Silicon coating on very dense tungsten particles by fluidized bed CVD for nuclear application

Abstract

Tungsten particles were coated by silicon from silane (SiH4) at 645 °C using fluidized bed chemical vapor deposition (FB-CVD). The coated particles were made from a surrogate tungsten powder and showed a median diameter (75 μm) and density (19,300 kg/m3). We studied the behavior of these tungsten particles due to their similarity with U(Mo) particles which are being investigated for use as fuel in research reactors. The results show that the fluidized bed's thermal gradient and pressure drop both increase as soon as silane is injected into the reactor. These phenomena are linked to the reactivity of silane and the very high density of the tungsten powder, and they represent a risk of bed agglomeration. A low inlet molar fraction of silane (0.5%) makes it possible to limit this risk. An increase in the fluidizing gas velocity and a decrease in the reactor diameter from 3.8 to 3.0 cm, have little impact on these phenomena for the operating range studied. Characterizations (EDX, SEM, XRD, etc.) show that all the tungsten particles are uniformly coated by a continuous film of silicon, mainly crystallized and of nodular morphology. The thickness of this silicon layers varies between 0.1 and 1 μm, which corresponds to the target. As the feasibility of the coating has been demonstrated, new experiments are in progress with fissile particles.