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73-653: The UNGG is along with the Magnox the main type of gas cooled reactor (GCR). It was developed independently of and parallel to the British Magnox design, both to meet similar requirements of simultaneous production of electric power and plutonium. Although the French and the British models both used natural uranium and the same moderator and coolant, there were differences in design. In France, each next built reactor had

146-537: A heat exchanger to generate steam to drive conventional steam turbine equipment for power production. The core is open on one end, so fuel elements can be added or removed while the reactor is still running. The dual-use capability of the magnox design led to the UK building up a large stockpile of fuel-grade (reactor-grade) plutonium, with the aid of the B205 reprocessing facility . The low-to-interim burnup feature of

219-403: A 25 or 100-year decommissioning strategy should be adopted. After 80   years short-lifetime radioactive material in the defuelled core would have decayed to the point that human access to the reactor structure would be possible, easing dismantling work. A shorter decommissioning strategy would require a robotic core dismantling technique. The current approximately 100-year decommissioning plan

292-645: A different design. The first UNGG reactors at Marcoule used horizontal fuel channels and a concrete containment structure. Chinon A1 used vertical fuel channels and a steel pressure-vessel. The fuel cladding material was a magnesium - zirconium alloy in the UNGG, as opposed to magnesium-aluminium in Magnox. As both claddings react with water, they can be stored in a spent fuel pool only for short periods of time, making short-term reprocessing of fuel essential, which requires heavily shielded facilities. The programme

365-604: A few dozen reactors of this type were constructed, most of them in the UK from the 1950s to the 1970s, with very few exported to other countries. The first magnox reactor to come online was Calder Hall (at the Sellafield site) in 1956, frequently regarded as the world's first commercial nuclear power station, while the last in Britain to shut down was Reactor 1 in Wylfa (on Anglesey ) in 2015. As of 2016 , North Korea remains

438-402: A large number of fuel channels . Uranium fuel was placed in aluminium canisters and pushed into the channels in the front, pushing previous fuel canisters through the channel and out the back of the reactor where they fell into a pool of water. The system was designed to work at low temperatures and power levels and was air-cooled with the help of large fans. Graphite is flammable and presents

511-506: A loose-fitting magnox shell and then pressurized with helium . The outside of the shell was typically finned in order to improve heat exchange with the CO 2 . Magnox alloy is reactive with water, which means it cannot be left in a cooling pond after extraction from the reactor for extended periods. In contrast to the Windscale layout, the magnox design used vertical fuel channels. This required

584-445: A new beryllium -based cladding, but this proved too brittle. This was replaced by a stainless steel cladding, but this absorbed enough neutrons to affect criticality, and in turn required the design to operate on slightly enriched uranium rather than the magnox's natural uranium, driving up fuel costs. Ultimately the economics of the system proved little better than Magnox. Former Treasury Economic Advisor, David Henderson , described

657-465: A non-oxidising covering to contain fission products. Magnox is short for mag nesium n on- ox idising. This material has the advantage of a low neutron capture cross-section, but has two major disadvantages: Magnox fuel incorporated cooling fins to provide maximum heat transfer despite low operating temperatures, making it expensive to produce. While the use of uranium metal rather than oxide made reprocessing more straightforward and therefore cheaper,

730-418: A power station, high fuel burnup is desirable for: It is also desirable that burnup should be as uniform as possible both within individual fuel elements and from one element to another within a fuel charge. In reactors with online refuelling , fuel elements can be repositioned during operation to help achieve this. In reactors without this facility, fine positioning of control rods to balance reactivity within

803-431: A quarter of UK's generating needs. Although Sir John Cockcroft had advised the government that electricity generated by nuclear power would be more expensive than that from coal, the government decided that nuclear power stations as alternatives to coal-fired power stations would be useful to reduce the bargaining power of the coal miners' unions, and so decided to go ahead. In 1960 a government white paper scaled back

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876-476: A serious safety risk. This was demonstrated on 10 October 1957 when Unit 1 of the now two-unit site caught fire. The reactor burned for three days, and massive contamination was only avoided due to the addition of filtering systems that had previously been derided as unnecessary " follies ". As the UK nuclear establishment began to turn its attention to nuclear power , the need for more plutonium for weapons development remained acute. This led to an effort to adapt

949-444: A solid form for use or disposal in a further step if desired. If tritium has not been removed from the fuel in a step prior to this aqueous extraction, the water used in this process will be contaminated, requiring expensive isotope separation or allowing the tritium to decay to safe levels before the water can be released into the biosphere. Burnup is one of the key factors determining the isotopic composition of spent nuclear fuel ,

