3 April 1960 Westmoreland County, Pennsylvania Core melt

A core melt accident occurred at the Westinghouse Waltz Mill test reactor. From what information remains of the event, one fuel element melted, resulting in the disposition of 2 million gallons of contaminated water generated during the accident. At least a portion of the water was retained on site in lagoons, a condition which eventually led to detectable 90Sr in groundwater plus contaminated soil. The site is currently undergoing cleanup.

It dose not have an offical INES rating, but was probably a level 4 incident.


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