At 3:05pm on New Year’s Eve 2006, Sizewell A nuclear power station arrived at the end of an era. Forty years of electrical generation ended as the plant’s operator, British Nuclear Group (BNG), took the site into the next phase of its lifecycle – decommissioning. Located on the UK’s Suffolk coast, Sizewell A was the world’s second oldest operating nuclear power station, behind the Dungeness A plant which also reached the end of its generating life on the same day.
Sizewell A’s twin Magnox reactors
The Central Electricity Generating Board (CEGB) placed the main contract for construction of Sizewell A with Nuclear Design and Construction in November 1960. The construction of Sizewell A was pioneering in several ways. The design of the station was the first to incorporate a ‘common pile cap’. By using this configuration, the station was able to use a charge machine to fuel either of the two reactors, giving an extra degree of contingency against breakdown, as well as having a single fuel storage pond. Similarly, the site was able to use common reactor servicing equipment for maintenance of in-core components on both reactors.
The twin Magnox reactors, so-called because of the magnesium non-oxidising cladding that encases the natural uranium fuel elements, were designed with a thermal output of 950MW per reactor and a combined electrical generation of 650MW produced from two English Electric turbo-generators. In 1969, the station’s generating capacity was reduced to 420MWe to arrest the rate of oxidation of reactor core components. Both reactors have 3784 channels, each of which hold seven fuel elements, equalling around 26,500 fuel elements, or 321 tonnes of uranium in each reactor.
Sizewell A was also the first of the Magnox group of stations to be commissioned with a single turbine set per reactor. A crossover system enabled the station to run either turbine set from either reactor, continuing generation in the event of mechanical failure or scheduled maintenance programmes.
Electricity was first produced commercially in January 1966. The final cost of construction was £65 million and an estimated 20,000 working years of local employment were created during the construction of the station.
Lifetime performance
The design and construction of this reactor has been proven by the exceptional operational performance of the station during its lifetime. In 1980 the station became the first nuclear power station in the world to generate 50 billion kWh of electricity, at a rate 10 million kWh per day (around 0.5% of the UK electricity generating capacity).
The longest uninterrupted generation run was reactor 2, which remained at power for 653 days between August 1975 and 1977 before it was shut down for overhaul. This established a civilian nuclear reactor reliability record. The longest uninterrupted run of reactor 1 was nearly 500 days between 2004 and April 2006.
On top of its generation performance, the site has had remarkable successes on the safety and environmental fronts. In April 2005 it became the first industrial site anywhere in the world to be awarded level 10 on the International Safety Rating System developed by consultancy Det Norske Veritas. It followed this with level 10 on the International Environmental Rating System, to establish a unique double first.
The final fuel elements were loaded into both reactors in August 2006, marking the end of routine refuelling operations at the site. Sizewell A’s reactors were designed to be refuelled ‘on load’, enabling operators to maintain the nuclear reaction without having to halt electrical output. After routine refuelling was completed in August 2006, the opportunity was taken to undertake maintenance on the fuel route in preparation for defuelling, including the overhauling of the pile cap crane, fuelling machines, hoist units, flask crane and skip handling crane.
When the reactors were finally shut down at staggered times throughout 31 December 2006, they were still running smoothly. A small group of senior company staff, who had been to Dungeness A to watch its reactor 1 being shut down at 9:00am, then travelled to Sizewell to see shift charge engineer Dick Smith and his staff complete the shut down of its second reactor on schedule just before 3:00pm. Cameras in the main control room broadcast the event to the offsite Sizewell Sports and Social Club where current and former employees and their families were able to watch the final moments of generation. A spontaneous round of applause rang out when the reactor control engineer tripped the reactor for the final time, and the dials dropped back to zero.
Smith, who worked at Sizewell for 33 years, said: “It was sad, it’s the end of an era although it has not really sunk in yet that we will not be starting up again.” One of those watching was Nigel Turner, a shift engineer who was there when the station started producing electricity in 1966 and who left the site in 1991. He said: “It is sad and there is a lot of emotion about; that’s hardly surprising. The plant was still working well and making money. There’s a few of us who started onsite in the ’60s who still meet up regularly.”
