Structural and phase evolution in U$_3$Si$_2$ during steam corrosion

TL;DR

This study investigates the structural and phase changes in U$_3$Si$_2$ nuclear fuel during steam corrosion using neutron diffraction, thermodynamic modeling, and site-specific hydriding analysis to understand corrosion mechanisms and phase evolution.

Contribution

It combines in situ neutron diffraction with density functional theory and thermodynamics to elucidate hydriding behavior and phase transformations in U$_3$Si$_2$ during corrosion.

Findings

H absorbs in the 2$b$ site and moves to 8$j$ at higher hydrogen content

Hydriding causes lattice expansion and alters the $a/c$ ratio

Formation of UO$_2$, U$_3$Si$_5$, and USi$_3$ occurs above 375°C

Abstract

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 $x\ge 0.5$. Hydriding forces lattice expansion and change in $a/c$ ratio linked to site preference. Rietveld refinement tracks the corrosion reactions at 350-500 \deg C and preference for the 8j site. Above 375 \deg 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.