The Sizewell nuclear site consists of two nuclear power stations, one of which is still operational, 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. Sizewell B has a single pressurised water reactor (PWR) and is the UK's newest nuclear power station, and is due to close in 2055. A third power station, to consist of twin EPR reactors, is planned to be built as Sizewell C.

Sizewell A

Site The site of Sizewell A occupies north of Sizewell. It is on a low plateau above flood level. The geological foundation comprises Norwich Crag Formation and Red Crag Formation bedrock of Pleistocene age above Eocene London Clay. The Crag deposits predominantly consist of medium dense and dense sands with thin layers of clay and silt and fossiliferous shelly horizons. The Crag strata extend to a depth of below ground level. In 1972/73, Sizewell A was awarded the Christopher Hinton trophy in recognition of good housekeeping.

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. Nuclear fuel was handled by a crane loading facility at the end of the Leiston/Sizewell branch railway. This is connected to the East Suffolk line via a south-facing junction to the north of Saxmundham station. The line was extended and used for the delivery of construction materials for Sizewell B power station in the 1980s. The line is now only rarely used for transportation of spent fuels from Sizewell B since the completion of decommissioning of Sizewell A in 2006. A new branch line off the Leiston branch has been proposed for rail terminals for the construction of Sizewell C power station to the north of the existing B power station.

Construction In March 1958, East Suffolk County Council selected the Sizewell site from shortlist to be earmarked for a nuclear power station, and in September the Central Electricity Generating Board announced it would apply for consent to build one there. The following February 1959, the planning committee of Suffolk County Council recommended approval of the plans, and that no public inquiry was necessary. They received two objections (from the headmaster of Sizewell Hall girls school who feared pupils would be subject to "annoying remarks" from workmen, and from Ipswich Natural History Society). In January 1960 the Minister of Power gave consent for the project to proceed.

The main construction contract was awarded to British Nuclear Design and Construction Ltd ('BNDC'), a consortium of English Electric, Babcock & Wilcox and Taylor Woodrow Construction, in November 1960. The initial budget was £56million, but due to inflation this figure rose to £65million. During its 40-year operational lifetime, it had produced of electricity, which would have been sufficient to meet the domestic needs of England and Wales for six months. Unit 1 was commissioned on 21 March 1966, and Unit 2 on 15 September. The station was officially opened on 7 April 1967 by the Lord Lieutenant of Suffolk, the Earl of Stradbroke.

Design The designed net electrical output of the station was 652MWe. The power station originally had a single 324.75MW turbo-alternator, and a gross generating capability of 327.7MW. In 1967 a second turbo-alternator was commissioned with a rating of 275MW.CEGB Statistical Yearbook (various dates). CEGB, London. The total generating capacity was reduced to 490MW in 1969, and then 420MWe in 1973, to arrest the rate of oxidation of internal reactor-core components. At full load, 70MWe were used in providing works power from the gross electrical output of 490MWe.

The main plant consisted of two 1,010MW (thermal) 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 325MW, but which operated at a reduced capacity of 250MW from 1969.

The reactors and turbines were both supplied by English Electric.

Building specification

The foundations for the reactors and associated boilers are provided by a reinforced concrete raft thick, founded on the sand with a designed net bearing pressure of 3.5tons per square foot. The biological shields are high and vary between thick. The composite steel and reinforced concrete cap above each reactor is 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 long, wide and high, with a reinforced concrete basement 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 supplied the main turbines with of cooling water per hour drew sea water from an intake structure about offshore via twin diameter tunnels. This water was returned to the sea through similar tunnels discharging offshore.

Electricity output Electricity output from Sizewell A power station over the period 1966-84 was as follows.

Decommissioning The power station was shut down on 31 December 2006. The Nuclear Decommissioning Authority (NDA) subsidiary Nuclear Restoration Services, formerly Magnox Ltd, is responsible for placing contracts for the decommissioning of Sizewell A, at a budgeted cost of £1.2billion. Defuelling was completed in 2014. The turbine hall was demolished in 2025. Removal of most buildings is expected to take until 2034, followed by a care and maintenance phase from 2034 to 2092. Demolition of reactor buildings and final site clearance is planned for 2088 to 2098.

On 7 January 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 without activating any of the alarms. It is estimated that up to of radioactive water had leaked from a 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 had caught fire, it would have resulted in an airborne off-site release of radiation.

Sizewell B

Sizewell B is the UK's only commercial pressurised water reactor (PWR) power station. Its single reactor was built and commissioned between 1987 and 1995, and first synchronised with the national grid on 14 February 1995. The main civil engineering contractor was John Laing. The power station is operated by EDF Energy. The architectural design was carried out by Yorke Rosenberg Mardall.

EDF's strategic target was for a 20-year life extension for Sizewell B, beyond the initial accounting closure date of 2035. In 2026, Sizewell B received a 20-year lifetime extension to 2055 through a Contracts for Difference from the government with an inflation-linked strike price of £70.50 per MWh (2025 prices) from 2035.

Design 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 (National Nuclear Corporation – bought by Amec Foster Wheeler in 2005) in conjunction with Bechtel.

The Wolf Creek and Callaway plants each have single half speed, 1,800RPM (60Hz), steam turbine-alternator sets which use the steam produced from the heat generated in the reactor to produce about 1,200MW of electricity at the US grid frequency of 60Hz. 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, 3,000RPM (50Hz), nominal 660MW turbo-alternator sets similar to those used at the AGR power stations Hinkley Point B, Heysham 1, Hartlepool and Torness, and at some fossil-fuel power stations elsewhere, 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 fossil-fuel power stations, and the high- and intermediate-pressure stages of the steam turbines have to be designed cope with this. Sizewell B can run at half power using one turbo-alternator.

The major components were supplied by: Reactor system: Westinghouse Reactor vessel: Framatome (main shell: Japan Steel Works, domes and other components: Le Creusot Forge) Core internals: Westinghouse Steam raising: Doosan Babcock Turbines: GEC-Alsthom Civil works: John Laing Fuel: Springfields Nuclear Fuel Architect: Yorke Rosenberg Mardall

A distinctive white hemisphere envelopes the outer shell of the twin-walled containment building that protects the pressurised water reactor and its steam generators.

History 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 the Nuclear Installations Inspectorate (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 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 16million 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 cost was revised upwards three times to 135% of the original cost, providing a cost-performance of £2,250/kW (2000 prices) not including first of a kind costs and £3,000/kW including them. A post-startup evaluation estimated generating cost were about 6p/kWh (2000 prices, equivalent to £/MWh in ), excluding first-of-kind costs but using an 8% discount rate for the cost of capital, much higher than the expected cost in 1995 of 3.5p/kWh (2000 prices, equivalent to £/MWh in ).

Sizewell B was built and commissioned between 1987 and 1995, and first synchronised with the National Grid on 14 February 1995. The cost of Sizewell B has been quoted as £2billion (1987 prices) with a quarter of that cost being related to civil engineering works.