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. Deposition is mainly concentrated where the temperatures of the wall and fluid decrease strongly, i.e. just above the bundle where the fluid cools from ∼2000 °C to ∼700 C and, in the hot leg of the steam generator, from ∼700 °C to ∼150 °C. In the vertical line, simultaneously developing flows and changes in geometry (successive reductions in tube diameter) enhance deposition. The main processes in the circuit are: chemical transformation of vapours, vapour condensation onto structures as well as nucleation to form aerosols, or onto aerosols, aerosol agglomeration (primarily by diffusion), and thermophoretic deposition. The lower flows in FPT2/3 led to deposition being displaced upstream, e.g. onto the upper parts of the rods and also in the upstream part of the steam generator, where the presence of a large partial boron-rich blockage was deduced in FPT3. Revaporisation was observed in FPT1/2, notably for Cs, leading to delayed transport into the containment after reactor shutdown, also with some evidence for resuspension in FPT0. Post-test revaporisation experiments carried out externally on circuit samples from FPT1/3/4 aid in interpretation of the integral results. Also, the fact that temperatures fall from ∼2000 C at the exit of the bundle to ∼150 C at the exit to the containment, with fast transit times, implies that some reactions are kinetically limited so the composition at containment entry partly reflects that at higher temperatures. This applies particularly for iodine, where early presence of gaseous iodine is found in the containment (e.g. in FPT0, FPT1 and FPT3), and for carbonaceous gases resulting from B4C oxidation in FPT3, where the methane is lower than detection limits (reflecting high temperature bundle conditions, over 2000°C), whereas a non-negligible fraction would be expected under equilibrium, low temperature, containment conditions (typically about 150°C). © 2013 Elsevier B.V. All rights reserved