PLUTO was a nuclear reactor at the Atomic Energy Research Establishment at Harwell, Oxfordshire in the United Kingdom, one of five reactors on the site.
It was based on the design of DIDO and 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.
PLUTO was the third DIDO class reactor to become operational, after DIDO itself and HIFAR.
The 26 MW PLUTO reactor went online in 1957 and was shut down in 1990. It is expected to be completely dismantled by 2024.
Both PLUTO and DIDO were used initially to test materials for use in commercial reactors, but latterly as intense neutron sources for diffractometry and the investigation of crystal structures.