Numerical study on the influence of simplified spray boundary conditions for the characterization of large industrial safety spray systems used in nuclear reactors

Introduction During the course of a severe accident in a Pressurized Water Reactor (PWR), hydrogen can be produced due to reactor core oxidation, leading to potential combustion and deflagration, as observed in Three Mile Island and Fukushima accidents. In some reactors, spray systems are placed at the top of the containment to prevent overpressure. Spray modelling is thus part of thermal-hydraulic containment codes. The two major phenomena involved in spray behaviour under such accidental conditions are the thermodynamical effect of a spray (steam condensation on droplets, leading to a local increase of hydrogen concentration) and the dynamical effect (mixing of gases, leading to a decrease of hydrogen concentration). The competition of these two coupled phenomena is an important issue for nuclear safety and can be assessed using CFD codes. For nuclear reactor (containment vessel of around 60 000 m), simplifications have to be done to simulate a nuclear accident in the containment where gas mixture (steam, hydrogen and air) is mixed by the spray systems. Up to now, no CFD calculations are available in the open literature on spray systems in a real-scale nuclear containment, using detailed spray initial conditions, accurate droplet modelling and droplet-gas momentum interaction. Many simplifications can be performed in the computer simulation to reduce the computational time of such sprays induced flow in a very large containment: atomization zone is neglected, considerations of only one droplet size and velocity at one single injection point, consideration of so-called ‘dynamical equilibrium’ between gas and droplet, etc. [1]. The objective of this paper is to evaluate the influence of several simplifications performed on spray boundary conditions, on some selected ‘output’ parameters that can influence the overall gas mixing in nuclear reactors. This evaluation is performed on a real-scale PWR spray nozzle (hollow cone) having an outlet diameter of 9.5 mm and a maximum diameter of the induced spray envelope of about 2 m. CFD calculations are performed using the ANSYS code (lagrangian approach) and the EDF NEPTUNE_CFD code (eulerian approach).