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 2,000,000 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.