Investigations on the release of mercury from liquid eutectic lead–bismuth alloy under different gas atmospheres

Eutectic lead–bismuth alloy (LBE) samples with mercury concentrations similar to those expected for a liquid metal spallation target were prepared. The release of mercury from these samples under a continuous flow of gas was studied as a function of temperature, using γ-spectroscopy and 203Hg as a radioactive tracer. Various gas compositions ranging from a reducing atmosphere (Ar/7%H2) to oxidising conditions (moist air) were applied. Under reducing conditions, substantial release of mercury from LBE started at temperatures around 473 K. At about 571 K, half of the mercury present in the samples was released within 1 h. Oxidising conditions led to a strong decrease in mercury release. In 1 h-experiments, no release of mercury was observed for temperatures up to 935 K within the limits of experimental error when humid air was used as a carrier gas. Apparently, the formation of an oxide layer on the surface of the melt inhibits the evaporation of Hg. This effect is of importance with respect to accident scenarios in liquid metal spallation targets, where liquid LBE could be exposed to air as well as cooling water. In such a case, a substantial decrease in the evaporation rate of mercury can be expected due to the formation of an oxide layer.