Caithness in 1955 was the right kind of place to put an experimental nuclear reactor: remote enough that no one would be near it if something went wrong, served by a railway and a harbour for moving materials, and with a small population that needed work. The Nuclear Power Development Establishment opened on the site of a wartime airfield, RAF Dounreay, which had operated as HMS Tern (II) under naval control from 1944 but had seen no action and been mothballed in 1949. The first reactor, the Dounreay Materials Test Reactor, went critical in May 1958 - a DIDO-class research reactor used to test how metals behaved under intense neutron bombardment. The second, and the one inside the sphere, was the Dounreay Fast Reactor, which achieved criticality on 14 November 1959 and began exporting power to the National Grid on 14 October 1962. By the time it was shut down for decommissioning in March 1977, it had supplied 540 million kilowatt-hours of electricity. The whole installation, in 1950s money, had cost £15 million - around £370 million in 2021 terms.

A fast breeder reactor is not just a power station. It is, in theory, a power station that produces more fuel than it consumes - turning non-fissile uranium-238 into fissile plutonium-239 as a side effect of generating heat. In an age when uranium was thought to be scarce, breeders looked like the long-term answer to nuclear power. The Dounreay Fast Reactor proved that the concept worked. Its successor, the Prototype Fast Reactor, was a serious commercial-scale attempt: 250 megawatts, designed to test the components of a full-scale future plant. The PFR ran from 1974 to 1994 with persistent problems in its sodium-water steam generators and a lifetime load factor of only 26.9 percent - meaning it produced about a quarter of the electricity it theoretically could have. In 1988 the British government cut fast breeder funding from £105 million a year to £10 million. The world had not run out of uranium; the breeder dream had outlived its economic case. The PFR was shut down in 1994 and never restarted.

Dounreay's hardest legacy is a 65-metre vertical shaft. It was originally dug as part of a tunnel for the sea discharge pipe; afterwards, with the tunnel finished, it was simply convenient to drop intermediate-level nuclear waste into the empty shaft, and so this is what was done. There were no proper records. There was no engineered containment. At one point, workers fired rifles into the shaft to sink polythene bags that were floating on the water at the bottom. In May 1977 a hydrogen explosion in the shaft - caused by sodium and potassium wastes reacting with water - blew radioactive particles into the surrounding area. The particles were milled into shards, some of which washed into the sea as the cooling ponds were later drained. In 2012, a particle two million becquerels in activity was found on Sandside beach, twice the radioactivity of any previously discovered. In 2007 the UKAEA pleaded guilty to four charges relating to activities between 1963 and 1984 - illegal dumping, releasing fuel particles into the sea - and was fined £140,000. The full cleanup of the shaft is among the most difficult tasks remaining.

All five reactors at Dounreay are shut down. The site is now operated as NRS Dounreay - Nuclear Restoration Services Dounreay - having passed through several corporate restructurings since the Nuclear Decommissioning Authority took ownership in April 2005. The current schedule brings the site to an interim care and surveillance state by 2036 and to a brownfield site, fit for other uses, by 2336. Yes, three centuries from now. The plutonium has already been moved to Sellafield, the highly enriched uranium has been sent to the United States, and the dismantling of the PFR core began in 2016. Workers at the site are still finding parts of the plant being entered for the first time in fifty years, and the surprises are rarely pleasant. The adjacent Vulcan Naval Reactor Test Establishment - a Ministry of Defence facility where Britain's submarine reactors were prototyped - is being decommissioned in parallel under a separate ten-year contract that began in 2023. The sphere on the coast, the visible signature of an era when Britain meant to lead the world in atomic power, will outlast almost everyone alive today and the most of their grandchildren too.