Safestore | Decontamination & decommissioning
Berkeley starts 60-year wait9 May 2011
A total of 26 first-generation gas-cooled magnox reactors were built in the UK in the 1950s and 1960s. In 1988 Berkeley was the first of them to shut down. In December 2010 its two reactors became the first to start a 60-year wait for core radioactivity to degrade before final decommissioning.
Work at the Berkeley civil nuclear site in the UK reached a milestone on 16 December 2010 when its two reactors were formally placed into passive state, known as Safestore. The reactors will remain in this state until the site is finally cleared in the years 2074-2083. The reactor buildings were ceremonially closed and locked, with access to them now only available for staff carrying out routine maintenance.
This is the first such decommissioning milestone in the UK nuclear industry. Starting in 2074, the reactor vessels, heat exchangers and primary circuit of the reactors will be dismantled, and the reactor buildings demolished, along with infrastructure built to facilitate these activities. Within nine years final site clearance would then be completed, the nuclear site licence withdrawn and the land released for alternative use.
Berkeley Power Station was the third unit of the first generation of nuclear reactors in the UK, but the first whose primary purpose was electricity generation. The two earlier stations – Calder Hall and Chapelcross – were built to create plutonium for defence purposes, with electricity generated as a useful by-product.
Magnox reactors like Berkeley’s two were named for the magnesium alloy (magnox) metal can that covered natural uranium fuel rods. The fuel rods were inserted in a graphite lattice that acted as a moderator, and cooled by CO2 gas. Berkeley’s two reactors came into service in 1962. They supplied four turbogenerators with a total output of 276MW of electricity and generated over 43TWh over their 26-year life.
Defuelling commenced in summer 1989, and was completed three years later. Then preparations for the care & maintenance period began. As with all UK decommissioning work, the rate of progress was affected by availability of funding. While pressing ahead with decommissioning reduces the burden of future work, it requires upfront resources to be available. Leaving the work for later has some advantages: cooling and radioactive decay takes place and advances in technology and understanding may bring new techniques to bear on the issues. However, there are disadvantages: safety and security measures on nuclear licensed sites are stringent and ongoing maintenance of essential systems is nonetheless required. The so-called ‘hotel costs’ mean that the site continues to require expenditure without appreciable progress being made in reducing the outstanding work. Thus, ultimately delaying decommissioning means continuing to spend, instead of deferring all the costs.
The Berkeley site is managed by EnergySolutions EU Ltd through its wholly-owned Site License Company, Magnox Ltd for the site owners, the Nuclear Decommissioning Authority (NDA). The NDA’s budget for Berkeley in 2009-10 was GBP 40m, dropping to 32m in 2010-11.
The programme of work first began in 1993 with the decommissioning of both reactors’ eight primary gas circuits. This required the removal of top and bottom gas ducts, sealing of reactor vessel inlets and outlet ducts, removal of the boilers and removal of the gas circulators and motors. It also saw the installation of new temperature and humidity monitoring instrumentation as part of a planned long-term reactor condition and monitoring programme.
From 1993, redundant plant and equipment from inside the building, including electrical work, control cubicles and apparatus was removed. The fuel handling machines were dismantled and the cooling water system demolished. The emphasis was on reducing the reactor to a smaller envelope.
The design of magnox reactors evolved at each site where they were built: consequently no two sites have identical reactors. Unlike other magnox stations, Berkeley’s main boilers were outside the reactor buildings and connected by long top and bottom gas ducts, requiring more extensive decommissioning work than at other sites. Once the boiler houses were dismantled, the 70 feet tall (22 metres) boilers, each weighing 310 tonnes, were lowered and laid down in a horizontal position. The boilers could have remained in this position during the Care and Maintenance (C&M) period, but a trial decontamination of one of the boilers led to 85% being successfully removed from site as scrap. Further options for disposal of the remaining boilers are now being considered.
The turbine hall has been dismantled and the spent fuel ponds have been decontaminated and dismantled (another first within the UK nuclear industry).