1022-437: A turbine to generate electricity, or as process heat in the nearby Windscale works, was seen as a kind of free by-product of an essential process. The Calder Hall reactors had low efficiency by today's standards, only 18.8%. The British government decided in 1957 that electricity generation by nuclear power would be promoted, and that there would be a building programme to achieve 5,000 to 6,000   MWe capacity by 1965,

1095-429: A very long half life, constitutes 5% of fission products, but can be alloyed with uranium and transuranics during fuel recycling, or used in zircalloy cladding, where its radioactivity is irrelevant. The remaining 20% of fission products, or 1% of unprocessed fuel, for which the longest-lived isotopes are Cs and Sr , require special custody for only 300 years. Therefore, the mass of material needing special custody

1168-482: Is 1% of the mass of unprocessed used fuel. In the case of Cs or Sr this "special custody" could also take the form of use for food irradiation or as fuel in a radioisotope thermoelectric generator . As both the native elements strontium and caesium and their oxides—chemical forms in which they can be found in oxide or metal fuel—form soluble hydroxides upon reaction with water, they can be extracted from spent fuel relatively easily and precipitated into

1241-403: Is a key element of the design. In magnox, the neutrons are moderated in large blocks of graphite . The efficiency of graphite as a moderator allows the magnox to run using natural uranium fuel, in contrast with the more common commercial light-water reactor which requires slightly enriched uranium . Graphite oxidizes readily in air, so the core is cooled with CO 2 , which is then pumped into

1314-399: Is called Safestore. A 130-year Deferred Safestore Strategy was also considered, with an estimated cost saving of £1.4 billion, but not selected. In addition the Sellafield site which, amongst other activities, reprocessed spent magnox fuel, has an estimated decommissioning cost of £31.5   billion. Magnox fuel was produced at Springfields near Preston ; estimated decommissioning cost

1387-431: Is difficult to measure, so the alternative definition is preferred. This can be computed by multiplying the thermal power of the plant by the time of operation and dividing by the mass of the initial fuel loading. For example, if a 3000 MW thermal (equivalent to 1000 MW electric at 33.333% efficiency, which is typical of US LWRs) plant uses 24 tonnes of enriched uranium (tU) and operates at full power for 1 year,

1460-432: Is not a linear function of enrichment, it is more expensive to achieve higher enrichments. There are also operational aspects of high burnup fuels that are associated especially with reliability of such fuel. The main concerns associated with high burnup fuels are: In once-through nuclear fuel cycles such as are currently in use in much of the world, used fuel elements are disposed of whole as high level nuclear waste, and

1533-689: Is not closely related to burnup. High-burnup fuel generates a smaller volume of fuel for reprocessing, but with a higher specific activity . Unprocessed used fuel from current light-water reactors consists of 5% fission products and 95% actinides (most of it uranium), and is dangerously radiotoxic, requiring special custody, for 300,000 years. Most of the long-term radiotoxic elements are transuranic, and therefore could be recycled as fuel. 70% of fission products are either stable or have half lives less than one year. Another six percent ( I and Tc ) can be transmuted to elements with extremely short half lives ( I : 12.36 hours; Tc : 15.46 seconds). Zr , having

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1606-530: Is often referred to as the heavy metal to distinguish it from other metals present in the fuel, such as those used for cladding . The heavy metal is typically present as either metal or oxide, but other compounds such as carbides or other salts are possible. Generation II reactors were typically designed to achieve about 40 GWd/tU. With newer fuel technology, and particularly the use of nuclear poisons , these same reactors are now capable of achieving up to 60 GWd/tU. After so many fissions have occurred,

1679-448: Is possible in principle to remove plutonium from used fuel and divert it to weapons usage, in practice there are formidable obstacles to doing so. First, fission products must be removed. Second, plutonium must be separated from other actinides. Third, fissionable isotopes of plutonium must be separated from non-fissionable isotopes, which is more difficult than separating fissionable from non-fissionable isotopes of uranium, not least because

1752-417: Is predominantly from medium-lived fission products , particularly Cs (30.08 year half life) and Sr (28.9 year half life). As there are proportionately more of these in high-burnup fuel, the heat generated by the spent fuel is roughly constant for a given amount of energy generated. Similarly, in fuel cycles with nuclear reprocessing , the amount of high-level waste for a given amount of energy generated

