Water Chemistry

Technetium to go

11 May 2004

A plant-scale trial of a new method of reducing technetium discharges from Sellafield has been successful. The UK’s Environment Agency and Nuclear Installations Inspectorate have granted BNFL permission to begin routinely using this method.

Technetium-99 (Tc-99) is a fission product present in spent nuclear fuel. When this fuel is reprocessed through the Magnox reprocessing facility at BNFL’s Sellafield site, a liquid effluent known as medium active concentrate (MAC) is produced. In its infancy, reprocessing operations at Sellafield discharged MAC directly into the Irish Sea.

In 1980, direct sea discharge of MAC was halted pending the construction and operation of a discharge treatment facility, the Enhanced Actinide Removal Plant (EARP), designed to remove the most significant radionuclides from discharges. Prior to the building and commissioning of EARP, MAC was stored onsite until this new facility was ready to operate.

At a cost of £160 million, EARP began treating MAC discharges in 1994. Although successful in removing 99.9% of radioactivity from discharges, EARP could not remove

Tc-99 from the cleansed and treated MAC discharged to sea. The technology to abate Tc-99 simply did not exist at the time of design and construction. Moreover, Tc-99, as a low beta emitter, was not considered to be of any radiological significance.

The operation of EARP meant that Tc-99 would again be discharged to sea in significant quantities after a 14-year virtual hiatus. The annual discharge limit from Sellafield was set at 200TBq per year by the UK’s Environment Agency (EA).

In 2000, this level was reduced to 90TBq per year before the EA launched a public consultation in November 2000 into the options for the future management of Tc-99 from Sellafield. This lengthy consultation resulted in a number of options being placed before government ministers, who decided in December 2002 to direct BNFL to divert future arisings of MAC from Magnox reprocessing away from treatment in EARP and eventual sea discharge towards Sellafield’s highly active liquid waste stores for vitrification.

The government direction also required BNFL to process the historic backlog of MAC stored on the Sellafield site through EARP by 2006, whilst operating within a discharge limit of 90TBq per year. This would enable the annual discharge level of Tc-99 from Sellafield to be cut from 90 to 10TBq per year by around 2006.

Finally, the government also required that BNFL undertake a further extensive programme of research and development into methods of abating Tc-99 discharges.

Scandinavian concern

Tc-99 discharges from Sellafield have acquired a high profile in recent years causing interest and concern throughout northern Europe and Scandinavia. Norway, whose second largest export industry is seafood, expressed particular concerns about Tc-99 discharges as minute traces of Tc-99 became detectable in Norwegian seafood – a concern shared by some other countries heavily reliant upon their fishing industries.

Over a number of years, politicians and NGOs from Norway consistently raised the issue of Tc-99 discharges from Sellafield with support from

Ireland. More recently, Norway has sought a meaningful engagement with BNFL over the discharge issue in the shape of the Bellona Foundation and local group Lofoten mot Sellafield (see NEI March 2004, p8), as well as the Norwegian government and the Norwegian Radiation Protection Authority.

A series of visits to the Sellafield site by Norwegian journalists, politicians from across the Norwegian political spectrum and NGOs enabled such a dialogue to take place. John Clarke, Sellafield’s head of environment, health, safety & quality, was one of those involved in brokering a factually based dialogue with the interested Norwegian parties. “We had understood for some time that technetium was a source of some anxiety in Norway, though we were never in any doubt that discharges of technetium posed any health risk whatsoever to people

living in and around Sellafield, let alone people living further afield in either Norway or Ireland – a point accepted by the radiation protection bodies in both countries.

“BNFL’s operations clearly have a high profile and that means that we have a responsibility to explain our activities to interested parties and whenever possible, find common ground. This engagement resulted in a meaningful dialogue and an eventual resolution to the technetium discharge issue.”

After almost two years of dialogue, the Tc-99 issue was resolved last month when BNFL received permission from the Nuclear Installations Inspectorate and the EA to begin using the tetraphenylphosphonium bromide (TPP) chemical treatment process in EARP.

