Benchmarking of CFD and LP codes for sprays systems in containment applications: spray tests at two different scales in the TOSQAN and MISTRA facilities
Congress title :CFD4NRS Benchmarking of CFD Codes for Application to Nuclear Safety
Congress location :Munich
Congress date :05/09/2006
During the course of a hypothetical severe accident in a Pressurized Water Reactor (PWR), hydrogen can be produced by the reactor core oxidation and distributed into the reactor containment according to convection flows and water steam wall condensation. In order to prevent the overpressures in the event of a steam break, spray systems are used in the containment. However, the effect of a spray could even increase the risk of hydrogen explosion by condensing steam and therefore locally enriching gaseous mixtures in hydrogen.
The TOSQAN (IRSN) and MISTRA (CEA) projects have been created to perform separate- and coupled-effect tests that are representative of typical post-accident thermal-hydraulic flow conditions in the reactor containment. Both facilities have been designed with the objective of providing very detailed data suitable for CFD code validation.
The main thermal-hydraulic phenomena when water sprays are used in a containment are the mixing induced by spray entrainment and the heat and mass transfers between the gas, the droplets and the walls.
In order to improve the modelling of the latter phenomena, a global benchmark exercise is being organized in the framework of the SARNET Network of Excellence on severe accidents (6th FP project of EURATOM). This benchmark is based on test data provided by IRSN and CEA, namely two TOSQAN tests (test 101 and test 113), and two MISTRA tests (test MARC2 and test MASP).
The paper will first focus on the results of the first TOSQAN benchmark exercise (test 101). In this test a cold water spray is injected into the steam/air-filled vessel, resulting in partial depressurization. The first phase in 2004 was blind, and the second phase in 2005, was open. The codes involved in this exercise are either lumped-parameter-codes or Multi-Dimensional codes: ACACIA-1D (IRSN), ASTEC/CPA-0D (IRSN), GASFLOW-MultiD (FZK), GOTHIC-MultiD (AECL), TONUS-CFD (CEA) and TONUS-LP (CEA). Results of the two phases will be presented. The major observations of this benchmark concern the comparison between the modeled phenomena and the real phenomena involved when spray is injected in the TOSQAN vessel. It is shown that results, such as the depressurization curve, are very sensitive to the modelling, depending on how the vaporization of droplets, in the gas or on the structures, is taken into account.
The second part of this paper will present the status of the on-going benchmark activities on TOSQAN 113 and MISTRA tests. TOSQAN test 113 is a cold' test for which a cold spray is injected into a helium-air stratified mixture. It is proposed to evaluate the mixing effect of a spray on stratification. MISTRA test MARC2 and MISTRA test MASP are respectively similar tests to the TOSQAN 113 and TOSQAN 101, but at a larger scale (100 m3 for the volume of the MISTRA facility and 7 m3 for TOSQAN), thereby giving some insight into possible scaling effects.