Magnox Reactors

Magnox reactors were some of the world’s first commercial nuclear reactors

The Magnox Story

Magnox reactors hold a significant place in the history of nuclear power, marking the early days of commercial nuclear energy. Developed in the United Kingdom during the mid-20th century, these reactors were pivotal in demonstrating the feasibility of nuclear power for electricity generation and supporting military research.

The Calder Hall nuclear power station in the United Kingdom, the world's first commercial nuclear power station.
Calder Hall, the world's first commercial nuclear power station. The station was officially opened on the 17th of October 1956 by Queen Elizabeth II.
Hinkley Point A and Hinkley Point B as seen across a moor at dusk
Hinkley Point A (left) and B (right). Hinkley Point C, the UK's only nuclear new build, is set to commence operations in 2027.
Wylfa, on the shores of Anglesey, was the last and largest of the UK’s Magnox reactors, finally shutting down in 2015.

Development and Design

The first Magnox reactor, Calder Hall, was commissioned in 1956 at Sellafield, England. It is often celebrated as the world’s first commercial nuclear power plant. The name “Magnox” comes from the magnesium-aluminium alloy (Magnesium Non-Oxidising) used to clad the natural uranium fuel rods. These reactors utilized graphite as a neutron moderator and carbon dioxide as a coolant, which were innovative choices at the time.

Operational History

Between 1956 and 1971, the UK built 26 Magnox reactors, with a few exported to countries like Italy and Japan. These reactors were designed to produce both electricity and plutonium for military purposes. The dual-purpose design was a strategic decision during the Cold War era, reflecting the geopolitical climate of the time.

Berkeley, an early magnox power station, under construction in 1962
Berkeley under construction in June 1958
Dungeness A under construction around 1962. The magnox reactor buildings are partially finished and surrounded by cranes
Dungeness A under construction around 1962

Advantages and Challenges

Magnox reactors were known for their simplicity and the ability to use natural uranium, which eliminated the need for expensive enrichment processes. This made them economically attractive in the early days of nuclear power. However, they also had several drawbacks. The reactors produced a significant amount of radioactive waste and had lower thermal efficiency compared to later reactor designs. Additionally, the magnesium alloy cladding was prone to corrosion, which limited the lifespan of the fuel rods.

Legacy and Decommissioning

The last Magnox reactor in the UK, Wylfa, was shut down in 2015. Despite their eventual phase-out, Magnox reactors played a crucial role in the development of the British nuclear industry. They provided valuable operational experience and laid the groundwork for more advanced reactor designs such as Advanced Gas-cooled Reactors. The decommissioning of these reactors is an ongoing process, involving the safe disposal of radioactive materials and the dismantling of reactor structures.

How do Magnox Reactors work?

A Magnox reactor is a type of gas-cooled, graphite-moderated nuclear reactor that uses natural uranium as fuel. In a Magnox reactor, carbon dioxide gas is circulated through the reactor core to transfer heat from the fuel rods to a steam generator, where water is converted to steam to drive turbines and generate electricity. The graphite moderator slows down neutrons to sustain the nuclear chain reaction.

Schematic of a typical Magnox reactor

Natural Uranium Fuel

One of the most notable aspects of Magnox reactors is their use of natural uranium as fuel. Unlike many other reactor types that require enriched uranium, Magnox reactors could operate with uranium in its natural isotopic composition. This eliminated the need for expensive and complex uranium enrichment processes, making the reactors more economical and accessible during their time.

Magnox Alloy Cladding

The reactors are named after the Magnesium-Aluminum Non-OXidising alloy, known as Magnox, used to clad the fuel rods. This alloy was chosen for its excellent corrosion resistance and ability to withstand high temperatures. The cladding played a crucial role in containing the fission products and preventing the uranium fuel from oxidising, which was essential for the safe and efficient operation of the reactor.

Magnox fuel came in several variants as reactor designs evolved. This design uses spiral heat fins to improve heat transfer. The protruding plates keep the fuel element centred in the fuel channel.

