It was built between 1955 and 1960 to make plutonium for the UK's atomic arms and make some electricity for civil on the side. It was located near Annan Airfield in Dumfries-shire.
In 1967 there was a partial meltdown in reactor No 2 after a fuel rod broke and ignited, after which other fuel rods then caught fire. Radiation spewed all over and inside the plan, as well as Possibly in to the local environment. A long turn cover-up then issued and the truth would finally only come out after several decades had passed.
It has no official INES rating, but it was probably a Level 4 INES Scale event. Some anti-nuclear scare mongers have put it as level 5.