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This article summarizes the results obtained studying the interaction between polonium and stainless steel 316L in different atmospheres by the thermochromatography method. This issue is particularly important in terms of licensing of the MYRRHA prototype ADS reactor, where highly radiotoxic α-emitting polonium isotopes are produced in lead-bismuth eutectic used as both spallation target material and coolant, while stainless steel 316L is going to be the main structural material. The polonium adsorption enthalpy on stainless steel 316L in inert atmosphere was measured as −147±6 kJ mol−1. The findings obtained using reducing atmospheres show that the affinity of polonium for stainless steel 316L increases with the metallic character of the stainless steel surface. The presence of moisture in inert and reducing atmosphere does not significantly influence the polonium adsorption. Oxidizing atmospheres induce the formation of polonium dioxide which has a stronger interaction with the stainless steel 316L, with an adsorption enthalpy of −225±6 kJ mol−1. A prolonged exposure of the stainless steel to strong oxidants, such as pure oxygen, reduces the metallic character of the stainless steel surface and, consequently, its affinity to the polonium oxide. Furthermore, it was found that in all experiments at least 5 ppm of the total polonium present in the starting material were not retained on the stainless steel 316L surface at all.

Oxygen control is one of the key technologies to limit corrosion or coolant oxidation in lead bismuth eutectic cooled nuclear reactors. In this paper, we show that a stirred bubble column reactor can be an efficient tool for addition and extraction of dissolved oxygen from lead-bismuth eutectic. The design criteria and the resulting layout of the HELIOS3 reactor are detailed and typical results obtained are described. Dissolved oxygen extraction rates are only given at high oxygen activity, as dissolved corrosion products and oxides can strongly influence the rate of addition or extraction of dissolved oxygen at low oxygen activity.

Qualitative and quantitative understanding of Po volatilization under different conditions is of key importance for safety assessments of lead-bismuth eutectic (LBE) based nuclear reactors, spallation targets and accelerator driven systems. In this work we explore the possibilities of the transpiration method in combination with simple models to study the equilibrium and kinetics of Po evaporation from highly diluted solutions in lead-bismuth eutectic between 600 and 1000 ℃ in Ar/5% H2 and Ar. On the basis of evaporation experiments at various carrier gas flow rates, we identified the conditions of vapor saturation allowing the determination of equilibrium constants. From the limiting behavior at high flow rates, values for the maximal evaporation rate of Po from LBE were estimated. Measurements of evaporation as a function of time were consistent with the assumption that polonium dissolved in LBE obeys Henry's law. A theoretical analysis furthermore suggested that diffusion of polonium in LBE was not a rate limiting factor for evaporation under vapor saturation conditions. Newly determined values for the Henry constant of Po in LBE between 600 and 1000 ℃ were consistent with previously derived correlations.

The evaporation behavior of polonium as one of the most hazardous radionuclides produced in spallation based neutron sources with liquid lead–bismuth targets has been quantified in this study. The normalized apparent vapor pressure, i.e. the Henry constant of polonium over liquid lead–bismuth eutectic was determined in the temperature range relevant for operation of such targets, i.e. 164–500 °C. For comparison and better fundamental understanding, the Henry constant of polonium over pure liquid bismuth was determined in a temperature range of 300–500 °C. The Henry constants of polonium in this temperature range were found to be orders of magnitude higher than expected from earlier studies at higher temperatures. Possible mechanisms responsible for this unexpected behavior are discussed.