Graphite Moderator

Magnox reactors used graphite as a neutron moderator. The graphite slowed down the fast neutrons produced during fission to thermal energies, which are more effective at sustaining the chain reaction. The use of graphite allowed the reactors to maintain a stable and controlled fission process, contributing to their reliability and safety.

Carbon Dioxide Coolant

Another unique feature of Magnox reactors is their use of carbon dioxide gas as a coolant. The carbon dioxide circulated through the reactor core, absorbing the heat generated by the fission reactions. This hot gas was then used to produce steam in a secondary circuit, which drove turbines to generate electricity. The choice of carbon dioxide as a coolant was innovative and provided several advantages, including chemical inertness and low neutron absorption.

Graphite, typically in the form of blocks, is used as a moderator in some reactor types, such as Magnox and AGRs.

Dual-Purpose Design

Some Magnox reactors (Calder Hall and Chapelcross) were designed to serve both civilian and military purposes. In addition to generating electricity, they were capable of producing plutonium for nuclear weapons. This dual-purpose design reflected the geopolitical climate of the Cold War era and demonstrated the versatility of nuclear technology.

Operational Simplicity and Safety

The design of Magnox reactors emphasised simplicity and safety. The use of natural uranium and graphite moderation reduced the complexity of the reactor core. Additionally, the reactors incorporated multiple safety features, such as robust containment structures and redundant cooling systems, to manage the risks associated with the revolutionary technology.

A glowing orange pellet of radioactive plutonium decaying and thereby generating large amounts of energy in the form of heat and light
A glowing orange pellet of radioactive plutonium decaying and thereby generating large amounts of energy in the form of heat and light

Differences in Designs

Magnox reactors, while sharing a common basic design, exhibited several differences across various installations. Here are some key distinctions:

Pressure Vessel Material: Early Magnox reactors, like those at Calder Hall and Chapelcross, used steel pressure vessels. Later designs, such as Oldbury and Wylfa, employed prestressed concrete pressure vessels, which allowed for higher operating pressures and temperatures.

Fuel Cladding: The Magnox alloy used for fuel cladding varied slightly in composition and thickness across different reactors. This affected the thermal efficiency and the maximum burn-up of the fuel.

Cooling Systems: While all Magnox reactors used carbon dioxide as a coolant, the configuration and efficiency of the cooling systems evolved. Later reactors had improved gas circulators and heat exchangers, enhancing overall thermal efficiency.

Reactor Core Design: The arrangement of graphite moderator blocks and the design of control rod channels differed among reactors. These variations influenced the neutron economy and the ease of refuelling.

Operational Lifespan: The design improvements in later reactors, such as those at Oldbury and Wylfa, allowed them to operate for longer periods compared to earlier reactors.

These differences reflect the evolutionary nature of Magnox reactor technology, with each new station incorporating lessons learned from its predecessors to improve performance and safety.

Where are the Magnox Reactors?

Calder Hall

Calder Hall, located at Sellafield in Cumbria, England, was the world’s first full-scale commercial nuclear power station. It began operation in 1956 and was primarily designed to produce weapons-grade plutonium for the UK’s nuclear weapons programme, while also generating electricity for the National Grid. The station featured four Magnox reactors, each with a capacity of 60 MWe, and operated until 2003. 

Go behind the scenes at the beginning of the civil nuclear age with our ‘Life at Calder Hall‘ page.

The Calder Hall nuclear power station in the United Kingdom, the world's first commercial nuclear power station.

Chapelcross

Chapelcross, located in Annan, Dumfries and Galloway, Scotland, was the country’s first commercial nuclear power station. Commissioned in 1959, it featured four Magnox reactors, each with a capacity of 60 MWe. Initially, its primary purpose was to produce weapons-grade plutonium for the UK’s nuclear weapons program, but it also generated electricity for the National Grid. Chapelcross ceased operations in 2004 and completed defueling by 2013. The site is currently undergoing decommissioning.

An image of the four cooling towers at Chapelcross Nuclear Power Station in Scotland, with the four reactor buildings sitting behind (each with two chimneys rising from their boilers).

