The predicted effects of selected Phebus FPT1 sump constituents on iodine volatility

Abstract

The analytical results of containment sump samples in the Phebus FP1Test show the presence of Cd, Sn, U, Ag, Re (destroyed thermocouples) as well as Ag at certain concentrations. An exploratory study was conducted at PSI to study the potential effect of these constituents by modelling different reactions and by conducting specific experimental programmes. Respective reactions are modelled in the PSI's iodine behaviour prediction code, PSIodine (Cripps et al., 2011a), by supplementing the aqueous phase iodine reactions of the code with reaction sets for Cd, Sn, U, Ag and Re. Their reaction rate constants with iodine species were either taken from the available literature or estimated using PSI's experimental data or made by ‘engineered guesses driven from similar reactions or diffusion controlled reactions.We show in four test cases, the effects of these measured FPT1 sump constituents on the fractional I2 yield released from the aqueous solution. They were studied in a comparative approach by including or excluding the reactions of iodine with these constituents. In Test Case 1 (without considering additional models but with the model for AgI dissolution and radiolytic decomposition), only 0.2% I2 is predicted to release. In Test Case 2 (which considers Case 1 reactions and silver species reactions). Excess and isolated Ag atoms have significantly increased I2 yields to 22% by shifting AgI and AgCl dissolution equilibria. In Test Case 3, considered are Case 2 reactions and reactions for Cd, Sn, UO2+ and ReO4− in estimated speciation have further increased AgI oxidation to ∼70% I2. Test Case 4 considered Case 3 reactions and reduction reactions of I2 and IO3− with metal species. The additional reduction reactions of metal species and ReO42− ions with I2 have suppressed the I2 release to only 0.2%. PSI experiments have confirmed the model prediction that irradiated and sparged CsI solutions at pH 4.6 produce lower %I2 yields with increasing perrhenate ion concentration.In conclusion, these results show that predicted iodine chemistry in the aqueous phase can be significantly affected by including additional models to simulate reactions between iodine and metallic species, which were reported in the Phebus FPT1 post-test sump. These models are first attempts and although most of the corresponding kinetic data were literature-sourced, some were “informed estimates”. Thus no claim is made for direct insertion of these models into established iodine prediction codes. Nevertheless, the authors hope that the results highlight the need for in-depth experimental and modelling studies of these and possibly other metallic sump constituents or suppression pools, that is, in research areas, which were hitherto relatively unexplored.’