BBC environment analyst Roger Harrabin explains what has happened at the Fukushima plant:
“The power plant is supposed to be earthquake-proof and shut down automatically in response to the quake,” he says. “But this starved power from the stations’ cooling systems. Then the back-up diesel cooling system also failed. Reactor number 1 overheated, and it is said that hydrogen released exploded, causing the concrete roof of the plant to blow off. Now that’s been repeated at Number 3 reactor, Numbers 2 and 4 have problems with cooling.”
Repeated ? I think not.
Need some clarification on specific Daichi and Daini plants now. Fukushima 1 has reactors 1 to 4 in the one block and two further reactors (5&6 ?) in a second block immediately north. Fukushima 2 has four further reactors 12/15 km south of Fukushima 1. (Update F1 is Daichi, F2 is Daini)
Anyway, key point whatever the existing and ongoing difficulties with cooling water systems, and whether all the plants were actually shut down successfully (control rods fully home) before these cooling water difficuties …. the two explosions so far were quite different.
F1-Daichi-R1 was a very clean and fast explosion initially – Hydrogen ? – with all the smoke appearing to be concrete dust, with lighter weight panels flying away from the building. (See the initial shock wave rising vertically above the building, before the smoke, and no fire or subsequent visible emissions.)
F1-Daichi-R3 was not. There was a hydrocarbon yellow flash and a plume of black smoke, with large heavy pieces falling quickly back to earth around the building. And the live footage seems to show steam and ongoing fire escaping from that building ?
F1-Daichi-R2 (and R4 ?) now seems to be having cooling difficulties.
[Update: 4, 5 & 6 were already shutdown before the quake / tsunami, with fuels rods in the holding ponds as the reactors underwent work. The other good news is that Daini / Fukushima 2 do not appear to have had the post-shut-down cooling failures (cooling pump electric power and water supply failures), so in principle the design is earthquake safe. This one will run and run.
So, the real problem now is that Daichi-R2 explosion seems to have cracked primary containment – how did that happen ?!? The pressures involved however seem miniscule, so the residual heating energy in the shutdown state must be small – don’t panic and maintain ad-hoc cooling seems the order of the day ?]