Berkeley

Berkeley nuclear power station, located on the eastern bank of the River Severn in Gloucestershire, England, was one of the UK’s first commercial nuclear power stations. Commissioned in 1962, it featured two Magnox reactors with a combined capacity of 276 MW. Berkeley operated until 1989, after which it entered the decommissioning phase. The site has since been managed by the Nuclear Decommissioning Authority, with significant progress made in dismantling and decontaminating the facility.

Aerial view of one of the magnox reactors at Berkeley nuclear power station

Bradwell

Bradwell nuclear power station, located on the Dengie peninsula in Essex, England, was one of the UK’s early Magnox reactors. Commissioned in 1962, it had two reactors with a combined capacity of 242 MW. Bradwell operated until 2002 and was the first UK nuclear power station to enter long-term care and maintenance in 2019. 

At one point there were plans for a new nuclear power station, Bradwell B, but the project has since been discontinued.

Bradwell nuclear power station in 1963 with both Magnox reactor building visible

Hunterston A

Hunterston A, located in Ayrshire, Scotland, was one of the UK’s early Magnox nuclear power stations. Commissioned in 1964, it featured two reactors, each capable of generating 180 MWe. The station operated until 1990, after which it entered the decommissioning phase. The site is undergoing a lengthy process of dismantling and decontamination. The decommissioning process is expected to continue for several decades.

Hunterston A, with its two magnox reactors, in front of the the Firth of Clyde

Hinkley Point A

Hinkley Point A, located on the Bristol Channel coast in Somerset, England, was one of the UK’s early Magnox nuclear power stations. Commissioned in 1965, it featured two reactors with a combined capacity of 500 MW. The station operated until 2000, generating over 103 TWh of electricity during its lifetime.

Trawsfynydd

Trawsfynydd nuclear power station, located in Snowdonia National Park, Gwynedd, Wales, was the UK’s only inland Magnox power station. Commissioned in 1965, it featured two reactors with a combined capacity of 470 MW. The station operated until 1991 and has since been undergoing decommissioning. Unique for its inland location, Trawsfynydd used water from the artificial Llyn Trawsfynydd reservoir for cooling. The site has been designated as the lead project for decommissioning former Magnox stations in the UK.

Trawsfynydd nuclear power station as seen from across the reservoir. A dam can be seen on the mountain behind the station.

Dungeness A

Dungeness A, located on the Dungeness headland in Kent, England, was one of the UK’s early Magnox nuclear power stations. Commissioned in 1965, it featured two reactors with a combined capacity of 438 MW.

Dungeness A and B nuclear power stations at dusk with a lighthouse in the front of the picture

Sizewell A

Sizewell A, located on the Suffolk coast in England, was one of the UK’s first generation Magnox nuclear power stations. Commissioned in 1966, it featured two reactors with a combined capacity of 420 MW. The station operated until 2006, after which it entered the decommissioning phase.

Oldbury

Oldbury nuclear power station, located on the south bank of the River Severn in South Gloucestershire, England, was one of the UK’s early Magnox reactors. Commissioned in 1967, it featured two reactors with a combined capacity of 434 MW. Oldbury was notable for being the first UK nuclear power station to use prestressed concrete pressure vessels. The station operated until 2012 and is currently undergoing decommissioning.

Close-up of Oldbury nuclear power station under construction with a crane in the foreground

Wylfa

Wylfa nuclear power station, located on the island of Anglesey in Wales, was the last and largest of the UK’s Magnox reactors. Commissioned in 1971, it featured two reactors, each with a capacity of 490 MW. Wylfa’s Reactor 2  was shut down in 2012 – to make best use of the remaining Magnox fuel, which was no longer being manufactured). However, Wylfa’s Reactor 1 operated until 2015 – making it the longest-serving Magnox station and the world’s final first generation reactor to shut down. Wylfa was recently chosen as the preferred site for a new large scale nuclear power station.

