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Measurements of the Radiolytic Oxidation of Aqueous CsI Using a Sparging Apparatus

C. B. Ashmore, D. Brown, S. Dickinson, H. E. Sims

Nuclear Technology

Volume 129 / Number 3 / March 2000 / Pages 387-397


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Radiolytic oxidation is considered to be the main mechanism for the formation of I2 from aqueous CsI in the containment of a water-cooled reactor after a loss-of-coolant accident (LOCA) in a pressurized water reactor. Despite the amount of study over the last 60 yr on the radiation chemistry of iodine, there have, until recently, been few consistent sets of experiments spanning a wide enough range of conditions to allow models to be verified with confidence.

The results from a set of experiments carried out to remedy this deficiency are described. In this work the rate of evolution of I2 from sparged irradiated borate solutions containing CsI labeled with 131I was measured on-line over a range of conditions. This work involved the measurement of the effects of pH, temperature, O2 concentration, I- concentration, phosphate concentration, dose-rate, and impurities on the rate of evolution of I2. The range of conditions was chosen to span as closely as possible the range of conditions expected in a LOCA, but also to help to elucidate some of the mechanisms especially at high pH.

The pH was found to be a very important factor in determining iodine volatility. Over the temperature range studied, the extent of oxidation decreased with increasing temperature, but this was counteracted, to a greater or lesser extent, by the decrease in partition coefficient. The oxygen concentration was more important in solutions not containing phosphate. The fractional oxidation was not particularly dependent on iodide concentration, but GI2 was very dependent on [I-]. There was no effect of added impurities, Fe, Mn, Mo, or organics although, in separate work, silver was found to have a very important effect.

During attempts to interpret the data, it was found that it was necessary to consider the iodine atom as a volatile species with a partition coefficient of 1.9 taken from thermodynamic data. The modeling work is described in a separate paper.

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