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.

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 the reactor design would become responsible for changes to US regulatory classifications after the US–UK reactor-grade plutonium detonation test of the 1960s.

As of 2016[update], North Korea remains 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The magnox design was superseded by the advanced gas-cooled reactor, which is similarly cooled but includes changes to improve its economic performance.

[2][3] The reactors consisted of a huge cube of this material (the "pile") made up of many smaller blocks and drilled through horizontally to make a large number of fuel channels.

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".

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.

Magnox alloy is reactive with water, which means it cannot be left in a cooling pond after extraction from the reactor for extended periods.

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.

Due to the size of the pile, only the reactor core itself was placed within the steel pressure assembly, which 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 CO2 gas outside 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.

Later units at Oldbury and Wylfa replaced the steel pressure vessels with prestressed concrete versions which also contained the heat exchangers and steam plant.

This caused some problems as the Nimonic springs used contained cobalt, which became irradiated giving high gamma level when removed from the reactor.

As the magnox design was being rolled out, work was already underway on the advanced gas-cooled reactor (AGR) with the explicit intention of making the system more economical.

Former Treasury Economic Advisor, David Henderson, described the AGR programme as one of the two most costly British government-sponsored project errors, alongside Concorde.

[18] The magnox reactors were considered at the time to have a considerable degree of inherent safety because of their simple design, low power density, and gas coolant.

Loss of coolant accidents (at least those considered in 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.

[19] North Korea also developed their own magnox reactors, based on the UK design which was made public at an Atoms for Peace conference.

[24] 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.

[31] The Nuclear Decommissioning Authority (NDA) announced on 30 December 2015 that Wylfa Unit 1 – the world's last operating Magnox reactor – was closed.

[32] The small 5 MWe experimental reactor, based on the magnox design, at Yongbyon in North Korea, continues to operate as of 2016[update].

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 a non-oxidising covering to contain fission products.

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.

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.