Cutting decontamination costs

Above: Sellafield is working on radionuclide, mineral and microbial behaviour in waste pond environments, and radionuclide and mineral behaviour during effluent treatment processes (courtesy of Sellafield Ltd)

Work involving Professor Katherine Morris at the University of Manchester Dalton Nuclear Institute reduced both the cost of decommissioning at Sellafield and discharges into the Irish Sea. The research impact was cited in the university’s Research Excellence Framework submission.

The Dalton Nuclear Institute at the University of Manchester is associated with the Sellafield Effluent and Decontamination Centre of Expertise (E&DCE), supported by Sellafield Ltd and the UK’s National Nuclear Laboratory. It carries out research into management and treatment of radioactive effluents. Formed in 2012, the E&DCE’s aim was to “provide fundamental understanding of the underlying processes impacting on effluent management... which in turn provides direct cost savings and risk reduction and therefore increases stakeholder confidence in operational activities”. Technical meetings are held between the partners to plan research and discuss results as they are obtained. It has been the route to transfer state-of-the-art research methodologies from academia (including Transmission Electron Microscopy, preparation methods and microbial ecology characterisation) directly into industry.

Kath Morris is Professor of Environmental Radiochemistry and Environment and Waste Lead at the Institute. The team explored radionuclide, mineral and microbial behaviour in waste pond environments, and radionuclide and mineral behaviour during effluent treatment processes.

The research findings were shared between industry and academic partners, enabling Sellafield Ltd to improve efficiency and safety. Talking about how input from partners at Sellafield and NNL help translate fundamental research into practical actions on site, Morris told NEI: “The careful exchange of information across the Centre of Expertise, with frequent meetings between academic researchers and subject matter experts has really enabled optimised translation of research from the lab scale through to plant scale.”

What is the process for deciding on what projects to take forward? Morris says, “this is all about communication of challenges by the subject matter experts coupled to careful listening and refinement of research ideas across the Centre of Expertise, which has led us to define and refine successful projects together”.

The group’s work was cited in the Research Excellence Framework, the UK’s five yearly assessment of impact made by its higher education and research organisations. The Framework’s aims include providing accountability for public investment in research and produce evidence of the benefits of this investment; and to provide insights into the health of research in the UK’s higher education institutions. The citation is a testament to the impact of the E&DCE between September 2015 and July 2020. Its outcomes included:

• Reducing radioactivity within the effluent treatment system at Sellafield’s legacy ponds by more than 69%, with an estimated cost saving of more than £10 million (US$12.6 million).
• Microbial growth research that informed in-pond treatment settings and enhanced fuel retrieval operations enabling a 40% increase in fuel retrieval operations, with savings of around £2 million (US$2.5 million).
• Research into iron oxide floc formation and binding with radionuclides, resulting in optimised treatment protocols for radioactive reprocessing effluents, allowing Sellafield to achieve a 50–90% reduction in actinide discharges during targeted periods of plant operations and significantly reducing alpha radioactivity discharges to the Irish Sea.

The challenge at Sellafield

At the UK’s complex and challenging Sellafield site, operations and decommissioning are under way simultaneously. The work addressed two types of radioactive effluents that are generated on site. Firstly, acidic radioactive effluents, from reprocessing spent nuclear fuel and the subsequent clean-up of legacy reprocessing facilities, are treated in the enhanced actinide removal plant (EARP). Secondly, alkaline to neutral radioactive effluents from the highest nuclear risks and hazards at Sellafield – its legacy ponds and silos – are treated in the Site Ion Exchange Plant (SIXEP).

Since 2013 a programme of research has covered three areas:

• Iron oxide flocculation and radionuclide removal during reprocessing effluent treatment.
• The EARP process neutralises the acidic, radioactive effluents to form an iron floc that scavenges the radionuclides from solution. The treated aqueous effluent is then discharged to the environment under authorisation.
• Research on iron oxide floc formation pathways was developed and discussed with NNL and Sellafield.

