From arc-melted ingot to MTR fuel plate: A SEM/EBSD microstructural study of U3Si2

Abstract U3Si2/Al fuel plates are widely used in Material Testing Reactors (MTRs) as Low Enriched Uranium driver fuel. In this paper, a reinvestigation of the microstructure of U3Si2 particles is proposed to take full advantage of the new capabilities offered by Electron BackScattered Diffraction (EBSD) techniques. Using EBSD we demonstrate that most particles are single crystalline in as-fabricated plates. To understand this characteristic, linked to the microstructure of the starting material, an in-depth study of U3Si2 ingots produced by arc-melting was performed at a laboratory scale; they were extensively characterized by EBSD, scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. It is shown that U3Si2 grain may be large (up to several thousands of micrometers) and that they exhibit a strong preferential orientation, linked to the axial thermal gradient created in the arc-melting chamber. A significant impact of the cooling rate after arc-melting on the ingot microstructure is noticed: grains are smaller and more columnar when the cooling rate is high. A deviation from 3U/2Si stoichiometry caused for example by impurities induces the formation of a secondary phase, which exhibits a square spiral morphology. We then demonstrate that the cooling rate of U3Si2 ingots has a direct influence on the characteristics of the powders obtained by crushing these ingots. Indeed the powder obtained from “slow” cooled ingots is found very close to the powder used for industrial MTR plates. On the contrary particles obtained from “fast” cooled ingots, are polycrystalline and more resistant to crushing. Thus, this work provides significant advances both in the characterisation of technological products like U3Si2/Al MTR plates and in more basic knowledge about the U3Si2 phase formation.