Towards a better understanding of iodine chemistry in RCS of nuclear reactors

The Phebus FP in-reactor integral experiments provided new insights into iodine transport through the primary circuit. Indeed, in these tests transported iodine was often found not associated with caesium as generally postulated up to now. Several iodine species were experimentally shown to have been transported in the hot leg at 700 deg. C, while a fraction was also suspected to be in a gaseous form in the cold leg at 150 deg. C. For a better estimate of the iodine source term to the containment, both in terms of speciation and quantity, it becomes thus necessary to reconsider iodine species behaviour along their pathway in the reactor coolant system (RCS). This paper presents the current understanding, mainly based on SOPHAEROS equilibrium chemistry calculations of Phebus FP tests performed within the I-RCS technical circle of the SARNET network of excellence in the EU 6th Framework programme. The suspected connection existing between Cs, Mo, Cd and I chemistry and the strong influence of both their release kinetics and related species thermodynamic properties on the iodine speciation in different environments (reducing/oxidizing) are highlighted. Potential explanations for the predicted iodine volatility and the level of association of I to Cs are also discussed.