The PSIodine Code: A computer program to model experimental data on iodine and other species in irradiated CsI solutions sparged with argon, air, or nitrous oxide

Abstract

Experimental study programmes were carried out at Paul Scherrer Institute (PSI), Switzerland on iodine behaviour under conditions relevant to postulated severe-accidents in NPP containments. To interpret the results obtained from bench-scale, gas-sparged and irradiated iodide solutions, a mechanistic computer code (PSIodine) was developed using the FACSIMILE Software to provide data for comparison. The code models reactions for the iodine oxidation states −1 to +5 in solution under strong (N 2O-saturated) and weak oxidising (argon- and air-saturated) conditions. An empirical model was developed to transport I 2 and other species from solution to the gas space by gas bubbles (sparging). By using measured I 2 mass transfer rates for specific reaction vessels, the need to apply assumptions, e.g., uniform and estimated bubble sizes and concentration, diffusion coefficients, was circumvented. By using the same I 2 transfer rate for irradiation of CsI solutions with and without additional ions, data for % I 2 yields for initial chemical conditions can be compared. Reaction rate changes due to solution evaporation are also modelled. The predicted and experimental data (I 2 fractional releases, pH changes and H 2O 2 formation) correlate well for initial CsI concentrations from 4.0 × 10 −5 to 1.0 × 10 −3 mol dm −3 and for pH 4.6–7.1 in weak oxidising systems (argon- and air-sparged solutions). Data correlations for strong oxidising conditions (N 2O-saturated CsI solutions) are also satisfactory. Irradiated containment atmospheres can generate oxides of nitrogen, which form nitrate and nitrite ions in the sump. Nitrate concentrations up to 5.0 × 10 −3 mol dm −3 in irradiated and argon or air-sparged CsI solutions can lower fractional I 2 release ( Cripps et al., 2011). The model confirms the observed trend of fractional I 2 releases for the pH range 4.6–7.1. Silver aerosols from control rods are postulated in PWR severe accidents to form AgI. The decomposition of aqueous colloidal suspensions of AgI was investigated using 188Re as an in situ beta-radiation source ( Güntay et al., 2005). Under strong oxidising conditions, large I 2 fractions were released. The described model predicts data which support the interpretation of the direct oxidation of AgI particle surfaces.