Conditioning of Cs/Sr and rest waste fraction in Synroc and waste form durability: ASOF WP3, Deliverable D3.1.1, D3.1.2, D3.2.1 and D3.2.2

The separation of minor actinides and short-lived fission products from spent nuclear fuel as studied in the ASOF project results in the production of a waste stream containing the heat-emitting Cs and Sr, and a rest fraction waste stream containing the other fission, activation products and small amounts of lanthanides and actinides. In WP3 of ASOF, the immobilization of these waste streams was investigated using thermal and non-thermal treatment routes. This report presents the results for the thermal route. Synroc, a composite ceramic waste form composed of four titanate minerals, was proposed for the conditioning of both waste streams. Synroc batches to immobilize the Cs/Sr waste stream with a low (5 wt.%) and high (9 wt.%) waste loading were produced at the University of Sheffield (UK). Batches for conditioning of the rest fraction waste stream with a waste loading of 18 wt.% or 30.5 wt.% were synthesized at ANSTO (Australia). The chemical durability of the Synroc samples was studied at SCK CEN by performing leaching experiments with a KOH solution at pH 12.5 and 90 °C using a sample surface area to solution volume of 100 m-1. To compare the results with the leaching behavior of vitrified waste, simultaneous tests were performed with SON68 glass, which is the inactive reference composition of the R7/T7 glass used for the immobilization of spent fuel reprocessing waste from the Belgian nuclear reactors. The leaching tests suggest that the release rate of safety relevant radionuclides (Mo-93, Cs isotopes, Sr-90) would be one or two orders of magnitude lower for Synroc than for a well performing waste glass like SON68.