Ideas and iodine6 July 2016
Dr Mark Foreman, associate professor of Nuclear Chemistry and Industrial Materials Recycling at Chalmers University of Technology, challenges some long-held beliefs on organic iodine in the nuclear sector.
One of my deep fears as an academic is that scientific literature can become polluted by an error or a misunderstanding. It is in the best interests of the scientific community and wider society if these errors (great and small) are rectified quickly. Otherwise, memes containing incorrect ideas and concepts could spread and even displace more accurate ideas.
One important question in the nuclear industry is "what organic forms of iodine can form during a reactor accident?" Often the organic iodine is modelled and assumed to be methyl iodide, but a question hangs over this assumption.
If the assumption proves to be wrong, and the properties of the organic iodine compound produced are different from those of methyl iodide, then much of the work on the organic iodine associated with a reactor accident will be flawed and could be misleading.
It is an important fact that the chemical properties of different organic iodine compounds are not the same. For example, the rate at which the compounds release inorganic iodine as they react with electron- rich species (nucleophiles) differ greatly. This difference could alter the ability of accident mitigation systems to remove organic iodine from gas during containment venting and it could alter the efficiency of some sampling devices. The latter might sound innocuous but a failure to capture radioactive organic iodine compounds could result in erroneously low measurements of a radioactive iodine release being made. This could, in turn, result in underestimates of the thyroid doses for the general public, which could lead to poor radiological decisions being made.
Some years ago I considered whether an incorrect methyl iodide meme had spread through the scientific community. It is interesting that I met a person who had expressed a strong insistence that methyl iodide is the only important organic form of iodine emitted during a nuclear accident but was unable to offer any concrete evidence to support their belief. To my mind this blind faith in methyl iodide is similar to a religion. I choose to deal with this problem by looking for the source of the idea.
I found that at least two different groups were able to observe methyl iodide formation by gas chromatography using radiometric detection, in experiments that replicate some aspect of the nuclear industry. In 1964 British workers reported that carrier free radioactive sodium iodide forms methyl iodide when exposed to ordinary air. In 1971 Japanese workers reported the formation of alkyl iodides, including methyl iodide, when uranium metal is dissolved in nitric acid. In Japan it was also found that when carrier free iodide was reacted with nitric acid a range of organic iodine compounds were formed. Methyl, ethyl, propyl, butyl and vinyl iodides were detected, along with other organic iodine compounds. While the methyl iodide was the main organic iodine compound, detection of the others is important as it shows that other organic iodine compounds can form.
This formation of organic iodine compounds requires a very small amount of organic matter; the addition of a stable iodine (127I) carrier prevents the formation of these radioactive organic iodine compounds by competing for the organic carbon source.
This theme of tiny traces of organic matter reacting with iodine to form organic iodine compounds also appears in work on reactor chemistry. In experiments where neutron- irradiated uranium metal or its oxides were exposed to hot steam, some of the radioactive iodine released was in the form of organic compounds (mainly methyl iodide). This paper is very important as it suggests how the methyl iodide formation observed by Eggleton and Atkins (1964) could take place, using the methane present in normal air as the source of organic matter.
Because of the difference in the gas phase rates of the reaction of the hydroxyl radical with the different alkanes methane has a much longer lifetime in the atmosphere than the other alkanes (ethane, propane, butane etc). As a result, when the source of organic matter is normal air then methyl iodide will be likely to be the main organic iodine compound formed.
It has been shown by many research groups that the organic iodine compounds formed from iodine and organic matter depend on the nature of the organic matter used. Eggleton and Atkins (1964) found the distribution of the iodine radioactivity between the organic compounds varied. It was different for room air, the iodine released by lightly irradiated uranium (1 hour at 1012 neutrons cm-2 s-1) and the iodine released by more heavily irradiated uranium dioxide.
This early British work strongly supports the hypothesis that during a nuclear accident radioactive iodine in the form of methyl iodide will be released. However it clearly shows that while the majority of the organic iodine radioactivity released from the fuel will be methyl iodide, radioactivity will appear in the form of other organic compounds. This work, when taken in combination with that done at Chalmers, that of Schuler and Wojnarovits and others, suggests the division of iodine radioactivity between the different organic forms will be a function of many different parameters.
If we concentrate on methyl iodide to the exclusion of the other organic compounds, a danger exists that while we make perfectly good arrangements for dealing with the methyl iodide another iodine compound will provide us with a nasty surprise.
While a methyl iodide capture or abatement system will offer some protection to the public if another reactor accident, similar to the Fukushima event, occurs it could fall short if different organic iodides are released from the containment into the capture device.
It would be prudent never to regard the organic iodine profile of an accident as a closed book. With each change to the materials and likely conditions inside a reactor building the organic material needs to be reevaluated for its ability to form the different organic iodides. One method of doing so is to use pyrolysis gas chromatography to screen and identify the organic materials present in the buildings. A Chalmers / Manchester collaboration has already shown this method has good potential as a method of identifying paints, plastics and their pyrolysis products.
After the identification of key pyrolysis products, these compounds and the original paints, plastics and other organic materials should be tested for their ability to form volatile iodine compounds. Finally, both sampling systems and the individual and collective protection systems should be tested to discover how the different organic iodine compounds behave in them.
For example, in an experiment in the analytical pyrolysis equipment a paint which had been previously exposed to an electrophilic iodine reagent (iodine monochloride) was heated. This epoxy paint was found to form both methyl and ethyl iodides. While the ethyl iodide has similar chemistry to that of methyl iodide the formation of an organic iodine compound with very different chemistry would be more worrisome.
One particular organoiodide which should be watched for is vinyl iodide. This compound would be particularly troublesome. It will not react with the DABCO (TEDA) used on charcoals to trap methyl iodide, but if it is like vinyl chloride and vinyl bromide then in the liver it
will release the iodine as iodide into the blood. Both vinyl chloride and vinyl bromide are stable until they are activated by oxidation enzymes (P450), when they are converted into haloacetaldehydes which give up their halogen atoms. It has already been shown that mixtures of brominated plastics with PET (polyethylene terephthalate, polyester) form vinyl bromide when heated. If polyester was to be used inside nuclear plants then the importance of vinyl iodide could increase.
As it is impossible to predict, with certainty, what plastics and paints will be used in nuclear plants in the future we should maintain a careful watch on the organic materials used in nuclear plants and on the different organic iodine compounds.
The greater the range of organic iodine compounds we evaluate and protect ourselves against, the lower the probability will be that an unexpected organic iodine will result in either an overexposure of workers or the general public or an overestimate of the threat posed by radioactive iodine during an accident.
The views expressed in this piece are those of the author and do not necessarily reflect those of the publication. References are available upon request.