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

Then

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

Then

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

Then

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

Then

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.

Plutonium in soil near the japanese nuclear reactors

Dear Reader,

It has come to my attention that traces of plutonium has been detected in the soil close to the site of the reactor accident.

Before anyone asks, I will say that the detection of small traces of plutonium in soil is not an easy matter. I imagine some of you will have seen in films where someone waves a Geiger counter like a magic wand and instantly detects plutonium or whatever else. In real life things are not so easy. The reported levels of the plutonium in the soil is low, it is only about 10 times a limit for baby milk which is 1 Bq per kilo.

Typically soil samples have to be dried in ovens, burnt to ash and then subjected to a very harsh acid extraction. The plutonium has to be separated away from all the other stuff before it can be measured by either ICPMS, alpha counters or some other method.

This is not a quick and easy experiment to do ! One of the big problems is the fact that it is normally very hard to dissolve plutonium oxides.

http://www.nisa.meti.go.jp/english/files/en20110329-5.html

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110328e14.pdf