CFD modelling of hydrogen stratification in enclosures: Model validation and application to PAR performance

Transient Analysis of Passive Autocatalytic Hydrogen Recombiners Using CFD

Passive autocatalytic recombiners (PARs) are being used in most modern nuclear power plants (NPPs) for mitigating the hydrogen risk both in normal and accidental scenarios. Development of a systematic model of PARs based on computational fluid dynamics (CFD) is the subject of this paper. 2D simulations of AREVA recombiner have been performed to validate PARs data and then the developed methodology has been applied to a typical NPP recombiners to assess their performance. Two cases have been discussed; one with constant velocity at inlet and other one is devoted to the startup response of the PAR. Within the recombiner, the hydrogen and oxygen recombine on catalytic plates surface by an exothermic reaction to produce steam. Reaction heat is dissipated among the plates, surroundings and air inside the PAR. Flow inside the PAR will continue downwards until the gas absorbs enough heat to become lighter in weight than the gas outside the PAR. In addition, hydrogen accumulation in containment dome of a typical NPP has been modelled and the results have been compared with MELCOR results. Without PARs, hydrogen got buildup within the containment dome, but when PARs are activated, hydrogen concentration first started to rise until the recombination reaction activated at about ~2% hydrogen concentration. The comparison shows that the results obtained by the model agree well with the MELCOR results.

Hydrogen Dispersion and Ventilation Effects in Enclosures under Different Release Conditions

Hydrogen is an explosive gas, which could create extremely hazardous conditions when released into an enclosure. Full-scale experiments of hydrogen release and dispersion in the confined space were conducted. The experiments were performed for hydrogen release outflow of 63 × 10−3 m3/s through a single nozzle and multi-point release way optionally. It was found that the hydrogen dispersion in an enclosure strongly depends on the gas release way. Significantly higher hydrogen stratification is observed in a single nozzle release than in the case of the multi-point release when the gas concentration becomes more uniform in the entire enclosure volume. The experimental results were confirmed on the basis of Froud number analysis. The CFD simulations realized with the FDS code by NIST allowed visualization of the experimental hydrogen dispersion phenomenon and confirmed that the varied distribution of hydrogen did not affect the effectiveness of the accidental mechanical ventilation system applied in the tested room.

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.