TY - JOUR
T1 - Experimental thermochemistry of neptunium oxides
T2 - Np2O5 and NpO2
AU - Zhang, Lei
AU - Dzik, Ewa A.
AU - Sigmon, Ginger E.
AU - Szymanowski, Jennifer E.S.
AU - Navrotsky, Alexandra
AU - Burns, Peter C.
N1 - Funding Information:
This work was supported as part of the Materials Science of Actinides, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0001089 . Calorimetry and diffraction data were collected at the Materials Characterization Facility in the Center for Sustainable Energy at the University of Notre Dame.
Publisher Copyright:
© 2017 Elsevier B.V.
PY - 2018/4/1
Y1 - 2018/4/1
N2 - 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 973 K. The enthalpy of the decomposition reaction, Np2O5(cr) = 2NpO2(cr) + 1/2O2(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.
AB - 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 973 K. The enthalpy of the decomposition reaction, Np2O5(cr) = 2NpO2(cr) + 1/2O2(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.
KW - Formation enthalpy
KW - Neptunium oxides
KW - Thermochemistry
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U2 - 10.1016/j.jnucmat.2017.10.034
DO - 10.1016/j.jnucmat.2017.10.034
M3 - Article
AN - SCOPUS:85042262005
SN - 0022-3115
VL - 501
SP - 398
EP - 403
JO - Journal of Nuclear Materials
JF - Journal of Nuclear Materials
ER -