58 Table 5.1 Analysis condition ··· 74 Table 5.2 Hydrogen recombination pattern in the PARs of different speed variations ··· 76 Table 5.3 Steam production pattern in the PARs of different speed variations ··· 77 Table 5.4 Oxygen removal pattern in the PARs of different speed variations ··· 77 Table 5.5 The streamline pattern of the hydrogen velocity in the case of an upward-directed gas flow ··· 80 Table 5.6 The streamline pattern of the hydrogen velocity in the case of a side-directed gas flow ··· 81 Table 5.7 The streamline pattern of hydrogen velocity in the upward direction with side-directed gas flow · 82. PAR capabilities are ultimately subject to mass transfer limitations and may not keep pace with high hydrogen release rates in small volumes, for example, as might exist in close proximity to hydrogen release.
Introduction
- History background
- Overview of international hydrogen risk assessment
- Study outline
- Passive autocatalytic recombiner (PAR)
It aims to improve the effectiveness of the countermeasure against the hydrogen explosion scenario in nuclear containment building. The hydrogen recombination rate increases due to the increased surface area of the honeycomb catalyst.
Consideration on hydrogen explosion in APR1400
General
Die volume fraksie van die stoom is meer as 80% in die boonste gedeelte van die omhulsel soos in Fig. The hydrogen layer moves to the lower part of the casing, and some hydrogen is trapped between the compartments in the lower part of the inclusion as shown in Fig. In die laat van hierdie stadium vanaf 20 000's word 'n laag waterstofwolk bo-op die inperkingsgebou gevorm met waterstof wat opstyg.
It is assumed that the leakage failure occurs at the lower region of the enclosure. In the lateral air flow 3M/s scenario (Table 5.2), the severed recombination process occurred in the pair with a certain blue color area, which means that it was all the amount of hydrogen flow that was reduced. In this scenario (Fig. 8), the recombination efficiency among the four PAR types did not seem very different, but type 2 slightly leads the other new types of pairs.
The main risk occurs at Stage 5 (33,000 s), which runs at the top of the restraint temple.
Background introduction
Calculations grids and conditions
- Geometry and grids
- Mathematical model and calculation conditions
Results and condition
- Gas behavior and hydrogen explosion risk
- Gas concentration and explosion risk
- Hydrogen explosion scenario
In the lower part there is a lot of air under the hydrogen layer, therefore a small scale hydrogen explosion can occur at the lower part of containment. Therefore, it appears that there is almost no risk of hydrogen explosion occurring at the middle part of containment.
General
Introduction
Hydrogen is a combustible gas, which means it reacts chemically with water to form steam: 2H2 + O2 → 2H2O. 34;flammability limits" There is a "lower flammability (a minimum necessary concentration of combustible gas) and a "higher flammability. Therefore, the PAR is designed to convert hydrogen gas into containers, so that keep the hydrogen concentration below the "lower flammability limits" [36].
The PAR works spontaneously as soon as the hydrogen concentration in the atmosphere starts to increase [37]. The recombination reaction occurs spontaneously at the surfaces, and the water vapor as a product of reaction by the recombiner, as natural convective flow currents promote mixing of combustible gases in the envelope [38].
Proposal of a mitigation system
- Simulation conditions
The total number of grids is 2,700,000 and the grids are generated using NX7.5 and ICEM-CFD nodes. The grids are densely generated near the hydrogen removal to reduce high velocity and high pressure gradient errors.
Results and discussion
- Gas behavior in the shallow reservoir containment 35
Steam began to accumulate on the upper part of the container building since the 2000s, which is shown in red in the figure. By the 6000s, steam collected from the hydrogen removal section and also collected under the guide wall, while air remained in the air tank. 55-56% of the released hydrogen was removed in cases of 120 - 300 kg hydrogen release, but the reduction rate decreased rapidly to 25% in the case of 440 kg hydrogen release.
The conditions for steam discharge from the inlet opening are the same as for shallow ducts (type I). Type I vapor behavior Most of the hot vapor was collected by the guide wall and passed through the center hole and. the steam was collected from the top of the containment temple and the air remained in the air collector.
CFD analysis of the effect of different PAR locations
General
Introduction
The heat produced during the recombination process therefore creates strong driving effects that increase the inflow of surrounding gases into the PAR inlet [47]. For example, the PAR manufacturers such as AREVA and AECL used plate type catalyst while NUKEM invented a specialized cartridge containing pellet type catalysts. The KNT PAR is a stainless housing equipped with catalysts inside the lower part of the box.
