Correcting the myths about Fukushima

Updated 16/03/2011 09:30 GMT

There has been an astonishing warning from TEPCO that “The possibility of re-criticality is not zero.” This can only mean that they have put so much fuel in the storage pool that it could start a nuclear reaction outside the reactor. This would not explode, but could cause the fuel rods to release radioactive particles outside of containment. This would spread radiation throughout the area.

http://www.bbc.co.uk/news/science-environment-12762608

— Original article —

There is a lot of misinformation and fear being spread about the nuclear reactors at Fukushima. I would like to present the correct information here.

  • All the explosions have been of hydrogen, and outside the reactor. The visible damage is the metal outer shed. It is unknown but thought unlikely that the reactors themselves have been breached. Talk of a “nuclear explosion” by the press is highly misleading.
  • Most of the radioactive particles in the steam vented from the reactors have very short half lives and will decay to nothing after a few seconds. A tiny amount of more harmful materials may have been released.
  • The highest level of radiation recorded near a reactor, 400 millisieverts-per-hour, is less than half that required to cause nausea and vomiting and less than an eighth of a possibly lethal dose, (3.5 sieverts) and that was right next to the reactor. At the site entrance it was only 8 millisieverts.
  • The largest release of radiation so far and the main risk of more has come from spent fuel rods in the storage pond next to reactor 4. These have caught fire twice in the last 24 hours. Fuel must be kept in cooling storage ponds for several years after being removed from the reactor. Al Jazeera has more information.
  • Spent fuel rods in storage ponds will not meltdown, but radioactive particles may be released if they are not kept under water, and especially if they catch fire.
  • There is a danger of more highly radioactive particles leaking from fuel if any reactor containments are breached. So far none have been since explosions have been outside the containment.
  • If any reactor cores do reach a state of meltdown, that means that the fuel will become a molten pool of metal on the floor of the reactor. The containment is designed to keep this in and prevent a leak. It will eventually cool down and be removed.
  • All of the reactors at Fukushima Dai-ni are in a state of cold shutdown.
  • Reactors 4,5 and 6 at Fukushima Daiichi were already shut down for inspection at the time of the earthquake.
  • A hoax is circulating which informs people that radiation has spread to other parts of Asia. This is not yet the case. More info at the BBC.

The situation is serious, radiation has been leaked, and that is not good, however the people have been evacuated and issued with iodine tablets (Potassium iodide prevents radioactive iodine from lodging in the thyroid) and the radioactive particles should decay and dissipate quickly. The worst thing that could happen now is a breach of the containment, in which case the area could become irradiated and uninhabitable. This is unlikely to happen.

Disclaimer: I have not had time to provide references for any of this. I suggest that you read reliable sources on the subject, such as the IAEA and NOT a news agency. (Especially not Fox.)

For up to date information, please see the International Atomic Engergy Agency. The IAEA website is at www.iaea.org however it is under heavy load so they are publishing all information on Facebook at www.facebook.com/iaeaorg

You can also view press releases from the Tokyo Electric Power Company (TEPC) at  www.tepco.co.jp/en/press/corp-com/release/index-e.html however their website is also under heavy load and may not respond.

Note: The reactors at Fukushima are 40 years old and of very poor design compared to modern ones. New reactors can shutdown and cool themselves without power or human intervention. Germany and the USA were about to replace 40 year old reactors with modern designs but have suspended this in reaction to events in Japan. This means that 40 year old designs will be kept in operation.

In defence of nuclear power

Schematic of a Boiling Water Reactor
A Boiling Water Reactor. Image from Wikipedia.

When I was ten I remember being given some sort of exercise at school that required me to draw my answer. I drew a nuclear reactor. I drew it in some detail, including fuel rods and control rods and cooling system. This wasn’t actually unusual for me; I frequently drew machinery of many kinds, huge cogs and mechanisms and bizarre perpetual motion machines. Imagine my astonishment when the first thing my teacher said on seeing my drawing was “So you’re in favour of nuclear power then?”

I was astounded. It hadn’t occurred to me that anyone could be against nuclear power. It was a machine! It was THE ultimate machine! How could anyone not love it?

Twenty two years later I am still in favour of nuclear power, although this time for much more considered reasons.

