Final report on the evaporation, release and capture of Po: FP7 SEARCH DELIVERABLE D 6.4

The necessity of finding a suitable solution for the disposal of the radioactive waste has led to a growing interest in the last few decades in the development of accelerator‐driven systems (ADS) which could efficiently performed the transmutation of this waste. One of the most prominent ADS projects worldwide is MYRRHA (Multi‐purpose hYbrid Research Reactor for High‐ tech Applications). MYRRHA is currently under development at the Belgian Nuclear Research Centre (SCK•CEN). MYRRHA consists of a subcritical core coupled to a proton accelerator and cooled by lead‐bismuth eutectic (LBE). The use of LBE in ADS such as MYRRHA is supported by advantageous physicochemical properties. However it also presents certain challenges. One important concern with the use of LBE as nuclear coolant is the formation of large quantities of 210 Po due to neutron capture reaction with Bi. Polonium (Po) is critical to safety of ADS due to its elevated radiotoxicity and propensity to evaporate. A qualitative and quantitative understanding of Po evaporation from LBE is thus required for safety assessments, design and licensing of LBE‐based nuclear systems. In this work the evaporation of Po from LBE was investigated with the so‐called transpiration method under different conditions relevant to MYRRHA. Firstly, the evaporation of Po from LBE between 600 °C and 1000 °C was studied. In this temperature range the evaporation of Po was quantitatively described by simple models and a single Henry constant‐temperature correlation. Through the performance of experiments in which the flow rate of the carrier gas was varied and the time dependence of the evaporation was measured, it was shown that saturation conditions were effectively achieved and that the Henry's law was applicable in the studied Po concentration range (ca. 10‐13 ‐10‐10 Po mole fraction). Furthermore, at these high temperatures, the release of Po from LBE was found to be insensitive to factors such as the diffusion of Po in the bulk of the LBE, the oxygen content in LBE and the nature of the carrier gas. The extension of these investigations to lower temperatures, including the operating temperature range of MYRRHA (200‐400 °C), revealed that the physicochemical mechanisms of Po evaporation from LBE are different and more complicated than those observed at higher temperatures. In various circumstances, below 500 °C, much larger Po release from LBE was observed than expected on the basis of its behavior at high temperature. Evidence was found that two characteristics affect the increased evaporation of Po from LBE at low temperatures.