According to one document[1] > The decay heat produced by a reactor shutdown from full power is initially equivalent to about 5 to 6% of the thermal rating of

According to one document[1] > The decay heat produced by a reactor shutdown from full power is initially equivalent to about 5 to 6% of the thermal rating of the reactor. This decay heat generation rate diminishes to less than 1% approximately one hour after shutdown. However, even at these low levels, the amount of heat generated requires the continued removal of heat for an appreciable time after shutdown. At 400 MW it takes about 4 hours to boil the water in an olympic swimming pool[1], so there should be some safety margin for the power to drop. Then once it is down to 1% decay heat it could boil for a couple of weeks before running dry. [1] https://web.archive.org/web/20110316042315/http://www.hss.doe.gov/nuclearsafety/ns/techstds/standard/hdbk1019/h1019v2.pdf https://web.archive.org/web/20110316042315/http://www.hss.do... [2] http://www.wolframalpha.com/input/?i=(specific+heat+of+vaporization+of+water)+*+(2500+m%5E3)+*+(density+of+water)+%2F+400+MW http://www.wolframalpha.com/input/?i=(specific+heat+of+vapor...