Dragon was an experimental high temperature gas-cooled reactor at Winfrith in Dorset, England, operated by the United Kingdom Atomic Energy Authority. Its purpose was to test fuel and materials for the European High Temperature Reactor programme, and it was built and managed as an Organisation for Economic Co-operation and Development/Nuclear Energy Agency international project. Dragon used helium gas as the coolant and graphite as the neutron moderator. Fuel was formed into tiny spherical pellets and then coated with ceramics. These were then mixed with the graphite and pressed together to form blocks of various shapes and sizes. 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. Helium was used due to its low nuclear cross section which led to higher neutron economy, as well as its chemical inertness allowing it to operate at higher temperatures without fear of eroding the reactor materials. Higher temperatures also allow for more efficient steam turbine operation and make it more suitable for direct use as process heat. In the case of a power failure, natural convection of the helium provided emergency cooling. Dragon was a test machine only, and did not attempt power extraction. Many designs of the 1960s and 70s were based on this general TRISO fuel concept, including "prismatic" designs with fixed fuel layouts like Dragon, and the pebble-bed reactor designs being developed in Germany. none of these concepts have been used for commercial reactors.
The reactor
The Atomic Energy Establishment at Winfrith was built for the construction and operation of experimental and research nuclear reactors. The site extended to of heathland in rural south Dorset, and nine different experimental reactors were located there. Dragon was a helium-cooled reactor and was a multi-national collaboration in nuclear energy production by several nations, a European inter-governmental research and development project. The fuel used in the reactor was coated particles, consisting of micro-pellets of a fissile material surrounded by a ceramic outer layer. The reactor resembled an enormous bottle, with the larger area at the bottom containing the active fuel within the reflector, and the smaller area on top holding additional fuel elements for reloading. The Dragon complex consisted of the reactor and various buildings constructed in the late 1950s/early 1960s to support operations. Construction started in 1959 and was completed in 1962. Operation started in 1965 with a power output of 20MW. The reactor operated until 1976 when it was placed in care and maintenance. The facility was high and in diameter. It was partially decommissioned in 2005. Of the nine experimental reactors at Winfrith, only the Dragon Reactor and the Steam Generating Heavy Water Reactor remain, and they are in the process of being decommissioned. During decommissioning, the reactor vessels will be placed in reactor safestores, other structures will be dismantled and stored, any remaining waste will be sent to allocated storage sites, and the soil will be removed as necessary to a suitable Low Level Waste Repository. Finally, the site will be declassified as a nuclear licensed site, landscaped and returned to normal use. The contract for decommissioning the site has been awarded to Costain Nuclear, and the final decommissioning phase has been deferred for twenty years.
