Diffusion defused

The Capenhurst Diffusion Plant was built in the early 1950s by the Atomic Energy Division of the Ministry of Supply, at which time it was the largest industrial building in Europe under a single roof, 1200m long and 150m wide. The plant consisted of a cascade of 4800 stage units connected by 1800km of process gas pipe work up to 550mm diameter, with numerous valves and associated process services. It was a technical feat of engineering that used electricity at an enormous rate, and at one time took almost 4000 personnel to operate and maintain.

It was originally built to produce highly enriched uranium for defence purposes, but in the early 1960s the high enriched section was isolated, emptied and put under care and surveillance by the on-site operations team. This area was decommissioned in two phases, firstly with a plant strip-out and secondly with residue clean-up and processing. The rest of the plant was then modified to produce low enriched uranium for civil use.

The plant operated for three decades before a more efficient method using centrifuge technology to enrich uranium was developed and implemented. The focus for the diffusion plant then changed to decontamination and decommissioning, ultimately including a strong emphasis on meticulous waste characterisation and disposals.

Phase one

Decommissioning commenced almost immediately from the date of closure utilising the knowledge and skills available from the operating workforce. The progress of the decommissioning project was slow and erratic given that it was not considered to be a core activity on site at that time – the manufacture and operation of centrifuge enrichment plants took priority.

The focus was on large-volume, low-activity material. By 1986 most of the smaller units had been removed from the plant and stored awaiting further processing when the decommissioning priority changed to the removal of the larger compressor stage units.

The main dismantling task was to break down the process plant into manageable units (up to 1m3), many of which were then stored pending the availability of size reduction and decontamination facilities. Economical methods for safe penetration and

in situ cutting were developed. Units were then removed, sealed and placed into managed outdoor storage of up to 7000t of material.

Much of the main building, with the cells which had originally housed the plant and ancillary structures such as 11 cooling towers, pump houses and an electrical substation were demolished, monitored and sold as clean scrap. In addition to the stage units, each of which consisted of a compressor, a cooling unit and a membrane, around 200,000 lengths of piping, 3500t of electric motors and 800t of process valves were removed and treated.

As removed from the plant most items were too large and complex to be monitored or decontaminated without cutting – the decision on whether or not to cut was not down to size but to do with the geometry of exposed surfaces. For each type of item, a cutting plan specified the method of dismantling to render the item suitable for onward processing through decontamination and melting.

There were various size reduction techniques.

1. Robotic plasma cutting

The size and complexity of the large aluminium compressor stage units posed particular problems and after study it was decided to develop a facility based around two large industrial robots adopted for plasma arc cutting. A purpose-built ventilation system dealt with the large volumes of fumes generated and a fully remote operation minimised operator exposure. Given the uniform geometry, the process lent itself to robotics and the use of plasma arc technology. The arc was used to cut through several millimetres of metal and was deployed on the end of a robotic arm.

2. Remote gas cutting

Almost 1500 nickel-plated stage units were cut up using semi-automatic gas cutting, which required an element of manual handling of the plasma gun, again using special ventilation and remote operation.

3. Large bandsaw

Over 2200t of aluminium pipe up to 450mm diameter was satisfactorily size reduced to a maximum of 1m3 to allow it to feed into a melter, using a large capacity horizontal bandsaw with rollerbed feeding.

4. Tube trepanning and stripping

A total of 2500 heat exchangers contained a total of 98,000 uncontaminated cupronickel tubes sheathed with contaminated aluminium fins. Special equipment was developed to trepan the tubes out of the tube plates and then strip the clean tubes away from the fins without any cross contamination. The equipment was used to bore the aluminium metal and separate the clean cupronickel tubes, which were then sold on as clean scrap.

5. Hand stripping

A large number of plant items, particularly compressor motors, were either hand stripped or disassembled using conventional machine tools in a workshop environment.

Decontamination

The success of the project depended on the development of a decontamination process to remove surface contamination from a wide variety of metals and surface textures, ranging from bright wrought aluminium to heavily rusted steel.

Following extensive laboratory and pilot plant investigation, a full-scale wet decontamination plant was developed and completed in 1989. Suitably size-reduced items were loaded into baskets which then automatically passed through successive stages of washing. This was a 10-stage process, with each operated in succession. The specially formulated processing liquors, including sulphuric acid, citric acid and hydrogen peroxide, entrained the nuclides for subsequent transfer into ion exchange resins. The decontamination process used the sulphuric acid to leach contaminants from the base metal, whilst the citric acid solutions were used to bind the uranium isotopes and keep them in solution. The ion exchange resins, which were used to remove radionuclides such as technetium and neptunium, as well as uranium, represented relatively a manageable 46m3 of low-level waste that was subsequently encapsulated and disposed of.

There were some components, following decontamination, which could not be released to the open market because of uncertainties regarding weld quality and hidden voids where undetected contamination could still be present. In order to prove those metals suitable for free release, a metals melting facility was installed in 1994.

The melting facility primarily aimed to produce ingots which were below free-release levels for unrestricted re-use. This was achieved by first decontaminating the material as described above, and then using melting to produce homogenous ingots which could then be radiochemically analysed to confirm for free release. If the free release levels were not achieved then the process was still valuable in reducing the volumes for burial.

Phase two

In 1993, BNFL’s civil enrichment business was transferred to the newly-formed Urenco (Capenhurst) Ltd and the site was split into two licensees. Urenco was the owner-operator of the centrifuge business as a completely separate entity, while BNFL retained its status as owner-operator of the formerly operational diffusion plant. It was responsible for the subsequent decommissioning of that section of the site. With this split a lot of the resources were moved from the decommissioning project and progress slowed down considerably.

