CFD modeling of nuclear aerosol transport and decay heat distribution in containment flows

by Manohar Kampili (Forschungsinstitut Jülich)

Wednesday 29 Aug 2018, 14:00 → 15:00 Europe/Zurich

OHSA/E13 (Paul Scherrer Institute)

Nuclear aerosols generated in a severe accident are the main carriers of the radioactive fission products besides the noble gases. In addition to the radiological source term, the radioactive elements in the aerosol particles act as heat sources because of the decay heat associated with them. Up to now, Containment flows are analyzed without considering this heat source, which can be airborne, deposited on surfaces or accumulated in water pools. In containment typical flows, system codes estimate that the number density of aerosols is in the order of 1013/m3 with a mean decay power in the order of 100 W/m³. It is expected that this will influence the buoyant flows and water-steam balance in containment. A Computational Fluid Dynamics (CFD) model based on Eulerian-Lagrangian approach is developed to simulate the nuclear aerosol transport. For the Eulerian phase, a URANS approach closed by buoyant k-ω SST model is used. In the Lagrangian phase, drag, gravitational, lift, thermophoresis and turbulent dispersion are considered. Additionally, for particles below 1 μm, the effect of Brownian diffusion is also taken into account. The ISP40 STORM experiment SD 11 (deposition phase) is used for validating the CFD model. It is found that the default models in OpenFOAM for turbulent dispersion over-predict the particle deposition. To allow a first assessment of the effect of decay heat associated with the aerosol particles in buoyant flows, a buoyancy driven cavity flow with Rayleigh number (Ra) of 109 is considered. Tin dioxide (SnO2) particles with a typical concentration 0.1 g/m3 and log-normal distribution are considered as representative core melt aerosols. The decay heat is modelled as a volumetric heat source on the particles. It is found that additional heat transfer to the fluid induces local temperature and density changes significantly. In order to improve the accuracy of particle tracking, continuous random walk model for turbulent dispersion in OpenFOAM-6 is developed. Validation of the complete model using Differentially Heated Cavity (DHC) experiments with aerosol particles is the next step. The final goal is to assess the overall effect of decay heat associated with particles on fluid flow at containment scale.