Two studies were conducted on leakage and dispersion in nuclear hydrogen production systems. The first aimed to identify the worst-case leakage accident conditions of the nuclear hydrogen production system designed by the Institute of Nuclear and New Energy Technology at Tsinghua University (Gao et al., 2022a). The second study compared the effectiveness of obstacles with different shapes in preventing hydrogen diffusion (Gao et al., 2022b). The active scheme involved establishing a fan array downstream of the chemical plant to create a reverse airflow, purging the hydrogen jet and preventing its diffusion. The passive scheme focused on constructing obstacles along the hydrogen jet to alter its trajectory or dilute the concentration. Notably, two classifications of passive schemes were considered: decentralized and centralized arrangements. The decentralized approach entailed dividing the open space between the nuclear power plant and the chemical plant into ecological forest areas, serving as a natural barrier for large-scale leakage accidents while also maintaining a green environment. Alternatively, cylindrical mounds were manually piled downstream of the jet to create small obstacles, offering cost-effective diffusion prevention. The concentrated arrangement involved constructing a large obstacle with superior performance downstream of the jet, incurring a certain construction cost.

Figure 1 illustrates the security improvement scheme in this study, featuring a high-pressure hydrogen storage tank with a diameter of 2.5 m, a height of 8 m, and a calculation area of 600 m × 600 m × 200 m. Scheme 1 adopted a passive distributed arrangement, with each conical mound having a bottom surface diameter and height of 2 m. The distance between the centers of cone bases was 4 m, and the first row of soil mounds was positioned 10 m away from the hydrogen storage tank. Scheme 2 employed a passive centralized arrangement, utilizing semi-cylindrical obstacles with a curved surface diameter and width of 20 m, positioned 30 m away from the hydrogen storage tank. Scheme 3 utilized an active fan array with a height and width of 10 m, positioned 30 m away from the hydrogen storage tank.

The process of leakage and diffusion involves the exchange of mass, momentum, and energy between hydrogen and air, with the entrainment of hydrogen by air also involving component transfers. The related equations are as follows:

The continuity equation is: ∂ ∂ x i ρ U i = 0 (1)

The momentum equation is: ∂ ∂ x j ρ U i U j = − d P d x i + ∂ ∂ x j τ i j − ρ u i u j ¯ + ρ g (2) τ i j = μ ∂ U i ∂ x j + ∂ U j ∂ x i − 2 ∂ U j 3 ∂ x j (3)

The energy equation is: ∂ ∂ x j ρ U j h = ∂ ∂ x j λ ∂ T ∂ x j + U j ∂ P ∂ x j (4)

The species transport equation is: ∇ ⋅ ρ U i Y i = − ∇ ⋅ J i (5) where U _i is the velocity (m/s), ρ is the density (kg/m³), P is the pressure (Pa), μ is the molecular viscosity (kg/(m·s)), h is the fluid enthalpy (m²/s²), λ is the thermal conductivity (W/(m·K)), T is the temperature (K), Y _i is the local mass fraction of species i, J _i is the species diffusion flux.

In addition, the kinetic energy of the hydrogen jet changes drastically in the initial stage of leakage and diffusion, and the Shear Stress Transport (SST) k-ω turbulence model, known for its superior calculation accuracy and applicability (Yu and Thé, 2016; Adegboye et al., 2021), is employed for more accurate representation (Menter, 1994): ∂ ∂ x i ρ k U i = ∂ ∂ x i μ + σ k μ t ∂ k ∂ x i + S k − α ρ ω k (9) ∂ ∂ x i ρ ω U i = ∂ ∂ x i μ + σ ω μ t · ∂ ω ∂ x i + ρ γ μ t S k − β ρ ω 2 + 2 1 − F 1 ρ σ ω 2 ω ∂ k ∂ x i ∂ ω ∂ x i (10) μ t = ρ α 1 k max α 1 ω , Ω F 2 (11) where k is the turbulence kinetic energy (m²/s²), ω is the specific dissipation rate (s^-1), σ is the turbulent Prandtl number, μ _t is the eddy viscosity (kg/(m·s)), Ω is the vorticity magnitude (s^-1), and the remaining variables are the empirical parameters of the model.

This study uses the SIMPLEC algorithm to solve the velocity field and pressure field. Since the distance between the leak port and the Mach Disk is very small, the hydrogen gas quickly expands to atmospheric pressure at the Mach Disk location after leaving the leak port. Therefore, this study uses the conditions of the Mach Disk position instead of the conditions of the leakage port to simplify the complex calculation of the expansion area and thereby improve the convergence. In terms of calculation assumptions, this study uses a full buoyancy model, ignores interphase interactions, and performs calculations based on the ideal gas assumption and constant pressure specific heat. In addition, this study assumes that the upstream pressure is a constant value and the air flow direction does not change with changes in air pressure.

