Organic impurities in containment water, originating from various painted structural surfaces and organic containment materials, could have a significant impact on iodine volatility following an accident. To determine the effect of these impurities on iodine volatility under accident conditions, literature, experimental, and modeling studies have been conducted on
1. the radiolysis of organic compounds in the aqueous phase
2. thermal and radiolytic formation and decomposition of organic iodides
3. dissolution of organic solvents from various painted surfaces into the aqueous phase
4. hydrolysis and aqueous-gas phase partitioning of organic iodides
5. iodine deposition on painted surfaces.
The experimental studies consist of intermediate-scale "integrated effects" tests in the Radioiodine Test Facility and bench-scale "separate effects" tests. Recent findings from these studies and implications of these studies on the safety analysis of an accident in a nuclear power station are discussed.
The studies have shown that organic impurities will be found in containment water as a result of the dissolution of organic compounds from various surface paints. These compounds can have a significant effect on iodine volatility following an accident. The main influence of containment paints on iodine behavior will arise as a result of the aqueous-phase radiolysis of dissolved organic solvents, which are leached from the painted surface by the water. The radiolysis products will decrease the sump pH and dissolved oxygen concentration, consequently increasing the overall rate of conversion of dissolved I- to volatile I2. It appears that the rates of these processes may be controlled by the dissolution kinetics of the organic compounds from the surface coatings. Moreover, organic compounds may also react thermally and radiolytically with I2 to form organic iodides in the aqueous phase. Our studies have shown that the formation of organic iodides in the aqueous phase from soluble organic compounds such as ketones, alcohols, and phenols will have more impact on the total iodine volatility than the formation of CH3I from CH4 and I2 from either the gas or the aqueous phase.