The design of core containment may cause some hydrogen quantity to become trapped in the containment and cannot be reduced by the mitigation equipment. This study suggested installing the PAR at different locations in the nuclear confinement to investigate and compare the difference of hydrogen reduction.
Mathematical modeling
A Direct Numerical Simulation of this flow would require significantly more computing power than is available now or in the future. Therefore a mixture of the k-ω model near the surface and the k-ε in the outer region was made by Menter which led to the formulation of the BSL k-ω turbulence model. Outside the boundary layer and on the edge of the boundary layer, the standard k-ε model is used.
The k-ε and Wilcox k-ω turbulence models do not account for the transport of the turbulent shear stress, resulting in an overprediction of eddy viscosity. F2 is a mixing function which limits the limiter to the wall boundary and S is the invariant measure of the strain rate.
KNT PAR calculations
- Mesh and conditions
- Results
A manhole and penetration ports are located on the side for instrumentation and air and hydrogen injection. The motion of the gases in the containment was in a swirling direction and most of the hot steam collected at the top of the containment. The recombination process continued continuously throughout the experiment, but a small amount of hydrogen accumulated in the lower part of the inclusion.
The hydrogen in the containment was successfully reduced to 4%, the lower flammability limit, and the remainder was assumed to be oxygen. The experimental network was found to have a slightly higher recombination rate compared to KNT PAR, but still achieved the same target at the end of the simulation.
PAR installed locations
- Mesh and conditions
- Results and discussion
We have seen that the container in which the PAR is placed 2.0 meters from the inlet has the maximum remaining amount of hydrogen gas. The container in which the PAR was placed at the very bottom of the case has the least amount of hydrogen gas remaining. The average amount of hydrogen gas in the nuclear temple has decreased by 75% of the original concentration.
But in the cut plane of the nuclear temple, a large amount of hydrogen gas still remained at the bottom. If the PAR was placed further away from the hydrogen induction source, there was more residual hydrogen gas at the end.
Proposal and analysis of flow considered-design new
General
Introduction
The standard form of the hydrogen recombination code correlation for the honeycomb PAR was determined as. The red colored area in the figure represents the original concentration of hydrogen before it is removed. In the new concept designed PAR, the conduit wall promoted the H₂ induced area, which allowed more amount of gas in the PAR.
In the actual scenario, the distance of the guide from the base of the PAR plays a role in influencing the hydrogen recombination rate. In the upward with lateral case (Fig. 5.7), type I and type II work well, but type I is slightly better compared to the other 2 types of PAR. While the other 4 lines of PAR flux in show the mass flux into PAR at the heat release generated in the given hydrogen concentration.
In various scenarios, the modified PARs show better performance compared to the original honeycomb PAR.
Mathematical model and calculation condition
Model description
The honeycomb PAR catalyst has a design feature of improved hydrogen removal efficiency by increasing the surface area and increasing the flow rate through the catalyst at the same time. The guide wall has a pyramidal shape with an opening at the top and base, which allow gas to pass through the catalyst. The guide wall is designed to act as a reflector, reflecting induced gas from any direction and channeling them to the narrowing head and then forcing them into the catalyst.
The difference between these three types of PAR is the distance of the guide wall from the base of the PAR, which is 150 cm, 75 cm and 0 cm (attached to the base of the PAR body). 1 shows the analytical grids of the model (5 m high x 5 m long x 5 m wide) and the PAR placed exactly in the middle.
Results and discussion
- Flow induction
- Gas distribution variations scenario
- Hydrogen induced area
- Maximum PAR recombination performance
The gas distribution in the case of 4 vol.% hydrogen is shown in Table 2 – 4, where the hydrogen and oxygen are reduced while the steam increases as the final product of the recombination process. It was due to the shape of the original honeycomb PAR, a weak point that limits the influx of hydrogen under the hydrogen recombination performance. That resulted in a very low hydrogen recombination rate compared to other newly designed PARs, and it is more important in the case of higher speed, such as 3 m/s (Fig. The mass flux of the hydrogen recombination rate at a hydrogen concentration of a side-directed flow case.
The other part of hydrogen went directly up to the top of the containment through the gap at the building wall, where hydrogen remained at the top after the hydrogen release. Malliakos, Analysis of hydrogen depletion using a scaled passive autocatalytic recombiner, Journal of Nuclear Engineering and Design, vol. Cao." A study on the evaluation of a passive autocatalytic recombiner PAR system in the PWR large-dry containment." Nuclear Engineering and Design 238, no.
Royl, Procedure and tools for the deterministic analysis and control of the behavior of hydrogen in severe accidents.
Conclusion
