graduate student
Group: Dr. Xiaofeng Guo
Title: Heat capacities of actinide orthosilicates for nuclear waste applications
Abstract Actinides from the spent nuclear fuel (SNF) and defensive plutonium waste are proposed to be immobilized in ceramic waste forms. Their fates in the deep geological disposal site are dictated by thermodynamics, which impacts evaluations of potential release of actinides to the biosphere in the long term. Zircon structure mineral, such as coffinite (USiO4) and it has been found in multiple uranium mining site and the Oklo natural nuclear fission reactor. Due to its abundance in nature and lab evidence of alteration,1 zircon has been considered as an important waste host of actinides and also alteration phase of disposed UO2 SNF. Thermodynamic property (ΔH, ΔS, ΔG) of coffinite is vital to estimate its stability under conditions that are relevant to deep geological sites. Formation enthalpy of coffinite has been measured by high temperature drop calorimetry, and the formation Gibbs free energy has been calculated by solubility tests.2, 3 By that, standard molar entropy was derived as 136 ± 25 J/mol/K at 298.25K. However, there has been debate over the standard molar entropy due to the big difference with the entropy value of thorite (ThSiO4), which is iso-structural to coffinite. A direct entropy measurement has been done by adiabatic pulse method with adiabatic calorimetry. We found that the ΔS (298.15K) = 127.15 ± 1.27 J/mol/K, which agrees well with our computational result, ΔS (290K) = 103.3 J/mol/K, that was obtained by performing Density-Functional Perturbation Theory (DFPT) calculation. Furthermore, I also calculate the phonon structure of CeSiO4, which is an analog of PuSiO4. The calculated structure helps elucidate our high pressure experiments on CeSiO4, particularly revealing a hidden phase transition indicated by phonon dispersion curve softening.
1. Szenknect, S.; Alby, D.; López García, M.; Wang, C.; Podor, R.; Miserque, F.; Mesbah, A.; Duro, L.; Zetterström Evins, L.; Dacheux, N., Coffinite formation from UO2+ x. Scientific reports 2020, 10 (1), 1-10.
2. Guo, X.; Szenknect, S.; Mesbah, A.; Labs, S.; Clavier, N.; Poinssot, C.; Ushakov, S. V.; Curtius, H.; Bosbach, D.; Ewing, R. C., Thermodynamics of formation of coffinite, USiO4. Proceedings of the National Academy of Sciences 2015, 112 (21), 6551-6555.
3. Szenknect, S.; Mesbah, A.; Cordara, T.; Clavier, N.; Brau, H.-P.; Le Goff, X.; Poinssot, C.; Ewing, R. C.; Dacheux, N., First experimental determination of the solubility constant of coffinite. Geochimica et Cosmochimica Acta 2016, 181, 36-53.