Interaction between iodine and liquid lead bismuth

In this report we give an overview of the behaviour of iodine in an lead-bismuth eutectic (LBE) cooled reactor. We start with the position were most of the radioactive iodine will be formed: within the fuel pin, and study the interaction between gaseous I2 and the two most common stainless steels (SS) in the MYRRHA-reactor: 15-15Ti for the cladding of the fuel and 316L for the structural components. This through experiments performed at Chalmers University of Technology. The experiments show clear deposition of I2 on to the steel, at 500°C even a new material was formed. In the next step we imagine a leak in the fuel pin, leading to experiments that study the combination of iodine and LBE with SS, separated by noble gas (Chalmers). This time no iodine seem to have been transported, suggesting that the presence of LBE within the fuel pin might actually halt interaction between the iodine and the cladding. In experiments where the interface between the LBE and the solid iodine was maximized, the top layer had a golden look at 500°C. As the fuel pin further deteriorates iodine will enter the LBE surrounding the fuel pin, but iodine will also be present in the LBE through spallation. Recently an article was submitted by the Paul Scherrer Institute (PSI) on the behaviour of iodine within the irradiated LBE spallation target MEGAPIE, which describes perfectly where to expect iodine within MYRRHA. It was observed that the bulk LBE is largely depleted of iodine. Actually, most of the expected iodine is found at the LBE-steel interface, but enrichment was also found in a lesser degree at the free surface. These first two parts of the story is what is entailed in task T4.4.1, "Segregation and deposition of iodine from LBE on structural materials". Although only little the expected iodine (0.004%) were recovered from the Au- and Pt-foils, placed in the gas phase above the LBE and only traces were found in the gas phase during the actual irradiation, it is still of primary importance to know precisely how much iodine can be expected in the gas plenum above the LBE. This is exactly the aim of task T4.4.2. To answer this we report on transpiration experiments performed at PSI and at SCK•CEN. The experiments differed at both institutes in sample preparation, measurement methods and were also focussed on different temperature ranges, yet there is clear agreement between both independent studies. It is difficult to dissolve iodine, or lead-iodide, homogeneously in LBE, as surface enrichement has been found at increased temperatures below 500°C. The henry constant for iodine in LBE is also higher than was expected from the vapour pressur of PbI2, the most probable iodine specie above LBE, and from earlier experiments performed at PSI. For the small samples that were used, significant losses of iodine were observed above 400°C.