Erosion of a confined stratified layer by a vertical jet – Detailed assessment of a CFD approach against the OECD/NEA PSI benchmark

The 33rd German CFD Network of Competence Meeting: 20 years of advances in the numerical 3D simulation of reactor relevant flows

The 33rd German CFD Network of Competence Meeting was held in March 2022 at the Gesellschaft für Anlagen-und Reaktorsicherheit (GRS) gGmbH in Garching, Germany. In 2022 the meeting celebrates its 20th anniversary with 17 scientific presentations, distributed in two main sessions: “Simulation of Reactor Cooling Circuit Flows” and “Simulation of Reactor Containment Flows”. This paper gives an overview of the different contributions, presented at this anniversary meeting, and also provides information on the background and the objectives of the German CFD Network of Competence.

The Tailored CFD Package ‘containmentFOAM’ for Analysis of Containment Atmosphere Mixing, H2/CO Mitigation and Aerosol Transport

The severe reactor accident at Fukushima Daiichi Nuclear Power Plant (2011) has confirmed the need to understand the flow and transport processes of steam and combustible gases inside the containment and connected buildings. Over several years, Computational Fluid Dynamics (CFD) models, mostly based on proprietary solvers, have been developed to provide highly resolved insights; supporting the assessment of effectiveness of safety measures and possible combustion loads challenging the containment integrity. This paper summarizes the design and implementation of containmentFOAM, a tailored solver and model library based on OpenFOAM®. It is developed in support of Research & Development related to containment flows, mixing processes, pressurization, and assessment of passive safety systems. Based on preliminary separate-effect verification and validation results, an application oriented integral validation case is presented on the basis of an experiment on gas mixing and H2 mitigation by means of passive auto-catalytic recombiners in the THAI facility (Becker Technologies, Eschborn, Germany). The simulation results compare well with the experimental data and demonstrate the general applicability of containmentFOAM for technical scale analysis. Concluding the paper, the strategy for dissemination of the code and measures implemented to minimize potential user errors are outlined.

Analyses of Gas Stratification Erosion by a Vertical Jet in Presence of an Obstacle Using the GOTHIC Code

The GOTHIC code was validated using three experiments carried out in the PANDA facility in the framework of the OECD/NEA HYMERES project. These tests addressed the mixing of an initially stratified atmosphere by means of a vertical jet in the presence of on obstacle (circular plate). This paper reports on the simulations of three experiments, and complementary, quasi-steady-state tests without stratification, where the flow structure above the impingement plate could be observed by means of particle image velocimetry (PIV) velocity measurements in a region larger than that considered in the transient experiments. Moreover, simulations of similar tests without obstacle conducted during the OECD/SETH-2 project are also discussed. The reference, best-estimate model used for the analyses of the three experiments with different flowrates and initial and pressure boundary conditions was built on the base of a multistep approach. This was based on mesh and modeling sensitivity studies mostly performed for the complementary tests, to assess the capability to represent the flow structure produced by the jet–plate interaction with different meshes around the plate. Generally, the results show that the use of a coarse mesh and the standard k–ε turbulence model permits a reasonable representation of the erosion process, but with a systematic over prediction of the mixing time. The results with the reference model were more accurate for two experiments with two flowrates and same initial conditions and all complementary tests. For the third test with different initial and boundary conditions, however, poor results were obtained with the reference model, which could only be improved by further refining the mesh. These results indicate that a model “qualified” for certain conditions could be inadequate for other cases, and sensitivity studies are necessary for the specific conditions considered in the analyses.

Large Scale Experiments Representing a Containment Natural Circulation Loop during an Accident Scenario

The assessment of hydrogen release, distribution, and mitigation measures in the containment of a nuclear power plant is increasingly based on code calculations. These calculations require state-of-the-art experiments to benchmark the codes against them. Two of these experiments are presented in this paper. These experiments were conducted in the PANDA facility (Switzerland) in the framework of the OECD/NEA HYMERES project. The experiments consider natural circulation flow in a two-room type containment where flow loops can form between the inner and the outer zones. During normal operation these zones are separated and in the case of an accident they become either connected by the opening of rupture disks, convective foils, and dampers or connected by bursting of doors and opening of other connections between compartments. For the experiments considered here one lower PANDA-vessel represents the steam generator (SG) tower and the inaccessible area whereas the other vessel represents the outer room area. The lower vessels are isolated from one another except for a small aperture that represents the damper. The two upper vessels—representing the containment dome—are connected to the lower vessels through tubes. The scenario consisted of four phases. In phase 1, a high steam mass flow rate was injected in the vessel representing the SG tower. After the relaxation phase 2, helium (representing hydrogen) was injected in the same vessel (phase 3). Finally in phase 4 no active interventions were done until the end of the test. Two tests were conducted to evaluate the developing helium transport by the natural circulation flow: one with and one without damper (by closing the aperture). The results showed that a two-room containment (TRC) mixing scenario can be well represented with the PANDA facility. It is found that, with the mixing damper open, a global natural circulation loop develops over all four vessels, whereas with closed damper the natural circulation loop is established only between the three vessels representing the inner zone and the upper dome. It is shown that the presence of the damper has a strong effect on the resulting helium content in the inner zone with 3 times less helium at the end of the test compared with the configuration without damper. The formation of a stable helium stratification in the upper vessels was observed in the presence of the open damper.

Reactor safety research within the Helmholtz Association

The reactor safety research of the Helmholtz Association is an integral part of the national provident research. It focuses on the safety of nuclear power plants in Germany and abroad as well as on safety aspects of internationally developed innovative reactor concepts. The research in the three involved Helmholtz centers Forschungszentrum Jülich, Helmholtz-Zentrum Dresden-Rossendorf and Karlsruhe Institute of Technology covers important areas of design basis and beyond design basis accidents. A unique combination of code and model development supported by own large-scale experiments ensures the active preservation of the reactor safety competences. The research that is embedded in a strong international co-operation will be continued after the completion of the national phase-out from the use of nuclear energy for electricity production in 2022.