While the containment plays a critical role in the most severe nuclear reactor accidents , it is only designed to contain or condense steam in the short term ( for large break accidents ) and long term heat removal still must be provided by other systems . In the Three Mile Island accident the containment pressure boundary was maintained , but due to insufficient cooling , some time after the accident , radioactive gas was intentionally let from containment by operators to prevent over pressurization . This , combined with further failures , caused the release of up to 13 million curies of radioactive gas to atmosphere during the accident
Three Mile Island accident
Three Mile Island accident
Containment building