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The isotope signature of the water in the turbine building

It has come to my attention that the turbine building of unit 1 has some radioactive water in the basement. Here is a bar chart of a radiochemical examination of the water.

Radioisotopes in the basement water

The cesium isotope signature is right for a power reactor, an atom bomb will form next to no Cs-134 and Cs-136. These isotopes are not normally fission products. The reason is Xenon-136 is a very long life which prevents the formation of Cs-136 by simple beta decay. Here is some data for the neutron rich isotopes with a mass of 136.

Element

Half life

Decay mode

     

Sn

Very short

Beta (electron emission)

Sb

0.82 seconds

Beta (electron emission)

Te

17.5 seconds

Beta (electron emission)

I

83.4 seconds

Beta (electron emission)

Xe

9.3 x 1019 years Double beta (electron emission)

However what can happen is that neutron rich isotopes with a mass of 135 can decay to form Cs-135. When an atom bomb explodes the isotopes do not have the time required to decay to the Cs-135, while if a power reactor is critical for a long time then the Cs-135 has plenty of time to form.

The long lived Cs-135 can be neutron activated to form Cs-136. Here is the table of data for these isotopes.

Element

Half life Decay mode Neutron crosssection (barns)
     

(n,gamma) thermal neutrons

       

Sn

Very short Beta (electron emission)

Sb

1.68 seconds Beta (electron emission)

Te

19 seconds Beta (electron emission)

I

6.57 hours Beta (electron emission)

Xe

9.14 hours Beta (electron emission)

2647580

Cs 2300000 years Beta (electron emission)

8.7

So in common with Cs-134, the Cs-136 is an activation product which shows that the radioactivity is from a power reactor.

The chlorine-38 is a bit of a puzzle to me, it has a very short half life and yet it appears in high concentrations. If salt was irradiated in a critical (switched on) power reactor then some long lived chlorine-36 (300000 years) and some short lived chlorine-38 (37 minutes) will be formed. Also some sodium-24 (half life 15 hours) would be formed from the salt. So if the reactor had been critical when the salt water was added at the moment on day one when the reactor was SCRAMed  then I would expect that the chlorine-38 would have decayed away over next day. Maybe the data for chlorine-38 is a  misprint, I hope we will find out soon what has happened.

PS. For the original data see the following web page.

http://www.nisa.meti.go.jp/english/files/en20110325-6.pdf

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