79Se is considered as a key mobile fission product for nuclear waste disposal in the Boom Clay, the reference host formation presently studied in Belgium. Therefore, a good understanding of selenium migration behaviour through the clay barrier is essential to underpin its transport parameters selected for the performance assessment of a deep repository for spent fuel and high-level waste. Under the reducing conditions prevailing in Boom Clay, selenide, [Se(-II)], is the predominant thermodynamically stable chemical form of selenium. However, selenium may suffer of severe redox disequilibrium and selenate is very reluctant to reduction while a kinetically controlled sorption-reduction-precipitation behaviour is observed for selenite.
Due to the large uncertainty on the speciation of selenium in the waste form, and taking the stability of selenate versus reduction into account, it is necessary to consider the non-solubility limited SeO42– as a possible migrating species. However, when a lower oxidation state [Se(IV), Se(0), and Se( II)] is present in the waste form, a solubility limit may also contribute to delay and attenuate the 79Se release. Therefore, two sets of transport parameters are proposed to PA: a first set for selenate (without solubility limit) and a second set for selenide (with solubility limit).
|Number of pages
|Published - May 2010
|Studiecentrum voor Kernenergie