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Radioactive iodine activity in the water coolant sharply grows during shutdown of the working WWER or RBMK reactors. Therefore, it is marked that radioactivity of 131I in water coolant grows in 30-200 times during shutdown of the RBMK reactor (the Leningrad and Smolensk NPPs) (Kriytskii et al., 2005). Authors of the given paper explain such increase of activity in water coolant as follows. During shutdown of a reactor the temperature of fuel inside fuel elements is reduced, that results in reduction of volume of fuel and occurrence of free space inside fuel elements. At the same time, water from coolant penetrates inside fuel elements through defects in fuel cladding and dissolves radioactive iodine and caesium, which were diffused in the fuel and accumulated in a gap between fuel and cladding at a high degree of the fuel burnup. As a result of dissolution of radioactive iodine and caesium, they are passed as ions into a water coolant. Radioactive iodine can be present in a water coolant as ionic (Iand IO3-) and molecular (I2) forms due to the oxidation-reduction reactions. Furthermore, because of a radiation destruction of the ion-exchanged resins the organic form (CH3I) can be also formed in a water coolant. At the same time the content of radioactive iodine in a gas phase grows more than by the order as molecular and organic forms of radioactive iodine make the basic contribution to pollution of a gas phase since their distribution factors between gas and water phases are higher on some orders than similar for ionic forms. The similar effect of increase in the radioactive iodine content in the water coolant during shutdown can be also observed as on working (PWR, LWR, BWR), and on new developed (EPR /Framatome ANP/, SWR-1000 /Framatome ANP/, AP-1000 /Westinghouse/, ESBWR /General Electric/, APWR /Mitsubishi/, ABWR /Toshiba and Hitachi/) NPPs (Krebs, 1999; Reactor Design, 2002; Tomorrow's reactors, 2003). Usually for decreasing of the radionuclide content in a water coolant the cationand anionexchange resins are used on NPPs during normal operation. Moreover, the 50% decreasing in the power capacity are used on the Leningrad NPP for decreasing of the radioactive iodine content in the water coolant during shutdown of a reactor, i.e. the reactor works on 50% of the power capacity during 2-5 days (Kriytskii et al., 2005). The given approach allows to lower radioactivity of iodine in the water coolant due to short-lived radionuclides decay. However, these actions do not influence the 131I behavior, which makes the basic contribution to activity of the water coolant even in 5 days.

New Composite Materials for Decreasing of Radioactive Molecular Iodine in the Water Coolant in Working and New Developed NPPs