First-principles design of next-generation nuclear fuels
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Introduction The ultimate performance of nuclear fuels is influenced by a host of materials properties, including thermomechanical strength, chemical stability, microstructure, and lattice defects. Overlaying this multitude of properties, nuclear fuels are exposed to a severe irradiation environment, which causes a continuous change of materials properties. Nuclear fuel behavior is therefore exceptionally rich, and a predictive understanding poses significant challenges. Experimental investigations of nuclear fuel materials, however, are very costly and, due to the risk of toxicity and radioactivity, are often restricted. Computational simulation and modeling can play a decisive role in the research on nuclear fuel materials, as first-principles modeling is now capable of providing precise, valuable insights and property data from an atomistic perspective. In fact, firstprinciples studies based on density functional theory (DFT) have already greatly contributed by supplying fundamental electronic-structure knowledge1–8 and the material properties of defect-containing nuclear fuels.9–20 Among all fuel materials, UO2 and PuO2 have been most extensively studied both experimentally and computationally. This is especially true for UO2, the standard nuclear fuel for light water reactors. PuO2 has attracted interest because of its importance in mixed oxide fuel, (U,Pu)O2, and as a radiotoxic
ingredient of spent fuel. Various actinide compounds, including UC and U-Pu-C alloys, have also recently been investigated as candidate fuels for next-generation reactors.20,21 The intricate nature of the 5f electrons in the actinides is crucial to understanding nuclear fuel properties such as chemical bonding, heat conductivity,22 fission gas behavior,11,12,14–19 and the surface interaction with molecules23 relevant to fuel corrosion.24 Hence, understanding actinide electronic structure is a prerequisite for understanding the behavior of nuclear fuel materials. The local spin density approximation (LSDA) and the generalized gradient approximation (GGA) are most widely used in DFT-based first-principles calculations to estimate the exchange and correlation energy of electrons. The DFT provides an exact mapping of a many-electron system to a single-electron system, in which the electron moves in an effective potential; however, its exact form is not known. The unknown exchange-correlation potential is locally approximated in the LSDA by that of an electron in a homogeneous electron gas having the same density. The GGA is also a local approximation, but in addition, it takes the gradient of the electron density into account. Both approximations have been successfully applied to describe many of the fuel materials properties, despite their known limitation in capturing strong correlation effects of f-electrons.25,26 In defect energy calculations, using DFT-LSDA/GGA Hamiltonians have
Younsuk Yun, Laboratory of Reactor Physics and Systems Behaviour, Paul Scherrer Institut, Switzerland; [email protected] Peter M. Oppeneer, Department of Physic
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