Iodine-Metal Surface Interactions under Severe Accident Conditions in a Nuclear Power Plant

Doctoral thesis, 2005

In the event of a severe nuclear reactor accident, iodine will be released from the fuel and eventually enter the containment. Radioactive iodine isotopes constitute both an acute and a long-term threat to humans, owing to its half-life and its accumulation in the thyroid gland. To produce reliable models predicting the possible environmental exposure, it is crucial to consider the chemical reactions involving iodine in the containment. This work has been an effort to extend the knowledge of iodine chemistry during severe accidents by studying reactions between iodine and reactive metals present in the containment, metals that have as yet been little studied. In addition, the influence of radiation on iodine-metal interactions and the formation of iodine radiolysis products in the gas phase have been studied experimentally. Reactions between I2, CH3I and three metals (Cu, Zn and Al) were studied in a humid nitrogen atmosphere. I2 had a great affinity for all three metals, and adsorption rates as well as activation energies for the reactions could be determined. CH3I had an affinity only for Cu above 50ºC, and the adsorption rate was then several orders of magnitude lower than in the I2 case. Surface analysis showed that I2 and CH3I were chemisorbed as metal iodide. I2 was also retained on Cu in acidic solutions, with an adsorption rate about two orders of magnitude lower than the gas phase reactions. I2 was converted to I- on Al and Zn surfaces in acidic solutions. Conversion rate constants were determined for temperatures up to 140ºC. Mass spectrometry methods failed to identify iodine species formed through radiolysis reactions in air or nitrogen.