A considerable proportion of the UK’s LLW and ILW radioactive waste inventories arise in a range of physical and chemical forms with varying degrees of radioactive / toxic hazard. They include PCM, IEX resins and decommissioning wastes such as concrete, bricks, sludges and contaminated soils. These wastes are considered unsuitable for treatment using existing vitrification and cementation technologies at Sellafield, owing largely to their heterogeneity and variable organic / inorganic compositions. Consequently, alternative thermal treatment technologies to form glass / glass-ceramic / glass-composite wasteforms are being considered and developed to treat these wastes. These include the GeoMelt technology currently being trialled by NNL; also plasma and other technologies. A key issue thermal treatment processing is the (currently) limited ability to (a) monitor off-gas emissions in real-time; and (b) control, constrain or inhibit emissions. In particular, emissions of volatile radionuclides (e.g. 137Cs, 129I, 36Cl, 99Tc, 106Ru) and toxins (e.g. dioxins, furans, flammables) must be minimised and monitored, in order to provide maximum safety and control. Monitoring is currently carried out post-treatment through analysis of gas filtration media, with off-gas calculations performed retrospectively. From the perspective of safety and process control, on-line, real-time monitoring of off-gas emissions, in addition to other thermal treatment parameters such as melt temperature / viscosity, melt pool conditions and melt rate, are key developments needed to present the most robust methodology for safe thermal treatment of UK LLW / ILW.
This PhD project will address these issues in two ways. The primary objective is to develop new, more responsive methods for on-line, real-time monitoring of off-gas emissions. The secondary objective will be to develop new methods of controlling, limiting or inhibiting off-gas emissions of volatile compounds during thermal treatment. The student will carry out a thorough desktop survey and investigate a range of potential on-line monitoring technologies, which may include recently developed THz (Okhoshi et al, Sci. Rep. 7 (2017) 8088) and γ-spectroscopies (Maekawa & Oshima, J. Nucl. Sci. Technol. 55 (2018) 181-189) in addition to more established chemical / process / biological engineering routes. Design and development of new or modified technologies will be carried out at desk-scale then at lab-scale. The multidisciplinary approach taken will be reflected in the supervisory team, comprising experts in radioactive waste vitrification, (bio) chemical and process engineering; and instrumentation and data processing. On-line gas monitoring technologies used in the nuclear industry will be considered; also expertise from across manufacturing, chemical, bio-chemical and food industries will be enabled through the team’s existing industrial links. New developmental technologies will also be considered and modest budget has been included for rental of on-line off-gas sensing technologies for benchmarking and to aid technology development and testing.
The student will establish selection criteria for candidate technologies, and will use them to down-select candidate technologies for development and testing. Lab-scale testing will be carried out in parallel with lab-scale development of methods for reducing off-gas emissions (expected to include glass melt chemistry manipulation, temperature control, physical and chemical controls and addition of inhibitors such as barrier layers). Following initial development and testing at lab-scale, it is envisaged that the most promising candidate technologies, addressing both primary and secondary objectives, will be up-scaled and trialled using inactive and then active facilities. The preferred active facility is the NNL Central Laboratory GeoMelt test facility, where it is expected that the student will receive training and access to active areas to prepare for, and execute, active trials. In addition, discussions are underway with Veolia to access its’ inactive GeoMelt facility in Richland, USA. Other candidate thermal treatment technologies including plasma vitrification will also be explored, to provide further diversity and risk mitigation for management of the project. Close consultation with key stakeholders including RWMD, NDA, NNL, Sellafield Ltd and TRANSCEND consortium members and associates, will be maintained to ensure the wastes, technologies and approaches used provide tangible scientific and process engineering advances and increase the TRL of thermal treatment of UK LLW / ILW. The first 18 months of the project will prepare and train the student with the skills and clearance needed for active area access, and the second 18 months of the project will include preparation, execution and data analysis from full-scale inactive / active trials. A target of 3 experiments at the NNL active laboratory is envisaged (the first for benchmarking and the second and third for trials of candidate technologies and materials), averaging one active experiment per year throughout the project. However, it is acknowledged that any active experiments will need to align with other experiments and demands upon the active facility, so flexibility will be maintained throughout. Close communication with stakeholders will ensure all trials are well-planned in advance.
Academic Lead: Paul Bingham
Researcher: Alex Stone
Location: Sheffield Hallam University