The Phebus Fission Product Programme studies key phenomena and phenomenology of severe accidents in water-cooled nuclear reactors. The FPT3 test, which was the fifth and last test of the programme, was especially devoted to study the impact of a boron carbide control rod on fuel degradation and fission product transport and speciation in water-cooled reactors. It used uranium dioxide enriched to 24.5 GWd/tU, re-irradiated in situ for 8.4 days to about 180MWd/tU before the test transient to create a representative short half-life fission-product inventory. The experiment showed clear effects of the B4C control rod on bundle degradation, on the transport of fission products in the circuit and on their behaviour in the containment.
Some of the results were rather unexpected, such as the large deposition of boron-containing materials between the hot and cold legs, responsible for a large dip in fission-product transmission in the cold leg, and the large amount of gaseous iodine transported through the primary circuit up to the containment vessel. Despite the high concentrations of gaseous iodine arriving in the containment, the overall behaviour of this gaseous form did not significantly depart from that in previous Phebus FP tests and is characterised by a fast depletion due to adsorption on containment surfaces at short term and constant low residual concentrations in the long term. Analyses of the test results with ASTEC code modules are on-going. They especially highlight that the overall iodine behaviour in the containment can be satisfactorily well captured by assuming a significant injection of gaseous iodine from the primary circuit that remains under-predicted.