"Where and when?"
1979: Three Mile Island, nr. Harrisburg, Pa.: one of two reactors lost its coolant, which caused overheating and partial meltdown of its uranium core. Some radioactive water and gases were released. This was the worst accident in U.S. nuclear-reactor history. (approximately 2.5 million curies of radioactive gases, and approximately 15 curies of iodine-131 plus 40,000 gallons of radioactive waste into Susquehanna River). Cleanup cost - $1 billion.
That's two major accidents. There were 8 others smaller accidents with deaths or steam release.