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Magnox sludge immobilisation in bespoke acid-based geopolymers

About This PhD Project

Project Description

Disposal of magnesium-bearing intermediate level waste sludge arising from reprocessed spent fuel from Magnox reactors in the UK presents significant decommissioning challenges. This project aims to develop bespoke acid based geopolymer cements for the disposal of Magnox sludge. Specific objectives are to develop a suite of acid-based geopolymer mixes with high contents of Magnox sludge simulants, elucidating correlations between mix design parameters, microstructural features development, and their pore structure evolution, as well as their structural stability under conditions relevant to a Geological Disposal Facility (GDF).

The acid-based geopolymers to be created will be produced using UK calcined clays, in combination with other additives. Fresh state properties (e.g. hardening time, rheology and reaction kinetics) will be assessed. In hardened grouts, detailed nano and microstructural characterisation will be carried out to determine factors controlling the binder’s ability to immobilise Magnox sludge components. This will be achieved by applying advanced characterisation techniques including microscopy, spectroscopy and x-ray imaging.

The successful applicant will work in the UKCRIC Centre for Infrastructure Materials at the University of Leeds. You will have access to brand new laboratories for the development, characterisation and performance assessment of cementitious materials, including access to our own spectroscopy and microscopy facilities including a unique X-ray microtomography instrument for in-situ monitoring of pore structure changes and microcraking under controlled environments (e.g. temperature, gas atmosphere, humidity).

New Knowledge

The main plan for the disposal of legacy Magnox sludge in the UK is via cementation, using conventional grouts based on blends of blast furnace slag or fly ash with commercial Portland cement. However, the amount of Magnox sludge that can be encapsulated in these cementitious systems is relatively low. This can have significant consequences on the cost of the final disposal in a Geological Disposal Facility.

In a novel approach, this project seeks to use the Magnox sludge as part of the binder, by producing acid-based geopolymer cements. Conversely to conventional geopolymers, which are produced using highly alkaline solution, acid-based geopolymers are manufactured via a chemical reaction of phosphoric acid and an aluminosilicate source (e.g. calcined clays). The chemical environment achieved in acid based systems can promote high dissolution of Mg(OH)2 (main component of Magnox sludge). The dissolution of Mg(OH)2 will promote formation cementitious phases rich in magnesium, making the sludge a fully integrated part of the grout.

Developing New Skills

Geopolymer materials based on calcined clays provide highly desirable performance in the immobilisation of heavy metals, and key radionuclides including 137Cs and 90Sr. These materials have been identified by the International Atomic Energy Agency (IAEA) as being attractive for waste immobilisation, but this requires detailed testing for full validation.

Specific geopolymer formulations have been commercialised for radwaste treatment including as a sealing and dust-reduction agent in construction of the sarcophagus protecting the damaged reactor core in Chernobyl, and radioactive waste treatment in the Czech and Slovak Republics, and in Germany. The French nuclear agency CEA is active in development in this area, for immobilisation of oily and metallic wastes, and the Australian nuclear agency ANSTO has published extensively on geopolymers. Extensive research is currently conducted in the UK on this topic by academia and industry.

Additional to the skills gained by the PhD researcher and supervisory team in developing these novel geopolymer cements, we will apply a multi-technique approach for the nano- and microstructural assessment of these materials. We will taking full advantage of the new opportunities that the Bragg Centre of Materials Research brings in terms of access to world-leading microscopy technique (e.g. cryo STEM-EDX which is ideal for the analysis of beam sensitive materials, such as the ones to be produced in this project), and also the UKCRIC Centre for Infrastructure Materials, in the School of Civil Engineering. We will particularly aim to develop detailed studies of pore structure evolution of the grouts to be developed using our own X-ray microtomography instrument, where the conditions during testing (e.g. temperature, humidity and loading) can be modified.

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