We now have TWO confirmed LOCA's and subsequent H2 explosions at the Fukushima complex. To make a big H2 boom, you have to make a lot of H2, which evidently comes from this reaction:

Zr + 2 H2O  -->  ZrO2 + 2 H2

So this means that lots of Zr fuel cladding has turned into zirconia and lots of H2, and that the really hot UO2 fuel pellets (self heating due to daughter products radioactive (via beta emission) decay) are now exposed to hot steam and some nasty "daughters" are puking out of the system via the steam vents. Just to add spice to the gumbo, Unit 3 has about 5% Mixed Oxide Fuel (plutonium 239 based, but also some Pu240).

Supposedly the daughters can provide 5 to 7% of the thermal energy of a fissioning facility, but since the Unit 1 was evidently about to get changed out, this could be more like 15%. For a 480 MW unit, this would mean that 36 to 72 MW of heat had to be removed, assuming that all fission reactions were stopped by the control rod insertion.

Well, that's a lot of heat. Translated, 7.5% residual heat generation is 36 MW is 122.8 MBtu/hr (millions of Btu/hr), and that is close to 126,000 lbs/hr of steam generation at atmospheric or 15,136 gallons/hr = 252 gpm water evaporation). For the metrically inclined, the 7.5% decay heat removal corresponds to an evaporation rate of 57 tonnes/hr of water. At 15%, you can double that. That's a lot of water buckets...... On the other hand, a gasoline powered water pump with a 2" pipe outlet (50 mm) might be able to do the trick, if they have them handy, and if they have a way to pump this water into the system. Power wise, they need about 25 kw engines to pump this water, or about 35 hp ones.

And that is needed for EACH active reactor.

Oh, BTW, they also need lots of cooling for those swimming pools where all those spent fuel rods are "cooling" off. While no where nearly as hot, take those out of water and they will also start glowing cherry red after a few minutes to a few hours. And when those catch on fire, well, that's just another massive load of stuff to hit the fan.