Mark Morant, managing director of BNG’s Reactor Sites business, led the small group of senior company representatives who visited both sites on 31 December. He said: “These two power stations have fulfilled a vital role in providing electricity for the south east of England and both their operational and safety records have been exemplary. These sites now enter the next phase of their operational lives which is defuelling of the reactors and consequent removal of the vast majority of the radiological hazard.”
By ceasing generation, BNG has achieved the target date outlined by the Nuclear Decommissioning Authority (NDA) in its strategy document.
The future
In May 2006, the Health and Safety Executive gave formal permission for the site to decommission, based on outline decommissioning proposals and their expected environmental impact. Following cessation of generation and approval of the post-operational defuelling safety case by the Nuclear Installations Inspectorate, work is set to begin on defuelling the reactors.
The next scheduled milestone at Sizewell will be to completely defuel both reactors by removing all fuel elements and delivering them to Sellafield for reprocessing by August 2009, in line with the long-established Magnox Operating Plan (MOP) schedule. This is a task the site is familiar with performing.
A significant difference in operating conditions will be the permanent disablement of the control rods in their inserted position, ensuring nuclear safety. Initially, gas circulators will be used to forcibly cool the fuel in the reactors, whilst passive cooling tests will indicate when reactor temperatures remain below 100°C for 30 days, without any plant running. To facilitate this state of passive cooling, some of the lagging which normally insulates the boiler system will be removed.
Various decommissioning projects are due to run simultaneously on the site. In the short term (12-18 months), three of the larger projects include: electrical overlay works, modification of site buildings to accommodate the workforce, and a post-operational clear out.
The main objective for the electrical overlay project is to provide secure electrical services for the remaining operational plant, situated away from the location of any demolition work. It will also isolate the reactor building supply, making conventional buildings and plant available for dismantling and demolition as soon as practicable. The contract for demolition of conventional buildings is due to be awarded to a single contractor who will also be responsible for similar activities at the Dungeness A and Bradwell sites.
In preparation for this demolition work, the workforce relocation project aims to vacate the existing office buildings and workshops as soon as feasible. Arrangements include assembly of a modular building onsite to accommodate around 170 project delivery personnel, conversion of the site’s long-term store into a new fire station and emergency control centre, and conversion of areas inside the reactor building into office accommodation. The concept is that by the time a demolition contractor is appointed, the workforce will be concentrated in one area of the site away from demolition areas.
Decommissioning work on the site, as with all the Magnox stations operated by BNG, will continue to present some unique challenges. However, learning and best practice from other sites will be shared in delivering Sizewell A’s decommissioning programme and continuing the transition from an operating to a decommissioning environment.
To reflect this approach the site has a new director, Bob Kury, who brings specialist expertise to the defuelling and decommissioning work from his previous roles at Hinkley Point A, and in the USA and Japan.
“I’m very proud to come to Sizewell,” he said. “It’s a site with an excellent record for safety and electrical generation. But, decommissioning is a very different business and I will be drawing on my previous experience to help the site deliver safe, efficient and cost-effective decommissioning for our customer, the NDA.”
Sizewell A’s twin Magnox reactors Sizewell A Archive image of an operator working on the Sizewell A heat exchangers Sizewell A heat exchanger Sizewell A unit 2 pile cap Sizewell A pile cap 1965-1989 Central Electricity Generating Board (CEGB)
1989-1996 Nuclear Electric (following the part privatisation of the CEGB)
1996-1998 Magnox Electric (following the transfer of the AGR and PWR nuclear power stations to British ESizewell A ownership 2007-2009 Defuelling
• Removal of fuel from the reactors and storage ponds and despatch to Sellafield in Cumbria for reprocessing.
• Isolation of redundant plant and systems.
• Enabling of retrieval, containerisation and storage of exSizewell A current decommissioning programme The Sizewell A workforce will shortly be relocated into this (now complete) new modular building. The Sizewell B PWR can be seen in the background Modular Building The twin turbine sets could be run from either reactor, giving greater defence against breakdown Sizewell A turbine Author Info:
Dan Gould, British Nuclear Group, 1100 Daresbury Park, Daresbury, Warrington WA4 4GB, UK
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