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Rapid and slow neutron processes

I was looking at the data for the plutonium which was found on soil samples at the Fukushima nuclear power plant, the good news is that the level of plutonium at the sites which have been sampled is quite low but the bad news is that any pollution with plutonium is bad. Also the fact that plutonium pollution has been detected is a sign of something horrible which has occurred on the site. 

Now one of the great problems with the plutonium is working out where it came from, one way is to look at the isotope signature. The ratio of 238Pu to 239/240Pu can say something about where it came from and how it was made.

The 238Pu to 239/240Pu ratio for one site at the nuclear reactor site is 2:1 while fallout plutonium from bomb tests in Japan is normally 0.026:1. So we can tell that some plutonium which is very 238Pu rich has been released.

Years ago it was explained to me that plutonium is not just plutonium, he said you need to know which plutonium it is. My coworker held the view that by comparison that 239Pu was not a nasty isotope to work with while 238Pu was a nasty horror isotope. He explained to me that a well made sintered pellet of 239PuO2 if left for days on a surface will not contaminate the surface. While if you leave a 238PuO2 pellet inside a glovebox then within a few days every surface in the box will be crawling with radioactivity.

We might then want to know where does 238Pu come from, the answer is that it forms in a power reactor in the following way.

235U + n -> 236mU -> 236U


236U + n -> 237U -> 237Np (beta decay, half life of 6.75 days)


237Np + n -> 238Np -> 238Pu (beta decay, half life of 2.1 days)

What has happened is that after neutron activation the isotopes have time to undergo beta decay. However during an atom bomb detonation or a supernova the uranium atoms will be bombarded with neutrons at such a high rate that they do not have time to undergo beta decay.

Now we get reactions like

235U + 3n -> 238U

If the uranium-238 which is formed in an excited state loses the excess energy then it will form a very long lived uranium isotope which will undergo alpha decay.

An alternative route to 238Pu would be the alpha decay of 242Cm (half life of 163 days)

Any uranium-238 present in the bomb or nearby can be activated.

238U + 4n -> 242U


242U -> 242Np (beta decay, half life 17 minutes)


242Np -> 242Pu (beta decay, half life 2 minutes)

Now we have formed a very long lived plutonium isotope which slowly decays by alpha emission to 238U

Also the plutonium can be neutron activated during the atom bomb blast, but again if 242Pu is formed then this can not decay to make the 242Cm.

One last route to 238Pu exists, this is the n,2n reaction for 239Pu. This is a very minor reaction when compared with fission or the conversion of 239Pu into 240Pu. So during a bomb detonation very little 238Pu forms while in a power reactor due to the fact that the neutron flux is lower and longer lived the slow reactions which have time to form 238Pu can occur.


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