Transport and deposition in the Phébus FP circuit

In a nuclear reactor severe accident, fission products, after release from the fuel, along with related structural materials and actinides, are transported and partly deposited in the reactor coolant system, finally being injected into the containment. Both in the reactor coolant system and containment, they undergo physical and chemical processes that govern their behaviour and their airborne concentration in the containment atmosphere, which mainly determines the potential source term to the environment. The Phébus FP series provides a wide range of integral data on these processes, which are summarised in this account that covers the whole bundle geometry test series (FPT0 to FPT3). The transport in the circuit depends on the thermal hydraulic conditions (gas composition, temperature, flow rate). FPT0 and FPT1 were carried out under steam-rich conditions, while FPT2 and FPT3 were carried out with lower steam flows, leading to a period where hydrogen formed a large majority of the gas (90%) in the circuit during the main oxidation phase (oxidation of the Zircaloy cladding). The composition of the material transported in the circuit was dominated on average by the fission product noble gases, the volatile fission products Cs and Mo (except FPT0), the structural material Sn (from the cladding), the control rod materials Ag, In and Cd (FPT0/1/2) or B (FPT3), W and Re from the thermocouples, then other volatile fission products such as Te, Rb and I, the lower volatile Ba, and the fuel material U. For FPT2 and FPT3, whose main difference was in the control rod material, the proportions by mass are very similar if the control rod materials are excluded. Most of the materials were transported in the hot leg of the primary circuit as multi-component aerosols, with the notable exceptions of iodine (mainly in gaseous/vapour form) and Cd. In the cold leg, all materials are transmitted in aerosol form,with the notable exception of iodine in FPT3, where a large fraction reaches the containment in gaseous/vapour form. The size of aerosols is characterised by unimodal log–normal distributions; in FPT3 the results can also be interpreted in terms of bimodal distributions.JRC.E.2-Hot cell