In the UK 26 Magnox reactors were built at 11 sites between 1956 and 1971.1 The first Magnox reactor was built at Calder Hall, next to Sellafield, and opened by the Queen on 17th October 1956. It is a British designed reactor – only two were sold abroad – one to Japan and one to Italy. The last Magnox reactor opened at Wylfa on the Isle of Anglesey in 1971.
Reactor 1 at Wylfa – the last remaining Magnox reactor closed on 30th December 2015.
As with most nuclear reactors Magnox reactors are fuelled by uranium. When a uranium atom is split, two or three neutrons fly off at a tremendous speed, splitting more uranium atoms causing a chain reaction. During this chain reaction, the uranium fuel becomes very hot. In Magnox reactors carbon dioxide gas is used to carry the heat away and heat water into steam, which is then used to generate electricity in turbines, just as in a conventional power station.
In order to keep the reaction under control, Magnox reactors have a graphite core or ‘moderator’, which slows down the neutrons. Uranium fuel elements are inserted into vertical channels in this graphite core. (The uranium fuel is clad in a special magnesium alloy hence the name ‘Magnox’). Control rods can also be inserted to absorb the neutrons and stop the chain reaction. These control rods are raised when the operator wants to start up the reactor again. The core is enclosed inside a pressure vessel made out of steel or concrete.
Unlike most other reactors, Magnox reactors use natural uranium. Uranium dug out of the ground, only contains about 0.7% of the split-able (fissile) type of uranium (uranium-235), so 99.3% of the atoms are non-fissile (uranium-238). Most of the world’s nuclear reactors use fuel containing ‘enriched uranium’, which means the uranium-235 content has been increased to about 2 or 3%.
Magnox Ltd is a wholly-owned subsidiary of the Nuclear Decommissioning Authority. It is the legal entity which holds the nuclear licence, under the Nuclear Installations Act 1965. It manages and is responsible for 11 nuclear sites (all Magnox sites apart from Calder Hall plus the Winfrith SGHWR site) and one hydroelectric plant (Maentwrog). The Magnox sites are Wylfa, Chapelcross, Oldbury, Sizewell A, Hunterston A, Berkeley, Bradwell, Dungeness A , Hinkley Point A and Trawsfynydd.
Bradwell, in Essex, is the first Magnox reactor in the UK to enter its ‘care and maintenance’ (C&M) state. The station ceased generating in 2002 – 16 years ago. Since then all the fuel has been removed from the site, and the reactors have been prepared for the C&M phase. De-fuelling took until 2005. Many of the other Magnox reactors will take longer to reach the care and maintenance stage.
The NDA’s current strategy is to defer reactor dismantling for around 85 years following shutdown. This means that on current plans there will be a period of around 30 years when all Magnox sites are in C&M before reactor dismantling begins at the first site. Deferred reactor dismantling means workers can benefit from radioactive decay enabling dismantling to be undertaken with significant worker access, and reduced dose rates. However, the NDA is increasingly questioning this strategy. The lengthy deferral period means there is likely to be a loss of skills, knowledge and capability to carry out final site clearance. With advances in robotics that have been made in recent years perhaps the lengthy deferral period is no longer necessary. The NDA says:
“…advances in remote decommissioning techniques and international experience demonstrate that nuclear power reactors can be dismantled promptly without the need for significant worker access.”
|Ceased Generation||Expected to enter C&M||Time spent preparing for C&M||No. Jobs at April 2017||Jobs Jan 2018|
|Dungeness A||2006||2025||19 years||161||156|
|Hinkley Point A||1999||2027||28 years||162||165|
|Hunterston A||1990||2024||34 years||150||146|
|Sizewell A||2006||2027||21 years||201||202|
Advanced Gas-cooled Reactors (AGRs)
The Advanced Gas-cooled Reactor design was an attempt to improve on the earlier Magnox design, aiming to achieve higher gas temperatures to improve efficiency. AGRs use uranium in which the fissile part of the uranium (U-235) has been increased or enriched to 2 per cent. Like Magnox reactors, this was a peculiarly British design and no AGRs were sold abroad. Also like Magnox reactors, AGRs have a graphite core. Like the last two Magnox stations (Oldbury and Wylfa), AGRs have a pre-stressed concrete pressure vessel, and use carbon dioxide gas as a coolant.
