Immobilisation of toxic and radioactive waste in phosphate glasses

This project concerns the development of new and novel phosphate-based and phosphate-containing glasses for waste vitrification. Current topics are:

i) Investigation of the effects of glass composition and preparation conditions on the solubility, speciation and structural environment of "problem" waste components including plutonium and sulphur.

ii) Study of composition / structure / property relations in novel phosphate and silicate glasses for radioactive waste immobilisation.

iii) Environmentally-friendly re-use opportunities for waste materials in commercial glassmaking.

This project is examining synthesis and characterisation of zirconolite (CaZrTiO₇), pyrochlore (Gd₂Zr₂O₇) and monazite (GdPO₄) ceramic matrices for the immobilisation of PuO₂, using Ce⁴⁺ as a stimulant species for Pu⁴⁺.

Processing is being optimised to enable development of ceramics suitable for disposal. Crystallisation of ceramic phases in CaO-Al₂O₃-SiO₂ based glasses are also being examined. The main ceramic phases of interest are zirconolite and pyrochlore. The glass and crystal morphologies, waste species incorporation and properties will be studied using a range of characterisation techniques.

Dr Nick Collier

Immobilisation of Intermediate Level Nuclear Wastes in Cement – Keeping the Nuclear Option Open (KNOO) Project

A number of the legacy wastes in the UK arising from reprocessing of nuclear fuel cannot be readily immobilised prior to disposal. This KNOO project addresses the disposal of specific waste streams and the development of novel cementing systems. Two aspects are being investigated:

1) The immobilisation of Magnox sludges resulting from reprocessing Magnox fuel rods. Using a simulant waste material, reactions within appropriate cementing media are being investigated to select the most appropriate formulation of immobilisation in terms of chemical stability and wasteform durability whilst optimising processing parameters.

2) Composite cements where secondary cementitious materials are used to partially replace ordinary Portland cement are being developed to provide new immobilisation matrices for intermediate level nuclear waste (ILW). Present work focuses on the activation of composite cements made up of blast furnace slag (BFS).

Gillian Parkin

Vitrification of “oldside” high activity liquors

Supervisor: Dr Neil Hyatt

This project aims to investigate the feasibility of immobilising High Level Waste (HLW) produced during early reprocessing of low burn up Magnox fuel. The presence of high levels of transition metal oxides is expected to result in a viscous melt at high waste loadings (≥ 25wt %) and the formation of transition metal oxide spinel phases which may result in potentially problematic heel formation on a large scale. Therefore the key aim is to optimise the process to allow a high waste loading with acceptable processing conditions.

The vitrified product will be characterised by XRD and SEM/EDS to establish chemical homogeneity and identify crystalline phases. The durability of the compositions will also be assessed through the PCT test to establish short term dissolution rates and the identity of alteration phases formed.

NF-PRO Joint Integrated Test with SCK•CEN Belgium

Supervisor: Dr Neil Hyatt

This project is part of the European NF-Pro framework of tests and is investigating the interaction between a current Magnox Blend Glass and the near field in a repository environment. The project is looking at the dissolution of the glass when placed in contact with canister corrosion product and backfill clay. The experimental set-up allows monitoring of the water composition at the glass surface and solid analysis of the near field layers.

Additionally long-term PCT tests are being conducted on the same glass to confirm secondary phase formation and dissolution rates.

Daniel Reid

Adrian Covill

Novel Encapsulants for ILW in the UK Nuclear Industry

Supervisors: Dr Neil Hyatt, Dr Nicholas Collier and Dr Joanne Hill (AMEC)

This EngD project comprises two distinct areas of waste management with two distinct aims, firstly to evaluate the potential of a novel cementing system being magnesium phosphate cement to encapsulate problematic ILW wastes in the UK, and secondly to assess the chemical durability of a novel GeoMelt glass formation intended for the immobilisation/encapsulation of ILW in the UK.

Large amounts of problematic wastes arising from early power generation, for example wastes containing metals such as aluminium, uranium etc, cannot be effectively isolated using conventional cement systems. Conventional cements would corrode these metals creating issues with long-term stability. A cement system that may provide the long-term stability needed for encapsulation could be magnesium phosphate cement. The main technical objective is to investigate the suitability of two magnesium phosphate cement formulations to encapsulate natural uranium. Assessments of suitability will be based upon the reactions that occur between the cement and uranium.

Leach tests such as MCC-1 (monolithic test) and PCT (powder test) are commonly used to investigate chemical durability of glasses. Soon after repository closure the ILW vaults are expected to become re-saturated. The ILW vaults will be backfilled with a cement material. This cement would condition any passing groundwater to a pH of at least 12 via leaching of calcium hydroxide. The main technical objective is to investigate the behaviour of GeoMelt in a Ca(OH)₂ solution which would resemble that of a re-saturated ILW repository.