Analysis of the gas phase chemistry of impurities in the vapor above lead-bismuth
During operation of MYRRHA [1,2], radionuclides are produced in the lead-bismuth eutectic coolant by nuclear reactions. In addition, radionuclides formed in irradiated fuel may be released into the coolant in case of leaks in the fuel cladding. Quantitative understanding of the evaporation of these radionuclides from the LBE is required for the safety assessment of MYRRHA. The source term of evaporated radionuclides must be known as accurately as possible in order to predict the short and long-term health and environmental effects of the release during various postulated accidents.
State of the art
The retention of radioactive material in heavy metal coolants such as LBE strongly differs from that in water. It is generally considered that heavy liquid metals form a good barrier for radionuclide release and therefore have a significant mitigating effect on the source term . In addition to their different retention properties, heavy metal cooled systems also lead to the production of a number of radioactive elements not typically encountered in water cooled reactors. Importantly, significant amounts of polonium will form when LBE is used as coolant . When the heavy metal is also used as spallation target in accelerator driven systems, a broad range of additional radioactive elements is formed . Since all these elements are formed in the primary coolant, the accurate assessment of the retention properties of the coolant in such HLM based systems is a critical step in the determination of the source term. Although in safety analyses one could in principle make the conservative assumption of complete release, this assumption in many cases leads to unacceptable radiological consequences. Therefore the release needs to be quantitatively known.
In a first approximation, one can assume for source term calculations that the release of radioactive elements from LBE/Pb is governed by an equilibrium that effectively exists between a single, highly diluted dissolved species and a single vapor species. The partial pressure of the radioactive element is then given by Henry’s law, using a simple correlation for the Henry constant Kh, which only depends on temperature T, and the concentration of the dissolved element in the LBE. In the 2015 edition of the OECD-NEA handbook  on HLM technology, such correlations are recommended for a selection of important radionuclides including the mobile fission products I, Cs and Te dissolved in LBE. These correlations, typically of the form log Kh = A/T+B, were in most cases derived from the analysis of experimental results obtained by the so-called transpiration method. A significant drawback of this method, and the derived Henry constant correlations, is that it does not give any information on the nature of the molecular composition of the impurity vapor in equilibrium with LBE.
Recently at SCK-CEN, a comprehensive thermochemical modeling approach was developed for a system consisting of an LBE, gas and solid phases with impurities . This model was applied to re-evaluate empirical Henry constant correlations obtained by the transpiration method. The output of the model provides, besides the total vapor pressure, the molecular composition of the vapor. The approach is currently used to obtain best estimates of the release of radionuclides for MYRRHA in various conditions which are not necessarily covered by experimental campaigns.
 Abderrahim, H.: Contribution of the European Commission to a European Strategy for HLW Management Through Partitioning & Transmutation. In: Nuclear Back-end and Transmutation Technology for Waste Disposal; Springer Japan, 2015; p 59-71.
 Fazio, C.; Sobolev, S.P.; Aerts, A. et al., OECD Handbook on Lead-bismuth Eutectic Alloy and Lead Properties, Materials Compatibility, Thermal-hydraulics and Technologies, Nuclear Energy Agency Organisation For Economic Co-operation And Development, 2015.
 Gonzalez Prieto, B.; Lim, J.; Rosseel, K.; Martens, J. & Aerts, A. Polonium evaporation from liquid lead-bismuth eutectic with different oxygen content Journal of Radioanalytical and Nuclear Chemistry, Springer Netherlands, 2015, 1-9.
 Aerts, A.; Modeling the release of radionuclides from irradiated heavy liquid metals, 1st IAEA workshop on challenges for coolants in fast spectrum neutron systems, IAEA, Vienna, July 2017.
In order to better validate the thermochemical model of MYRRHA, it is proposed that the molecular composition of impurity vapor above lead-bismuth should be determined experimentally. Most promising are mass spectrometry methods. This will result in improved analysis of existing experimental results obtained by the transpiration method and will lead to more accurate predictions of the release of radionuclides from LBE in MYRRHA.
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