1825-511: Is reprocessed on-site, as proposed for the Integral Fast Reactor , opportunities for diversion are further limited. Therefore, production of plutonium during civilian electric power reactor operation is not a significant problem. One 2003 MIT graduate student thesis concludes that "the fuel cycle cost associated with a burnup level of 100 GWd/tHM is higher than for a burnup of 50 GWd/tHM. In addition, expenses will be required for

1898-414: Is £371   million. The total cost of decommissioning magnox activities is likely to exceed £20   billion, averaging about £2   billion per productive reactor site. Calder Hall was opened in 1956 as the world's first commercial nuclear power station, and is a significant part of the UK's industrial heritage. The NDA is considering whether to preserve Calder Hall reactor 1 as a museum site. All

1971-491: The CEGB and operated on commercial fuel cycles. However Hinkley Point A and two other stations were modified so that weapons-grade plutonium could be extracted for military purposes should the need arise. In early operation it was found that there was significant oxidation of mild steel components by the high temperature carbon dioxide coolant, requiring a reduction in operating temperature and power output. For example,

2044-521: The Latina reactor was derated in 1969 by 24%, from 210   MWe to 160   MWe, by the reduction of operating temperature from 390 to 360 °C (734 to 680 °F). The Nuclear Decommissioning Authority (NDA) announced on 30 December 2015 that Wylfa Unit 1 – the world's last operating Magnox reactor – was closed. The unit had generated electricity for five years longer than originally planned. Two units at Wylfa were both scheduled to shut down at

2117-453: The advanced gas-cooled reactor (AGR) with the explicit intention of making the system more economical. Primary among the changes was the decision to run the reactor at much higher temperatures, about 650 °C (1,200 °F), which would greatly improve the efficiency when running the power-extracting steam turbines . This was too hot for the magnox alloy, and the AGR originally intended to use

2190-494: The fuel rods inside the reactor. Like most other generation I nuclear reactors , the magnox was designed with the dual purpose of producing electrical power and plutonium-239 for the nascent nuclear weapons programme in Britain . The name refers specifically to the United Kingdom design but is sometimes used generically to refer to any similar reactor. As with other plutonium-producing reactors, conserving neutrons

2263-434: The AGR programme as one of the two most costly British government-sponsored project errors, alongside Concorde . Source: The first magnox reactors at Calder Hall were designed principally to produce plutonium for nuclear weapons . The production of plutonium from uranium by irradiation in a pile generates large quantities of heat which must be disposed of, and so generating steam from this heat, which could be used in

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2336-476: The Nimonic springs used contained cobalt, which became irradiated giving high gamma level when removed from the reactor. Additionally, thermocouples were attached to some elements and needed to be removed on fuel discharge from the reactor. The dual-use nature of the magnox design leads to design compromises that limit its economic performance. As the magnox design was being rolled out, work was already underway on

2409-742: The UK's magnox reactor sites (apart from Calder Hall) are operated by Magnox Ltd , a subsidiary of the NDA. Reactor Sites Management Company (RSMC), a NDA Site Licence Company (SLC), originally held the contract to manage Magnox Ltd on behalf of the NDA. In 2007, RSMC was acquired by American nuclear fuel cycle service provider EnergySolutions from British Nuclear Fuels . On 1 October 2008, Magnox Electric Ltd separated into two nuclear licensed companies, Magnox North Ltd and Magnox South Ltd. Magnox North sites Magnox South sites In January 2011 Magnox North Ltd and Magnox South Ltd recombined as Magnox Ltd . Following procurement and management issues with

2482-445: The actual energy released per mass of initial fuel in gigawatt -days/ metric ton of heavy metal (GWd/tHM), or similar units. Expressed as a percentage: if 5% of the initial heavy metal atoms have undergone fission, the burnup is 5%FIMA. If these 5% were the total of U that were in the fuel at the beginning, the burnup is 100%FIFA (as U is fissile and the other 95% heavy metals like U are not). In reactor operations, this percentage

2555-457: The average burnup of the fuel is (3000 MW·365 d)/24 metric tonnes = 45.63 GWd/t, or 45,625 MWd/tHM (where HM stands for heavy metal, meaning actinides like thorium, uranium, plutonium, etc.). Converting between percent and energy/mass requires knowledge of κ, the thermal energy released per fission event. A typical value is 193.7  MeV ( 3.1 × 10   J ) of thermal energy per fission (see Nuclear fission ). With this value,

