Treating a 50-year-old legacy of radioactive sludge waste
Ben Irons describes how some of Sellafield’s 50-year-old radioactive sludge waste is being made safe.
In March 2005a newly-designed £90m (US$165m) radioactive waste retrieval plant was started up at the UK’s Sellafield nuclear reprocessing site now operated by British Nuclear Group. A patented process was used to successfully mobilise and transfer more than 1500m3 of radioactive sludge waste from a 50-year-old storage tank to high integrity modern containmentso that the waste could be made safe for long-term safe storage.
Since Sellafield began operating in the early 1950sliquid effluent has been treated using a flocculation process to remove radioactivity prior to sea discharge.
Historicallyacidic effluents were neutralised with ammonium hydroxide. Dissolved metals in the effluent reacted with the alkali to form a metal hydroxide flocculent precipitatecommonly referred to as floc. This process was effective at removing radioactive actinidestrapping them within the floc as it formed. The resultant suspension was transferred to storage tanks within the site's sea tank complex. The floc precipitate would settle and the clarified liquorreferred to as supernatewould be decanted using a floating boom system and discharged to sea. Over the yearssignificant volumes of settled floc (approximately 4500m3) were collected in six large storage tanks (each 15m diameter10m height and 1700m3 volume).
In the 1990s the historic settling process was superseded by the operation of the Enhanced Actinide Removal Plant (EARP). In a similar way to the historic processEARP removes actinides using flocculation. The acidic effluent is neutralising with an alkaliin this case sodium hydroxide. The overall effectiveness of the activity removal process is enhanced by the addition of reagents that remove soluble radioactive species. British Nuclear Group’s improvements to this process to increase technetium 99 removal were described in August 2004 edition of tce.
Rather than using a settling processEARP uses cross-flow ultrafiltration to de-water the floc. The resulting permeate has very low residual activity levels and is suitable for sea dischargefollowing confirmatory sampling and analysis. The concentrated radioactive floc is transferred to a separate downstream plantthe Waste Packaging and Encapsulation Plant (WPEP). Within WPEP the concentrated floc is blended with cement powders in an encapsulation process that produces a stable concrete product within a stainless steel container suitable for long-term safe storage.
Risk reduction strategy
With the operation of EARP and other effluent treatment plants constructed at a similar timethe floc storage facilities and former sea tank complex became redundant.
Howeverthere was no provision to empty the historic floc waste from the storage tanks and no route to treat it.
To address this problemthe Sellafield Floc Retrieval Project was initiated. British Nuclear Group’s aim was to retrieve and treat the historic waste. A target was set to retrieve in excess of 95 per cent of the inventory from the six storage tanks and to treat it through the EARP and WPEP processes. Importantlythis would:
- Reduce the risks associated with storing a liquid waste.
- Produce a stable product suitable for long-term safe storage.
- Open up the opportunity to begin making detailed plans for decommissioning the former sea tank complex.
Understanding the floc waste material
When the project began approximately 4500m3 of settled floc with approximately 3000m3 of attendant supernate was stored within the former sea tank complex.
Like similar historic waste materials stored at various nuclear sites across the worldthe exact inventory was not accurately known. The floc was generated over many years from the treatment of various effluent streams. Whilst some flowsheet information was availabledescribing the chemical and radiochemical content of streams routed to the facilitythere was doubt about the accuracy and completeness of data. Development of a suitable waste retrieval technique would also required physical property data and an understanding of how the material had aged since it was consigned to storage. At the start of the projectthis information was not available.
A number of samples were taken over many years using different techniques. These gave some understanding of the material but were not adequate to underpin the development of future flowsheets. In 1993the project started a comprehensive sampling and characterisation campaign. We took samples from a number of locations and all depths within each tank. The sampled material was analysed to understand the chemical and radiochemical propertiesas well as the physical properties such as densitysettling behavioursand rheology.
Chemical analysis identified many speciesboth inorganic and organic. The metal hydroxide floc was known to be predominantly Iron hydroxide but other metals such as aluminiumcalcium and chromium were also found. Within the attendant ammonium nitrate supernate liquorthere were also many other soluble species.
