Chemical and Physical Nature of Fallout I131 and... : Health Physics

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Abstract
Fallout from Russian nuclear tests in the autumn of 1961 was sampled weekly at Harwell, England, with a composite sampler capable of distinguishing I131 in various forms. It consisted of a high-efficiency asbestos filter for collection of particulate iodine, followed by brass gauze for elemental iodine and by charcoal-impregnated filter paper for removal of certain compounds of iodine. It was backed up by a 1-in. bed of activated charcoal. The results showed that during the period October, 1961, to December, 1961, an average of 75 per cent of I131 was in particulate form. The remainder was distributed approximately equally between the brass gauze and the charcoal paper. Concurrently, measurements were made in a wind tunnel of dry deposition velocity (VG) of fallout I131 and of total fission products to grass. VG values varying from 0.1 to 1.5 cm/sec were obtained. There was, however, a close correspondence between the values for I131 and those for total fission products during individual sampling periods, showing that deposition of particulate matter was occurring in each case. Measurement of I127 in the atmosphere from natural sources showed a different behaviour. Air samples gave concentrations ranging from 0 to 7 microg/m3 with an average of approximately 0.1 microg/m3. Only a small fraction of the iodine was particulate. This was supported by measurements of the wash-out ratio in rain, (mass of iodine/kg rain) /(mass of iodine/kg air) when values in the range 10–50 were obtained for I127 compared with 300–1500 for I131 and other particulate fission products in fallout. Attempts to identify certain compounds of iodine produced when carrier-free I131 was released by oxidation in air of both NaI131 and irradiated uranium showed that two groups of substances were formed, one of which consisted of alkyl iodides, predominantly methyl iodide.

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