Investigation of the Thermal Treatment of Higher Activity Waste

Considering the high overall costs of radioactive waste disposal and the growing requirements for improved quality of the final waste form, the benefits offered by thermal processing become very significant. Key drivers for the application of thermal treatment processes include the reduced volume, improved passive safety, and superior long term stability of the vitrified wasteform products. Currently, a fundamental lack of scientific knowledge and understanding significantly hinders the uptake of thermal treatment processes for the immobilisation and disposal of plutonium contaminated material waste. The project will contribute to accelerating the acquisition of knowledge and experience required to support the Nuclear Decommissioning Authority (NDA) in deploying thermal technologies as a national asset for intermediate level waste (ILW) treatment. Plutonium contaminated materials (PCM) waste is a sub category of ILW. The current treatment method for PCM is supercompaction and cement encapsulation, however, there are significant concerns regarding the reliability of the treatment method to consistently deliver a waste form suitable for long term performance. This project follows on from previous work which provided proof of concept studies for thermally treating PCM waste. This work uses a soda lime silica (SLS) glass cullet as the glass forming additive to aid vitrification, providing substantial benefits in terms of costs saving compared to previous work. The thermal treatment experiments showed no violent reactions between the waste simulant and the glass additives. The Ce (acting as a Pu surrogate) was effectively partitioned within the slag fractions of the waste form, with crystalline regions present in certain formulations. The Ce was found as trivalent species providing confidence that the slag component of the wasteforms developed here could incorporate Pu at the concentrations expected from treatment of PCM wastes. The materials produced here are broadly comparable, in terms of durability, to other simulant UK ILW glass products considered potentially suitable for geological disposal. The project also investigates the potential to use the glass forming oxides found within the ILW itself to aid vitrification. Using suitable ternary phase diagrams, it is possible to use the waste and any additional additives to create a formulation to ensure a glass is successfully formed at a reasonable operating temperature whilst always maintaining compatibility with currently available technology platforms. This method was utilised to vitrify representative pond scabbling waste. The simulant waste contained high levels of SiO2 which, in combination with glass forming oxides, were successfully vitrified. Characterisation studies were performed to understand the relationship between Na2O and B2O3 and the effect this had on the microstructure of the resultant structure of the glass. Glass dissolution experiments were also performed to test the performance of the glass in conditions expected within a geological disposal facility (GDF). The thesis also includes the results of samples acquired from a three month student placement working with Kurion’s geomelt facility at the Workington site (UK). Contained within this PhD are experiments which aim to provide significant information into the mechanism that drive glass alteration. However this data has only been applicable to short-term alteration. The research presented in Chapter 7 aims to understand the long term mechanism of the vitrified PCM waste by using 250 year old slag samples as a glass alteration analogue. The results presented provide evidence for the long term durability of the vitrified PCM waste samples.