The Phebus Fission Product (FP) program studies key phenomena and phenomenology of severe accidents in water-cooled nuclear reactors. In the framework of the Phebus program, five in-pile experiments were performed that cover fuel rod degradation and behavior of FPs released via the coolant circuit into the containment vessel. Analyses of FP behavior were performed using standard stand-alone versions of codes with input data mainly taken from measured boundary conditions.
The FPT2 test used 33 GWd/t uranium dioxide fuel enriched to 4.5%, reirradiated in situ for 7 days to a burnup of 130 MWd/t. This test was designed to study low-pressure FP release and transport through a primary cooling system that included a noncondensing steam generator, with release into the containment vessel in steam-poor conditions. This test also investigated how diluted boric acid in the injected steam influenced FP speciation. In the containment vessel, the objective was to study iodine chemistry in an alkaline sump under evaporating conditions.
The analytical approach consisted of progressive studies to explore and explain the main disagreements between base-case calculations and experimental results. Regarding releases in slightly degraded fuel zones, the fission gas behavior and characteristics are shown to be satisfactorily reproduced by the calculations. Electron microprobe analyses also validate the mechanisms for Mo and Ba releases, while the Cs mechanism requires further investigation.
Concerning the transport of FPs, a strong connection is shown to exist between Cs, I, Mo, and Cd that substantially impacts vapor-phase chemistry in equilibrium and iodine volatility. Most of the cesium released from fuel is shown to rapidly convert into cesium borates and then into cesium molybdates when the molybdenum release becomes significant at the end of the hydrogen production phase. The main predicted iodine vapor species is cesium iodide. A low fraction of gaseous hydrogen iodide is also calculated at low temperature, but this fraction was found to be strongly dependent on the Cd release kinetics. Hydrogen iodide is the main candidate predicted by equilibrium chemistry calculations to explain the persistence at low temperatures of volatile iodine. Nevertheless, potential limitations on chemical kinetics in the primary circuit zones, characterized by a sharp decrease in temperature, could also be an explanation and are currently under investigation.
In the containment, gas-phase reactions were found to predominate in governing iodine chemistry. As for the previous Phebus tests, the gaseous iodine fraction measured in the containment early in the test is thought to come from the primary circuit. However, this low gaseous iodine was hardly tractable by the few dedicated samplings mounted in the primary circuit cold leg upstream from the containment entrance. By favoring hydrolysis reactions of volatile iodine species, the alkaline sump is shown to act as an iodine trap despite the evaporating conditions that prevail during the long-term chemistry phase. However, during this latter phase, a persistent, low-level concentration of gaseous iodine was reached in the long term, as during the previous FPT0/1 tests, indicative of a competition in the containment between iodine traps and sources. Aside from the aerosol particles injected by the primary circuit, in situ iodine oxide particles were found to be continuously forming from the decomposition of I2 and ICH3 by air radiolysis products. These particles are suspected to be fine, implying that they predominantly deposit by diffusion on all the containment surfaces. Therefore, in the long term, both the persistence of gaseous iodine and the survival of iodine oxide particles are shown to exist in the containment.