2628-399: The basic Windscale design to a power-producing version that would also produce plutonium. In order to be economically useful the plant would have to run at much higher power levels, and in order to efficiently convert that power to electricity, it would have to run at higher temperatures. At these power levels, the fire risk is amplified and air cooling is no longer appropriate. In the case of

2701-632: The build-up of fission products poisons the chain reaction and the reactor must be shut down and refueled. Some more-advanced light-water reactor designs are expected to achieve over 90 GWd/t of higher-enriched fuel. Fast reactors are more immune to fission-product poisoning and can inherently reach higher burnups in one cycle. In 1985, the EBR-II reactor at Argonne National Laboratory took metallic fuel up to 19.9% burnup, or just under 200 GWd/t. The Deep Burn Modular Helium Reactor (DB-MHR) might reach 500 GWd/t of transuranic elements . In

2774-485: The building programme to 3,000   MWe, acknowledging that coal generation was 25% cheaper. A government statement to the House of Commons in 1963 stated that nuclear generation was more than twice as expensive as coal. The plutonium credit which assigned a value to the plutonium produced was used to improve the economic case, although the operators of the power stations were never paid this credit. Once removed from

2847-524: The complete gas circuit, are much lower. In all, 11 power stations totalling 26 units were built in the United Kingdom where the design originated. In addition, one was exported to Tōkai in Japan and another to Latina in Italy. North Korea also developed their own magnox reactors, based on the UK design which was made public at an Atoms for Peace conference. The first magnox power station, Calder Hall ,

2920-496: The contract, Magnox Ltd will become a subsidiary of the NDA in September 2019. Burnup In nuclear power technology, burnup (also known as fuel utilization ) is a measure of how much energy is extracted from a primary nuclear fuel source. It is measured as the fraction of fuel atoms that underwent fission in %FIMA (fissions per initial metal atom) or %FIFA (fissions per initial fissile atom) as well as, preferably,

2993-438: The core, and repositioning of remaining fuel during shutdowns in which only part of the fuel charge is replaced may be used. On the other hand, there are signs that increasing burnup above 50 or 60 GWd/tU leads to significant engineering challenges and that it does not necessarily lead to economic benefits. Higher-burnup fuels require higher initial enrichment to sustain reactivity. Since the amount of separative work units (SWUs)

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3066-487: The costs of the magnox programme. Later reviews criticised the continuing development project by project instead of standardisation on the most economical design, and for persisting with the development of a reactor which achieved only two export orders. A retrospective evaluation of costs, using a low 5% discount rate on capital, estimated magnox electricity costs were nearly 50% higher than coal power stations would have provided. The magnox reactors were considered at

3139-486: The design) would not cause large-scale fuel failure as the Magnox cladding would retain the bulk of the radioactive material, assuming the reactor was rapidly shutdown (a SCRAM ), because the decay heat could be removed by natural circulation of air. As the coolant is already a gas, explosive pressure buildup from boiling is not a risk, as happened in the catastrophic steam explosion at the Chernobyl accident . Failure of

3212-492: The development of fuels capable of sustaining such high levels of irradiation. Under current conditions, the benefits of high burnup (lower spent fuel and plutonium discharge rates, degraded plutonium isotopics) are not rewarded. Hence there is no incentive for nuclear power plant operators to invest in high burnup fuels." A study sponsored by the Nuclear Energy University Programs investigated

3285-475: The differences between the stations; for example, nearly every power station used a different design of magnox fuel element. Most of the magnox builds suffered time overruns and cost escalation. For the initial start up of the reactor neutron sources were located within the core to provide sufficient neutrons to initiate the nuclear reaction. Other aspects of the design included the use of flux shaping or flattening bars or controls rods to even out (to some extent)

3358-564: The dome, connected through piping. Although there were strengths with this approach in that maintenance and access was generally more straightforward, the major weakness was the radiation 'shine' emitted particularly from the unshielded top duct. The magnox design was an evolution and never truly finalised, and later units differ considerably from earlier ones. As neutron fluxes increased in order to improve power densities problems with neutron embrittlement were encountered, particularly at low temperatures. Later units at Oldbury and Wylfa replaced

3431-628: The end of 2012, but the NDA decided to shut down Unit 2 in April 2012 so that Unit 1 could continue operating in order to fully utilize existing stocks of fuel, which was no longer being manufactured. The small 5   MWe experimental reactor, based on the magnox design, at Yongbyon in North Korea , continues to operate as of 2016 . Magnox is also the name of an alloy —mainly of magnesium with small amounts of aluminium and other metals—used in cladding unenriched uranium metal fuel with