In 1998, a review of the remaining decommissioning liabilities was carried out which resulted in a number of options being presented to the BNFL board. The preferred option was to continue and complete the decommissioning of the diffusion plant. The integrated decommissioning project was then set up using existing experienced resources at Capenhurst and, from both internal and external resources, professional project and team members.

The late 1990s had seen only a skeleton level of resources on the BNFL site and therefore the new team faced a considerable challenge to restart this work. This challenge extended to upgrading the infrastructure, such as power supplies, building fabric and waste processing equipment, identifying new waste processing technologies and, fundamentally, characterising the considerable amounts of wastes still left in the diffusion plant.

Waste Characterisation

In order for waste to be processed and consigned, it was first required to be characterised. Two techniques were predominantly used to accomplish this task: chemical analysis and high resolution gamma spectroscopy.

As part of the first phase of decommissioning a ‘museum’ was set-up showing the main components of the diffusion plant. These components respresented individual items of the various stage units, thus providing excellent coverage of the radionuclide fingerprint of the diffusion plant. For each component a sampling methodology was developed which entailed taking samples of the base metal, the surface oxide layer and any residual uranic material. Both this methodology and the subsequent analysis data were approved by off-site disposal routes and therefore provided the foundations for future waste disposals. While its attraction as a tourist hot spot was limited, these items allowed for chemical analysis and a ‘fingerprint’ to be developed for future consignments to waste repositories.

The samples taken were chosen to provide as much as possible of the interface between the uranium hexafluoride gas stream when the diffusion plant was in operation and the vessel wall/internal components. Each sample therefore contained some breakdown product deposited on the surface, the oxide layer interface and the underlying metal. Samples were taken by abrading the surface with a sabre saw blade, providing a sample of small swarf pieces.

Completion of this work allowed for the determination of the radionuclides that were present in the diffusion plant and thus aided greatly the subsequent waste processing and disposal regimes.

In addition to chemical analysis, high resolution gamma spectroscopy (HRGS) was used extensively to characterise individual waste streams.

The primary purposed of the HRGS facility was to measure uranium mass and enrichment of blended residues. The secondary purpose was to use HRGS to enable optimum loading of ISO freight containers destined for off site disposal.

The HRGS facility was an enclosed operating environment where drums and containers filled with a variety of high and low enriched uranium wastes and residues, could be analysed and characterised. The main components of the facility included a rotating turntable, on which drums and plastic containers were mounted, that could then be raised and rotated vertically between a radioactive transmission head and a detection system. An electrical control panel governed the speed and direction of the travel of the turntable as well as controlled all the safety interlocks and safety mechanisms. The signal from the detector was processed electronically and fed to a computer terminal in which calculations were performed and presented in the form of on-screen readouts and computer printouts. A database held all the results, cross-referenced to container identification numbers and their weight. The systems were configured to measure various combinations of container types and residues.

Building Demolition

The aircraft-hanger scale of F, G and H bays within the diffusion plant was the main centre of clean-up and dismantling activities as decommissioning work commenced in 1982. The equipment within the cells was dismantled and replaced with the previously-mentioned robotics and cutting machinery to provide the area with the capacity to characterise, process and then dispose of materials.

The 4800 aluminium stage units from the diffusion plant were also dismantled and processed both by hand and with the aid of plasma cutting torches mounted on two robots, with the aluminium being recycled back on to the open metals market.

Once this work had been completed, the area was used to underpin the storage capability of the site, with spare bays now being used to safely store uranic materials.

Prior to demolition, 100% of the available surfaces in the bays had to be monitored for radioactivity, a task which took four people a year to complete. Also services in the building – mainly high and low voltage electricity supplies and domestic and raw water supplies – had to be mapped and provisions made for their disconnection or diversion. This was a huge challenge as they were mostly shared with operational areas. In addition, the structural integrity of the neighbouring bays had to be maintained. This was achieved with the use of additional roof bracing and an innovative use of strengthening bolts whose design was carried out by Risley-based support teams.

Once the team had re-housed a bat colony that had made its home in the rafters of the condemned bays, three 360-degree excavators were drafted in to dismantle the building, a task that took around three months. While the upright parts of the structure were relatively straight forward demolition tasks, the roof was a different story. Parts of the bays were joined to operational enrichment facilities that are sensitive to vibrations, so demolition was completed by a mixture of both work by hand involving a substantial amount of work at height, and machine. At no point were there any lost time accidents, nor were there any interruptions to surrounding plants.

The demolition was officially declared a success on 15 February 2008. Since then, the rest of the plant was demolished, and the team is presently focusing on waste disposal.

The main diffusion facility was not demolished, and is now used as a store for hex

Capenhurst’s new future

Sellafield Ltd’s team at Capenhurst is now investigating ways in which the site can maximise assets and grow its business. It has recently secured enhanced funding from Sellafield Ltd and the Nuclear Decommissioning Authority for the development of two new projects that will deal with 28,000 drums of Magnox depleted uranium (MDU) and about 11,000 containers of uranium hexafluoride, or hex.

The cylinder handling facility will be designed to sample, characterise and repack into transport-compliant containers the entire stock of hex at the site. The export capability project will focus on two areas: first, it will ensure the MDU currently stored in mild steel drums is repackaged to provide robust options for transport and storage. Second, it will provide a new goods import/export facility and railhead on the site to reduce some of the current and possible future road journeys at the site, both for Sellafield Ltd and potentially for the site’s other licensee, Urenco UK Ltd.

Ian Mason, head of operations, Sellafield Ltd Capenhurst site, near Chester, West Cheshire, CH1 6ER, UK