Densification of Nuclear Waste Incinerator Ashes via Hot Isostatic Pressing

The utilisation of Hot Isostatic Pressing (HIP) for the safe, efficient densification of loose simulant radioactive ashes has been investigated. As part of a multinational project looking at pre-disposal thermal treatment of organic material (PREDIS), the University of Sheffield is collaborating with CEA to determine the wasteform characteristics of specific HIPed ashes.<br/><br/>These ashes arise from processing organic materials surrogates (simulating materials contaminated by α-emitting actinides) within the IRIS process (Installation for Research on Incineration of Solids) in France. It is a multi-step process able to treat high chloride containing wastes via a combined pyrolysis and calcination process. Simulant inactive ashes arising from the IRIS process are comprised of a calcium-zinc aluminosilicate rich material, with a very low level of residual carbon – making these ashes ideal candidates for HIP processing.<br/><br/>HIP trials were undertaken by hydraulically compacting IRIS ashes into stainless steel cans, with no additives or additional components, resulting in a 100 % waste loading. These cans were then processed at 1250 °C with a maximum pressure of 100 MPa. The resultant material was a densified, solidified wasteform, safely contained within the stainless steel canister. Post-processing characterisation revealed the formation of a polycrystalline material consisting of anorthite, diopside, chlorapatite, leucite, and spinel. Wasteform-can interactions were observed as a result of the thermal processing, leading to some internal can corrosion and a chromium rich altered wasteform interface abutting the can wall.<br/><br/>These trials have demonstrated the suitability of HIP towards the processing of such ash materials, resulting in a solidified product with a substantially reduced volume. Though forming a solid product, substantial porosity remains within the final product, creating potential for wasteform improvements. Further wasteform optimisation is ongoing to investigate the impact of inclusion of glass forming additives to lower the wasteform melting temperature and reduce wasteform-can interactions, along with studies into the long-term aqueous durability of these materials.