Things have been moving so quickly this morning, that it’s been difficult to find time to update the blog.

First, there was another massive explosion at 11:01 local time inside Fukushima’s reactor 3. Unit 3 suffered a cooling system failure on Saturday, and was flooded with seawater in the hours leading up to the blast. According to Japan’s Nuclear and Industrial Safety Agency (NISA), the explosion was caused by hydrogen build-up inside the reactor hall. “The containment vessel of the unit was not broken,” NISA says. Tokyo Electric Power Company (TEPCO) reports that 11 people were injured the blast, and the broadcaster NHK was reporting that at least some of those hurt were from the Japanese Defence Force.

The explosion appears similar to an explosion on Saturday at the power station’s unit 1 reactor. You can learn more by reading yesterday’s post, but the hydrogen is most likely created by the zirconium fuel cladding reacting with steam in the core to create large quantities of hydrogen (it’s also possible for the steam to react with the fuel itself, so this too is a possibility). When the pressure in the vessel was released, the hydrogen reacted with oxygen and exploded, blowing the building apart.

Meanwhile, Fukushima unit 2 has lost its cooling pumps. According to a report on NHK world, the fuel rods inside unit 2 were “fully exposed” meaning that they was no water left in the core to cool them. This makes it pretty likely that unit 2 has now suffered serious damage to the core. Accordingly, at 8:00PM local time, TEPCO began injecting seawater into the unit 2 reactor. The latest I heard on NHK was that about 2m of the reactor’s 4m-high core is now back under water.

Pressure from steam in the unit 2 reactor is rising, so it may be that TEPCO will be unable to complete covering the core until they vent. This, again, raises the possibility of a massive explosion sometime in the coming hours.

The Japan Atomic Industrial Forum (JAIF) has begun providing very helpful colour-coded charts showing the status of each reactor. One thing the chart makes clear is that, while the situation remains stable, there’s a lot that’s yet unknown about conditions inside the cores.

For full coverage of the Fukushima disaster, go to Nature’s news special.