Sellafield's cell challenge29 March 2013
Decommissioning of Sellafield's Primary Separation Plant comprises a patchwork of different projects. Work began in 1989, but the UK plant is not due to be demolished for over half a century. By Ali McKibbin and Mark Nealy
Sellafield's Primary Separation Plant was constructed between 1947 and 1951. Its purpose was to carry out reprocessing of Windscale Pile Reactor fuel, but it was also utilised to reprocess early magnox fuel. The Windscale Pile reactors produced materials for the defence industry and the by-product heat was discharged through the pile chimney rather than be used to generate electricity. The Primary Separation Plant was replaced by the Magnox Reprocessing Plant in 1964, whereupon it was shutdown and underwent clean-out and washdown. Part of the plant was later converted to reprocess oxide fuel from 1969 until 1973.
Essentially the facility is a 61 metre high, ten-storey brick building consisting of an outer annulus and an inner cell structure. It was structured around two identical reprocessing lines that occupied the north and south halves of the building. Each production line was contained within a Medium Active (MA) cell for plutonium and uranium separation and two accompanying Highly Active (HA) cells for fission product removal. The cell walls were constructed of thick shielding concrete and were surrounded by the operating annulus of the building, which housed the control and inactive services. On top of the building is a 61m high ventilation stack that is still used for aerial discharges from several key facilities.
The south process lines -- High Active South Outer (HASO), High Active South Inner (HASI) and Medium Active South Cell (MAS) -- were utilised for the dissolution and separation of uranium oxide fuel. As part of these modifications, a shear cave and supporting facilities were installed in the upper floors whilst a silo for waste stainless steel hulls (waste from the oxide fuel dissolution process) was constructed within the base of the Medium Active South (MAS) cell. The oxide fuel reprocessing campaigns ceased operation in 1973.
A strategic decision to defer decommissioning and adopt a regime of surveillance and maintenance was made to allow the beneficial effects of natural radioactive decay prior to commencement of decommissioning operations in 1989.
Windscale piles and magnox reprocessing was conducted between 1952 and 1964. The metal uranium fuel bars were hoisted up to top of the process cells in a shielded flask, where they were fed by hand-held tongs through an underwater charge chute to the fuel dissolver. The resultant hot dissolved fuel solution then overflowed a weir into a stirred vessel where it was cooled and transferred down to five packed columns to commence the separation process, from which the liquor was transferred to separate dedicated purification and finishing plants.
The south side of the plant was later converted to better reprocess magnox fuel, with the addition of a shear/leach head end unit.
The uranium oxide fuel was firstly sheared and then dissolved in 100°C 5M nitric acid. Existing solvent extraction was used for the primary separation of fission products, before being sentenced to the Magnox Reprocessing facility. The stainless steel fuel hulls dropped to a silo at bottom of MAS cell and the zirconium fuel shipped out to the Magnox Swarf Storage Silo.
Between 1969 and 1973 four magnox fuel campaigns were completed comprising a total of 91 teU. During the fifth campaign on 26 September 1973 an exothermic reaction occurred in the constant volume feeder when butex feed contacted hot, dried-out fission product and zircalloy fines. The plant was shut down as a consequence of this incident.
Decommissioning studies commenced in 1989. Originally, decommissioning was divided into nine phases covering a period of 20 years.
The original strategy to decommission the Primary Separation Plant in nine distinct phases of work did not move very far beyond the third phase. Due to the amount of plutonium contaminated material (PCM) discovered during early decommissioning in the MAN Cell, appropriate disposal routes were not available. In fact, decommissioning is not scheduled to start in the plant until 2017, to align with the site strategy for PCM disposal. Also, the discovery of severely-corroded support steelwork in the High Active North Outer (HANO) cell prevented further progress.
In outline, the first three phases of work (starting in 1989) covered:
- Construction of a management centre
- Construction of a basic waste handling facility
- Decommissioning design studies
- Testing of remote equipment
- Upgrading the ventilation system to modern standards
Following this early work a number of decommissioning projects have been carried out at the Primary Separation Plant, including modification of ventilation systems, decommissioning of some of the cells as well as emptying of waste from the silo.