Other Magnox Reactors

Japan

The Tōkai Nuclear Power Plant, located in Tōkai, Ibaraki Prefecture, Japan, holds the distinction of being Japan’s first commercial nuclear power station. The plant consists of two units: Tokai I and Tokai II. Tokai I, commissioned in 1966, was based on the British Magnox design and had a capacity of 166 MW. It operated until 1998 and was the first full-scale reactor in Japan to be decommissioned. The decommissioning process began in 2001 and involved dismantling the reactor and safely managing the radioactive waste.

Italy

The Latina Nuclear Power Plant, located in Lazio, Italy, was the country’s first commercial nuclear power station. Commissioned in 1963, it featured a single Magnox reactor with an initial capacity of 210 MW, later derated to 153 MW due to operational adjustments. Latina operated until 1987, when it was shut down following a national referendum on nuclear power prompted by the Chernobyl disaster. The plant’s decommissioning process began in 1999 under the management of SOGIN, Italy’s state-owned company responsible for nuclear decommissioning.

Latina nuclear power plant in Italy. The white dome of the magnox reactor building is visible in the centre of the image

North Korea

The Nyongbyon Nuclear Scientific Research Center, located in Nyongbyon County, North Pyongan Province, North Korea, is the country’s primary nuclear facility. Established in the 1980s, it houses North Korea’s first nuclear reactors and has been central to the nation’s nuclear weapons programme. The site includes a 5 MWe experimental Magnox reactor, a fuel fabrication plant, a reprocessing facility, and a short-term spent fuel storage facility. The Magnox reactor was initially disabled in 2007 as part of the six-party talks agreement but resumed operations in 2015.

Decommissioning the Magnox Reactors

The Decommissioning Process

Defueling: The first step in decommissioning a Magnox reactor is the removal of all spent nuclear fuel. This process is critical as it significantly reduces the radioactivity and heat within the reactor. The spent fuel is transported to facilities such as Sellafield, where it is managed and stored, prior to reprocessing.

Decontamination: After defueling, the reactor and associated systems undergo thorough decontamination. This involves cleaning surfaces and removing radioactive contaminants to reduce radiation levels and make the environment safer for workers.

Dismantling: The physical dismantling of the reactor structures and components is a complex and lengthy process. This includexs the removal of the reactor pressure vessel, control rods, and other internal components. Specialised equipment and techniques are used to safely cut and remove these materials.

Waste Management: Decommissioning generates a significant amount of radioactive waste. This waste is categorised based on its radioactivity level and managed accordingly. Low-level waste is often compacted and stored, while intermediate and high-level waste require more secure containment and long-term storage solutions.

Site Restoration: Once the reactor and associated facilities are dismantled, the site undergoes restoration. This involves demolishing remaining structures, remediating any contaminated soil, and preparing the site for potential future use. The goal is to return the site to a condition that poses no risk to the public or the environment.

Chapelcross Cooling Towers being demolished in 2007

The two magnox reactor building of Bradwell nuclear power station encased in scaffolding as seen from a nearby field
Bradwell undergoing decommissioning in 2014
Close-up shot of the two containment buildings of Bradwell nuclear power station
Bradwell in its current long-term care and maintenance state

Challenges

Safety: Ensuring the safety of workers and the public is paramount. Strict safety protocols and continuous monitoring are essential to prevent radiation exposure and accidents.

Technical Complexity: The decommissioning process involves intricate engineering challenges, particularly in dismantling radioactive components and managing waste.

Cost: Decommissioning is a costly endeavour. The Nuclear Decommissioning Authority (NDA) estimates that the total cost for decommissioning Magnox sites will be between £6.9 billion and £8.7 billion.

Time: The entire decommissioning process can take several decades as most Magnox stations were not designed and built with disassembly and decommissioning in mind.

Case Study: Trawsfynydd

Trawsfynydd in Gwynedd has been designated as the lead project for decommissioning former Magnox stations in the UK. Plans include the removal of the twin reactors, making it the first site in the UK to be fully decommissioned. This project serves as a model for other Magnox sites, demonstrating the feasibility and challenges of complete decommissioning. 

Trawsfynydd nuclear power station as seen from across the neighbouring reservoir

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