Hinkley Point A nuclear power station

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Hinkley Point A nuclear power station
Hinkley Point A nuclear power station was a Magnox power station located on a 19.4-hectare (48-acre) site in Somerset on the Bristol Channel coast, 5 miles (8 km) west of the River Parrett estuary.

The construction of the power station, which was undertaken by a consortium backed by English Electric, Babcock International Group and Taylor Woodrow Construction, began in 1957. The reactors and the turbines were supplied by English Electric. The power station, which is currently being decommissioned, had twin Magnox reactors, each supplying steam to a turbine alternator set which was designed to produce 500 MWe but, after de-rating of the reactor power output due to corrosion concerns, producing 221 MWe. The design followed the principles established by the Calder Hall nuclear power station, in that it used a reactor core of natural uranium fuel in Magnox alloy cans within a graphite moderator, all contained in a welded steel pressure vessel. The core was cooled by CO 2 pumped by six nominal 7,000 hp (5.2MW) gas circulators, which transported the hot gas from the core to the six Steam Raising Units (boilers) via steel ducts. The gas circulators could be driven by induction motors supplied with mains electricity or, when steam was available, dedicated variable speed turbo alternator sets. The design pressure of the gas circuit was 185 psig, and the temperature of the gas leaving the reactor was 378°C, although this was later reduced when the hot CO 2 was found to be corroding the mild steel components of the gas circuit more quickly than had been anticipated. Like all Magnox reactors, Hinkley Point A was designed for on-load refuelling so that exhausted fuel elements could be replaced with fresh without shutting down the reactor.

Gas circulator design problem
In August 1963 during a hot run test on the first reactor, which had not then been loaded with nuclear fuel, problems were encountered due to noise from the single stage axial flow gas circulators. This could be heard up to five miles away, and personnel working at the station had to wear ear defenders. After unexplained drops in the mass flow rate and motor driving current in number 3 and 5 gas circulators, the hot run tests were stopped and the gas circuit opened up. Severe mechanical damage to the blading and diffuser sections of the number 3 and 5 gas circulators was observed. Large sections of the diffusers had broken away, and extensive fatigue cracking was found in the outer tapering shell and central axial cone. Large pieces of the diffuser casing had entered the gas circulator blades and caused heavy impact damage, and large amounts of debris had been transported down the gas duct. The Inlet Guide Vanes (IGVs), which were provided to enable the performance of individual gas circulators to be "trimmed," were found to be extensively damaged, and the rotor blades and outlet guide vanes also had extensive impact and fatigue damage. Large numbers of the nuts and bolts involved had been shaken loose. The subsequent investigation determined that the noise was caused by interaction between the IGVs and the rotor blades. The sound pressure levels generated by this noise were high enough to cause rapid fatigue failure in gas circuit components, and major re-design of the gas circulators and associated components was required. The IGVs were scrapped and flow straighteners introduced to smooth the flow of gas into the gas circulator intakes. Much pioneering experimental laboratory work on resonance and sound pressure levels was performed at English Electric's Gas Turbine and Atomic Power Division (APD) facilities at Whetstone, Leicestershire, to support the redesign work, and instrumentation to measure stress and sound pressure levels in the gas circuit during testing was developed. The delay caused severe financial difficulties for the consortium and set the construction schedule back; the station began generating electricity two years late in 1965.

Both reactors were shutdown in April 1999 to carry out reinforcement work following a Nuclear Installations Inspectorate Periodic Safety Review. Reactor 2 was returned to service in September 1999, but shut down on 3 December 1999 because of newly identified uncertainties in the reactor pressure vessel material properties. Because of the cost of remedying these problems, it was announced on 23 May 2000 that Hinkley Point A would be shutdown. Hinkley Point A was one of eleven Magnox nuclear power stations commissioned in the United Kingdom between 1956 and 1971. During its 35 years of operation, Hinkley Point A generated more than 103 TWh of electricity. There are two nuclear power stations at Hinkley Point, the other is Hinkley Point B, operated by British Energy. There are also two other power stations nearby at Oldbury and Berkeley on the banks of the River Severn. A new Hinkley Point C nuclear power station is under consideration.