University of Manchester research into the EARP floc formation pathway identified that, counter to classic nucleation models, Fe-13 Keggin moieties were present under very acidic conditions (pH <0.15). This highlighted that effluent streams in EARP with higher initial pH values may be less effective at removing radionuclides from effluents than previously thought. As a result, Sellafield changed the acid dosing of EARP effluents, increasing the level of acidity in the effluent prior to neutralisation in order to enhance Fe-13 Keggin formation and radionuclide sorption. This change further reduced the low-level alpha radioactivity environmental discharges from this effluent treatment facility to the Irish Sea. Specifically, in high challenge liquor batches there has been up to 90% further reduction in alpha activity discharge to the sea. The reduction in Am-241 after the change in acid dosing on EARP also has regulatory significance as it forms part of Sellafield’s demonstration that it applies ‘Best Available Techniques’ – part of its legal consent to operate.

As the Sellafield site continues in its decommissioning activities, it will treat new effluents with different chemical compositions. Additional research on radionuclide removal within EARP provided detailed understanding of the retention mechanisms of key radionuclides on plant. These results directly informed predictive models for radionuclide behaviour, including plutonium, in the EARP system that will be used to plan future operations and to ensure radioactivity is abated. Sellafield confirmed this research has “significantly improved effluent treatment processes in EARP... assisting the decommissioning process, overall leading to reduction in discharge and assisting in the clean-up of Sellafield site. These improvements support the optimisation of site decommissioning, which is a multi-billion pound project.”

Colloid stability and radionuclide behaviour in spent fuel pond effluents

Effective effluent treatment at SIXEP before discharge supports decommissioning the ponds and leads to reduced radionuclide discharges to the environment. Research at Manchester provided fundamental understanding of uranium colloid stability and speciation in conditions directly relevant to the Sellafield nuclear fuel ponds.

During pond retrievals settling must occur, to separate highly radioactive solids liquids from radioactive liquors, which are treated within SIXEP. Previously, these liquors were collected in an effluent collection vessel where the radioactivity in the liquors increased with each retrieval. With new understanding of dynamic colloid behaviour in 2017, Sellafield adopted new protocols for plant operations and pond effluents management. They include new mixing regimes implemented to reduce colloid concentrations, which further reduced both alpha activity and turbidity in the system by over 95% and total beta activity concentrations by 69%. These plant level changes have also reduced the processing time in the facility and the level of monitoring and surveillance required on site.

The underpinning research directly informed decisions to reconfigure the effluent discharge route to sustain visibility and allow continued retrievals. The outcomes are now integrated into the Sellafield Alpha Guidelines Document, the primary information source on the behaviour of alpha emitting radioactivity in the legacy ponds and silos.

Biomass characterisation and control

In order to decommission the legacy ponds at Sellafield, nuclear materials must be removed for safe treatment and storage. Microbial biomass growth, otherwise known as algal blooms, can reduce pond visibility and slow the rate of pond retrievals.

Research provided detailed information on the microbial ecology before, during and after bloom periods, and identified that photosynthetic microorganisms were responsible for the loss in visibility during blooms. Once these species had been identified using DNA analysis, the ultrasonic control units used in the ponds were tuned to optimise biomass control.

Sellafield amended these ultrasonic settings in April 2019 and pond visibility increased by up to 40% more days compared with 2018. Each day that in-pond retrieval operations are impacted by biomass is estimated to cost the decommissioning programme around £39,000 (US$49,000). Given the retrievals and dewatering programme is likely to last more than 10 years, the benefit of targeted algal bloom control will be millions of pounds.

DNA sequencing was also used to characterise several hydraulically connected ponds including the First Generation Magnox storage pond and the fuel handling pond showed that each pond has a distinct microbial community adapted to live in that facility. Sellafield therefore concluded “that there were minimal risks of cross contamination [of biomass] causing visibility problems”. This negated previous concerns that movement between ponds potentially seeds bloom-causing microorganisms, helping justify fuel transfer between facilities.

Enabling the decommissioning programme to run on time increases confidence in the programme of key stakeholders including the site owners (NDA), regulators (ONR and Environment Agency) and the general public.

Overall, the E&DCE body of underpinning research coupled to the subject matter expertise at Sellafield and NNL has informed plant-level operations used in the decommissioning of key nuclear fuel storage ponds and liquid waste treatment systems at Sellafield – a top priority for the reduction of risk on site.

What are the current lines of research and what is on the horizon? Morris explains: “We have a couple of ongoing mineralogy and geomicrobiology projects on effluent treatment in the ponds and we are starting to explore further areas for future research with subject matter experts at Sellafield and NNL.”