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Cleaning drinking water and what are carrier free radioisotopes

The last news which I have heard suggests that while some radioactive iodine has appeared in drinking water it is only at a low concentration. While it went over the limit for very young children it did not go over the limit for adults.

I have recently seen on the internet some advice issued by a person who was posting under a pen name whose real name or background is a mystery to me. In this advice the person suggests distillation of water as a means of removing the radioactivity, while distillation is able to remove some substances it is not a suitable treatment.

Firstly it is very energy intensive, you need a lot of energy to boil the water and then some means of cooling is needed.

Secondly distillation is not able to remove radioactive iodine from water. Iodine in the form of elemental iodine is volatile. It is likely to evaporate from the heated water.

While experiments with high chemical concentrations of iodine / iodide might suggest that the distillation works, these would be misleading. This is because elemental iodine (diiodine) reacts with iodide to form a triiodine anion (I3-). This is why iodine is soluble in potassium iodide solution but not in water. When dealing with low chemical concentrations of iodine (but can still be high radioactivity levels) it is important to understand chemicals can behave in a different way at super low concentrations.

Effects such as absorption onto the surfaces of filters, plastic and glass tubes can make carrier free radioisotopes vanish from solution. Carrier free means that the only atoms of the element present in the sample are radioactive. In real life it is often impossible to get perfectly carrier free radioisotopes but in real life it is possible to get radioisotopes where the “chemical concentration” of the radioisotope is close to zero. A 1960s book held the view that many strong stock solutions of radioisotopes have such low concentrations of the radioactive elements that all chemical tests would suggest that the stock solution was pure water.

When a stable isotope of the radioactive element is added then it is known as adding a carrier, for example if tube of sea water was spiked with a drop of sodium-22, then the radioactive sea water sample would contain a carrier in the form of stable sodium-23.

In the same way if sea water was neutron irradiated to form sodium-24 then because the sea water contains plenty of unchanged sodium-23 it is still a solution which has a sodium carrier.

In general if a radioisotope is formed by the neutron activation of a stable isotope of the same element as the wanted radioisotope then the radioisotope will have a carrier unless a radiation induced reaction can be used to separate the neutron activated atom from the bulk. Many neutron capture reactions are ones where the nucleus swallows up the neutron and then dumps its excess energy by gamma emission. This is a n-gamma reaction.

For example Co-59 + n gives an excited state of Co-60 which will emit a gamma ray and become ground state Co-60

However if the nuclear reaction forming the radioisotope causes a change of atomic number then the radioisotope can be carrier free, For example

n + Mo-98 –> Mo-99 –> Tc-99m

The Tc-99m can be separated from the Mo to give pure Tc-99m

Also when a np reaction occurs then we have the chance to form a carrier free radioisotope.

For example 32S + n –> p + 32P

is a reaction which makes phosphorus-32, the phosphorus-32 then decays by a beta decay back into sulphur-32

Also when nuclear fission occurs it is sometimes possible to get a radioisotope which is close to carrier free, the thing is that for many elements the fission process forms isotopes which decay very quickly into stable isotopes of many elements, so the iodine from the damaged fuel will contain stable iodine-127 in addition to the radioactive iodines. But the ratio of the number of the radioactive atoms to stable atoms is better in radioactive atoms than stable iodine which has been bombarded with neutrons.

The radioactive iodine in the environment is not perfectly carrier free it may be close to carrier free. I would expect when the chemical iodine concentration is low (carrier free) then any iodide which is oxidized to form iodine will be more volatile than in a system where the iodide concentration is high.

Cresson Kearny’s book “Nuclear War Survival Skills” in chapter eight (http://www.oism.org/nwss/s73p919.htm) explains that distillation does not remove radioactive iodine from water. (http://www.oism.org/nwss/)

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