More Fun at Fukushima

Following Saturday's (12/3/11) explosion at the number 1 reactor today (14/2/11) there has been another explosion at the number 3 reactor at Japan's Fukushima-Daiichi nuclear power plant. Both explosions appear to have occurred because of attempts to cool the reactors with sea water rather then the normal chemical coolant.

Nuclear reactors normally operate at temperatures above 500C which is hot enough both to generate steam and actually break water (H2O) down into it's constituent parts such as Hydrogen and Oxygen both of which are highly explosive. In both number 1 and number 3 reactor some of these gases appear to have seeped out into the concrete building that houses the concrete encased reactor and exploded blowing the roofs of the buildings. Although this is not how a nuclear power station is supposed to operate it isn't actually much of a problem. The concrete buildings surrounding a nuclear reactor are designed to operate much like a cars crumple zone absorbing the energy of any explosion and in the process protecting the reactors concrete skin and the core containment vessel preventing any release of radioactive material. While you can't exactly just stick your head into a reactor core to check it for damage the plant operators aren't reporting any damage to either the reactor skin or core containment vessels on either number 1 or number 3 reactor. As there have been no reports of significantly increased radiation levels following either event I believe them.

What is slightly more alarming is the situation at the number 2 reactor where it has been reported that the cooling system has either partially or totally failed leaving the reactor core exposed on at least two occasions. Even this is nowhere near as dramatic as it sounds though because when they say the core has been exposed they mean that the water levels have dropped exposing the nuclear fuel rods to the core containment vessel not the outside world. If the reactor was functioning normally rather then being cooled you would expect the fuel rods to be exposed to the core containment vessel. However without a functioning coolant system the temperature of the solid state fuel rods will continue to rise until they eventually melt down into a radioactive sludge at the bottom of the containment vessel. This is known as a nuclear meltdown and creates the risk that the radioactive sludge will then melt through the containment vessel.

Long before this happens though the plant operators may attempt to remove the fuel rods from the core and place them into individual cooling caskets that are then placed into cooling ponds to cool down. As nuclear fuel rods need to be replaced every 6-12 months this is part of the normal operation of a nuclear power plant and something the plant operators will have done hundreds of times before. It is though a very complicated process that places the individuals carrying it out at extreme personal risk. As aftershocks are still striking Japan and because the temperature and state of the fuel rods is constantly changing the plant operators may decide that it is safer to opt for a controlled meltdown. This involves just letting the fuel rods melt down to sludge because it is highly unlikely that they will actually melt all the way through the containment vessel and if they do at Fukushima they will only melt into another containment vessel that has been designed for this exact purpose. However to prevent the reactor core from exploding while this happens the operators will have to vent gases containing radioactive particles from the reactor.

As Fukushima is a predominately plutonium fuelled reactor all but a tiny fraction of these radioactive particles will be beta type radiation which is electromagnetically charged. This means that if the right equipment is in place the plant operators should be able to filter most of these radioactive particles out of the gases before they are released into the air around the plant. If the radioactive particles are released into the air they will immediately start smashing into other, non-radioactive, particles much like balls on a snooker table. Once enough collisions have taken place the radioactive particles will lose their energy, stop being radioactive, and fall harmlessly too the ground. While the particles they've collided with will become ionized they won't become radioactive and will also fall harmlessly to the ground. Exactly how long this will take and how far the radioactive particles will travel depends on the specific electron voltage (eV) of the specific particles but I think we're talking in terms of hours and tens of metres rather then years and kilometres. The Japanese authorities have already evacuated everybody from within 20km of the Fukushima plant.

By contrast what happened at Chernobyl is that the Soviets didn't want to admit that there was a problem so took no action to fix it. This resulted in an uncontrolled meltdown and a massive explosion which sent highly radioactive core materials hundred of km's into the sky. To make matters worse the Chernobyl reactor was built from flammable materials meaning that it then caught fire sending up plumes of radioactive smoke for several days. The Fukushima plant is not made out of flammable material so I don't think it's even possible for this to turn into a disaster of the scale of Chernobyl.

Or to put it another way I'm so not alarmed by the situation at Fukushima that I found time to go to the gym, have lunch and read the newspaper before writing this up.