Is trapping on charcoal reliable as a means for sampling radioactive iodine ?

What is it about?

Charcoal is used to capture radiaoctive iodine both in devices designed to take samples for measurement in air and in the "gas mask" filters of people who need to work in areas contaminated with radioactive iodine. The paper considers the question of how well will a charcoal containing a standard additive work on different forms of radioactive iodine.

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Why is it important?

There are two problems which interconnect, during a series nuclear accident (and also under some other conditions) multiple forms of radioactive iodine can be released into the air. If we are to be able to make rational and good choices needed to protect the general public (and workers) we will need to make good estimates of the amount of radioactive iodine present in the environment. Also when we use air purification devices ranging from the "Air Purifiying Respirator" (APR) to large scale filter systems designed to provide collective protection. After the 1957 Windscale fire there was a problem that the methods used to collect air samples were not reliable for the collection of radioactive iodine in air. The method used would have worked well for solid particles containing iodine. After the horrible fire it was understood that a better system for taking a sample of radioactive iodine from air was needed. This lead to the development of the "May pack" which was invented by a person called "May". The original maypack was a pad of charcoal with an asbestos paper filter in front of it. The back of the charcoal pad is connected to a vacuum pump which sucks air through the two sampling layers. The idea is that after sucking for a given time the charcoal pad and the filter layer are then both examined for radioactivity. It is possible to then discover how much iodine was captured and also to determine the fraction of the iodine which is in the form of particles and gases. By the time of the Chernobyl event the maypack had been improved further, the Japanese workers who detected Chernobyl iodine in Japan used a more complex may pack which was able to distingish between particles, elemental iodine gas (using silver gauzes), HOI with a thin charcoal filter soaked with phenol and a pad of DABCO impregnated charcoal to capture methyl iodide. The DABCO is an additve which is able to improve the ability of a charcoal to capture acidic gases and volatile organic halogen compounds such as methyl bromide and methyl iodide. What happens is that the methyl compound reacts with the DABCO to form the N(C2H4)3NCH3 cation and the halide anion. This ionic compound is not volatile and will remain in the charcoal. This is an chemical reaction which converts the methyl iodide or methyl bromide into a substance which can not be released from the charcoal pad. We have already shown by subjecting mixtures of iodine and paint solvent to radiation that many different organic iodine compounds can be formed. We reason that during a serious nuclear accident that rather than only methyl iodide that a vast host of different compounds can be formed and then escape the accident site. The compound which worried us most was ethyl iodide, this is a voltaile compound which has a boiling point lower than 100 oC. It will also react with DABCO about 30 times slower. Our concern was that it would react too slowly with the DABCO to be fixed in the charcoal pad. We were concerned that this could result in an under estimate of the amount of radioactive iodine in the environment or for radioactive iodine to pass through respirator filters with greater ease than we would expect. So we prepared different organic compounds containing radioactive iodine. These were applied as vapors to one end of a glass tube packed with charcoal granules which had a flow of nitrogen gas going through it. We found that in our experiments that all the chemical forms of iodine which we prepared were well captured by the charcoal.

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