The recent earthquake in Japan has triggered an actual nuclear emergency at the Fukushima Daini nuclear power plant. According to IAEA, the explosion earlier today originated in the Unit 1 reactor building and was not the result of explosive breach of the primary containment. One character interviewed on CNN called it a six sigma event.
If memory serves, water dissociates at ~2300 C. The cracking of coolant water by overheated fuel elements would result in the generation of noncondensable gases (H2 and possibly O2) that would add to the pressure excursion. Venting is the only option at that point. This was an issue at TMI. The explosive concentration range of hydrogen is very wide.
IAEA goes on to say that Units 1,2,and 4 are experiencing increased pressure, but Unit 3 is in a safe cold shutdown condition. Tokyo Electric Power Company received permission to inject boronated seawater into the Unit 1 reactor.
This is very ominous news. Plainly, if the cooling loops were dumping enough energy out of the reactor they would not inject corrosive sea water into it.
There is a lot of talk about a meltdown. As of this post, nothing has been disclosed about the actual state of the Unit 1 reactor core. There has been no word on the state of the fuel elements or the state of the coolant loops. I assume that the reactor design has a negative reactivity coefficient that will attenuate the reactivity with water coolant loss or void space formation. The link on reactivity coefficients delves into a number of interesting and perhaps not-so-intuitive effects on reactivity during an upset condition.