Cobalt-60, a major contributor to occupational radiation exposue in BWRs, is formed from neutron absorption by elemental cobalt, which can be found in a variety of alloys, including those used in control rod blades, turbines and some valves. Corrosion can lead to the release of cobalt and its corrosion products into a reactor’s primary circuit. Ion-exchange resins are used for primary water treatment to remove corrosion products and their radioactive isotopes. However, in order to maintain their effectiveness, resin precoats must be applied to the demineralizer filters regularly.
Terry Heller, sales manager and senior technical specialist for nuclear applications at Purolite, says there are three key factors that determine the frequency that a resin needs to be replaced. Resins can last three to four weeks, but are typically renewed after a set time period determined by the plant. This could be as often as every 10-14 days. Corrosion products that move into the filters can coat the elements, causing a pressure drop, and a need to backwash. Finally, ions other than cobalt can exhaust the resins, prompting a need for reapplication.
Getting resins to last longer could decrease occupational exposure, lower manpower requirements and reduce the volume of radioactive waste generated – all contributing to cost savings.
Back in 2009, the US-based Electric Power Research Institute (EPRI) started development of a powdered overlay material – CoSeq® – that could be applied to conventional demineralizer filters to remove elemental and activated cobalt from cooling water streams. Dr. Paul Frattini, a nuclear fuels chemical expert, was the EPRI scientist who developed the resin.
CoSeq® has been designed with speciality ligand chemistry to target and pick up only cobalt, leading to higher removal efficiency.
“The material is revolutionary in the sense that…CoSeq® has moved away from standard functional groups and has brought in a whole new technology for treating cobalt," noted Heller.
Following its development, the cobalt sequestration resin was tested in the laboratory and at nuclear power plants through an EPRI programme. During the first demonstration at La Salle 1 in April 2012, EPRI reported that the CoSeq® powdered resin operated for 36 days with "high removal efficiencies and no negative impacts on plant operation." The comparison test bed, which used La Salle’s normal resins, had to be backwashed twice during the 36-day test period. A demonstration at Peach Bottom 2 the following year also showed CoSeq® offers higher removal efficiency than the standard resin.
In April 2013, EPRI exclusively licensed the CoSeq® technology to Purolite for large-scale production and commercialisation. That process took less than two years to complete.
“It was imperative that all requirements were met prior to commercialisation of CoSeq® to ensure that the removal performance achieved using laboratory- produced resin could be achieved using resin produced at a manufacturing scale," said Neil Wilmshurst, EPRI vice president. "Now that Purolite has met these requirements, we are confident CoSeq® can be successfully deployed to the nuclear industry."
Purolite made its first sale of the CoSeq® precoat material in Spring 2015 and now has a worldwide licence for all nuclear applications.To date, five plants in the USA have worked with the new resin precoat.
Future outlook
Currently, CoSeq® is available as powdered precoat material that can be used in BWR reactor water cleanup systems and potentially radioactive waste precoat demineralization filters. EPRI is currently working on a whole bead cobalt sequestration resin that will open up new areas of application.
The whole bead material could be used in pressurised water reactor chemical volume control systems for outage and/ or online activities, or in spent fuel pool demineralizer filters.
Heller says Purolite’s primary objective was to build inventory of powdered material for use in BWR reactors. Development of the whole bead product for cobalt sequestration in PWR systems is underway. EPRI has already developed the whole bead product at laboratory scale and is working with Purolite to move into commercial production. At least two locations have expressed interest in evaluating the whole bead material, according to Heller.
Looking beyond CoSeq® there is potential for development of new resins to target other radioisotopes.
“As systems get cleaner and cleaner, other isotopes will become more significant," says Heller, noting that Ag-110 is becoming of more interest. EPRI is also working on another product that could selectively capture other radioisotopes.
