Postmortems

Nuclear reactors are complicated, and sometimes people make mistakes.

Nuclear reactors are complicated, and sometimes people make mistakes. (Many good stories (explanations) on this site.)
“”“When equipment, training, and procedure problems causing minor events are tolerated instead of being fixed, these pre-existing impairments can shorten the list of “what if’s” needed for future failures to result in more serious outcomes.
While it may seem intuitive that a shut down reactor is inherently “safer” than an operating one, this is not necessarily the case.
Shutdown cooling pump 12 was designated as the “protected train” for cooling the water inside the reactor pressure vessel at Nine Mile Point Unit 1. But that designation only protected it from being intentionally turned off. It remained vulnerable to being inadvertently turned off.
The operators restored cooling of the water inside the reactor pressure vessel at 4:17pm. In the 31 minutes that cooling had been lost, the water heated up to 145°F, a heatup rate of about one degree per minute. Had cooling not been restored, the water would have begun boiling around 5:30pm.
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http://allthingsnuclear.org/nine-mile-points-pyramid-scheme