Advanced Gas-cooled Reactor - AGR Design

AGR Design

The design of the AGR was such that the final steam conditions at the boiler stop valve were identical to that of conventional coal-fired power stations, thus the same design of turbo-generator plant could be used. The mean temperature of the hot coolant leaving the reactor core was designed to be 648°C. In order to obtain these high temperatures, yet ensure useful graphite core life (graphite oxidises readily in CO₂ at high temperature) a re-entrant flow of coolant at the lower boiler outlet temperature of 278°C is utilised to cool the graphite, ensuring that the graphite core temperatures do not vary too much from those seen in a Magnox station. The superheater outlet temperature and pressure were designed to be 2,485 psia (170bar) and 543°C.

The fuel is uranium dioxide pellets, enriched to 2.5-3.5%, in stainless steel tubes. 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 the higher neutron capture losses of stainless steel cladding. This significantly increased the cost of the power produced by an AGR. The carbon dioxide coolant circulates through the core, reaching 640°C (1,184°F) and a pressure of around 40 bar (580 psi), and then passes through boiler (steam generator) assemblies outside the core but still within the steel lined, reinforced concrete pressure vessel. Control rods penetrate the graphite moderator and a secondary system involves injecting nitrogen into the coolant to hold the reactor down. A tertiary shutdown system which operates by injecting boron balls into the reactor has been proposed 'as retrofit to satisfy the Nuclear Installations Inspectorate’s concerns about core integrity and core restraint integrity'.

The AGR was designed to have a high thermal efficiency (electricity generated/heat generated ratio) of about 41%, which is better than modern pressurized water reactors which have a typical thermal efficiency of 34%. This is due to the higher coolant outlet temperature of about 640 °C (1,184°F) practical with gas cooling, compared to about 325 °C (617°F) for PWRs. However the reactor core has to be larger for the same power output, and the fuel burnup ratio at discharge is lower so the fuel is used less efficiently, countering the thermal efficiency advantage .

Like the Magnox, CANDU and RBMK reactors, and in contrast to the light water reactors, AGRs are designed to be refuelled without being shut down first. This on-load refuelling was an important part of the economic case for choosing the AGR over other reactor types, and in 1965 allowed the CEGB and the government to claim that the AGR would produce electricity cheaper than the best coal-fired power stations. However fuel assembly vibration problems arose during on-load refuelling at full power, so in 1988 full power refuelling was suspended until the mid-1990s, when further trials led to a fuel rod becoming stuck in a reactor core. Only refuelling at part load or when shut down is now undertaken at AGRs.

The AGR was intended to be a superior British alternative to American light water reactor designs. It was promoted as a development of the operationally (if not economically) successful Magnox design, and was chosen from a plethora of competing British alternatives - the helium cooled High Temperature Reactor (HTR), the Steam Generating Heavy Water Reactor (SGHWR) and the Fast Breeder Reactor (FBR) - as well as the American light water pressurised and boiling water reactors (PWR and BWR) and Canadian CANDU designs. The CEGB conducted a detailed economic appraisal of the competing designs and concluded that the AGR proposed for Dungeness B would generate the cheapest electricity, cheaper than any of the rival designs and the best coal-fired stations.

There were great hopes for the AGR design. An ambitious construction programme of five twin reactor stations, Dungeness B, Hinkley Point B, Hunterston B, Hartlepool and Heysham was quickly rolled out, and export orders were eagerly anticipated. However, the AGR design proved to be over complex and difficult to construct on site. Notoriously bad labour relations at the time added to the problems. The lead station, Dungeness B was ordered in 1965 with a target completion date of 1970. After problems with nearly every aspect of the reactor design it finally began generating electricity in 1983, 13 years late. The follow on stations all experienced similar problems and delays. The financing cost of the capital expended, and the cost of providing replacement electricity during the delays, were enormous, totally invalidating the pre-construction economic case.

The small-scale prototype AGR at the Sellafield (Windscale) site is being decommissioned. This project is also a study of what is required to decommission a nuclear reactor safely.