It was a major hydrogen gas explosion that damaged 3 power station buildings and forced shutdown of Millstone-1 Boiling Water Reactor on June 10th, 1977. It has on official INES rating, but was probably a level 4 event. 15 peole died.
Later, a leaking valve forces shutdown Millstone Nuclear Power Plant Units 1 and 2 on February 20th, 1996 revealed multiple equipment failures found. 1 person died and it has no offical INES rating, but was probably a lever 3 event.