Radioactivity in seawater near the Japanese reactor site

Dear Reader,

I have found some details of the radioactivity in the sea water close to the Japanese plant, these results are from an examination of water with a germanium detector. Now first the results, in the following bar chart the left hand blue bar is the measured value while the right hand purple bar is the limit for that radioisotope.

Radioactivity in the sea water near the Japanese reactor site

It is clear that for cesium and iodine that the sea water contains far more radioactivity than the limit. You might ask “what is a germanium” detector. Firstly it is not a geranium detector, it does not have a special ability to detect those nice delicate flowers.

A germanium detector is a crystal of germanium, it is common to use a very pure germanium crystal. The crystal is a semiconductor which has a very low back ground current due to its high purity and the fact that it is normal to cool them to a very low temperature.

What happens is that when a gamma photon is absorbed by the crystal it causes the formation of charge carriers (free electrons and holes) these are able to flow through the crystal under the influence of an electric field (A bias voltage is applied to the crystal), the greater the energy of the photon the more charge carriers it forms and therefore the more charge which passes.

What happens is that the electronics attached to the germanium crystal will measure how much charge passes, as a result it is possible to measure how much energy was in the gamma photon.

The Cobalt-58 is an activation product which is formed from Ni-58 by a np reaction, the nickel will be present in the form of stainless steel in the reactor. SS316L is the classic stainless steel for making nuclear reactor parts and also bits of reprocessing plants. SS316 is mostly iron, nickel and chromium. I imagine that a SS316 part which has been neutron irradiated will also contain nickel-63 (long lived beta emitter), iron-55 (electron capture), iron-59, cobalt-57, cobalt-56 and chromium-51.

while the other

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