Figure 2 illustrates the mesh division of the model, while Table 1 outlines the diverse boundary conditions employed in the study. Meshing utilizes a structured grid. Our study assessed the changes in jet centerline concentration across different grid numbers—166,000, 254,000, 398,000, and 546,000. Notably, at 254,000 grids, the concentration changes stabilized, satisfying the prerequisites for subsequent calculations.

Hu et al. (Hu et al., 2018) conducted experimental measurements involving the concentration field of a helium jet impinging on an obstacle. The experimental setup included an initial pressure of 15 MPa and a 0.5 mm nozzle. The axial distance between the obstacle and the nozzle was varied at 0.2 m, 0.3 m, and 0.4 m. A thermal conductivity sensor mounted on the obstacle’s surface measured the concentration on the vertical centerline. Figure 3 illustrates the comparison between the simulation method employed in this study and the experimental results. The comparison revealed that the concentration change trend on the vertical centerline under different working conditions was consistent between the simulation and experimental results. Furthermore, the maximum absolute error was within 2%. This alignment indicated the suitability of the current model for calculating high-pressure hydrogen leakage scenarios.

In the examination of this security improvement scheme, we classified the placement of hydrogen storage tanks and conical mounds, conducting calculations for four distinct cases as depicted in Figure 4. The variations between these cases were twofold. Firstly, it involved the orientation of the hydrogen storage tank leak, either facing a specific mound in the first row or directed towards the gap between the mounds in the first row. Secondly, it considered whether the mounds were arranged in a linear alignment or a staggered arrangement.

Figure 5 shows the calculated results for the four cases under different wind speeds. Table 3 provides a summary of the diffusion distances of combustible hydrogen in these cases. Notably, the kinetic energy of the hydrogen jet yielded a prolonged impact at higher wind speeds, resulting in a more extensive spread. Observations revealed that, at wind speeds of 1 m/s and 15 m/s, the shortest diffusion distance of hydrogen occurred when soil mounds were arranged in a staggered manner. This outcome could be attributed to the soil mound effectively dividing the hydrogen jet into multiple streams upon impact, and the staggered arrangement continues this division, continuously diluting the overall concentration. Conversely, when the hydrogen jet was directed towards the gap and the mounds were arranged in an aligned manner, the diffusion distance is the longest. In this scenario, the gap did not shield the diffusion of the jet, and the kinetic energy decreased more gradually, leading to a more extensive spread.

Figure 6 shows the calculation results of the passive centralized arrangement scheme. Notably, the kinetic energy of the hydrogen jet experienced a rapid attenuation upon encountering the obstacle, a phenomenon that substantially diminished the diffusion distance. In this respect, under low wind speeds, the buoyancy effect of hydrogen became significant in proximity to the obstacle, causing the majority of combustible hydrogen to manifest above the obstacle. Conversely, under high wind speeds, the kinetic energy of the hydrogen jet underwent continuous replenishment from the swiftly moving air currents in the environment, thereby resulting in an augmented diffusion distance. It is worth noting that our investigation involved the utilization of half-cylindrical surfaces with a consistent structure. However, a notable distinction from previous studies lies in the straight edge of the semi-cylindrical surface upon ground contact, deviating from the previously employed arc-shaped configuration. This alteration introduced a distinct dynamic. For instances where the wind speed was 1 m/s and 15 m/s, the diffusion distances of combustible hydrogen gas were 56 m and 100 m, respectively.

Within this section, the influence of the active fan array blowing, specifically the fan array purge, was examined. The fan’s wind speed varied at 5 m/s, 10 m/s, and 15 m/s. Illustrated in Figure 7 are the distinct flow fields and isosurfaces corresponding to these different operational states. The semi-transparent blue region delineates the boundary of the hydrogen cloud, characterized by a volume concentration of 4%. Table 4 provides a summary of the diffusion distances of combustible hydrogen across diverse operational conditions. When the ambient wind speed was 1 m/s, the fan’s wind speed surpassed the ambient wind speed, instigating a pronounced reverse airflow that could impede hydrogen diffusion while concurrently expanding the range of diffusion. With escalating fan speeds, the entrainment of hydrogen by the air intensified, leading to a continuous contraction of the isosurface depicting combustible hydrogen. In instances where the ambient wind speed reached 15 m/s, even though the fan’s speed did not exceed the ambient wind speed, the impact of ambient wind remained significant, thereby prolonging the diffusion distance. However, the accelerated fan speed contributed to concentration dilution, presenting a scenario where, in comparison to lower fan speeds, the diffusion distance could be diminished.