Westinghouse have developed the AP1000 PWR, which is capable of passively cooling itself in the event of something like a primary cooling loop failure like what happened at Three Mile Island. TMI apparently boiled its reactor dry and reached criticality in 2.5mins, it can be that fast. The AP1000 is a significant leap forward compared to the units that were in operation in Japan.
http://www.power-technology.com/proj...s/5-ap1000.jpg