3504-587: The first reactor had been in use for nearly 47 years. The first two stations (Calder Hall and Chapelcross ) were originally owned by the UKAEA and primarily used in their early life to produce weapons-grade plutonium , with two fuel loads per year. From 1964 they were mainly used on commercial fuel cycles and in April 1995 the UK Government announced that all production of plutonium for weapons purposes had ceased. The later and larger units were owned by

3577-431: The fuel canisters were left in the reactor as long as possible, while for plutonium production they were removed earlier. The complicated refuelling equipment proved to be less reliable than the reactor systems, and perhaps not advantageous overall. The entire reactor assembly was placed in a large pressure vessel. Due to the size of the pile, only the reactor core itself was placed within the steel pressure assembly, which

3650-434: The fuel shells to lock together end-to-end, or to sit one on top the other to allow them to be pulled out of the channels from the top. Like the Windscale designs, the later magnox reactors allowed access to the fuel channels and could be refuelled while operating . This was a key criterion for the design because its use of natural uranium leads to low burnup ratios and the requirement for frequent refuelling. For power use,

3723-423: The magnox design, this led to the use of carbon dioxide (CO 2 ) as the coolant. There is no facility in the reactor to adjust the gas flow through the individual channels whilst at power, but gas flow was adjusted by using flow gags attached to the support strut which located into the diagrid . These gags were used to increase flow in the centre of the core and to reduce it at the periphery. Principal control over

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3796-399: The mass difference is one atomic unit instead of three. All processes require operation on strongly radioactive materials. Since there are many simpler ways to make nuclear weapons, nobody has constructed weapons from used civilian electric power reactor fuel, and it is likely that nobody ever will do so. Furthermore, most plutonium produced during operation is fissioned. To the extent that fuel

3869-453: The maximum burnup of 100%FIMA, which includes fissioning not just fissile content but also the other fissionable nuclides, is equivalent to about 909 GWd/t. Nuclear engineers often use this to roughly approximate 10% burnup as just less than 100 GWd/t. The actual fuel may be any actinide that can support a chain reaction (meaning it is fissile), including uranium, plutonium , and more exotic transuranic fuels. This fuel content

3942-442: The need to reprocess fuel a short time after removal from the reactor meant that the fission product hazard was severe. Expensive remote handling facilities were required to address this danger. The term magnox may also loosely refer to: The Nuclear Decommissioning Authority (NDA) is responsible for the decommissioning of the UK magnox power plants, at an estimated cost of £12.6   billion. There has been debate about whether

4015-459: The neutron flux density across the core. If not used, the flux in the centre would be very high relative to the outer areas leading to excessive central temperatures and lower power output limited by the temperature of the central areas. Each fuel channel would have several elements stacked one upon another to form a stringer . This required the presence of a latching mechanism to allow the stack to be withdrawn and handled. This caused some problems as

4088-586: The only operator to continue using magnox style reactors, at the Yongbyon Nuclear Scientific Research Center . The magnox design was superseded by the advanced gas-cooled reactor , which is similarly cooled but includes changes to improve its economic performance. The UK's first full-scale nuclear reactor was the Windscale Pile in Sellafield . The pile was designed for the production of plutonium-239 which

4161-426: The operational gas temperatures to 360 °C (680 °F), much lower than desirable for efficient steam generation. This limit also meant that the reactors had to be very large in order to generate any given power level, which was further amplified by the use of gas for cooling, as the low thermal capacity of the fluid required very high flow rates. The magnox fuel elements consisted of refined uranium enclosed in

4234-430: The others being its initial composition and the neutron spectrum of the reactor. Very low fuel burnup is essential for the production of weapons-grade plutonium for nuclear weapons , in order to produce plutonium that is predominantly Pu with the smallest possible proportion of Pu and Pu . Plutonium and other transuranic isotopes are produced from uranium by neutron absorption during reactor operation. While it

4307-402: The pressure vessel. Magnox Magnox is a type of nuclear power / production reactor that was designed to run on natural uranium with graphite as the moderator and carbon dioxide gas as the heat exchange coolant. It belongs to the wider class of gas-cooled reactors . The name comes from the magnesium - aluminium alloy (called mag nesium n on- ox idising), used to clad

4380-424: The reaction rate was provided by a number (48 at Chapelcross and Calder Hall) of boron -steel control rods which could be raised and lowered as required in vertical channels. At higher temperatures, aluminium is no longer structurally sound, which led to the development of the magnox alloy fuel cladding. Unfortunately, magnox is increasingly reactive with increasing temperature, and the use of this material limited

