Evaporation of polonium from lead-bismuth eutectic nuclear coolant

The neccessity of finding a suitable solution for the disposal of radioactive waste has led to a growing interest in the last few decades in the development of accelerator-driven systems (ADS) which could efficiently perform the transmutation of this waste transforming it to shorter-lived, less hazardous radioactive species. 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 coolant is the formation of large quantities of 210Po due to neutron capture reaction with bismuth (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. First, the oxygen content in the LBE seems to strongly affect Po evaporation. In experiments carried out with oxygen-saturated LBE samples, covered by a thin oxide layer, a fraction of Po is released very quickly. However, this rapid release was transient. It was shown that this fast release was due to a fraction of Po contained in the oxide layer at the surface of the LBE samples. It was also found that in addition to this fraction of fast-evaporating Po present in the oxide layer, the majority of the Po, present in the bulk of the LBE, evaporated according to the established high-temperature correlations for the Henry constant of Po in LBE. Secondly, the composition of the carrier gas above the LBE sample is another crucial factor for the evaporation of Po from LBE at low temperatures. The release of Po strongly increased in the presence of hydrogen and water vapor. An important finding was that the presence of water vapor not only resulted in an enhanced Po release but also the volatility of the formed gaseous Po species significantly increased. These results are highly important for the design and licensing of MYRRHA in which the cover gas in contact with LBE may contain hydrogen (e.g. during LBE conditioning) and water (e.g. after a steam generator tube rupture accident). Moreover these results emphasize the need of suitable filters for the immobilization of volatile Po compounds in MYRRHA. Exploratory capture studies revealed that traditional adsorbent materials such as activated charcoal are highly effective for the immobilization of even the most volatile Po molecules. The adsorption of these volatile compounds on the construction material of the MYRRHA reactor, stainless steel, was also studied. Interestingly the results also revealed that this material is a highly effective adsorbent for the volatile Po compounds which are formed in the presence of water vapor. This result opens up opportunities for the development of simple filters based on stainless steel that may contribute to the control of the Po problem in MYRRHA.