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Actinides (plutonium) in soil near the Japanese nulcear reactors

Dear Readers,

I have recently been supplied with some information about the soil near to the Fukuashima plant, these soil samples were taken about 500 meters away from the stricken plant. The results are mixed.

Some observers have been expressing a view that a horrible large scale release of nuclear fuel and plutonium has occurred from the spent fuel pond and the reactors. I know that at Chernobyl that a steam explosion occurred which scattered fuel over the accident site and high into the air. In Japan no similar large explosion has occurred, the hydrogen explosions were not good. These may have damaged the plant but the hydrogen air explosions occurred outside the containment. So I do not think as large release of plutonium has occurred in Japan as occurred at Chernobyl, it is also important to bear in mind that only a small minority of the plutonium in the Chernobyl reactor escaped into the environment. Any plutonium release is bad but not all plutonium releases are equal.

Also bear in mind that the isotopes of plutonium are different. (All plutonium is equal but some plutonium is more equal than others). The release of a gram of Pu-238 is much more important than a release of a gram of Pu-239 as the Pu-238 has a higher radioactivity for a given mass.

So these explosions have damaged the less important parts of the building, but it is important to bear in mind that the spent fuel ponds are outside the containment. Right now in general I do not want to start to consider who is to blame for what, but for the ponds I will make an exception. I think that placing the spent fuel ponds outside the containment, and not in its own containment was an unwise design choice.

Back to the results, the good news is that the alpha activity in the soil appears to be very low, it is mostly due to uranium. The uranium isotope signature matches one which I have calculated for natural uranium and the activity level is in the normal range for an unpolluted soil. Almost all soil contains some uranium, so it is impossible to find a soil which is totally free of radioactivity.

Here is a graph of the different levels of the radioisotopes in the soil at two points 500 meters from the reactor accident.

The levels of different isotopes in the soil near the Japanese reactor accident. I suspect that these levels were measured by an alpha spectrometer.

The bad news is that some of the actinides (elements such as plutonium, americium and curium) have escaped from the damaged reactors. I have included data for the two different sampling sites on the bar chart. I have also included the estimated isotope signature which has been predicted using the ORIGEN code for reactors 1,2 and 3. The Pu-238, Am-241, Cm-242 and Cm-243/Cm-244 ratio between the real samples and the ORIGEN prediction are very close.

The half life of Cm-242 only 163 days so the curium-242 release is likely to be due to a recent release of radioactivity rather than something which has built up slowly over many years. The Curium-242 is likely to have formed from Americium-241 by neutron activation followed by beta decay. But it is important to bear in mind that the release of the plutonium and minor actinides has been small when compared with the release of the iodine and cesium isotopes. My own view is that right now the beta / gamma emitters which are fission products are likely to pose the greatest threat to the general public.


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