Influence of Iodine Chemistry in I₂ Scrubbing Modeling

With the scrubbing of molecular iodine (I₂) from gas streams in water pools at an accident site (boiling water reactor wetwell, filtered containment venting systems, etc.), some discernible impact of the aqueous radiochemistry on the I₂ decontamination factor (DF) is expected, sometimes with hardly any effect on aerosol scrubbing. In the frame of the NUGENIA Integration of Pool Scrubbing Research to Enhance Source-Term Calculations (IPRESCA) project, the Central Research Institute of Electric Power Industry (CRIEPI) in Japan performed a series of tests to study the I₂ scrubbing behavior at high pH values, with changing pH in the course of the tests. We used these data for our simulation analyses, and we obtained sensible estimates of the experimental DF values in many calculated cases.

The employed tool for our simulations was a modified version of the SPARC code inside MELCOR. This program was used for the IPRESCA benchmark parametric calculations, where the pH-dependent effect of the iodine chemistry equilibria at the bubble-water interface was predicted to be significant. The same was true for our simulations in the CRIEPI experiments. We have now modeled not just the interface equilibria, but also some slower (kinetic-driven) chemical interactions in water, which would transform, at these high pH values, most of the I₂ into nonvolatile aqueous iodine species. The overall chemistry effect is quite pronounced, like in the experiments, though the description of the bubble thermal hydraulics was at least of the same importance for modeling. Many open questions remain, one of the trickiest of which being the employment of radiochemistry models (relevant to real accident conditions) for explanations of experiments driven by the normal (thermal) chemistry of iodine.