26 July 1959 Santa Susana Field Laboratory, Partial meltdown

A partial meltdown A partial core meltdown took place when the Sodium Reactor Experiment (SRE) experienced a power excursion that caused severe overheating of the reactor core, resulting in the melting of one-third of the nuclear fuel and significant releases of radioactive gases. The amount of radioactivity released is variously reported as 240 to 260 times worse than Three Mile Island. Over the succeeding years, the site was cleaned up and all buildings and contamination removed. The soil was removed and other soil brought in and now forms a portion of an area near the Simi Valley Adventist Hospital.

It dose not have an offical INES rating, but was probably a level 6 incident.


 * Sources: https://en.wikipedia.org/wiki/List_of_civilian_nuclear_accidents and https://www.wikiwand.com/en/List_of_civilian_radiation_accidents.