New method for iodine retention in the nuclear power plant venting filters faces crucial tests
These days, the Federal Office of Public Health distributes iodine tablets to the population living close to the Swiss nuclear power plants (NPP). The dispensing of iodine tablets within a radius of, now, fifty kilometres around NPP sites is aimed at protecting the residents from contamination with carcinogenic, radioactive iodine in the event of a severe nuclear accident.To make sure that as little radioactive iodine as possible gets into the environment as a result of a nuclear accident, researchers from the Paul Scherrer Institute PSI have for many years been developing a method that can be used in containment venting filters.
These days, the Federal Office of Public Health distributes iodine tablets to the population living close to the Swiss nuclear power plants (NPP). The dispensing of iodine tablets within a radius of, now, fifty kilometres around NPP sites is aimed at protecting the residents from contamination with carcinogenic, radioactive iodine in the event of a severe nuclear accident. By taking iodine tablets, the body – especially the thyroid – becomes saturated with the non-radioactive iodine that the tablets contain. This prevents the intake of radioactive iodine, which may be released during NPP accidents. The iodine radioactivity in NPP accidents primarily stems from the iodine isotope Iodine-131, which might only have a half-life of around eight days but can be carried far in the atmosphere and get into the body via water, food or the air.
Retaining iodine in the NPP
To make sure that as little radioactive iodine as possible gets into the environment as a result of a nuclear accident, researchers from the Paul Scherrer Institute PSI have for many years been developing a method that can be used in containment venting filters. The technique retains iodine in water pools very efficiently. The containment venting filters in which the method is now being tested were developed by the Thurgau-based company CCI AG. The iodine retention procedure patented by PSI was licensed to CCI AG in 2012.
Detlef Suckow, a scientist at PSI’s Laboratory for Thermohydraulics explains how the filters work:
“In the event of a severe accident, the pressure in the containment of a NPP may increase due to steam and gas (e.g., hydrogen, CO, CO2) generation. In order to relieve the pressure in the containment and prevent a massive containment failure, the accumulated mixture of vapour and gases needs to be released into the surroundings by so-called containment venting. To avoid release of radioactivity to the environment in the form of gas phase iodine and aerosols, the venting may be carried out through containment venting filters. Every NPP in Switzerland is equipped with such filters.”
Important: reduce bubble size
There are two types of containment venting filters: dry and wet. The wet filters used in all Swiss NPPs are essentially large water tanks that typically hold around 30 cubic metres of water. Additives are used in the water to control the pH and to bind the iodine preventing its release from the water pool. “The contaminated mixture of gas and vapour is directed into the filter and the radioactive aerosols and iodine are removed in several steps. The important thing with CCI AG’s filter technology is for the initially very large vapour and gas bubbles to quickly be reduced in size. This increases the total surface area of the bubbles and enables the radioactive iodine to be transferred into the water and to be bound more easily. Consequently, devices that decrease the size of the bubbles are fitted in the CCI filters,” adds Suckow.
In the event of an NPP severe accident, the most iodine is bound in solid fine particles. Gaseous iodine, however, is also present in two forms: as an inorganic molecule composed of two iodine atoms and as organically bound iodine, such as in the molecule methyl iodide. Contrary to aerosols which are efficiently removed in the wet filters, the gas phase iodine is very difficult to retain. Before it can be chemically retained in the filter, the iodine needs to be converted into a water-soluble form, which is where sodium thiosulphate, the additive contained in the filter water, comes in. While thiosulphate is very good at converting inorganic iodine into a water-soluble iodine ion, however, it is less effective for organic iodine. Consequently, the PSI method also uses a catalyst to help bind organic iodine.
High collection efficiency
In the laboratory, the PSI method for iodine retention has achieved a very high collection efficiency. The decontamination factor for organic iodine (represented by methyl iodide CH3I) is up to 1,000, which means that only around one iodine atom in 1,000 escapes from the filter. Or to put it another way: up to 99.9 per cent of the organically bound iodine atoms are captured long-term. In the ongoing tests, the scientists are measuring the decontamination factor on the larger technical scale of a filter prototype. The PSI researchers are confident that they can improve the retention of organic iodine considerably compared to previous methods.
Tests with non-radioactive iodine
The current tests at PSI are being conducted with non-radioactive iodine. In principle, the radioactivity does not affect the chemical behaviour of an element. In the water solution in the wet filter, however, the radioactive iodine and other radioactive substances produce decomposition products by radiolysis. These products might react with the iodine and influence its retention in the filter. The effect of irradiation has already been studied by the PSI scientists in laboratory tests, and has been found not to decrease the retention of organic iodine. “Consequently, our tests with non-radioactive iodine will also be relevant for radioactive iodine with regard to the retention in the filter,” stresses Suckow.
Text: Paul Scherrer Institute/Leonid Leiva