LEGACY POND TREATMENT | COVER STORY


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.


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


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.” ■


www.neimagazine.com | March 2024 | 33


Left: Reducing radioactivity within the effluent treatment system at Sellafield’s legacy ponds by more than 69%, has produced an estimated cost saving of more than £10 million (US$12.6 million)


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