A pair of air cooled graphite cored reactors built by the British to make plutonium for their weapons project. One of the cores caught fire in 1957. They got very lucky, the containment held, and it was put out with water. There was some release of radiation. Let’s call this one a near miss.
It was bad enough that both reactors were mothballed in place and no one has ever built another air cooled graphite reactor.

The site got renamed Sellafield, was used for weapons and then nuclear fuel manufacture and reprocessing. It is now the subject of an ongoing clean-up effort. To pick one example, there’s an open pool that was used for waste storage from the 1960s to the 1980s. Among other things, there is an estimated 1.5 tons of plutonium in it. It is a place so radioactive that two minutes near the edge of the pond is the limit for a human being. The pool attracts birds that land and take off, the walls have creaks permitting leaks, the poo spreads.

There are many different kinds of radioactive waste and each has its own half-life so, just to be on the safe side and to simplify matters, I base my calculations on the worst one and that’s plutonium.
–David R. Brower