Erosion of a helium-rich layer by a steam jet in the presence of an inclined grid: Comparison of the predictions by URANS, STRUC-URANS and LES

Abstract

Severe accidents in Light Water Reactors (LWRs) involve the production of hydrogen, which may accumulate in the upper regions of the containment and form flammable mixtures with the initial air inventory. To study phenomena related to light gas layer removal, the H2P1_10_2 HYMERES test (Paranjape et al., 2019, 2020) was conducted in the PANDA vessel with the aim of investigating the erosion of a helium-rich layer by a steam jet deflected by an inclined grid. In this paper, we make use of the test data to validate computational fluid dynamics (CFD) simulations of such a transient. The simulation models involve the standard k- ε URANS, the more advanced second generation STRUC- ε URANS, and a coarse Large Eddy Simulation (CLES). All simulations are conducted with the same numerical grid consisting of 3.5 million hexahedral cells. Results show that all models capture well the main physical quantities, namely: the helium erosion rate, the temperatures and the velocities downstream of the obstructing grid. Fine details such as the turbulent kinetic energy (TKE) are better reproduced by the CLES, although the latter miss somewhat the amplitudes due to lack of spatial resolution, especially in the near-field downstream of the grid region. The STRUC- ε model replicates qualitatively the flow field fluctuations in sections of the domain where the spatiotemporal resolution is adequate. Mesh refinement in the jet region shows a substantial quantitative improvement, demonstrating that the STRUC- ε model is superior to traditional URANS when spatial resolution is increased. Finally, the simulations show unmistakably that radiation modeling is necessary to achieve reasonable predictions of the temperature field, as the radiative flux can account for a substantial fraction of the heat transfer to the vessel walls, in particular when convective currents are weak.