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Saturday, March 12, 2011

Fukushima: A nuclear physics primer

As I am sure everyone has heard by now, there has been an enormous earthquake off the coast of North-East Japan, which (together with the tsunami caused by the quake) has lead to widespread distruction and loss of life. At the moment, however, there is concern over trouble at two nuclear power station. The amount of information coming out through the media is quite limited, and of course isn't easy to decrypt what experts are saying if you don't know what the terminology means.

Caveat: Although I am a physicist, I am the wrong type of physicist to be paricularly expert on nuclear safety. I am writing this simply so this to try to help non-physicists decipher what is going on at the moment in Fukushima.

The first thing you need to know is what a nuclear power plant actually does, and what radiation is. All atoms are composed of a cloud of electrons which surround a nucleous composed of neutrons and protons. The stability of the nucleus is what determines radioactivity, and is determined by the number of protons and neutrons contained in the nucleus. The interaction between these particles determine how tightly they are bound together.



The above diagram shows the relative stability of different atomic elements, running from lighter to heavier elements. As you can see, iron (Fe) has the highest binding energy (meaning that its nucleons are the most tightly bound). Nuclear reactions in which larger nuclei split apart are known as fission reactions (moving towards iron from the right), while reactions where smaller nuclei are combined to form a larger nucleus are called fusion reactions (moving towards iron from the left). All commercial reactors are at present fission reactors. These work by capturing the energy released by large nuclei splitting, usually using uranium as fuel. The reactors at Fukushima are boiling water reactors, which capture this energy by using it to boil water to produce steam used to drive a turbine, which in turn generates an electric current (producing electricity).

Different nuclei come apart in different ways, depending on their composition. Which element a nucleus is is governed by the total number of protons only, and this determines it's chemical properties. The nuclei of the same element can have different numbers of neutrons, and these different varieties are known as isotopes. The chart I showed above is of the most stable isotope for each element. How a particular radioactive element comes apart depends not on only on which element it is, but rather which isotope of that element. Below is a diagram showing how different nuclei decay.



Here the important unexplained types of decay are
  1. β- corresponding to the emission of an electron (this is a neutron changing into a proton in the nucleus),
  2. β+ corresponding to the emission of an positron, which is a positively charged version of an electron (this is a proton changing into a neutron in the nucleus)
  3. α which corresponds to the emission of a group of helium nucleus (2 protons and 2 neutrons tightly bound together), and happens because of the huge spike in binding energy for helium nuclei which can be seen in the first diagram.
  4. Fission, where the nucleus splits into several large parts
These ejected byproducts (together with γ-rays which can also be emitted) are what is usually what nuclear radiation is used to refer to. This what is known as ionising radiation, particles sufficiently energetic to knock electrons free from other atoms and molecules, which can in turn lead to chemical changes. Such radiation is dangerous for humans primarily because it can cause chemical changes within our body which can lead to any number of problems. In general ingestion or inhalation is much more dangerous than other types of exposure.

As I mentioned earlier, both Fukushima I and II user boiling water reactors. They use what is called 'light water' which simply means they use purified water to cool the fission reactions. The word 'light' is used to distinguish them from 'heavy water' reactors which use water where the hydrogen is replaced by a heavier isotope called deuterium (which can be used to regulate reactions in some reactor designs).

At Fukushima I, it appears that when the earthquake struck some problem occured with the cooling system for one of the reactors failed. The nuclear reactions produce a lot of heat, and needs to be kept cool by adding water. Unfortunately, even though you can slow down the fission reaction, some byproducts of the decay of uranium are themselves radioactive, and (according to one expert who just appeared on the BBC) can contribute as much as 10% of the energy produced in the reactor, which is essentially impossible to stop. Without the cooling system working, there is a build-up of steam. It is also possible for the fuel to react with the water to produce hydrogen. These gases have to be vented in order for the pressure inside the reactor to be kept within safe limits (so that the reactor doesn't come apart). As long as the reactor is intack, the amount of radiation released should be low. Hydrogen is not radioactive, even if it absorbs a neutron (producing deuterium), the nucleus is stable. Tritium would be bad, but I can't see anyway for that to have been produced. However, Nitrogen 16 will also have been produced, which is extremely radioactive. This may sound bad, but it is infact so radioactive that it decays very quickly, meaning that it doesn't pose a danger any distance from the plant. The half-life is 7 seconds, meaning that 7 seconds after production half of the radioactive nitrogen has converted to safe oxygen. So after after 11 minutes there is on millionth the radiation. Give that wind speeds are relatively slow, the nitrogen will decay before it can cause much trouble (except perhaps for people actually in the plant). This is why we are hearing about high radiation levels in the control room, but little additional radiation right outside the perimeter.

There has also been an explosion at the plant which is causing significant, since it is not entirely clear what has happened. It sounds like this was probably caused by the hydrogen igniting, which has damaged the building, but it seems like the reactor core is still intact. If this is true, and they can keep the reactor cool enough that the fuel does not all melt (a meltdown, which makes it much harder to cool, and which would likely result in the release of much more nasty isotopes) then the amount of radiation released shouldn't pose to much of a health risk. Fortunately the wind seems to be blowing out to see, which also improves the situation.

The latest I have heard is that the authorities are considering using sea water to cool the reactor core, which they will be reluctant to do since it will make the reactor unusable in future, but which should cool the reactor core. Apparently the incident is currently rated as 4 ("Accident with local consequences") on the International Nuclear Event Scale, which is still one level below the Three Mile Island incident in the US, and 3 levels below the maximum level which corresponds to a Chernobyl-like event.

If anyone has any further information they would like to contribute, or any corrections to what I have written above (as I say, I am the wrong type of physicist), then please let me know, wither in the comments or by email.

1 Comments:

At 3/16/2011 03:27:00 a.m., Blogger kk said...

Thank you for the clear explanation. I am a student who just finished a freshman level environmental chemistry class winter term. I knew some of the fundamentals you mentioned and visited a nuclear reactor at Reed College here in Portland, Oregon a few weeks ago. I was interessted in the atomic structure of the elements used in the Fukushima reactors. Nice job!

 

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