Perception of danger

Nuclear power is much safer than you might think. Think of the safety of nuclear power as something like an aircraft. Cars have frequent accidents, killing quite a lot, injuring more often. Aircraft few accidents, but when they go wrong, they really go wrong and generally kill everyone on board. A car accident might make the local news if there’s something odd about it. An aircraft accident will probably make the international news. Despite the differing attention given between these two, road accidents kill and maim many more people than air accidents do. The safety record of electricity generation is much the same. Fossil fuelled power stations might have accidents, but generally they are of little consequence. Even so, fossil fuel waste causes a lot of damage to the surrounding area and people living nearby. Nuclear power stations have very few accidents, but when they do, they have really serious ones.

People imagine an accident in a nuclear reactor as something like a bomb. Say “nuclear accident” and they see mushroom clouds and flattened cities but it isn’t like that. A nuclear bomb makes use of a runaway chain reaction which requires the uranium to be dense enough and large enough to reach “critical mass.” Nuclear reactors have their fuel split up into smaller parts encased in fuel rods which are held too far apart for a nuclear explosion to occur. What can actually happen is a meltdown, where the fuel rods melt, run together into a pool at the bottom of the reactor, then burn down through the floor and leak radiation. This can happen when the fuel is not kept cool enough. In most reactor designs there are three levels of containment around the core, and molten fuel will be contained by a very thick concrete basin under the reactor and prevented from leaking.

Apart from meltdown, a reactor that uses water as coolant can also produce hydrogen when things go wrong, and evaporating coolant can cause a build up of pressure inside the reactor. If the coolant stops circulating, the heat of the reactor can crack the water into hydrogen and oxygen and fill the containment up with this explosive mixture. In old (or Russian) reactors it could explode inside the containment and break it open. In more recent designs, the hydrogen is vented from the core before it can explode and expose the fuel to the outside world. The vented gasses are themselves radioactive, but usually not very much and not for very long since the radioactive particles have a half-life measured in minutes. (That is, they break down very quickly.) A big flaw with old reactor designs such as those at Fukushima and all Russian reactors is that the coolant will stop circulating if the pumps lose power. Modern reactors are designed in such a way that if the power stops, the coolant will keep circulating through convection.

In summary, modern nuclear reactors are much safer than those from the 1970s. They have multiple level of containment to catch molten fuel. Coolant can keep circulating even without power. High pressure and explosive gasses are removed from the core before they can explode and destroy it. Instead of setting a reaction going and then restraining it, modern designs require human intervention to keep them going. If the people aren’t there, the reactor shuts down. The rules on dealing with accidents are incredibly strict, almost paranoid. Finally, there are many more advanced designs of reactor to choose from than just pressurised water reactors or boiling water reactors. Pebble Bed Reactors, in particular, are designed so that they produce less power as the temperature rises, and so are self-limiting and cannot overheat.

Dealing with waste

Radiation Warning SymbolI will admit, dealing with nuclear waste is a problem. I would like to make some points about this. First of all, fossil fuels also produce waste. That waste in the form of ash, CO2, sulfur dioxide, nitrous oxide and other gasses has traditionally been pumped into the atmosphere where it causes acid rain, smog and climate change. Pollution from fossil fuels affects the workers at the power station and the residents in towns nearby. Recent attempts to scrub pollutants out of smoke before releasing it have reduced this a little, but not enough. Carbon capture will improve things but is very difficult and hugely expensive. Secondly, fly ash from coal is actually radioactive! Not just radioactive, but during day-to-day operation a coal power station releases 100 times more radiation than a nuclear power station producing the same amount of electricity. People living near coal-fired power stations actually have more radioactivity in their bodies than people living near nuclear power stations.

Nuclear waste is usually buried deep underground. It will remain dangerous for millions of years. I think though, that I would much rather have waste that can be buried than pump smoke and ash into the atmosphere and destroy the planet.

We could actually produce far less nuclear waste than we do at the moment. Used nuclear fuel can be re-processed, and can be re-used to fuel Breeder Reactors. These reactors produce more fuel as they use traditional fuel and so they produce a lot more energy from the same fuel, and the waste is more effectively used up. They can also run on thorium, which is more readily available than uranium. Unfortunately Breeder Reactors are generally not used because they create plutonium and governments are terrified that it would be used to create nuclear weapons.

Thank you for reading this far. In my next post I will explain why I believe that nuclear power is neccessary and why renewable sources are not adequate. Please also note that I am not a nuclear physicist, I am a computer scientist that happens to be fascinated by nuclear power. If I am wrong and you can provide evidence, feel free to say so in the comments.

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