Isolation of 14C species from spent ion exchange resins and their stabilisation

Lead Research Organisation: University of Sheffield
Department Name: Materials Science and Engineering

Abstract

Organic ion exchange resins are utilised in many different areas of the civil nuclear fuel cycle, from uranium ore concentration and refinement and chemical control of coolant water composition in light water reactors and spent fuel storage ponds, to decontamination of radioactive element-containing effluents arising from fuel reprocessing and nuclear decommissioning operations. These materials are effective "sponges" for a wide range of radioactive elements, hence their widespread use. The UK has stockpiled approximately 600 m3 of spent (i.e., used) ion exchange resins (SIERs), which require disposal, and continues to produce between 2.5 to 13 m3 per year.
The disposal of SIERs is problematic; there are several key issues, which include:

1. The 14C inventory of the materials. This isotope has a half life of 5,730 years and is incorporated as 14CO32- and H14CO3-, which, if allowed to enter the environment are extremely mobile and biologically available. Release of 14C gas in a disposal environment provides a rapid 14C migration pathway to the biosphere;
2. The degradation of SIERs in a disposal environment through radioactive decay processes produces organic complexant molecules, which may facilitate rapid transport of radioactive elements from SIERs to the biosphere;
3. The degradation of SIERs in a storage environment may also yield chemically toxic gases such as benzene, phenol and ammonia, which make storage extremely problematic.

These issues require the SIERs to be treated so as to meet waste acceptance criteria for disposal. This is typically achieved by destruction using thermal or chemical processes. In this proposal, we aim to develop a promising chemical treatment route for the destruction of SIERs, known as wet oxidation.

Wet oxidation has been successfully trialled elsewhere for the destruction of non-radioactive surrogates for SIERs, however, the specific methods previously utilised do not give rise to by-product residues that are amenable to immobilisation in a material suitable for disposal in the UK. We propose two novel approaches to wet oxidation processes that will not only generate by-products more suitable for immobilisation, but that also have a greater destruction efficiency than those previously trialled. Furthermore, we will develop and optimise tailored cement, ceramic and glass waste forms for the immobilisation of SIER degradation. We will provide a robust scientific underpinning of the chemical speciation and local distribution of radionuclides in SIERs and the immobilisation matrices we develop, and understand their behaviour in disposal environments, to support the safe and timely disposal of SIER wastes.

A significant novelty of this research is the verification of our new treatment and immobilisation methods for SIERs using real radioactive materials. After optimisation of the processes described above using inactive SIERs, we will apply them to real radioactive SIER from the UK decommissioning programme. If successful, this work will be a significant step towards demonstrating an effective treatment option for the resin, allowing early site termination of a significant hazard.

Planned Impact

In this collaborative UK-ROK research programme, we will develop a substantive and unique body of knowledge, of international significance, in relation to treatment and disposal of spent ion exchange materials, which have ubiquitous application across the nuclear fuel cycle. The overarching impact arising from this strategy is to achieve sustainable civil nuclear power in the UK and ROK, with public acceptance, and safe cost-effective decommissioning and waste disposal. This research is timely in that it meets a number of the objectives in the UK Government's Nuclear Industry Strategy, as well as the Nuclear Sector Deal, for example, it will ensure "effective waste management and establish collaborations overseas on R&D and innovation" and develop "innovation-led growth" in nuclear decommissioning, which could lead to "a 20 % reduction in costs to the UK tax payer".

The research will deliver specific impact in the following ways:
- Development of knowledge, understanding, capability and experience of wet oxidation processes that could be applied to SIERs and other organic wastes in the UK radioactive waste inventory.
- Building confidence in the applicability of wet oxidation process to ion exchange resins of relevance to the UK radioactive waste inventory, with validation using real radioactive spent ion exchange resin.
- Determination of the expected waste volume reduction factors and demonstration of effective waste treatment of wet oxidation residues, to achieve immobilisation of the radionuclide inventory, including C-14, for final disposal.
- Quantification and understanding of the release mechanisms of radionuclides from the conditioned products under realistic disposal conditions, and its relationship to radionuclide speciation.

These impacts will assist in meeting the strategic aims and research needs of organisations producing SIER waste (e.g. MOD, EDF Ltd., Sellafield Ltd.), organisations responsible for safe disposal of such wastes (RWM Ltd. and LLWR Ltd.) as well as the Nuclear Decommissioning Authority with overall strategic responsibility in this domain. The research will assist in supporting strategic options assessment and decision making with respect to the treatment and disposal of SIERs by these organisations, several of whom, at the stage of submission, are engaged as project partners. Additionally, the development of new radioanalytical methods in the project will develop new markets for instrument applications, supporting the business growth of instrument vendors.

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