It was a tritium leak at the American Atomics facility in Tucson, Arizona, at the public school across the street from the plant on September 29th, 1979.
About $300,000 worth of food was found to be contaminated; the chocolate cake had 56 nCi/L (2,100 Bq/L). By contrast, the EPA safety limit for drinking water is 20 nCi/L (740 Bq/L) based on consumption of two liters per day.
It dose not have an official INES rating, but was probably a level 4 incident.