Dragon was an experimental high temperature gas-cooled reactor at Winfrith in Dorset, England, operated by the United Kingdom Atomic Energy Authority (UKAEA).

[2] Dragon's construction was followed by similar work in the US, leading ultimately to the much larger Fort Saint Vrain Nuclear Power Plant.

[3] While the TRISO assembly provides the fuel, a reactor also requires a neutron moderator and a cooling fluid to remove the heat and extract its energy.

In the case of Dragon, the fuel was produced in small spherical pellets and then pressed into larger blocks containing the graphite moderator formed into long hexagonal rods.

Rennie favoured the use of helium as it would reduce corrosion issues and had the added advantage of having a very low nuclear cross section which improved the neutron economy and meant the gas would not become highly radioactive over time.

Risley Nuclear Laboratories, developing the carbon dioxide (CO2) cooled Advanced Gas-cooled Reactor (AGR), argued that the supply would be a serious issue, while CO2, in spite of any technical downsides, was trivially available.

The OEEC team overseeing the project admitted that the helium issue "throw(s) doubt on the feasibility of employing it in an extensive power program.

The UKAEA, in particular, had already gone ahead with the AGR program and were noting the issues with corrosion due to the CO2, and raised concerns that helium might not be as inert as proponents suggested.

Criticality is only possible when the blocks are placed together in certain configurations within a neutron reflector, allowing additional fuel to be held in a ready area and loaded on-the-fly.

[10] The Winfrith site extended to 129.4 hectares (320 acres) of heathland in rural south Dorset, and nine different experimental reactors were located there.