The dominant radioactive species were actinidesparticularly americium 241. Some beta- and gamma-emitting species were also presentincluding caesium and strontium. Overallin radioactivity terms the material was classed as an intermediate level waste (ILW). We found significant variations between the tanks as well as variations with depthconsistent with material being added in layers. One storage tank held nearly half of the 2000 TBq of activity.
The floc was not particularly densealthough there were variations between the oldest material towards the bottom of the tanks and the material added last.
The rheology was complex. The material exhibits a yield stress in a similar way to a Bingham plastic but also has thixotropic shear thinning properties closer to those expected of ‘powerlaw’ fluid. On standingthe shear thinned liquid would regain its initial yield stress. In layman’s termsthe stored material could be described as ranging from a gel or thick ketchup consistency through to a thick mud or clay-like material.
When shearedthis material would become thin and mobile in a similar way to when non-drip gloss paints are applied with a brush
Preparatory work
To prepare for retrieval of the tank contentsredundant equipment was removed from the complex. This included the extensive pipework previously used to connect to other facilities.
The 50-year-old storage tanks were known to be showing signs of ageing. Ammonium nitrate is known to affect the strength of concrete and the tanks were judged to be unsuitable for long-term storage. Integrity assessments during the project design phase identified that tank containment needed to be improved prior to retrieval operations.
British Nuclear Group undertook a separate project to construct a steel jacket around each concrete tank supported by an additional concrete ring beam around the base and steel ring beam around the top.
Although the tank containment had been improvedaccess into the tanks remained difficultwith only one small opening near the edge of the roof. Assessment showed the roof structures were not capable of supporting the weight of the equipment required to retrieve the tank contents. Four additional openings were carefully created without further undermining the strength of the roofs. Whilst not idealthis would allow equipment to be deployed from above providing it could be separately supported.
A seismically qualified picture frame bund was constructed to provide secondary liquid containment around the tanks.
We constructed an overbuilding to provide secondary aerial containment. The overbuilding was a portal frame designproviding a suspended operating floor above the tanksfrom which the equipment required for the retrieval of the floc could be deployed. During construction of the overbuilding no crane movements were allowed over the tanks for safety reasons. Construction was undertaken in phases at one end of the complex and the building was incrementally slid into place. On the final slidethe building weighed approximately 2800t and was 110m long46m wide and 28m high.
A services building was built housing water supply systemscompressed air suppliespower distributiona control roomcontrol systems and nuclear ventilation systems. An 85m3 agitated sentencing vessel with dedicated sampling system was also housed within the services building.
We carried out considerable development work to support the retrieval process. This included theoretical workmodelling techniqueslab scale worktest rigsa full-scale rigactive work using sampled materialand work using specifically developed simulants to replicate either chemical or physical properties. One of British Nuclear Group’s most difficult challenges was to make an accurate simulant of material that is 50 years old and to develop a process robust enough to deal with the possible variations in physical properties.
Retrieval technique
The floc material was retrieved using a patented hydraulic jet mixing and transfer system. The retrieval equipment comprised of a submersible pumpthree ‘spillback’ pipes with diametrically opposed nozzles and interconnecting pipework. The spillback pipes were inserted into the floc bed and suspended from turntables mounted on the operating floor. During retrieval operation these would be rotated by electrically operated planetary gear mechanisms. Interconnecting pipework and high integrity valves were then connected to the spillback assemblies. Due to the radioactive nature of the process material all the pipework was provided within secondary containment systems complete with leak detection equipment. Finally a submersible pumpheld within a secondary containment flaskwas connected to the pipework and deployed into the first tank to be emptied.
To minimise the equipment provided the interconnecting pipework and the pump flask were designed to be mobileallowing them to be re-used on the other tanks.
Additional water was added to ensure pump submergence. The pump was then used to circulate the mobile contents of the tank at design flowrates of approximately 400 m3/h. The pumped liquor was routed through the interconnecting pipework and discharged under pressure into the settle floc bed via the spillback pipes. This effected localised resuspension of the settled floc into the recirculating liquor. As resuspension progressed the flow was directed to each spillback in turn to ensure resuspension across the whole tank was achieved. We encountered some initial pumping problemsbut once the planned pumping regime was established the total resuspension process took approximately two weeks.
