- On the 11th of March, 2011, Japan was struck by a "triple catastrophe": massive earthquake (9 on the Richter scale) that lead to an enormous tsunami, that both lead to the accident at the Fukushima nuclear power plant (NPP)
- The Fukushima NPP reacted exactly as it was supposed to during an earthquake: the control-rods went into the reactor and shut it all down (stopped all the fission reactions in the fuel) - it was actually what you could call a "flawless automatic shutdown"
- When a reactor is shut down, with all the control- and scram-rods inside it, it can't start again - so there is no (and it wasn't) danger for a chain reaction out of control (which is what happened in Chernobyl - an accident extremely different from this accident), but you still have all the fuel, which is radioactive, inside the reactor and this needs to be cooled. Even though the chain reaction has stopped, and heat caused by radioactivity isn't a big factor compared the hat that actually comes from fission (around 10% of the total), it's still more than enough that it will be extremely hot if it isn't continuously cooled properly.
- To cool something you need a medium that can carry away the heat, like water, and you need that medium/water to flow. After the earthquake and the shutdown of the reactor, this wasn't a problem, since there were diesel generators that came online and operated the coolant pumps - so that water was flowing and cooling and doing it's job.
- So part one of the triple catastrophe was the earthquake (and it killed a lot of people), but then the tsunami came (and it killed a lot of people), as part number two, and it immediately caused part three of the accident; wiping out the diesel generators. After this there were only some backup batteries, but they didn't last for long, and then there were no more cooling pumps, and in my opinion this is when the real problem at Fukushima started.
- No pumps = no flow of cooling medium/water = no cooling anymore = fuel rods heat. In nuclear engineering language this is called a loss-of-flow accident (short: LOFA), a little bit similar to the accident at Three Mile Island in 1979.
- Since the fuel rods are radioactive they produce heat, and since there are no cooling, they become extremely hot; and if the water then comes in contact with the zirconium cladding of the fuel (which it did), free hydrogen is produced from the water (there is a lot of hydrogen in water).
- Hydrogen is an explosive gas, and when it came out of the reactor tank it exploded. Such an explosion is definitely not a good sign, but I believe it looks even worse for some than it actually was... This was NOT a nuclear explosion. What blew up was NOT the reactor tank, but the building around it.
- Since it became so hot inside the reactor, the fuel melted (which is what we call a melt-down), and then radioactive materials were released from the inside of the fuel to the water (normally, there are no radioactive materials in the cooling water that flows around the fuel rods).
- No one died of radiation from Fukushima.
I realise it's impossible to only give 10 facts about this accident, and I will continue very soon (Friday perhaps....? 😉 ).
Now it's time to prepare for a talk I'm giving tomorrow, for a small bunch of science journalists, then I have to look at my actual PhD thesis - which is on the plan this week...