FAQs: Types of nuclear reactors

Wylfa magnox nuclear reactor

What's a Magnox reactor?

Magnox reactors are a British design. Only two Magnox stations were sold abroad - one to Japan and one to Italy. The first one was built at Calder Hall, next to Sellafield, and opened by the Queen on 17th October 1956. As with most nuclear reactors they 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%.

The remaining Magnox stations are at least 30 years old. BNFL freely admits that the they are loss-making. They are not very efficient, and produce more dangerous nuclear waste per unit of electricity than more modern stations, and radiation doses to members of the public living nearby receive are higher too.

What are AGRs?

An AGR is an Advanced Gas-cooled Reactor. The design was an attempt to improve on the Magnox design, aiming to achieve higher gas temperatures to improve the 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 reactors, 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.

What is a PWR?

Pressurized Water Reactors use enriched uranium oxide fuel elements and ordinary water is used as the coolant, but it is kept under pressure to prevent it from boiling. The reactor core is contained in a pressure vessel made of welded steel. The water inside the pressure vessel serves also as the "moderator" which slows down the neutrons released when uranium atoms are split, allowing them to hit and split more uranium atoms. This water, or primary coolant, is used to heat water in another cooling circuit, which is allowed to boil - the resulting steam is used to drive the turbines.

The major concern with the current generation of PWRs is with the reliability of systems used to shut down the reactor and remove decay heat, after a loss of coolant incident. In March 1978, just such an accident happened in a PWR at Three Mile Island, Pennsylvania, 240 kilometres from New York. An initial failure of a pump that sent water past the fuel was followed by failures of other equipment together with errors in judgement by the staff. This led to the overheating of the core and some of it actually melted. PWRs are the dominant reactor-type worldwide, although only one has ever been built in the UK at Sizewell B. The reactor at Sizewell was designed by an American company, Westinghouse, which has now been bought by British Nuclear Fuels Ltd.

If new stations are built in Britain, what type would they be?

The most likely reactor-type would be one known as the AP1000, designed by the American company, Westinghouse - now owned by BNFL. The AP1000 design has evolved from the PWR, but none have been built anywhere in the world . The smaller AP600 was awarded a Design Certification by the US Nuclear Regulatory Commission in 1999, but the AP1000 variant is still under review for certification.

Westinghouse says the AP1000 uses "advanced passive safety" systems which use forces of nature to ensure safety, rather than relying on the operators to intervene. The design places greater emphasis on gravity and convection to keep the reactor from overheating. Westinghouse claim that the AP units will require 50% less pumps, 60% fewer valves and pipes and 80% less control cable than PWRs. But another way of looking at it is that compared with the original PWR, the AP600/1000 design has many of the safety systems stripped out to save money.

The idea of "passive safety" is to avoid reliance on operators and power during an emergency situation, but this could also mean that when certain events occur, operators would be powerless to deal with them, and in some situations the passive systems could make the situation worse. In order to improve heat dissipation in the event of an emergency, the structural integrity of the reactor dome has been sacrificed - heat dissipation relies upon air-cooling using atmospheric air. This means that the "secondary containment system" that is supposed to stop radioactivity escaping in a meltdown actually has a huge hole in it. It also means the reactor is more vulnerable to malicious attack. After September 11th one might have expected new reactors to be designed to be less vulnerable to terrorist attack. With the AP600/1000 design, the reverse is the case.

Westinghouse claims that because it will use modular construction techniques, it will be possible to build advanced reactors in 30 to 44 months, compared to the typical 100 months required to build a PWR. The lower costs claimed also assume that an AP1000 will be able to run for 60 years, (compared to Sizewell B, which it is assumed will run for 40 years). Experience tells us to treat these claims with extreme scepticism.

BNFL is known to be keen on building at least two AP1000 reactors which would be fuelled with a 100% MOX (plutonium) fuel . This means that any community chosen as a site for an AP1000 can expect to see armed convoys travelling through them from Sellafield, because it would be relatively easy for terrorists to separate out plutonium for use in a nuclear weapon from MOX fuel.