DIDO was a 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:
HIFAR was the last to shut down, in 2007.