Alternative Processing Methods for the Thermal Treatment of Radioactive Wastes

The UK has large volumes of radioactive materials which are classified as Intermediate Level Waste (ILW). The baseline treatment for these wastes is encapsulation via cementation, however, this method is not ideally suited for numerous wastes, both in the UK and globally. Alternative thermal processing methods for these materials may be capable of producing wasteforms with improved properties. This thesis presents a series of scoping studies on the thermal treatment of a diverse range of ILWs in order to identify the potential benefits and pits falls of such processes. The wastes selected were Tri-Structural Isotropic (TRISO) Fuel Particles, Prototype Fast Reactor (PFR) raffinate, SIXEP sand/clinoptilolite ion exchange materials and SrTreat® Ion exchange material. The scoping studies performed showed promise for the thermal treatment of all selected waste streams. A summary of the main results for each waste stream are provided below; TRISO Fuel Particles: Immobilisation focused on encapsulation of the particles in highly durable glass matrices. Alumino-borosilicates were determined to be the most effective glass composition for the production of composites, in terms of both their physical and chemical properties. The ability of Hot Isostatic Pressing (HIPing) to improve composites was investigated. Unfortunately, this was shown to result in severe fracturing within the composite. This was hypothesised to occur as a result of excessive pressurisation during the cooling cycle. The HIP process did show some benefits in terms of particle coating properties and with small alterations to the HIP cycle utilised it should also be possible to eliminate the detrimental fracturing features. PFR Raffinate: The vitrification of PFR raffinate was investigated using the G73 glass composition, a glass which has been previously proposed for the immobilisation of other ILWs. This glass was proven to be capable of accommodating a waste loading of up to 20 wt% PFR raffinate. The glass produced was homogeneous with good waste retention, had no noted crystal formation, an aqueous durability comparable to currently employed HLW glasses and the thermal characteristics necessary for industrial scale up. Further study should be performed on the ability of this waste to retain Cs during processing and in aqueous solution. SIXEP Sand/Clinoptilolite Waste: SIXEP sand/clinoptilolite was processed at 95 wt% with a 50 % volume reduction by HIPing. The waste produced was a phase separated glass-ceramic. The wasteform was deemed to be suitable for ILW immobilisation and had an exceptional Cs ii retention in aqueous solutions. However, the presence of an alkali earth sulphate phase increased the Sr release to solution. Attempts to qualify the suitability of this wasteform for disposal, developed methodologies to investigate the properties of phase separated materials. A combination of vertical scanning interferometry (VSI), dissolution experiments and SEM imaging was shown to be capable of elucidating the dissolution behaviour based upon compositional variation. SrTreat®: SrTreat® was processed at 100% waste loading via HIPing. This aim was to investigate the potential for developing ion exchange columns which could subsequently be HIPed, as such, providing a complete waste treatment solution. The HIP process produced a monolithic, mixed phase sodium titanate ceramic. This ceramic was formed by the sintering of individual grain structures and retained the compositional variations seen in the granular waste stream. The wasteform was porous around the grain edges, determined to occur as a result of carbonate formation prior to HIPing. The carbonation of this material is likely to limit the potential to utilise HIPing as a disposal methodology for these wastes. However the aqueous dissolution behaviour of these wastes was still favourable and the process was shown to create a significant reduction in waste volume. The work performed in this thesis has shown that various methods for thermal treatment can be rapidly investigated to determine the potential benefits and pit falls. The application of thermal treatments was shown to be capable of producing significant improvements in wasteform quality by comparison with the cementitious alternatives.