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U 3 Si 2 nuclear fuel is corroded in deuterated steam with in situ neutron diffraction. Density functional theory is coupled with rigorous thermodynamic description of the hydride including gas/solid entropy contributions. H absorbs in the 2 b interstitial site of U 3 Si 2 H x and moves to 8 j for x ≥ 0.5. Hydriding forces lattice expansion and change in a/c ratio linked to site preference. Rietveld refinement tracks the corrosion reactions at 350–500 °C and preference for the 8 j site. Above 375 °C, formation of UO 2 , U 3 Si 5 and USi 3 take place in the grain boundaries and bulk. Hydriding occurs in bulk and precedes other reactions. [Display omitted] • In situ neutron diffraction measures U 3 Si 2 corrosion in deuterated steam. • Rigorous thermodynamic description of hydride using key entropy contributions. • H absorption at 2 b interstitial site of U 3 Si 2 H x , moves to 8 j site for x ≥ 0.5. • From 350 °C, U 3 Si 2 H x fragmented by 1–2% anisotropic lattice expansion. • From 375 °C, UO 2 , U 3 Si 5 and USi 3 form in bulk and in grain boundaries. [ABSTRACT FROM AUTHOR]