The roof height of both reactors has been lowered by 55ft (17m). Charge face cranes, heavy machinery and glazing have been removed and external structures have been clad to reduce their visual impact. Lowering the roofs has delivered a practical benefit as the high integrity aluminium cladding provides a more robust, maintenance-free roof during the long care and maintenance period.
Around 200,000 bags of asbestos have been removed from the site to a specialised licensed disposal facility. Approximately 26,000 tonnes of scrap was recycled.
A crucial element of safestore planning occurred in April 2008, as part of the site’s periodic safety review – a 10-year requirement of the nuclear site licence justifying the integrity of the plant to regulators. It was then that the project team gained approval to complete the remaining work packages to meet the safety case requirements, and which ultimately allowed the reactor buildings to be sealed. Access will normally only be required every five years for basic monitoring and maintenance checks until final site clearance.
Paul Oswald, Berkeley site head of projects, said: “Since 2008, when the project restarted, the site team has managed to complete all remaining physical works to the buildings including the removal of all redundant equipment, preparing the reactor vessels and the isolation of power supplies. It’s a huge achievement and we’re now keen to push onto the final decommissioning work needed on the site.”
Waste retrieval and storage
With the reactors safely entombed, the next decommissioning challenge is dealing with an assortment of intermediate level waste (ILW) in Berkeley’s vaults. The vaults contain legacy fuel element debris (FED), created when used fuel rods were prepared for transport off site for reprocessing. The Berkeley fuel element design includes magnox metal splitters or lugs and graphite struts. When irradiated fuel elements were removed from the reactors, the splitters, struts and sleeves were stripped from the elements before the uranium rods were sent off to Sellafield site for reprocessing. As at other magnox stations, the fuel element debris removed from the elements was tipped into a concrete vault at the site, awaiting treatment at some point in the unspecified future. That time has now arrived. The project is on the site’s decommissioning programme critical path.
Berkeley’s Active Waste Vault Retrieval (AWVR) project is concerned with characterising and clearing the FED from two of the four vaults and miscellaneous irradiated debris from the other two. The vaults also contain post-irradiated examination material from the neighbouring Berkeley research laboratories.
This challenging task (one that is faced by all magnox stations) would be carried out using remote handling apparatus, which makes for slow progress. Before retrieval and processing of the waste in the vaults can proceed, ILW storage facilities must be available. Berkeley’s decommissioning plan included building a shielded ILW store. In common with other UK decommissioning plants, Berkeley planned to construct a shielded store on site for Intermediate Level Waste (ILW), then encapsulate the vault material in concrete and transfer the container into the store pending ultimate disposal elsewhere. Foundations for the GBP 155 million ILW storage building have been completed.
However, a different approach may be more cost-effective. The site has regulatory agreement to develop a safety case for changing its baseline from concrete encapsulation to cast iron ILW Ministores. These large transportable containers weigh around 18 tonnes when empty and hold up to about 2.85m3 of waste. The containers have been used in Germany for 20 years where they are certified for final disposal. In 2010, Magnox sites signed a contract with German firm GNS, worth up to GBP 106 million, for container supply. An initial order of 69 boxes was to be delivered in 2010.
Because each package is, in effect, a shielded store, the packages do not need a shielded building to be constructed to house them – they could be accommodated in an existing building of the appropriate size. No ILW store would need to be engineered to provide radioactive shielding. Also, the cost of containers can be spread over time. Ministores have the potential to offer a more flexible and cheaper alternative to building a conventional ILW store on the site. However, their use must currently be approved by the UK regulators on a case by case basis, and in current trials (notably at Dungeness in 2009) they have only been certified for storage for 10 years from 2009.
Research into using Ministores started at Berkeley in April 2010 and samples from two of the vaults have been sent for analysis. The first MiniStore container is scheduled to be filled before March 2011.
Two other principal activities on site are decommissioning the Caesium Removal Plant and the decontamination and demolition of the Shielded Area Facility. When these activities are completed all remaining radiological waste on site will be passively stored and Berkeley will become the first UK nuclear site to enter the passively safe state of care & maintenance.
Related ArticlesStudsvik wins £15m contract for treatment of Berkeley boilers Studsvik signs Berkeley waste treatment contract
1957 Construction begins on the first UK commercial nuclear power station,7 January