The prediction of iodine behavior in the containment of a pressurized water reactor following a loss-of-coolant accident requires a reliable model of the chemistry of iodine in aqueous solution. The INSPECT model, which has been developed over several years, contains a large number of the relevant chemical reactions of iodine and water radiation chemistry. Since the reaction set was first assembled, new data on rate constants and mechanisms have become available. In addition, the application of the model to various small-scale experiments has revealed problems in the modeling of some reactions, leading to an underprediction of the iodine volatility at high pH, although the experiments have demonstrated that the high-pH volatility remains satisfactorily low.

The INSPECT model is described along with the recent modifications that have been made to take account of new data and to improve the modeling where appropriate. The most important of these were (a) changes to the H2O2 - I2 reaction mechanism, (b) the inclusion of an impurity-catalyzed first-order O2- disproportionation reaction, and (c) the treatment of atomic I as a volatile species. These modifications have led to an increase in the predicted iodine volatility under neutral and alkaline conditions. At pH 4.6, where the original model had been found to be satisfactory, the modifications did not result in a significant change in the predicted volatility.

The predictions of the revised model are compared with the results of a comprehensive series of experiments, which are described in a separate paper. The model predictions are in generally good agreement with the experiments for the range of conditions studied (pH 4.6 to 9, 10-5 to 10-4 mol/dm3 I-, 0.02 to 0.2 Mrad/h, 25 to 70°C). The results at neutral and high pH show a significant improvement over the previous version of the model, which underestimated the volatility at pH 9 by more than two orders of magnitude.