|Sizewell nuclear power stations|
Sizewell nuclear power stations shown within Suffolk
|OS grid reference|
|Fuel:||Nuclear 420 MW + 1,195 MW|
Sizewell nuclear power stations are two nuclear power stations located near the small fishing village of Sizewell in Suffolk, England. Sizewell A, with two magnox reactors, is now in the process of being decommissioned, while Sizewell B has a single pressurised water reactor and is the UK's newest nuclear power station. A third twin-unit EPR reactor is planned.[1 ]
The site of Sizewell A occupies 245 acres (99 hectares) north of Sizewell. It is on a low plateau above flood level. The geological foundation comprises low grade Norwich Crag topsoil above stiff London Clay. The crag predominantly consists of medium dense and dense sands with thin layers of clay and silt. This extends to a depth of 200 feet (60 metres) below ground level.
The site is reached by road, with the nearest railhead about one mile inland at Sizewell Halt. Sidings were installed at the railhead primarily to transport irradiated elements to the United Kingdom Atomic Energy Authority plant at Sellafield, Cumbria.
The Midlands Project Group of the Central Electricity Generating Board (CEGB) planned and managed the project. The main contract was placed in November 1960 and construction work on Sizewell A began on 1 April 1961. The main construction contract was awarded to British Nuclear Design and Construction Ltd (BNDC) a consortium backed by English Electric, Babcock International Group and Taylor Woodrow Construction. The initial budget was £56 million, but due to inflation this figure rose to £65 million. Unit 1 was commissioned on 21 March 1966, and Unit 2 on 15 September. The station was officially opened on 7th April 1967 by Commander The Earl of Stradbroke, RN, Her Majesty's Lieutenant of Suffolk.
The designed gross electrical output of the station was 650 MW. The total generating capacity was reduced to 500 MW in 1969 to arrest the rate of oxidation of internal reactor-core components. At full load, 80 MW were used in providing works power leaving a net electrical output of 420 MW.
The main plant consisted of two 1,000 MW Magnox reactors, which were natural uranium, carbon dioxide gas cooled, graphite moderated units. These supplied heat to eight boiler units, four associated with each reactor. The steam produced by the boilers was fed to two turbo-generators each rated at 325 MW, but which operated at a reduced capacity of 250 MW from 1969.
The reactors and turbines were both supplied by English Electric.
The foundations for the reactors and associated boilers are provided by a reinforced concrete raft 8 feet (2.4 metres) thick, founded on the sand with a designed net bearing pressure of 3.5 tons per square foot. The biological shields are 100 feet (30.5 metres) high and vary between 10 and 14 feet (3 and 4.3 metres) thick. The composite steel and reinforced concrete cap above each reactor is 12 feet (3.7 metres) thick. Both reactors were housed in a single building to achieve savings in building costs.
The turbine house is a steel framed, aluminium clad building 380 feet (115.8 metres) long, 160 feet (48.8 metres) wide and 90 feet ((27.4 metres) high, with a reinforced concrete basement 26 feet (7.9 metres) deep. The foundations are provided by isolated bases and strip footings with a designed maximum bearing pressure of 3 tons per square foot.
The pumphouse which supplies the main turbines with 27 million gallons of cooling water per hour draws sea water from an intake structure about 1,350 feet (410 metres) offshore via twin 10 feet (3 metres) diameter tunnels. This water is returned to the sea through similar tunnels discharging 350 feet (107 metres) offshore.
The power station was shut down on 31 December 2006. During its 40 year operational lifetime, it had produced 110 TWh (400 PJ) of electricity, which would have been sufficient to meet the domestic needs of England and Wales for 6 months. The power station had an impeccable safety record. The Nuclear Decommissioning Authority (NDA) is responsible for placing contracts for the decommissioning of Sizewell A, at a budgeted cost of £1.2 billion.
On January 7, 2007 a contractor working on the decommissioning of the station noticed water leaking on to the floor of the laundry where he was washing his clothes. The water was found to be cooling water from the pond that holds the reactor's spent nuclear fuel which had dropped more than a 1 foot (0.30 m) without activating any of the alarms. It is estimated that up to 40,000 gallons (151,500 l) of radioactive water had leaked from a 15 feet (4.6 m) split in a pipe with some spilling into the North Sea. According to the HM Nuclear Installation Inspectorate's report of the incident, without the chance intervention of the contractor, the pond could have drained before the next scheduled plant inspection, if the exposed irradiated fuel caught fire it would have resulted in an airborne off-site release of radiation.