Process of abatement

This permission followed years of scientific research and development undertaken by BNFL into abating the discharge of Tc-99 from Sellafield and culminated last year in a plant-scale trial of a unique chemical treatment process that removed more than 97% of Tc-99 from the EARP discharge stream. TPP enables Tc-99 to be taken out of liquid discharges and stored as a solid waste instead of being discharged to sea.

Following the completion of the trial in late 2003, the results were sent to the EA, the Health and Safety Executive’s Nuclear Installations Inspectorate (NII) and Nirex for assessment. Following this assessment, permission to begin using the TPP process was granted.

The implementation of this new waste treatment process meant that, with immediate effect, all Tc-99 discharges from Sellafield were reduced by 90%. Furthermore:

  • The annual operational level of Tc-99 discharges from Sellafield can be immediately reduced in line with national and international expectations and the wishes of government and the EA.
  • This paves the way for the EA and NII in collaboration with BNFL and Nirex to review the use of TPP in EARP with a view to reducing the annual limit of Tc-99 discharges from 90TBq to 10TBq earlier than expected.
  • The MAC storage facility at Sellafield can be emptied more quickly and clean-up progressed sooner.
  • Much lower concentrations of Tc-99 will be detected in the marine environment.

Commenting on the start of the TPP process at Sellafield, Clarke said: “We have worked closely with the EA, NII and Nirex throughout our research and development work into abating technetium discharges and this success is a landmark achievement.

“In addition to addressing issues of international concern and fulfilling government and regulatory requirements, this will also mean that the clean-up of certain facilities at Sellafield can be accelerated.”

The abatement of Tc-99 discharges from Sellafield, including MAC diversion and research and development work, will add a further £12 million to the £2 billion spent by BNFL on waste management and discharge reduction techniques in recent years.

Norwegian NGO Bellona praised the implementation of the new technology at Sellafield claiming that “cheers went through the roof” at the news of the permission being granted.

Norwegian prime minister Kjell Magne Bondevik praised the use of TPP at Sellafield. “I am very grateful that a successful solution has been found to this difficult issue in our otherwise close relationship with Britain,” he said.

Norwegian environment minister Borge Brende pointed to the constructive professional dialogue, which has taken place over recent years, particularly the cooperation between the EA and the Norwegian Radiation Protection Authority. “The close contact between Norwegian and British politicians, expert authorities and the industry has been a highly positive experience,” he said. “This dialogue led to new research on the British side, which again formed the basis for today’s successful outcome. I have come to believe firmly in this kind of cooperation in order to solve complicated international problems.”

Author Info:

Panel written by Phil Stones, LAEMG Technical Support, Sellafield, Seascale, CA20 1PG, UK. Main article by Jamie Reed, B582, Sellafield, Seascale, CA20 1PG, UK

Technetium abatement at Sellafield
Technetium abatement at Sellafield

During reprocessing of Magnox fuel at Sellafield the irradiated and decanned uranium fuel rods are dissolved in nitric acid. Plutonium and uranium are separated from the resulting solution by liquid extraction using odourless kerosene. One of the waste streams from this chemical separation process is an aqueous acidic medium active liquor. This liquor contains a range of radioactive species in solution such as caesium, strontium, plutonium and other beta and alpha emitters including technetium. This medium active liquor is concentrated by evaporation to produce a medium active concentrate (MAC).
MAC is collected and stored in 1200m3 tanks prior to treatment in the Enhanced Actinide Removal Plant (EARP). Sodium hydroxide and an ion exchange reagent are added to the MAC within EARP. Iron dissolved in the MAC forms an insoluble floc, which also contains the main alpha and
plutonium species. The ion exchange reagent is incorporated into the floc and removes additional species, mainly caesium. Thus most of the soluble radioactive species are separated into a solid floc. The floc is then dewatered by ultrafiltration prior to encapsulation in 500-litre steel drums with cement. The permeate which contains a tiny fraction of the radioactivity in the MAC is discharged to sea.
Technetium (Tc) is not removed into the floc by either of these processes and is discharged to sea in the permeate. This is intentional and was included in the agreed design specification for EARP. More than 90% of the Tc discharged from Sellafield site results from treatment
of MAC.
EARP has operated very successfully for approximately ten years in this manner and
Tc has been discharged to sea in a controlled manner within the corresponding discharge authorisation.
Search for a Tc abatement method
During this period there was external pressure to reduce Tc discharges by backfitting an abatement process to EARP for use during MAC
treatment. The company carried out extensive development and assessment work on Tc abatement. A range of technically feasible methods were identified. These included:

