Scientists at Russia’s DI Mendeleev University of Chemical Technology (MUCTR) have developed a new technology for the manufacture of a sorbent for the elusive form of radioactive iodine - methyl iodide.
A filter based on the sorbent retains up to 99.5% of the hazardous isotope, does not require a large amount of expensive raw materials and reduces the cost of adsorption by an order of magnitude.
Radioiodine, iodine-131, is a fission product of uranium and plutonium nuclei. It does not occur in nature, but it enters the environment from damaged man-made sources at nuclear power plants or pharmaceutical industries and after nuclear weapons tests.
Radioiodine has a high volatility, and quickly spreads over large areas. Its various organic derivatives easily contaminate the air, soil and water, but also penetrate the human body, where they are carried by blood to all organs and tissues, causing cell mutation and death. During the disasters in Chernobyl and Fukushima 1, it was radioiodine that caused the greatest damage to biological objects.
The unauthorised release of iodine-131 from reactors is closely monitored. There are techniques by which you can capture a significant part of the radioiodine vapour during leaks, but up to 70% of the total content of iodine-131 in the air of work rooms at nuclear plants is methyl iodide. Elemental iodine is easily absorbed by cheap sorbents due to physical interactions, but methyl iodide requires sorbing substances that can form chemical bonds with them or exchange isotopes.
There are already such products on the market, but they are not perfect. Generally, they are made from activated carbon, the granules of which wear out rapidly due to air currents, which leads to the formation of dust that clogs the channels, and therefore sharply increases the energy losses for the cleaning process. In addition, the best sorbents are made using carbon produced from imported raw materials - coconut shells. This greatly increases the cost of the material and a filter for one iodine adsorber can cost around RUB100,000 ($1360).
Now MUCTR scientists have developed a more efficient and more economical technology for the manufacture of methyl iodide sorbents.
“We also used coconut shell charcoal, but about ten times less than in traditional filters, and not in granules, but in the form of powder of different fractional composition applied to a highly porous polyurethane foam matrix, which can significantly reduce energy losses,” said Eldar Magomedbekov, head of the Department of High Energy Chemistry and Radioecology.
“To increase the efficiency of sorption, the powder was impregnated with 4% triethylenediamine, a substance that can enter into chemical reactions with methyl iodide. When selecting the components, we were guided by such parameters as specific surface area, porosity and mechanical strength of the substance,” he explained.
As a result, the optimal samples of the composite iodine sorbent were selected. To test them, a collapsible sectioned column made of stainless steel was developed, where variants of absorbers were placed in a special way, zigzag or twisted into a roll, and radioiodine was fed into it in the form of vapours mixed with the main gas flow. After the experiment, the column with the sorbent was disassembled and the activity of each section was measured on a gamma-x-ray spectrometer. The best samples showed a very high absorption efficiency of methyl iodide - 99.5%.
Scientists expect that the results of the work will interest the manufacturers of radioactive iodine sorbents, who will be able to expand their product range through a new efficient and cost-effective development that surpasses the analogues on the market in terms of price and quality.