A reader asked some interesting questions

I have been following the news about the Daiichi reactors, and the main stream reporting has been poor. Each time I read an update…it raises more questions that I hope you can clear up. For starters, 

What causes the hydrogen build up? Were does it build up? What exactly blows up?

The enriched uranium fuel is encased in Zircaloy. At high temperature when fuel cells are exposed above the liquid water level, there is an oxidation reaction between the zirconium in the Zircaloy and steam (H₂O) which produces zirconium dioxide and hydrogen gas.

Zr + H₂O -> ZrO₂ + H₂

     (For the chemists out there like myself, this is not intended to be stoichiometric)

This is produced in the containment vessel and builds up together with steam from evaporating water and radioactive Cesium and Iodine from the fuel rods.  When you have hydrogen produced in a reactor, you know you have a problem, because it signals that fuel rods are exposed.  At Three Mile Island a hydrogen bubble developed in the reactor because fuel rods were exposed (it was there when President Carter visited the plant and there was a lot of concern that it would blow up during his visit to the plant), and it took a long time to slowly vent the hydrogen to minimze risk of an exposion.

At Fukushima, the reactor vessels at Units 1,3 were vented at different times to reduce pressure.  When the mixture was vented it was vented into the top of the reactor building, where there was air (20% oxygen).  There was a built up of hydrogen in the building, and as you know from the Hindenberg disaster, hydrogen and oxygen react rapidly and violently. Hydrogen explosions happened at both Units 1 and 3.  The cladding on the upper part of the reactor building was blown out. Fortunately the explosion was not violent enough to damage the reactor containment.

If they have the ability to pump water to the reactors, why are they still having trouble keeping them cool? Is it that they can not pump enough volume?

As WNN explained, there are two things keeping the water levels from rising higher than about half way up the fuel rods in Units 1,3. As they pump water in, because of the heat being produced by the decay reactions in the fuel rods, the water is evaporating and it appears that they can’t pump water in faster than it is evaporating. Secondly, the steam and hydrogen pressure buildup as a result of evaporation and the zirconium oxidation reaction makes it increasingly harder to pump water in.  Thirdly, I suspect that they have very limited pumping capacity at the plant because it has not had and still does not have off-site power since the earthquake, nor has it had functioning backup diesel generators since the tsunami (except for possibly one at Unit 6).  Some mobile generation capacity has been brought in.  TEPCO says that off-site power may be restored tomorrow (Friday) JST.

Why is a retaining pond burning?

Fuel rods that are not installed in reactors are stored under water in spent fuel pools (SFP).  After they are removed from the reactor, they continue to generate heat from the decay of products that were produced by uranium fission. When they are stored in the SFP they are kept separated from each other at sufficient distance to prevent “recriticality”, or uranium fission restarting.

If the water level in the SFP drops so that the rods are exposed, then the reaction between the zirconium in the fuel rod casings and steam (H₂O) will produce hydrogen. If this is allowed to build up in a closed room with air (containing oxygen), the result is an explosion. This appears to be what caused the initial explosion on the 4th floor of Unit 4.  I suspect the subsequent fires were ignited by the hydrogen explosion, and combustible materials in the walls, floors, crane, electrical and other equipment in the SFP room were burning.