The AGR programme was a disaster. The first station to be ordered – Dungeness B – was 12 years late, and most of the others were late too with huge cost overruns. Different contractors were used to build different stations, and design changes meant that most of the stations are of a slightly different design.
A long campaign was run against the construction and opening of one of the last AGRs to be built – Torness in East Lothian, near Edinburgh – in the 1970s and 80s. After Torness opened in 1989, the Glasgow Herald quoted a Scottish Office ‘source’ who described it as a £2.5 billion mistake which should never have been built.
Unlike the Magnox reactors, the AGR reactors are all still generating electricity. EDF Energy own and operate 14 AGRs at seven sites. Whilst all of these reactors have closure dates ‘for accountancy purposes’ this does not necessarily mean this is when they will close. These dates are kept under review and can be adjusted (in either direction) at any time to take account of commercial, technical and safety issues.
|EDF Energy’s Stations||Station Type||Net Capacity (MW)||Date Commissioned||Closure date for accounting purposes||Next PSR|
On 4th December 2012 EDF Energy announced that it will extend the operating life of two of its nuclear power stations, Hunterston B and Hinkley Point B, by seven years. Both plants are now expected to remain operational until at least 2023.
Reactor 3 at Hunterston B was closed in March 2018 for a scheduled outage, but EDF Energy subsequently discovered a higher number of keyway root cracks than predicted by its computer models. Consequently in May
2018, it was announced that R3’s shutdown would be extended for further investigation and revised modelling. By the end of December 2018, EDF estimated that the number of cracks was around 370 – over the operational limit at the time of 350. Reactor 4 (R4) was closed down on 2 October 2018 with an estimated 200 cracks across the whole core. R4 was re-started on 25th August after EDF’s safety case was approved by the Office for Nuclear Regulation (ONR) but only for about four months (a core burn up of 16.025TWd). R3 remains closed at the end of 2019. Concern has now been expressed that the graphite cores of the two reactors have begun to crumble as cracks spread, According to ONR there is “significant uncertainty” about the risks of debris blocking channels for cooling the reactor and causing fuel cladding to melt. (See Safe Energy No.84)
The Office for Nuclear Regulation (ONR) says it is working with EDF Energy to extend the life of its nuclear power stations and that it is “content for the plants to continue to operate”, as long as they pass regular safety tests.
On 27 August 2018 Dungeness B shut down Reactor 22 for its planned statutory outage. On 23 September 2018 Reactor 21 was also shut down for the planned double reactor outage. Both reactors have been shut since. EDF Energy currently expects them to re-open on 20th and 31st January 2020. The regular inspections on the reactors in Kent in late summer 2018 identified the need for repairs on steam pipes. The inspections showed that seismic restraints, pipework and storage vessels associated with several systems providing a safety function were found to be “corroded to an unacceptable condition” according to ONR. Measures are being taken to eliminate the corrosion, including the upgrading of more than 300m of pipeline associated with reactor cooling systems and renewal of numerous seismic pipework supports and remediation of carbon dioxide storage vessels.
EDF Energy’s 15th reactor is the Sizewell B Pressurised Water Reactor (PWR) – the only one in the UK. PWRs have a steel pressure vessel and the primary coolant is water which is pumped under high pressure to the reactor core where it is heated by the energy generated by the fission of atoms. The water also acts as a neutron moderator. The heated water then flows to a steam generator where it transfers its thermal energy to a secondary system where steam is generated and flows to turbines which, in turn, spin an electric generator. In contrast to a boiling water reactor, pressure in the primary coolant loop prevents the water from boiling within the reactor. Although Sizewell is not due to close until 2035, EDF Energy has been discussing the possibility of extending its life for 20 years.