4453-472: The reactor design would become responsible for changes to US regulatory classifications after the US–UK reactor-grade plutonium detonation test of the 1960s. Despite improvements to the design in later decades as electricity generation became the primary operational aim, magnox reactors were never capable of competing with the higher efficiency and higher fuel burnup of pressurised water reactors . In total, only

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4526-422: The reactor shutdown system to rapidly shut down the reactor, or failure of natural circulation, was not considered in the design. In 1967 Chapelcross experienced a fuel melt due to restricted gas flow in an individual channel and, although this was dealt with by the station crew without major incident, this event had not been designed or planned for, and the radioactivity released was greater than anticipated during

4599-454: The reactor, the used fuel elements are stored in cooling ponds (with the exception of Wylfa which has dry stores in a carbon dioxide atmosphere) where the decay heat is transferred to the pond water, and then removed by the pond water circulation, cooling and filtration system. The fact that fuel elements can only be stored for a limited period in water before the magnox cladding deteriorates, and must therefore inevitably be reprocessed , added to

4672-530: The reactors. For example, the most exposed members of the public living near Dungeness magnox reactor in 2002 received 0.56   mSv , over half the International Commission on Radiological Protection recommended maximum radiation dose limit for the public, from direct shine alone. The doses from the Oldbury and Wylfa reactors, which have concrete pressure vessels which encapsulate

4745-400: The remaining uranium and plutonium content is lost. Higher burnup allows more of the fissile U and of the plutonium bred from the U to be utilised, reducing the uranium requirements of the fuel cycle. In once-through nuclear fuel cycles, higher burnup reduces the number of elements that need to be buried. However, short-term heat emission, one deep geological repository limiting factor,

4818-422: The site in all directions would be less than six times the 10-degree limits. Planning permission constraints would be used to prevent any large growth of population within five miles. In the older steel pressure vessel design, boilers and gas ducting are outside the concrete biological shield. Consequently, this design emits a significant amount of direct gamma and neutron radiation , termed direct shine , from

4891-408: The station design. Despite the belief in their inherently safe design, it was decided that the magnox stations would not be built in heavily populated areas. The positioning constraint decided upon was that any 10-degree sector would have a population less than 500 within 1.5 miles (2.4 km), 10,000 within 5 miles (8.0 km) and 100,000 within 10 miles (16 km). In addition population around

4964-411: The steel pressure vessels with prestressed concrete versions which also contained the heat exchangers and steam plant. Working pressure varies from 6.9 to 19.35   bar for the steel vessels, and 24.8 and 27   bar for the two concrete designs. No British construction company at the time was large enough to build all the power stations, so various competing consortiums were involved, adding to

5037-487: The time to have a considerable degree of inherent safety because of their simple design, low power density, and gas coolant. Because of this they were not provided with secondary containment features. A safety design principle at the time was that of the "maximum credible accident", and the assumption was made that if the plant were designed to withstand that, then all other lesser but similar events would be encompassed. Loss of coolant accidents (at least those considered in

5110-543: Was a succession of units, with changes to the design increasing power output. In the experimental phase they were built by the Commissariat à l'Énergie Atomique (CEA), and later by Électricité de France (EDF). The largest UNGG reactor built was Bugey 1 with a net electrical output of 540 MW. The earlier units, at Chinon and Marcoule, had heat exchangers outside the main pressure vessel; Later units (Saint-Laurent, Bugey and Vandellos) moved these heat exchangers to inside

5183-454: Was bred in multi-week reactions taking place in natural uranium fuel. Under normal conditions, natural uranium is not sensitive enough to its own neutrons to maintain a chain reaction . To improve the fuel's sensitivity to neutrons, a neutron moderator is used, in this case highly purified graphite . The reactors consisted of a huge cube of this material (the "pile") made up of many smaller blocks and drilled through horizontally to make

5256-454: Was the world's first nuclear power station to generate electrical power on an industrial scale (a power station in Obninsk, Russia started supplying the grid in very small non-commercial quantities on 1 December 1954). The first connection to the grid was on 27 August 1956, and the plant was officially opened by Queen Elizabeth II on 17 October 1956. When the station closed on 31 March 2003,

5329-401: Was then surrounded by a concrete confinement building (or biological shield ). As there was no water in the core, and thus no possibility of a steam explosion, the building was able to tightly wrap the pressure vessel, which helped reduce construction costs. In order to keep the size of the confinement building down, the early magnox designs placed the heat exchanger for the CO 2 gas outside

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