• Tc removal by ion exchange using ion exchange columns within a new treatment plant. This plant would treat the permeate from the EARP process to remove Tc prior to sea discharge.
• Removal by electrolysis using a new plant to treat the EARP permeate. Electrolytic cells would be used to deposit technetium on one of the electrodes for retention and long-term storage.
• Precipitation of the Tc in the EARP permeate by formation of an insoluble salt within a new treatment plant. The precipitate would then be removed by filtration for retention.
• Addition of tetraphenylphosphonium bromide (TPPBr) to the MAC feed to precipitate the Tc into the EARP floc. This could be performed
in EARP and would require no new plant and equipment.

The first three methods required additional plant and equipment to implement and were thus expensive in terms of capital cost, operating and maintenance costs and decommissioning costs. The fourth method could be implemented using existing plant and equipment within EARP and thus this was the most attractive method based on cost. However the resulting encapsulated waste form was initially unacceptable for long-term storage. Initial assessment predicted that the long-term release of Tc and TPP from the waste packages may have been a hazard during storage.
Trial use
The acceptability of using TPPBr to precipitate Tc has been reviewed more recently and the waste form is now considered acceptable for long-term storage. Two main factors have made this possible:

• The lifetime inventory of Tc in MAC is much less than it was previously. This is mainly due to diversion of MAC away from EARP for vitrification at Sellafield. This began in 2003 and will ensure that no further MAC arises for treatment in EARP. All of the MAC for treatment in EARP is now in buffer storage and the total Tc inventory has been quantified.
• The estimated release rate of Tc from the waste packages under fault conditions during long-term storage has been revised and a reduced impact on the public is predicted.

Waste packages of encapsulated floc containing Tc bound with TPP were assessed and found to be acceptable. Thus it was possible to perform a trial using approximately 250m3 of MAC treated with TPPBr in the EARP process. This was carried out successfully during October, November and December of 2003.
Key results from this trial are:

• Greater than 95% of the Tc in the MAC feed was removed and retained in the final encapsulated waste product. Without TPPBr addition all of the Tc in the feed would have been discharged to sea in the permeate.
• The addition of TPPBr had no measurable detrimental effect on the removal of other radioactive species by the EARP process. For example the same high degree of alpha and plutonium species removal was achieved with TPP as without.
• The addition of TPPBr did not adversely effect the subsequent encapsulation of the floc. The cement powders were able to be thoroughly mixed with the floc to form an homogenous waste form of acceptable quality.
• The use of TPPBr in EARP caused no detectable plant deterioration (for
example, corrosion).
• Not all of the TPP added to the MAC feed reacts with the Tc and some is discharged to sea. TPP is chemically toxic and thus the amount discharged was controlled to below the level of environmental harm.
• As already noted, the cost of this method is very much less than any of the other methods. With TPPBr there is no requirement for additional plant or equipment, there are minimal additional operating costs and no future additional decommissioning costs. Costs are limited to the cost of TPPBr reagent and a modest increase in the total number of waste packages produced.
Future programme
Approximately 1800m3 of MAC remain in storage for treatment in EARP. It is intended to treat this using TPPBr during the next few years. Thereafter there will be no major sources of Tc discharges to sea from Sellafield site.
Future MAC will be treated in discrete campaigns in EARP using TPPBr addition. The impact of using TPPBr on EARP performance, discharges
of Tc and TPP to sea and waste form product quality will continue to be monitored. The process will be optimised to use the minimum amount
of TPPBr (and hence minimise the discharge to sea) commensurate with good Tc removal.

Waste Packaging and Encapsulation Plant, Sellafield Waste Packaging and Encapsulation Plant, Sellafield
Lofoten, Norway Lofoten, Norway
Enhanced Actinide Removal Plant operating floor, Sellafield Enhanced Actinide Removal Plant operating floor, Sellafield

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