Introduction: Uranium silicide (U3Si2), a candidate accident tolerant fuel material, is reported to be stable from room temperature to the melting point without transformations in the solid state. Thermal expansion of the material must be well measured and predicted over the entire temperature range to ensure sufficient safety margins for fuel operation. The thermal expansion measurements of White, et al. (2015) by dilatometry observed a variation in the instantaneous U3Si2 thermal expansion coefficient between 900 and 1000 °C, which may indicate a phase transition and has not yet been explained. Methods: This work set out to verify directly the crystal structure and thermal expansion of U3Si2 by in-situ neutron diffraction during heating to the melting point. In-situ annealing was conducted on pure U3Si2 samples and also on high density fuel composite UN 20wt.% U3Si2. Results: These are the most accurate and detailed measurement of U3Si2 thermal expansion to date, showing a negative, linear temperature dependence of the instantaneous thermal expansion described by α(T) = 2.10×10-5 – 7.25×10-9× T (1/°C). Anomalous changes in the diffraction patterns were observed in U3Si2 above 1000 °C including formation of a new diffraction peak, unequal changes in peak intensity and onset of anisotropic thermal expansion, as measured from the tetragonal lattice parameters. The thermal expansion of U3Si2 only appears to be isotropic because increase in expansion in the c-axis is balanced, through development of internal stresses above approximately 1000 °C, by the reduction in the a-axis expansion. Conclusions: Our observations show that the earlier measurement was not an experimental anomaly, but is related to structural changes during heating, with implications to the use of U3Si2 in fuel applications. A possible explanation may be that U3Si2 enters a region of extended stoichiometry range above 1000 °C, in agreement with previous modelling results.

5th Nuclear Materials Conference; Seattle; 2018-10-14 - 2018-10-18