Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
7963432 | Journal of Nuclear Materials | 2018 | 23 Pages |
Abstract
Neptunium (Np) compounds are important in the nuclear fuel cycle because of the buildup and long half-life (2.14 Ma) of Np-237 in nuclear waste, especially during long-term disposal in a geological repository. Neptunium in environmental conditions exists mainly in two oxidation states (+5 and + 4) and can substitute for uranium and/or rare earths in solid phases. Yet thermochemical data for solid neptunium compounds are scarce, despite being critical for evaluating the environmental transport of this radioactive and toxic element. Although high temperature oxide melt solution calorimetry has proven very useful in obtaining thermodynamic data for the formation of uranium and thorium oxide materials, it has not yet been applied to transuranium compounds. Continuing a program at Notre Dame to study the thermodynamics of transuranium compounds, we report the first determination of the enthalpies of drop solution of well-characterized neptunium oxides (Np2O5 and NpO2) using oxide melt solution calorimetry in molten sodium molybdate solvent at 973â¯K. The enthalpy of the decomposition reaction, Np2O5(cr)â¯=â¯2NpO2(cr) + 1/2O2(g) at 298â¯K, is determined to be 7.70â¯Â±â¯5.86â¯kJ/mol, and this direct measurement is consistent with existing thermodynamic data. The calorimetric methodology is straightforward and produces reliable data using milligram quantities of radioactive materials, and can be applied to many other transuranium compounds.
Keywords
Related Topics
Physical Sciences and Engineering
Energy
Nuclear Energy and Engineering
Authors
Lei Zhang, Ewa A. Dzik, Ginger E. Sigmon, Jennifer E.S. Szymanowski, Alexandra Navrotsky, Peter C. Burns,