Advanced Gas-cooled Reactor

These are the second generation of British gas-cooled reactors, using graphite as the neutron moderator and carbon dioxide as coolant.

They have been the backbone of the UK's nuclear power generation fleet since the 1980s. The AGR was developed from the Magnox reactor, the UK's first-generation reactor design.

The first Magnox design had been optimised for generating plutonium,and for this reason it had features that were not the most economic for power generation. Primary among these was the requirement to run on natural uranium, which required a coolant with a low neutron cross section, in this case carbon dioxide, and an efficient neutron moderator, graphite.

The Magnox design also ran relatively cool gas temperatures compared to other power-producing designs, which resulted in less efficient steam conditions.

The AGR design retained the Magnox's graphite moderator and carbon dioxide coolant but increased the cooling gas operating temperature to improve steam conditions. These were made identical to those of a coal fired plant, allowing the same design of turbines and generation equipment to be used.

During the initial design stages it was found necessary to switch the fuel cladding from beryllium to stainless steel. However, steel has a higher neutron cross section and this change required the use of enriched uranium fuel to compensate.

This change resulted in a higher burnup of 18,000 MWt-days per tonne of fuel, enabling less frequent refuelling. The prototype AGR became operational at Windscale in 1962, but the first commercial AGR did not come on-line until 1976. A total of fourteen AGR reactors at six sites were built between 1976 and 1988.

All of these are configured with two reactors in a single building, and each reactor has a design thermal power output of 1,500 MWt driving a 660 MWe turbine-alternator set.

The various AGR stations produce outputs in the range 555 MWe to 670 MWe though some run at lower than design output due to operational restrictions