We found no way to accurately measure when all the contents of the tank were fully mixed; the various techniques we considered were all rejected as unsuitable for this application. Howevermeasurement of pumping parameters such as flowratepressure and current draw gave an indication that a mixing endpoint had been achieved (ie that the nature of the material being pumped was no longer changing).
At this point the flow through the resuspension pipework was reconfigured to direct flow to all three spillbacks simultaneously to sustain even mixing across the tank. A side stream of approximately 50m3/h flowrate was diverted through a cyclonic separatorused to remove coarse debrisprior to onward transfer to a buffer tank. The buffer tank is a refurbished tank of similar size to the storage tanksbut built to modern high integrity containment standards and lined with a protective chemical resistant coating.
As the retrieval progressed the pump was progressively lowered tracking the reducing liquor level. The overall transfer took about a day and a halfduring which approximately 98.5percent of the volumetric inventory of the first tank was successfully retrieved.
Once transfer to the buffer tank was completedthe resuspension system had to be washed out to remove residual activity and minimise the risk of blockages from any settled material. The pump was withdrawn into its containment flask. The flask could then be moved through the building allowing the pump to be redeployed.
Downstream processing
Buffer storage of the retrieved floc is required as downstream plants utilise a batch processhandling approximately 30m3 of floc per batch. The buffer tank is able to withstand the effects of the mixing process over the extended periods required for processing the batches of floc. Use of the buffer tank minimises the potential degradation effects of the jet mixing on the 50 year old storage tanks.
The resuspension pump previously deployed in the storage tank is also used at the buffer tankbeing connected to a similar but permanently installed resuspension pipework system. Resuspension is carried out in an analogous way within the buffer tankbut this time batches of approximately 30m3 are transferred forward to a sentencing vessel via a second cyclonic separator.
Within the sentencing vesselthe floc is diluted and mechanically agitated. The material is circulated through a further grit removal hydrocyclone system.
When the storage tanks were designed they incorporated a graded sand and gravel filter bed in their bases. This was intended to function as a filter system. Howeverit was found that the gel like floc quickly blinded the filter bed. The mixing process used to retrieve the floc disturbs the graded sand and gravel filter medium. Once entrained in the floc this has the potential to cause significant erosion of pipework and equipment within the downstream treatment plants. The multiple grit removal operations are therefore an essential part of the process.
Following grit removal the floc is circulated through a sampling system and a representative sample is taken. Analysis of this confirms acceptability for downstream processing and provides information to allow operational parameters to be set.
The floc is then pumped to the EARP facility via dedicated coaxial pipelines and processed in similar way to flocs normally prepared in the EARP process. The concentrated floc product from EARP is then encapsulated in a cement matrix formed within a stainless steel drum in the WPEP process.
Safety and environmental risks
Above allprior to operationBritish Nuclear Group needed to make a robust Safety Case for the project to demonstrate that the floc retrieval process met the required safety criteria and reduced risks levels As Low As Reasonably Practical (ALARP). The risks addressed included those associated with:
- Loss of containment of radioactive material.
- Presences of organic material and radiolytically-generated hydrogen that could potentially lead to an explosive atmosphere.
- Physical risks associated with the moving and deploying large mobile equipment within a nuclear plant environmentand
- The potential effects of external hazards such as seismic events and extreme weather on a nuclear plant.
The Safety Case included both risk analysis and substantiation of the adequacy of the plant and equipment. It went through a multi-stage approval process including internal reviewindependent peer review and submission to the necessary regulatory bodies.
Success
In excess of 1500m3 of historic intermediate level radioactive sludge waste has already been successfully retrieved to high integrity modern containment. This world-leading achievement by British Nuclear Group represents a significant step forward in managing historic legacy wastes on the UK’s largest nuclear site. This first retrieval has proved the process is suitable to use to empty the remaining five storage tanks within the complex and has opened up a long-term safe storage route for the material.
Ben Irons is with British Nuclear GroupSellafieldUK. www.bnfl.com
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