How an AGR reactor works

The AGR has no steel vessel inside a containment building: the entire reactor, with core, boilers and circulators, sits inside a prestressed concrete cylinder about 20 meters in inner diameter with walls around 5 meters thick. Thousands of tensioned steel tendons compress the concrete so that the gas at 40 bar never puts it in tension, and a water-cooled inner steel liner makes it leak-tight.

That vessel is at once the primary circuit and the containment. It cannot fail suddenly: if a tendon breaks, the others take up the load, and any leak would be slow and through cracks. The first Magnox stations used steel spheres; the last of that family, Oldbury and Wylfa, introduced the prestressed concrete the AGR adopted as standard.

The moderator is a stack of sixteen-sided graphite bricks with about 300 vertical fuel channels. In each channel hangs a stringer of eight elements: each carries 36 pins of UO₂ pellets enriched to 2.5 to 3.5%, in stainless steel cladding inside a graphite sleeve. The steel withstands the 640 °C gas that Magnox could not, but it absorbs neutrons, and that forces the uranium to be enriched.

The coolant is carbon dioxide at about 40 bar. It enters below the core at about 290 °C, rises through the channels and leaves at about 640 °C toward the hot gas dome; part of the flow first passes through the graphite to keep it below 450 °C. Over the years the gas slowly oxidizes the graphite, and since the bricks cannot be replaced, the state of the core is what sets the life of each reactor.

The boilers, which in a PWR would sit in separate buildings, go in the annulus between the core and the concrete wall. They are once-through heat exchangers: feedwater enters at about 150 °C at the bottom, rises through steel coils and leaves at the top as superheated steam, while the hot CO₂ flows down counter-current outside the tubes. Depending on the station there are four or twelve units around the core.

Below the boilers, eight gas circulators of about 5 MW each, with the motor outside the vessel, push the CO₂, now cooled to about 290 °C, back under the core. If they stop, natural circulation of the gas inside the vessel keeps carrying decay heat to the boilers: that is one of the design's classic safety arguments.

It is the figure that defines the AGR: steam leaves at about 170 bar and 540 °C, with reheat, the same conditions as a modern coal plant. That is why the 660 MWe turbines and generators are the same ones British industry already built for fossil stations, and the cycle efficiency is around 41%, against 33% for a light water reactor with saturated steam at 285 °C.

On the cold side, every AGR station is on the coast: the condenser uses seawater. Each station has two twin reactors of 550 to 660 MWe, and the turbine hall is a clean area, because the steam never passes through the reactor and the CO₂ stays locked inside the concrete.

Above the vessel is the charge hall, with a fueling machine of hundreds of tonnes that travels on a gantry crane and couples to the standpipes passing through the concrete lid. Pressurized to the reactor's own 40 bar, it pulls out the complete eight-element stringer and lowers a new one, without opening the vessel or, in principle, stopping the reactor.

In practice, full-power refueling caused vibration problems in the stringers, and the stations do it with the reactor at low power or shut down. Even so, changing a few channels at a time, without the long outage of a PWR, remains one of the advantages the AGR inherited from Magnox and the CANDU.

Calder Hall, connected to the grid in 1956, was the world's first commercial-scale nuclear power station: natural uranium metal in magnesium-alloy cladding, graphite and CO₂. The United Kingdom built 26 Magnox reactors and in 1965 bet on their evolution, the AGR, with Dungeness B as the first unit. That project took 18 years to generate, and the whole fleet, 14 reactors at 7 stations, entered service between 1976 and 1989 with the delays and cost overruns that marked the program.

Dungeness B, Hunterston B and Hinkley Point B closed between 2021 and 2022 because of the state of their graphite; Hartlepool and Heysham 1 are expected to run until 2027, and Heysham 2 and Torness until 2030. No other country adopted the design. Sizewell B, from 1995, is the only British PWR, and with Hinkley Point C the country returned to pressurized water.