Fukushima facts and fallout

Japan is placed on a very unfortunate position in the world as it lives on three different tectonic plates which are merging together on a convergent trajectory, and as a consequence we have an awful lot of earthquakes in Japan. Japan has chosen to have a lot of nuclear power, and the nuclear power stations are located all around the coast, all close to water where you would expect to find them, but also all very close to tsunamis and earthquakes.

On 11 March we had an earthquake, nine points on the Richter scale off the coast of Japan, about 150 kilometres away from Fukushima. Fifty minutes later a very large tsunami, estimated to be 14 metres high, hit the facility which was built to withstand a tsunami of six metres.

Japan provides about 27 per cent of its power from nuclear energy (Fukushima producing about 10 per cent of it) making it the fourth largest operator of nuclear power plants in the world. It is planning on increasing its nuclear power capacity to deliver 50 per cent of its needs by 2050.

The Fukushima reactor is a boiling water reactor, one of the first generation reactors designed by General Electric. There are not many in operation any more. This plant was 40 years old at the time of the disaster; it would have probably have had a life span left of about 10 years.

It’s a very simple plant. Basically you have a big boiler, water comes in, steam goes through a turbine, which will then produce electricity. The steam is cooled in a condenser, and then starts back into the loop in order to be reheated. The control is very simple; it’s essentially some control rods that moderate the reaction and the steam void in the reactor itself. This is the torus, which can be used in an emergency to cool steam down. The torus is also connected to the inner containment area and that’s important because one torus later explodes.

A few things to observe: you have a range of containments and secondary containments, you have a sealed inner containment that has holes in it by definition because you have to get steam out of the reactor, to go to the turbine, and you have to pump water back in. So despite the fact it’s called containment, it still has significant holes in there.

The whole design is based on a safety system which we call N-minus-2. Meaning if it takes three pumps to do something you build five.

Everything was designed for an 8.0 earthquake. Then we got this big earthquake, and almost immediately the reactor stopped. The reactor stopped because the turbine doesn’t like the vibrations and has to shut down very quickly, so you SCRAM (emergency shutdown) the reactor so as to not generate any new steam. Any steam that is in train will immediately go into the condenser or to the torus. The SCRAM down should normally take about seven seconds if everything goes well.

As a consequence of the accident the Fukushima plant went off the grid, which is normal behaviour, to preserve all its electricity internally to maintain operations under bad conditions. But 15 minutes later came something that they didn’t foresee and wasn’t part of the design, a 14 metre tsunami hit, which took out all of their diesel power generators. Instead of having the normal seven days of power diesel generators would have provided, they now had eight hours. Alarm bells went off everywhere in the control room, because they needed 20 at least to bring this power beast down to normal conditions. So they had to get power very quickly from somewhere else. Battery back-up kicks in, and it will run for eight hours. But eight hours is not enough.

In that evening the authorities made the right call. They order the first evacuation of 10 kilometres radius around the plant. That’s precautionary, and there is no danger yet, but it’s the correct call for a level three accident.

They ran out of battery power on Saturday morning, and almost an hour later, very predictably, this boiler is boiling at a fairly good rate, and water is evaporating basically becoming steam, and the rods are reacting with super heated steam starting hydrogen formation.

Steam pressure rises, venting becomes necessary, actually without intervention this reactor will start venting, and that happens basically over that day. Then by eight o’clock there was the first major explosion of hydrogen that took out a big part of the building.

There was another hydrogen explosion in unit three, and at that time the authorities called it quits and resigned themselves to these reactors never working again, and pumped seawater into them. That’s not a very pleasant thing to do. Who knows what reactions to the seawater are going to take place, or the corrosion that is happening as a consequence of that? It basically means these reactors are terminated, they’re just going for safety, because seawater is readily available.

On Tuesday, torus number two exploded, another hydrogen explosion, and most likely at this point in time we knew that something dramatic had happened and there was now a physical pathway from the reactor’s inner vessels to the outside world. And we saw this from the higher radiation activity, so most likely this was the first evidence that a fuel rod has melted partially or fully.

Things got bad essentially from that point in time, but nevertheless the core was clearly well contained given that they had very little to work with. Most of the building equipment was blown up with the hydrogen explosions. They put in as much coolant as they possibly could, either through seawater or freshwater, whatever is available. Radioactive materials were being released, that was unavoidable at this point in time. Essentially they were working within the limits of the capacities of the plant they had available to contain everything there, and from an engineering point of view with the little that they had, they were actually doing a reasonably good job.

They face a long clean up that will take a few years to complete. What also will happen is that people will look at all the design limits, why it was designed the way it was designed, and there will be lots of questions asked and lessons learnt. But nevertheless these were first generation reactors. In the meantime we’re about four generations further ahead, and so even those lessons might not actually make that much of an impact on the new design, because most of these lessons have already been learnt somewhere along the line.

Hear Professor Mareels’ full talk given at the public lecture “Fukushima: the Facts and the Fallout” at
http://www.eπng.unimelb.edu.au/nuclear/.