An Advanced Gas Cooled Reactor (AGR) is a type of nuclear reactor. These are the second generation of British gas-cooled reactors, using graphite moderator and carbon dioxide as coolant. The fuel is uranium oxide pellets, enriched to 2.5-3.5%, in stainless steel tubes. The carbon dioxide circulates through the core, reaching 650°C and then past steam generator tubes outside it, but still inside the concrete and steel pressure vessel. Control rods penetrate the moderator and a secondary shutdown system involves injecting nitrogen to the coolant or releasing boron ball shutdown devices.
The design of the AGR was such that the final steam conditions at the boiler stop valve were identical to that of conventional power stations. Thus the same design of turbo-generator plant could be used. In order to obtain high temperatures, yet ensure useful graphite core life (graphite oxidises readily in CO2 at high temperature) re-entrant flow is utilised, ensuring that the graphite core temperatures do not vary too differently than that seen on a Magnox station.
The AGR was developed from the Magnox reactor, also graphite moderated and CO2 cooled, and a number of these are still operating in UK. The Magnox used natural uranium fuel in metal form and magnesium based cladding.
The original design concept of the AGR was to use a beryllium based cladding. When this proved unsuitable, the enrichment level of the fuel was raised to allow for stainless steel cladding, and its associated neutron losses through capture in the cladding. This significantly increased the cost of the power produced by an AGR.
Like the Magnox and CANDU reactors, and in contrast to the light water reactors, AGRs are designed to be refuelled without being shut down first, though a number of nuclear safety issues were identified in relation to this and so all AGRs either refuel at part load or when shutdown.
The prototype AGR at the Sellafield (Windscale) site is in the process of being decommissioned. The project is also a study of what is required to safely decommission a nuclear reactor.
Currently there are seven nuclear generating stations each with two operating AGRs in the United Kingdom. They are all owned and operated by British Energy. These are located at Dungeness B, Hartlepool, Heysham 1, Heysham 2, Hinkley Point B, Hunterston B and Torness.
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