Experimental verification of thermal decomposition of lead polonide

The chemical form of polonium in lead-bismuth eutectic (LBE) is an important issue, considering the problem of polonium contamination in nuclear systems that use LBE as a coolant and/or an irradiation target. It has been thought that polonium exists as lead polonide in LBE. Polonium forms compounds with several metals, some of which decompose at high temperatures. Thermal decomposition of lead polonide was not confirmed experimentally, but the temperature of decomposition was foreseen to be around 600 °C. In this paper, the thermal decomposition of lead polonide and its decomposition temperature were confirmed using neutron-irradiated LBE. Neutron-irradiated LBE ingots containing polonium-210 were heated at temperatures of 550 ± 10 °C or 630 ± 10 °C in a vacuum. Polonium, lead and bismuth evaporated from the LBE ingots, and were deposited onto the surface of type 316 stainless steel (316SS) plates at various controlled temperatures between 220 ± 20 °C and 450 ± 20 °C. After heating, the number of alpha particles emitted from polonium-210 deposited on 316SS plates was measured. The experimental results showed a clear difference in the alpha particle count rate, which indicated that lead polonide decomposed at a temperature between 550 ± 10 °C and 630 ± 10 °C.