Charcoal filters are used in environmental sampling to estimate radioactive iodine both under normal operating conditions and during emergencies. They are used in protection systems such as air purifying filter respirators to protect against radioactivity. But they must be versatile. Iodine can exist in many forms during a nuclear accident.
One of the most common types is methyl iodide, which is why the charcoal filter is designed to retain this kind of iodine. But there may also be other kinds of radioactive iodine, and as the formation of other organic iodine compounds has been observed in nuclear plants it can be reasoned that a failure of a charcoal to retain other types of organic iodine than methyl iodide could have adverse consequences.
Researchers at Chalmers have investigated how different charcoals have the ability to capture radioactive organic iodine compounds other than methyl iodide.
– Of the compounds tested it has been found that methyl iodide is the compound which is most poorly retained by charcoal. The charcoal in our tests was more able to capture and retain ethyl iodide, isopropyl iodide and chloromethyl iodide than methyl iodide. This is an important finding as it indicates that we can better trust the charcoal based devices used to sample the radioactive iodine in the air and also that we can better trust respirator filters which are based on the charcoal with the standard methyl iodide fixing agent, says Associate Professor Mark Foreman.
Compared with many other radioactive elements, iodine has a particularly high ability to harm humans and other animals. All vertebrates have a thyroid, a small but vital gland which controls the metabolic rate and other important bodily functions. The thyroid gland needs iodine to work properly and it absorbs both radioactive and non-radioactive iodine, which may lead to thyroid cancer if it is exposed to the harmful kind.
A lot of radioactive iodine is formed by the fission of uranium and plutonium atoms in a nuclear reactor. During a serious nuclear reactor accident a large fraction of the radioactive iodine in the fuel can escape from the core and subsequently from the plant. The iodine also has the potential to become very mobile, it can form several gases and very low boiling point compounds. While the noble gases in reactor fuel are more mobile than iodine, the iodine is often of greater concern as its chemistry and biology causes it to be more radiotoxic.
Text: Mats Tiborn