Iodine behaviour in the containment in Phébus FP tests

Abstract

Amongst the fission products that can be released in the environment during a severe accident, iodine causes great concern because of its volatility and of its potential radiological impact on population. The Phébus FP program provides an extensive database on iodine behaviour in the containment of prime importance for a more realistic evaluation of the iodine source term that can be released in the environment in case of a severe accident.The four main results regarding the iodine behaviour in the containment are: an early presence of gaseous iodine in the containment, the importance of painted surfaces exposed to the containment atmosphere in adsorption/desorption of volatile iodine, the key role of silver in preventing the iodine volatilisation from the sump when in excess over iodine (i.e. in FPT0/1 tests) and the low steady state levels of gaseous iodine concentrations reached on the long term, independently of the amount of gaseous iodine present early in the containment.During the fuel degradation, an early presence of gaseous iodine in the containment was observed in all tests. The interpretation of this result suggests that this gaseous iodine was likely formed in the primary circuit (RCS), probably linked to non equilibrium chemical effects. In FPT3, performed with a B4C control rod instead of Ag–In–Cd control rod for the other tests, the gaseous iodine fraction was much higher pointing to the essential role of control rod material on iodine chemistry in the RCS. In all tests, more than half of the iodine fuel inventory reached the containment under aerosol and gaseous forms.The Phébus FP tests give evidence that the sump did not contribute as much as expected to the production of gaseous iodine in the containment. FPT0 and FPT1 results highlight the key role played by silver produced mainly by the control rod degradation. Silver rapidly reacts with iodine to form non soluble AgI. Thus, despite acidic pH, in so far as silver is in excess compared to iodine, it greatly reduces any gaseous iodine re-volatilisation from the sump either by radiolytic oxidation of iodide or by organic iodide formation from the immersed painted surface. For FPT2, the very low volatilisation of gaseous iodine is attributed to the alkaline pH. For FPT3, where the silver amount was negligible (Ag only present as FP) and the sump acidic, no significant iodine volatilisation from the sump was observed, due to low amounts of iodide aerosols transferred in it.Phébus FP results underline the importance of painted surfaces in the containment atmosphere through their interaction with gaseous iodine. Adsorption of gaseous iodine leads to the large decrease of the gaseous iodine concentration and to the formation of volatile organic iodides. In some conditions, organic iodides gain in importance and become the predominant volatile iodine species notably in the long term for FPT0 and FPT1. This is of particular significance for radiological consequences when operating filtered containment venting as organic iodide are difficult to remove by filtration.The Phébus FP experiments show that whatever the early gaseous iodine fraction was in the containment, low long-term steady concentrations are established. This indicates that an equilibrium between iodine formation/destruction mechanisms and/or reversibility of iodine adsorption/desorption processes yield a similar steady-state concentration in the long term in all the tests.The paper details the main results obtained and their up-to-date understanding.