Sizewell B is the UK's only commercial pressurised water reactor (PWR) power station: it was built and commissioned between 1987 and 1995. The reactors were supplied by PWR Power Projects Ltd and the turbines by GEC. The main civil engineering contractor was John Laing plc. The 'nuclear island' at Sizewell B is based on a Westinghouse '4-loop' plant known as SNUPPS (Standard Nuclear Unit Power Plant System) initially designed in the 1970s and used at Wolf Creek and Callaway but with additional redundancy and diversity in the safety systems, and other modifications such as the addition of a passive Emergency Boration System. The containment design was not based on SNUPPS however, but was designed by NNC in conjunction with Bechtel. The power station is operated by British Energy.
The Wolf Creek and Callaway plants each have single half speed, 1,800 RPM (60 Hz), steam turbine-alternator sets which use the steam produced from the heat generated in the reactor to produce about 1,200 MW of electricity at the US grid frequency of 60 Hz. Such large turbo-alternator sets were not available in the UK at the time Sizewell B was designed. So that orders could be given to UK manufacturers, and to avoid project risk in dealing with what were at the time newly designed very large turbo-alternator sets, Sizewell B uses two full speed, 3,000 RPM (50 Hz), nominal 660 MW turbo-alternator sets similar to those used at Drax coal fired power station, and at the last of the AGRs Heysham 2, but adapted to cope with the wetter steam conditions produced by the PWR steam supply system. PWR steam supply systems produce saturated steam at lower temperature and pressure than the dry superheated steam produced by AGR reactors or coal fired power stations, and the high and intermediate pressure stages of the steam turbines have to be designed cope with this.
First announced in 1969 as an advanced gas-cooled reactor (AGR) based power station, and then in 1974 as a steam-generating heavy water reactor (SGHWR), Sizewell B was eventually announced as a PWR power station in 1980. The initial design submissions to the CEGB and NII were based on the design of the Trojan plant at Portland, Oregon. Designed by Westinghouse, construction of Trojan began in 1970 and was completed in 1975. Westinghouse continued to develop the design they had used for the Trojan plant into the SNUPPS design, built first at Callaway, and SNUPPS was adopted as the basis for the design approved by the CEGB in October 1981.
Before construction commenced, the design of Sizewell B was subjected to a detailed safety review by the Nuclear Installations Inspectorate (NII), and a lengthy public inquiry. The Pre-Construction Safety Case was submitted to the NII in August 1981. The public inquiry was held between 1982 and 1985, and took over 16 million words of evidence, a record at the time. The chairman of the inquiry, Sir Frank Layfield, reported in early 1987 that, subject to a satisfactory safety case, there were no substantive reasons why the project should not proceed. The Nuclear Installations Inspectorate accepted the Pre-Construction Safety Case and issued a licence to proceed with construction in August 1987.
Sizewell B was calculated to be economically viable at a 5% discount rate and was approved financially on that basis. The project was managed by the CEGB Sizewell B Project Management Team, who declared that it was completed on time and to budget, the final out turn cost being £2,030 million. A 2000 post-startup evaluation estimated generating cost at around 6p/kWh (1.7p/MJ), excluding first of a kind costs but using an 8% discount rate for the cost of capital, or about 8p/kWh (2.2p/MJ) including first of kind costs.
The original rating was for a thermal power of 3,444 MW and gross electrical output of 1,250 MW, which after house load of 62 MW gave a net output to the grid of 1,188 MW, equivalent to 8.7 TWh (31 PJ) in the year of 2005. It was uprated by 1% in 2005 with a thermal power of 3,479 MWh (12.52 TJ) and an electrical output of 1,195 MW, though this is dependent on seawater temperature.
As with many other PWRs, Sizewell B operates on an 18-month operating cycle; i.e., at or near 100% output continuously for around 17 months, followed by a month's shutdown for maintenance and refuelling. Sizewell B was designed for a commercial life of 40 years (i.e., to around 2035) but similar stations elsewhere have been granted extensions to 60 years.
A distinctive white hemisphere envelopes the outer shell of the twin-walled Containment building that protects the pressurised water reactor and its steam generators.
On 27 May 2008, the Sizewell B plant had its first unplanned shutdown for over three years, cutting off its supply to the National Grid. A British Energy spokesman said that the fault involved conventional equipment at the plant rather than any part of the nuclear reactor.
Since the recent sale of British Energy to Électricité de France (EDF), plans for a further twin-unit reactor to be built at Sizewell look increasingly positive and EDF are said to be beginning planning for new reactors in earnest.[1 ] Sizewell already has a connection agreement in place for a new nuclear power plant to be built. The government revealed that the 1,600 MW projected units, to be called Sizewell C, would, together with the planned units at Hinkley Point, contribute 13% of UK electricity in the early 2020s.[1 ] EDF plans to use Areva's EPR design for any new build reactors in the UK; the design of reactors currently being built in Finland and France.[1 ]