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DIDO was a materials testing nuclear reactor at the Atomic Energy Research Establishment at Harwell, Oxfordshire in the United Kingdom. It used enriched uranium metal fuel, and heavy water as both neutron moderator and primary coolant. There was also a graphite neutron reflector surrounding the core. In the design phase, DIDO was known as AE334 after its engineering design number.
DIDO was designed to have a high neutron flux, largely to reduce the time required for testing of materials intended for use in nuclear power reactors. This also allowed for the production of intense beams of neutrons for use in neutron diffraction.
DIDO was shut down in 1990. The primary facilities decommissioning is expected to be complete in 2023 with the reactor decommissioning completed in 2031 and final site clearance achieved in 2064 
In all, six DIDO class reactors were constructed based on this design:
- PLUTO, also at Harwell, first criticality 1957.
- HIFAR (Australia), first criticality January 1958.
- Dounreay Materials Testing Reactor (DMTR) at Dounreay Nuclear Power Development Establishment in Scotland, first criticality May 1958.
- DR-3 at Risø National Laboratory (Denmark), first criticality January 1960. 
- FRJ-II at Jülich Research Centre (Germany), first criticality 1962.
HIFAR was the last to shut down, in 2007.
- "Harwell Decommissioning Update 3" (PDF). Retrieved 13 July 2012.
- "Nuclear Decommissioning Authority Harwell Site". Retrieved 13 July 2012.
- http://sundhedsstyrelsen.dk/da/sundhed/straalebeskyttelse/nukleare-anlaegatomanlaeg/udgivelser/~/media/41E3012B782547DE9711945FB39E5E52.ashx p.15, retrieved 19 February 2014
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