Studies on Properties and Aging Behaviour of Corium Generated Under Severe Accident Conditions

During a severe accident, large amounts of volatile fission products, such as caesium and iodine, are released during the core degradation. A significant fraction of the non-volatile ones, including actinides, lanthanides and some of the so-called semi-volatile ones (e.g. barium, strontium), remains trapped in the corium debris. The trapping of low- to non-volatile fission products in corium phases limits the short-term airborne source term during and after the accident. The long term decommissioning and remediation perspectives of the Fukushima Daiichi more than a decade after the accident highlight the importance of adequately assessing the ageing and corrosion behaviour of corium debris and their impact on the “delayed” aqueous source term. In this context, water/corium interaction has to be assessed to be able to estimate the amount and chemical form of fission product that can be released.<br/><br/>In this work, results of ongoing leaching tests on real corium samples from the FPT2 test of the Phébus FP Program are reported. The objective of the FPT2 test was to study the degradation of an irradiated UO₂ fuel bundle and the fission product behaviour under conditions of low steam flow that created reducing conditions. One of the main objectives of the present experiments was to compare to previous tests on TMI-2 corium samples. Indeed considering the “reducing” conditions imposed during the Phébus FPT2 test, a different behaviour of the corium compared to that of TMI-2 could be expected since much more oxidising conditions prevailed during the TMI-2 accident.<br/><br/>Sequential batch leaching experiments at room temperature under ambient atmosphere in static conditions were performed. The composition of the leaching solution was based on the boric acid containing water from the storage tanks (2-2.5g/L). The used vessels were rinsed with 1 M HNO₃ at room temperature. Subsequently, the leachates and the rinse solutions were analysed using ICP-MS to quantify the release kinetics of the fission products and the actinides. Results are discussed in terms of Fraction of Inventory in the Aqueous Phase (FIAP) and dissolution rates, and compared to similar experiments carried out on TMI-2 corium and on LWR UO₂ and MOX spent nuclear fuels. In addition, preliminary surface area characterisation by Raman spectroscopy was carried out.<br/><br/>The present experimental results are part of a programme dedicated to the analysis and the management of LWR after severe accidents. In particular, they will be used to support decommissioning and remediation strategies for the Fukushima Daiichi nuclear power plants.