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.
 
Description This project is focused primarily on the development of wet oxidation technology for treatment of spent ion exchange resins (SIERs). Radioactive wastes of this type are unbiquitous in the civil nuclear industry, being utilised for decontamination of aqueous effluents. SIERs pose a challenge for radioactive waste management since they present a dispersible and flammable hazard, may release their radioactive inventory by leaching, and undergo radiolytic degradation. Furthermore, the long lived and mobile C14 content of these resins presents a challenged for their disposal. Wet oxidation presents a potential low cost method for destruction of SIERs, enabling volume reduction and capture of radionuclides in a sludge product suitable for cement encapsulation or thermal treatment.
The project is at an early stage, and progress delayed by the impact of the Covid 19 pandemic, but has made some progress toward achieving its objectives. A comprehensive review of the available literature was undertaken, in collaboration with project partners in POSTECH, to understand the current state of the art and knowledge gaps. This review was submitted for peer reviewed publication. A wet oxidation rig was designed, components procured, assembled and commissioned to enable systematic studies of reaction kinetics as a function of temperature, pH, oxidant concentration, and catalyst concentration. These studies have laid the foundations for a mechanistic understanding of the degradation resins and the important role of functional groups in controlling the reactivity of the resin toward different catalysts. Early results from a joint investigation with our partners at POSTECH, established optimal conditions for wet oxidation of fresh IRN-150 resin, applied in Korean nuclear power plants, achieving up to 92% destruction and removal of 85% of total organic carbon; this work also evidenced the important role of the carbonate radical in inhibiting the wet oxidation reaction (derived from surrogate HCO3-, the primary source of 14C in real SIERs). This work produced a joint publication - J. Env. Chem. Eng., 9 (2021) 104740. These results led our work toward optimisation of a one pot wet oxidation method for mixed SIERs, which are commonly applied in the UK civil nuclear industry. In contrast, previous studies have generally considered only one type of cationic or anionic resin, which has limited confidence in the application and scale up of the approach to real mixed SIER wastes. For this purpose, the project has also successfully acquired real, partially degraded, mixed SIER low level wastes from a nuclear site licencee which will be used to validate the efficacy of our optimised one pot approach. If successful, the research may yield considerable impact in underpinning treatment of these wastes, reducing the volume disposed to the low level waste repository by ca. 90%, with considerable cost savings.
The project attracted interest from Europe, leading to award of a collaborative EC Horizon 2020 consortium grant, PREDIS. This grant provides resource to further develop the research undertaken in this project and a gateway to knowledge exchange with European waste producers.
The project was supported by bi-lateral visits in 2019.
Exploitation Route To validate the research outcomes, the project has successfully acquired real, partially degraded, mixed SIER low level wastes from a nuclear site licencee which will be used to assess the efficacy of our optimised one pot wet oxidation approach. If successful, the research may yield considerable impact in underpinning treatment of these wastes, reducing the volume disposed to the low level waste repository by ca. 90%, with considerable cost savings.
Sectors Chemicals,Energy,Environment,Government, Democracy and Justice

 
Description (PREDIS) - PRE-DISposal management of radioactive waste
Amount € 23,773,743 (EUR)
Funding ID 945098 
Organisation European Commission 
Sector Public
Country European Union (EU)
Start 09/2020 
End 08/2024
 
Description NSLS-II collaboration exploitation and development of X-ray Emission Spectroscopy 
Organisation Brookhaven National Laboratory
Department National Synchrotron Light Source
Country United States 
Sector Public 
PI Contribution This collaboration has developed valence-to-core X-ray emission spectroscopy of the transition metals with the first reported application to niobium. The valence to core region of the XES provides a fingerprint for ligand co-ordination species and is thus a particularly powerful non-contact technique for application to radiological materials. Our team prepared high quality XES specimens of air sensitive reference compounds for investigation by XES and XAS techniques at the Advanced Photon Source and collaborated on data analysis. The techniques developed in this collaboration are a forerunner for application to technetium, which is considerably more challenging given its high specific activity.
Collaborator Contribution Our partners secured beamtime at the Advanced Photon Source and developed bespoke data reduction tools for extraction of the XES and valence to core region, utilising a novel detector system, and also contributed Density Functional Theory calculations to provide theoretical underpinning to data interpretation.
Impact The research resulted in a peer reviewed publication, as a collaboration across the domains of materials science and physics, published as Ravel et al., Phys. Rev. B., 97, (2018), 125139.
Start Year 2017
 
Description PNNL 
Organisation U.S. Department of Energy
Department Pacific Northwest National Laboratory
Country United States 
Sector Public 
PI Contribution Investigation and understanding of phase separation and crystallisation in glass-ceramics for immobilisation of high level radioactive waste from future fuel cycles.
Collaborator Contribution Investigation and understanding of phase separation and crystallisation in glass-ceramics for immobilisation of high level radioactive waste from future fuel cycles.
Impact See publications section.
Start Year 2015
 
Description POSTECH University 
Organisation Pohang University of Science and Technology
Country Korea, Republic of 
Sector Academic/University 
PI Contribution Pohang University of Science and Technology (POSTEC) are the collaborating partners on our joint UK / ROK project. Our contribution to the joint research programme is the development of ceramic wasteforms for the immobilisation of key radionuclides arising from advanced nuclear fuel reprocessing (I, Tc, Cs, lanthanides and actinides). We have developed a new capability for Tc materials chemistry in the UK and a unique radiological hot isostatic pressing capability to support the joint research. We have hosted several secondments from POSTEC to carry out joint research and train collaborators in advanced materials characterisation techniques (e.g. Quantiative Phase Analysis from X-ray difffraction data, using the Rietveld method).
Collaborator Contribution Pohang University of Science and Technology (POSTEC) are the collaborating partners on our joint UK / ROK project. They have developed complementary development of glass wasteforms for the immobilisation of key radionuclides arising from advanced nuclear fuel reprocessing (I, Tc, Cs, lanthanides and actinides). In particular, POSTEC counterparts have developed a suite of aluminoborate glasses which show high solubility for lanthanide oxides (Ln2O3), which were transferred to our laboratory for investigation of dissolution behaviour.
Impact Several joint research publications have arisen from this research as detailed elsewhere in this submission, further joint publications are being developed.
Start Year 2015