Molybdenum – Silicon – Uranium

Abstract

Interest in low enriched uranium (LEU) proliferation resistant reactor fuel systems triggered investigations on phase equilibria and compatibility of U3Si and U3Si2 with Mo-metal. Irradiation tests on U-4Mo-0.1Si (mass%) alloys discovered high swelling resistance in solution-quenched structures with effective control of fission-gas nucleation and bubble growth [1968Far]. Early reports on the ternary system mentioned the existence and crystal structure of two ternary compounds: a Laves-phase UMo1.25Si0.75 with the MgZn2 type [1963Sik] and U2Mo3Si4 with a unique structure type [1978Sik]. The crystal structures of both compounds were confirmed from X-ray single crystal counter data [1993LeB, 1995LeB, 1996LeB1]. To supplement the fission product chemistry in U3Si2-fuel burn-up, partial phase equilibria in the U rich part of the diagram at 850°C were studied by [1994Uga] reporting the existence of three ternary compounds: U2Mo3Si4, “U3MoSi2”, “U4MoSi3”, without defining the crystal structures of “U3MoSi2” and “U4MoSi3”. Based on a phase diagram investigation at 1400°C [1993LeB], phase relations were reinvestigated by [2001Rog] providing (i) a partial isothermal section at 1400°C for concentrations smaller than 70 at.% U, (ii) a partial isothermal section at 850°C for concentrations smaller than 70 at.% Si; and (iii) an evaluation of crystal structures from X-ray data. Physical properties (i.e. magnetic susceptibility and electrical resistivity) of the ternary compounds were reported from several sources [1988Ali, 1993LeB, 1994Was, 1995LeB, 1996LeB1, 1996Pat, 2001Rog]. The various experimental activities related to the constitution of the ternary Mo-Si-U system are summarized in Table 1.