The original decommissioning strategies were based upon the stack being in place until full building demolition took place. With the decision to demolish the stack early (see box below), the decommissioning strategy of the Primary Separation Plant was less constrained and a new approach formulated. The mandate selected practical methods for decommissioning the plant, taking advantage of experience gained from previous projects.
The mandate now assumes a 'top down' approach to the decommissioning strategy with the MAN cell being utilised as a waste segregation and export route. Overhead travelling cranes will be built above the HA and MA cells, allowing plant items to be lifted directly from the cells below. It is assumed that the various waste streams will be processed in the proposed new waste handling plants on the Sellafield Site. Due to the now relatively stable condition of the building and the need to manage resources to higher priority areas elsewhere on site, decommissioning of the MAN, MAS and HANO Cells is now scheduled for the period 2017-2049, with the final demolition of the building by 2077.
A seismic assessment of the Primary Separation Plant and its associated stack was carried out in 2005. The 61m-high stack, located on top of the 61m high Primary Separation Plant, would not meet with modern seismic standards.
Before the historic stack can be demolished, it must first be replaced with a new Separation Area Ventilation (SAV) stack. This new replacement has been constructed and site-wide connections are currently being installed. Aerial discharges from the Primary Separation Plant stack will be routed to the SAV, which should start operating in 2014.
The historic stack will be demolished using an industry standard 'off-the-shelf' system deployed by a specialist contractor. A self-climbing platform, located circumferentially around the 6m-diameter stack, will be used to size-reduce the concrete from the inside, starting at the top and working downwards.
The stack comprises 25 tonnes of stainless steel and a further 600 tonnes of reinforced concrete. Demolition waste will be sorted and carefully categorised to maximise the volume of exempt wastes, thus avoiding expensive disposal costs. Due to the Primary Separation Plant's location in the midst of a number of nuclear facilities, modelling and extensive demolition calculations are to be carried out before demolition can commence. A detailed design phase for demolition is currently underway.
The demolition of the stack will significantly benefit decommissioning processes and will have a huge visual impact on the Sellafield skyline.
Over five tonnes of redundant ventilation ducting and associated equipment were removed from the plant as part of a project to modify the ventilation system. The exercise, completed in 2008, enabled parallel working to be carried out in a number of cells at any one time. It involved installing new dampers and flow/pressure measurement systems to facilitate continual re-balancing of the system as future decommissioning works progress.
The decommissioning of the building header duct and associated containment was completed in 2008. More than six tonnes of stainless steel ventilation ducting and associated structural components were removed as low-level waste (LLW). A thorough characterisation study of the header duct containment structure ensured that 40 tonnes of brickwork and concrete were removed as very low level waste (VLLW).
Due to the close proximity of the main building filter banks, the demolition of the external containment structure was carefully controlled. Pre-positioned bore holes located both vertically and horizontally in the structure meant that appropriate sections of masonry waste could be engineered to ensure a controlled demolition and transfer of waste.
Out-cell process systems
The decommissioning of the out-cell process systems and redundant services was undertaken over several campaigns. The final campaign to remove the solvent system was completed in 2007 with the removal of nine tonnes of contaminated stainless steel inventory and associated services.
The proven methodology for decommissioning was to stabilise internal and external contamination via a locally-deployed fixative prior to size reduction using conventional cold-cutting techniques to minimise the possibility of re-suspension of residual contamination.
Partial decommissioning of the Medium Active North (MAN) cell (23m long, 8.5m wide and 36.5m high) was completed in 1997. Entry to the cell is restricted with no internal access floors, however the radiation levels did allow both manual and remote dismantling techniques to be used.
A remotely-operated platform manipulator mounted on a telescopic jib was installed to carry out remote operation via a central control facility complete with in-cell CCTV.
A four-tonne overhead crane was installed to support both the regular relocation of the platform manipulator and waste handling operations.
