Inelastic neutron scattering measurements of individual phonon lifetimes and dispersion at 295 and 1200 K have been used to probe anharmonicity and thermal conductivity in UO2. They show that longitudinal optic phonon modes carry the largest amount of heat, in contrast to past simulations and that the total conductivity demonstrates a quantitative correspondence between microscopic and macroscopic phonon physics. We have further performed first-principles simulations for UO2 showing semiquantitative agreement with phonon lifetimes at 295 K, but larger anharmonicity than measured at 1200 K.

Abstract

Accurately predicting changes in the thermal conductivity of light water reactor UO 2 fuel throughout its lifetime in reactor is an essential part of fuel performance modeling. However, typical thermal conductivity models from the literature are empirical. In this work, we begin to develop a mechanistic thermal conductivity model by focusing on the impact of gaseous fission products, which is coupled to swelling and fission gas release. The impact of additional defects and fission products will be added in future work. The model is developed using a combination of atomistic and mesoscale simulation, as well as analytical models. The impact of dispersed fission gas atoms is quantified using molecular dynamics simulations corrected to account for phonon-spin scattering. The impact of intragranular bubbles is accounted for using an analytical model that considers phonon scattering. The impact of grain boundary bubbles is determined using a simple model with five thermal resistors that are parameterized by comparing to 3D mesoscale heat conduction results. However, when used in the BISON fuel performance code to model four reactor experiments, it produces reasonable predictions without having been fit to fuel thermocouple data.

Two new ternary materials NaGaS 2 (1) and the Fedoped phase of NaGaS 2 , NaFe 0.135 Ga 0.865 S 2 (2), have been synthesized by employing polysulfide flux. Single crystal XRD analyses of 1 and 2 show that the structure is built up of adamantane-like Ga 4 S 10 super tetrahedral fundamental building units. These admantane-like units are connected through their corners to form [GaS 2 ] ∞ − layers that are stacked one over the other with Na ions residing in between the layers to balance the charge. Both the materials have the remarkable ability to absorb atmospheric water molecules and moisture from undried solvents as verified by TG analysis and FT-IR and XPS studies. The process of water absorption leads to stable distinct material) with restacked layers different from original crystal structure. This structural transformation is reversible as the transformed structures 1•H 2 O and 2•H 2 O can be returned to their original structures 1 and 2, respectively, upon heating. DFT calculation study reveals that a spontaneous exergonic hydration reaction takes place as outlined in NaGaS 2 + H 2 O → NaGaS 2 •H 2 O with the energy release, ΔE of −73.9 kJ mol −1 . DFT calculation predicts an increase in the unit cell parameters of b and c directions and shrinkage along the a direction of hydrated phase 1•H 2 O with an overall volume increase of 36.6%. Structural transformation affects their physical properties as the pristine compound 1 possess Na + ion conductivity of 2.88 × 10 −7 S cm −1 at 22 °C, whereas the hydrated compound 1•H 2 O displays ∼40 times increased ion conductivity of 1.25 × 10 −5 S cm −1 at the same temperature. DRS studies show very similar optical band gaps of ∼4 eV for compounds 1 and 1•H 2 O, respectively, in reasonable agreement with the DFT(HSE) band gap estimation but more than 1 eV above the DFT(PBE)-predicted band gaps of ∼2.4 eV. A sorption study indicates selective adsorption of water over MeOH, EtOH, and CH 3 CN with a maximum water uptake of 2.6 H 2 O per formula unit at P/P 0 = 0.9. A Karl Fischer titration study shows that NaGaS 2 (1) is certainly capable of adsorbing water from wet methanol and can be useful as a fast desiccating agent.

Simulation has long since joined experiment and theory as a valuable tool to address materials problems. Analysis of errors and uncertainties in experiment and theory is well developed; such analysis for simulations, particularly for simulations linked across length scales and timescales, is much less advanced. In this prospective, we discuss salient issues concerning uncertainty quantification (UQ) from a variety of fields and review the sparse literature on UQ in materials simulations. As specific examples, we examine the development of atomistic potentials and multiscale simulations of crystal plasticity. We identify needs for conceptual advances, needs for the development of best practices, and needs for specific implementations.