Magnox is an abbreviation/acronym for Magnesium Non-oxidizing, and refers to both an alloy of these properties and to a general type of nuclear reactor built using this alloy to clad and contain the fuel elements. Magnox reactors were designed in the United Kingdom, with the first unit coming online in the mid-1950s at Calder Hall. The original purpose of these reactors was to produce plutonium from low-grade uranium. As a by-product, the process of irradiating uranium until some quantity of it turns to plutonium generates a great deal of heat, and the British Government decided that since they darn sure were going to make the Pu-239, this heat was a 'freebie.' As such, the best thing to do with industrial-size quantities of reliable heat was to use it to spin steam turbines, and hence to make electricity.

Calder Hall and its later design siblings could run either a high-intensity fuel loading intended to produce plutonium, or a lower-intensity commercial fuel cycle intended to maximize useful operating time between refuelings. In either mode, a Magnox reactor unit produced roughly 150-250 megawatt electrical(MWe) of power. The Magnox name came from the alloy used to encase the uranium fuel elements. Composed primarily of magnesium as the name suggests, doped with aluminum and other trace metals, the Magnox alloy provided a cladding material that would not oxidize even under the high heat conditions inside the reactor, and provided a low neutron capture tendency thus becoming radioactive relatively slowly and, more important, providing a less 'lossy' cladding for these lower-power reactors.

Magnox coolant loops used gas rather than water or other liquids. This had pros and cons. Since the coolant was already gaseous, a runaway reactor would not be able to boil its coolant and produce a sudden rise in pressure in the coolant loop. This meant that coolant leaks were much easier to prevent. With boron-steel moderating rods, the Magnox design could run on 'natural' (unenriched) uranium, advantageous if one goal was to produce weapons grade materials. Plus, the low power density resulting from this fuel means that the reaction is easier to control and maintain.

On the other hand, Magnox reactors are limited to lower temperatures due to the melting limits of their cladding alloy, which results in lower efficiency due to Carnot Cycle losses. Their containment designs caused trouble, too - early Magnox reactors had containment vessels of steel. Since manufacturing pressure-tight vessels of this size is extremely difficult, these early models had only their core and primary heat exchanger loop inside this vessel - the secondary heat exchanger was outside. As a result, these early models produced noticeable levels of direct radiation as the radioactive primary coolant would radiate while outside the containment vessel. Later models used steel reinforced concrete containment vessels, which could be made much larger and holder higher pressures. These allowed both exchangers to sit inside the containment vessel and produced much lower levels of direct radiation.

Later in their lifecycle, the Magnox reactors proved to have other problems. The mild steel used in the moderating rods and other components was found to oxidize badly due to exposure to the high-temperature carbon dioxide used as a coolant, causing the power rating of the reactors to drop as they aged. Later versions used stainless steel and follow-on designs such as the AGR, or Advanced Gas Reactor, used graphite.

The first two Magnox power stations in the U.K. - Calder Hall and Chapelcross - were used for both plutonium enrichment and commercial power cycles. After these were built, the construction of Magnox power stations was turned over to consortia and run by commercial entities rather than the British Government directly. Early stations produced around 200 MWe; later stations had multiple reactor units and were thus able to produce more. In all, eleven Magnox reactor stations were built in the UK. In the order of their eventual/expected closure date, they were (thanks to 'No2Nukes' for this data):

Name Output
Berkeley 200 MW
Hunterston A 200 MW
Trawsfynydd 390 MW
Hinkley A 470 MW
Bradwell 246 MW
Calder Hall 194 MW
Chapelcross 196 MW
Dungeness A1 450 MW
Sizewell A 420 MW
Oldbury 434 MW
Wylfa 980 MW

1 - Dungeness A was, at retirement, the oldest operating nuclear plant in the world.

Magnox designs were exported to Italy and Japan. The design of these reactors was made public under the Atoms for Peace program, and North Korea's reactor at Yongbyon is a Magnox design. In addition, it has been reported that the 'Box on the Euphrates' in Syria, which was destroyed by the Israeli Air Force in 2007, was a nuclear reactor sourced from North Korea and based on the Magnox design.

Sources

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