The majority of items removed were stainless steel process vessels, columns, pipework and mild steel supporting steelwork. Further decommissioning progress was halted because of stability concerns within the HANO cell which is located directly above the MAN Cell and the fact that waste routing was constrained due to the PCM inventory encountered being far greater than anticipated.
Now that the HANO cell stabilisation process has itself been completed, initial decommissioning work on the MAN cell is planned to start in 2017/18.
Residual stainless steel fuel hull waste created during oxide fuel reprocessing operations was deposited into a stainless steel silo via a dedicated transfer chute from the shear facility. Zircaloy hull waste was also generated during oxide reprocessing operations but this was collected and disposed of directly to disposal flasks. The stainless steel silo is located within the Medium Active South (MAS) Cell.
In-cell radiation and contamination levels were significant and precluded personnel access, so a scheme was devised that enabled a Remotely Operated Vehicle (ROV) access to the stainless steel silo to retrieve and deposit the fuel hull waste into transport flasks for removal and storage. The transport flasks were loaded from above via a waste loading chute to minimise contamination of the transport flask externals.
Several potential ROVs were considered, including a Brokk ROV with a manipulator arm, however in the end a smaller ROV used for bomb disposal was selected, as it was already fitted with a sufficiently-dexterous manipulator as standard. The control desk including the operating system for the ROV was situated external to the building. It provided the camera audio and video control and recording system for in-cell monitoring, together with controls for operation of the transfer tunnel shield doors, flask port and seals, as well as the flask transfer system.
Access to the stainless steel silo was created through a shield wall. Prior to commencement of retrieval operations, a full mock-up of the silo and associated transfer tunnel was constructed to trial the ROV. The ROV was modified in order to make it suitable for operating in a nuclear environment, including the installation of radiation-resistant cameras and electronics. The project to clear the stainless steel silo of bulk fuel hulls was completed in 2000 and included further modification of the ROV to enable rudimentary decontamination of the structure and waste transfer chute.
HASO cell decommissioning
HASO (High Active South Outer) cell decommissioning was completed in 2010. Originally used for metal fuel reprocessing, this cell was being modified in 1973 to process oxide fuel when a strategic decision to defer all oxide reprocessing was taken. The legacy of historic operations, incomplete plant modifications and plant records meant the task of effective decommissioning was far from straightforward. Despite these problems, the team delivered the project safely, removing in excess of 35 tonnes of process vessels, pipework and support steelwork.
This was the first high active cell in the Primary Separation Plant to be decommissioned. As such, significant dose identification and modelling activities were undertaken (some of which were novel and/or unique to the Sellafield Site) to understand the radiological status of not only the HASO cell but also the interface with the adjacent High Active South Inner (HASI) sister cell prior to the commencement of intrusive works.
Radiation scanning systems used laser and gamma scanning technology to characterise the condition of the cell. The radiation dose techniques utilised were the N-VisageTM and RadBallTM systems. Radiation detection paints were strategically applied to the aforementioned partition wall of the HASI cell. A 3D map was constructed to show the location of radioactive sources, which is then used to minimise the radiation dose uptake of the workforce.
The first large scale fogging application at Sellafield was undertaken in the HASO cell to help control airborne contamination. A cell void area of 370m3 was fogged using a glycerine-based solution atomised via ultrasonic transducers, and the resulting fine mist passively 'pumped' into cell. Once atomised, the fog adheres to airborne particulate thus grounding the contamination. The fog also adheres to all in-cell surfaces, helping to fix loose contamination and prevent re-suspension during operations. Airborne alpha contamination was reduced from 1600 to 140 mBq/m3 and airborne beta contamination reduced from 40,000 to 160 mBq/m3.
HANO cell stabilisation
For over 20 years the High Active North Outer (HANO) Cell was used as a ventilation route for various process streams in the separation area at Sellafield. The original ventilation system went straight up through the operating annulus of the building to the roof stack, and was causing a multitude of problems for day-to-day operations, so in the 1960s it was re-routed. Given that HANO was the only redundant cell by this time, it was chosen to do that job.
