A comment on the thermal conductivity of (U,Pu)O 2 and (U,Th)O 2 by molecular dynamics with adjustment for phonon-spin s
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A comment on the thermal conductivity of (U,Pu)O2 and (U,Th)O2 by molecular dynamics with adjustment for phonon-spin scattering M. W. D. Cooper*, C. R. Stanek, X.-Y. Liu, D. A. Andersson Materials Science and Technology Division, Los Alamos National Laboratory P.O. Box 1663, Los Alamos, NM 87545, USA
Abstract A new approach for adjusting molecular dynamics results on UO2 thermal conductivity to include phonon-spin scattering has been used to improve calculations on Ux Pu1−x O2 and Ux Th1−x O2 . We demonstrate that by including spin scattering a strong asymmetry as a function of uranium actinide fraction, x, is obtained. Greater degradation is shown for Ux Th1−x O2 than Ux Pu1−x O2 . Minimum thermal conductivities are predicted at U0.97 Pu0.03 O2 and U0.58 Th0.42 O2 , although the degradation in Ux Pu1−x O2 is negligible relative to pure UO2 .
1. Introduction Nuclear fuel has been UO2 -based for several decades due to its radiation tolerance, high melting point and ability to accommodate the significant chemical changes that it undergoes during reactor operation. UO2 can also be blended with PuO2 [1, 2] or ThO2 [3] to form mixed oxide (MOX) fuels. Alternatively, transmutation of U to Pu during reactor operation creates a (U,Pu)O2 solid solution as the host lattice. (U,Pu)O2 is also used as MOX fuel in fast breeder reactors and pressurized water reactors, providing a route for recycling Pu stockpiles that have built up over decades of nuclear reactor operation. By mixing the fertile isotope Th232 with fissile species, such as U235 , the relatively abundant Th deposits can be incorporated into the fuel cycle. Advanced fuel cycles such as these would greatly improve the longevity and sustainability of nuclear power as an energy resource. To underpin performance for advanced MOX fuels or for higher burn up conventional fuel it is important to understand how the thermophysical properties of these solid solutions deviate from the end members. Many important properties, such as elastic constants, thermal conductivity, fission gas mobility and specific heat, are temperature dependent. Thermal conductivity, therefore, plays a particularly important role as the primary factor in determining fuel pellet temperatures. Knowledge of the thermal conductivity is crucial to avoiding centerline pellet melting such that safety cases for MOX or high burn up fuel require a strong understanding of this property [1, 4]. Consequently, there have been significant efforts to investigate the thermal conductivity of nuclear fuel experimentally [5–10] and computationally [11–14, 16, 17]. Using the same many-body potential employed here Qin et al. [16] predicted a strong degradation in UO2 thermal conductivity due to disorder on the anion sublattice. To identify the role of the mixed cation lattice in MOX fuel, Arima et al. [11, 12] and Ma et al. [13] investigated the thermal conductivity of (Ux Pu1−x )O2−y as a function of 0 x 0.3 and 0 y 0.25. It was predicted that the oxygen to metal ratio has a much stronger effect on thermal conductivity
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