In 1959, thirteen OECD countries began the Dragon Reactor Experiment (DRE) to build a reactor at Winfrith in Dorset, United Kingdom. The Dragon was the first experimental high temperature gas-cooled reactor (HTGR) built in the 1960s; it was operated by the United Kingdom Atomic Energy Authority
The reactor, which operated successfully between 1966 and 1975, had a thermal output of 20 MW and achieved a gas outlet temperature of 750°C. The high temperature reactor concept, if it justified its expectations, was seen as having its place as an advanced thermal reactor between the current thermal reactor types such as the pressurised water reactor (PWR), the boiling water reactor (BWR), the advanced gas-cooled reactor (AGR) and the sodium-cooled fast breeder reactor. It was expected that the high temperature reactor (HTR) would offer better thermal efficiency, better uranium utilisation, either with low enriched uranium fuel or with high enriched uranium thorium fuel, better inherent safety and lower unit power costs. All these potential advantages were demonstrated to be achievable, in principle. This view is still shared today. In fact, a very high temporature reactors is one of the concepts retained for Generation IV. Projects on constructing modular pebble-bed reactors (PBRa) are also under way.