The cell was not designed for this purpose, and the in-cell mild steel support steelwork experienced significant to severe degradation due to the acidic ventilation gases. Remote inspection confirmed the poor condition of the in-cell steelwork process vessels and pipe work.
A project was initiated to stabilise the cell inventory which would then facilitate safe decommissioning of it and other related cells. Following extensive research and development, the project team, in conjunction with the University of Dundee and the local supply chain, worked to develop a lightweight foam grout. This lightweight foam grout is made of a cement grout base mix with a foaming agent added before deployment. It has an air bubble consistency to minimise the weight and seismic vulnerability of the building, and can be crushed to 30% of its original volume when full decommissioning of the building takes place.
Higher-density grouts have been used to backfill mines and voids; however, the use of a foam grout in radioactive decommissioning projects is novel. The project, which was completed in 2010, was carried out in two phases. Before the first phase, there was an initial deployment of a 1m-deep high-density concrete layer to provide a solid base and encapsulate a significant corrosion product/debris layer. The first phase consisted of 500m3 of lightweight foam grout and was completed late in 2007. The second phase was completed three years later following the development and deployment of a 12m-deep layer of an even lower-density grout to minimise weight loadings on the building core. The HANO cell is now stabilised and in the care and maintenance phase pending full decommissioning.
Shear cave characterisation
The shear cave facility and associated maintenance facilities in the Primary Separation Plant were added in the mid- to late-1960s as part of the plant modifications required to facilitate oxide fuel reprocessing.
Several decades on from the cessation of oxide operations, high in-cell radiation levels were still being recorded. This was suspected to be due to residual fuel fragments on or around the in-cell shear and transfer equipment, but this could not be confirmed due to personnel access being prohibited. However, this assumption was confirmed in 2010 when innovative laser and gamma scanning technology was used to characterise the radiological condition of the cell.
The results of the laser and gamma scanning activities, combined with facility data, were used to construct 3D images of the cell showing both location and intensity of in-cell radioactive sources. The output from this exercise will be utilised to formulate a decommissioning strategy, that is, plan the removal/shielding of localised high-dose areas via remote techniques to then facilitate semi-remote and manual decommissioning of the facility.
The facility is furnished with an optimum range of Master-Slave-Manipulator (MSM) ports and is readily de-contaminable since it is clad in stainless steel throughout. These factors have resulted in the shear cave being identified as a suitable waste handling facility for intermediate level waste (ILW) when final decommissioning of the building commences.
Instrument bulge removal
There were 73 sample bulges built on to the central core of the building at strategic locations, the average weight being circa 14 tonnes each. Their purpose was to divert small quantities of liquor from the process vessels at various stages of operation for sampling purposes. The liquor could then be sampled by in-situ instruments before being returned to the process flow.
The instrument bulges are of a brick and concrete construction, heavily shielded with lead and contain a 250kg stainless steel sluice arrangement at the centre. Uncertainty surrounding the seismic integrity of the bulges, in that there was an assumption the bulges were merely cantilevered off the main process core, was the main driver behind the removal project.
Each bulge was categorised prior to the start of decommissioning as either Medium Active (MA), High Active (HA) or Plutonium Contaminated (PCM) which predominantly reflected its specific location in the various process streams.
Following removal of the external monitoring equipment, a 100mm-diameter multi-media dry cutter was used to gain access to the centre of the bulge arrangement via the top plate. After insertion of an inflatable plug into the outlet line, a lightweight grout was used to fill the bulge internals to seal in any contamination. Once cured, the 100mm diameter cutter was used to core through the concrete and sever the inlet and outlet pipe work, allowing the entire stainless steel assembly to be lifted free before then demolishing the brick and concrete carcass using a combination of expanding grout and bursting techniques.
After removal of 17 LLW bulges it was apparent that each bulge was securely tied in to the process core and thus the risk of collapse during a minor seismic event was not credible. The project was closed in 2009 to provide redirection of funding for other higher-hazard reduction works. The decommissioning methodology utilised on the LLW bulges is now very well developed and will be applied to the future removal of the remaining HA and PCM bulges.