Neptunium-237 is of great concern for the long-term geological disposal of spent nuclear fuel because of its long half-life (2.14 Ma) and potential mobility. It can form various compounds with peroxide, sulphate, phosphate, arsenate, etc. The incorporation of neptunium into studtite or metastudtite (important alteration phases of spent UO2 fuel), in particular, must be considered when accessing its migration in groundwater. Therefore, it is crucial to understand the geochemical stability of Np containing materials. However, there are very limited reported thermodynamic data. We aim to develop high temperature oxide melt drop solution calorimetry at the University of Notre Dame to study the formation enthalpies of neptunium compounds. Neptunium oxides (Np2O5 and NpO2) have been synthesized and characterized for calorimetric study. 5 mg sample powders were made into pellets in a glove box and dropped into the calorimeter containing sodium molybdate solvent at 700 °C. The oxidation enthalpy of 2NpO2 + 1/2O2 = Np2O5 at 25 °C is determined to be -7.70 ± 5.86 kJ/mol. This value is in accordance to the literature one of -14.7 ± 10.73 kJ/mol. Meanwhile, thermochemical study of neptunium incorporated studtite or metastudtite is ongoing.

Thermochemistry of neptunium oxides and Np incorporated studtite

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.