Experimental thermochemistry of neptunium oxides: Np2O5 and NpO2

JOURNAL OF NUCLEAR MATERIALS

卷:501

Article

Zhang, Lei¹  Dzik, Ewa A.¹  Sigmon, Ginger E.¹  Szymanowski, Jennifer E. S.¹  Navrotsky, Alexandra^2,3  Burns, Peter C.^1,4 

[1] Univ Notre Dame, Dept Civil & Environm Engn & Earth Sci, Notre Dame, IN 46556 USA

[2] Univ Calif Davis, Peter A Rock Thermochem Lab, Davis, CA 95616 USA

[3] Univ Calif Davis, NEAT ORU, Davis, CA 95616 USA

[4] Univ Notre Dame, Dept Chem & Biochem, Notre Dame, IN 46556 USA

关键词: Thermochemistry;    Formation enthalpy;    Neptunium oxides;   

DOI  :  10.1016/j.jnucmat.2017.10.034

来源: Elsevier

【 摘 要 】

Neptunium (Np) compounds are important in the nuclear fuel cycle because of the buildup and long halflife (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 973 K. The enthalpy of the decomposition reaction, Np2O5(cr) = 2NpO(2)(cr) + 1/2O(2)(g) at 298 K, is determined to be 7.70 +/- 5.86 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. (c) 2017 Elsevier B. V. All rights reserved.

【 授权许可】

Free