CFD Modeling of Wall Steam Condensation: Two-Phase Flow Approach versus Homogeneous Flow Approach
The present work is focused on the condensation heat transfer that plays a dominant role in many accident scenarios postulated to occur in the containment of nuclear reactors. The study compares a general multiphase approach implemented in NEPTUNE_CFD with a homogeneous model, of widespread use for engineering studies, implemented in Code_Saturne. The model implemented in NEPTUNE_CFD assumes that liquid droplets form along the wall within nucleation sites. Vapor condensation on droplets makes them grow. Once the droplet diameter reaches a critical value, gravitational forces compensate surface tension force and then droplets slide over the wall and form a liquid film. This approach allows taking into account simultaneously the mechanical drift between the droplet and the gas, the heat and mass transfer on droplets in the core of the flow and the condensation/evaporation phenomena on the walls. As concern the homogeneous approach, the motion of the liquid film due to the gravitational forces is neglected, as well as the volume occupied by the liquid. Both condensation models and compressible procedures are validated and compared to experimental data provided by the TOSQAN ISP47 experiment (IRSN Saclay). Computational results compare favorably with experimental data, particularly for the Helium and steam volume fractions.
Cfd Modeling of the Test 25 of the Panda Experiment
The accurate modeling of gas distribution (air, steam and Hydrogen) in a PWR containment, which can be released after the hypothetical beginning of a severe accident leading to the melting of the core, concerns phenomena such as wall condensation, hydrogen accumulation, gas stratification and transport in the different compartments of the containment. Numerical simulations performed with CFD solvers NEPTUNE_CFD and Code_Saturne are compared with experimental data corresponding to the test 25 of the PANDA experiment. The overall dimensions of the experiment (Diameter~4 m, Height~8 m, Volume of the 2 vessels~180 m) are not yet representative of the right scale of the reactors, but they already provide valuable information when compared to smaller scales (as experience TOSQAN~7m). The obtained computational results compare fairly well with experimental data and with computational results obtained with other flow solvers. The formation of high concentration helium layers in the two vessels is predicted, as well as the earlier arrival of helium with respect to steam at the vent. The analysis of the different fields (velocity, concentrations, density, mechanical pressure) and their comparisons with experimental data permit to explain the observed stratifications and to understand the formation of the complex flow structures. Ongoing works concern mesh sensitivity studies comprising structured mesh (hexahedra) or unstructured mesh (tetrahedron) and multiphase flows simulations.
Protecting Marine Biodiversity on the Continental Shelf Beyond 200 Nautical Miles
States are expending significant effort to chart the extent of their continental shelves where these extend beyond 200 nautical miles. As more is understood about marine biodiversity on the outer continental shelf, states may wish to regulate the use of biodiversity for the purposes of conservation or for future exploitation. This article identifies potential threats to marine biodiversity on the continental shelf, explores whether conservation is a legitimate purpose for exercising coastal state rights over the outer continental shelf under the Law of the Sea Convention, and considers the various legal rules that coastal states may use to protect marine biodiversity. The article concludes that the continental shelf regime is undesirably vague in some instances but that coastal states have a legal basis for taking action to regulate activities that impact the marine biodiversity of the outer continental shelf.