According to the timeline of the Fukushima nuclear accident, the Earthquake triggered an immediate shutdown of the reactor, and also disconnection from the main power grid, meaning that there was no mains electrical power available on site. Diesel-powered pumps cooled the reactors for around 20 minutes until they were disabled by the tsunami, and then back-up batteries kept the cooling systems to the three reactors going for around another 12 hours. Clearly, this was not quite enough, but together these back up systems certainly prevented a much much worse outcome. I have estimated some of the numbers involved and – for me at least – they make interesting reading.
The three working reactors each generated around 500 MW of electrical power, which means with typical efficiency, they probably released around 1500 MW of thermal power. Based on data from Wikipedia for a different type of reactor, activating the control rods immediately reduces this to around 7% of this value i.e. 0.07 × 1500 ≈ 100 MW. This is still an enormous amount of heat. Now I don’t know the dimensions of this reactor, but guesstimating it is a block 5 m × 5 m × 5 m this block will have a mass of around 625 tonnes. Assuming it has a heat capacity similar to steel, 100 MW of heating will cause its temperature will rise by around 0.3 °C per second. So that’s ≈18 °C per minute or ≈1080 °C per hour. Now these are only guesstimates, but they are probably not too far off. Given that the reactor was hot (probably ≈500 °C) immediately prior to shutdown, this means that without cooling the reactor would turn into a molten mass of intensely radioactive ‘corium‘ within an hour. Additionally pressures would rise in many internal spaces causing warping of components and the overall pressure may approach the safety limits for the containment vessel.
This residual heating is caused by the radioactive disintegration of the daughter products of the nuclear fuel, and there is no way to avoid it. However, it quickly decays away. As the graph at the head of this article illustrates, within half a day, the decay heating falls by a factor ≈10, and will diminish further with time. Now, given that this has been a disastrous week as Fukushima staff struggled to remove heat from each reactor, just imagine how much worse things might have been if they had had to remove 100 MW of heat from each reactor, instead of ‘only’ 10 MW. The calculation suggests the thermal power 9 days after the accident should now be ≈ 3MW and falling.
So the fact that the back-up systems, and the people operating them, worked for the first 12 hours has certainly averted a much more serious accident. Phew…