Hydrogen Risk Assessment Studies: A Review Toward Environmental Sustainability

Abstract

The transition to hydrogen as a clean energy source is critical for addressing climate change and supporting environmental sustainability. This review provides an accessible summary of general research trends in hydrogen risk assessment methodologies, enabling diverse stakeholders, including researchers, policymakers, and industry professionals, to gain insights into this field. By examining representative studies across theoretical, experimental, and simulation-based approaches, the review highlights prominent trends and applications within academia and industry. The key focus is on evaluating risks in stationary and transportation applications, paying particular attention to hydrogen storage systems, transportation infrastructures, and energy systems. By offering a concise yet informative summary of hydrogen risk assessment trends, this paper aims to serve as a foundational resource for fostering safer and more sustainable hydrogen systems.

1. Introduction

The global shift toward eco-friendly fuels is critical for addressing climate change and achieving environmental sustainability. Among various alternatives, hydrogen has emerged as a promising energy source due to its high energy density and environmentally friendly characteristics, emitting only water and heat during combustion. According to the study by Janardhanan et al. (2024), hydrogen energy plays a critical role in addressing the limitations of renewable energy sources like solar and wind [1]. These sources suffer from intermittency and variability, which hinder stable energy supply and large-scale promotion. By storing excess electrical energy as chemical energy through hydrogen production, such as water electrolysis, hydrogen enables the efficient utilization of renewable energy and supports large-scale decarbonization efforts. Also, as highlighted by Li et al. (2024), hydrogen’s unique role as a carrier for renewable energy extends beyond its use as a fuel. It facilitates the stabilization of energy grids by mitigating the effects of renewable energy variability and enables efficient long-term storage of energy. This positions hydrogen as a key enabler for achieving global energy sustainability and deep decarbonization [2]. These attributes make hydrogen a critical component of the transition away from traditional fossil fuels, particularly in sectors where decarbonization is challenging, such as transportation, energy storage, and industrial processes. The urgency to adopt sustainable energy sources has driven substantial investments in hydrogen infrastructure worldwide, encompassing production, storage, and transportation.

However, the widespread adoption of hydrogen is accompanied by significant safety challenges that must be addressed to realize its potential as a sustainable energy carrier. Hydrogen presents unique hazards due to its wide flammability range (4–75%), low ignition energy (0.02 mJ), and high deflagration index (K), which significantly increases the risk of vapor cloud explosions compared to traditional fuels. Moreover, its colorless and odorless nature complicates leak detection, raising the stakes for accidents during storage, transportation, and utilization. High-profile incidents, such as the Stockholm explosion in 1983, involved 13.5 kg of hydrogen leaking from industrial pressure vessels, resulting in injuries, vehicle damage, and shattered windows within a 90 m radius [3]. Similarly, the Sandvika hydrogen refueling station explosion in 2019, caused by faulty plug sealing, resulted in debris projection and window damage up to 65 m away [4].

Hydrogen safety research has progressed substantially in recent years, employing diverse methodologies to address these risks. For example, Hubert et al. (2014) developed advanced sensors to enhance hydrogen leak detection in fuel cell vehicles, improving safety across various operating conditions [5]. Singh et al. (2015) evaluated hydrogen’s potential as an energy carrier by examining its environmental, safety, and economic implications across the entire hydrogen value chain [6]. These studies, along with others, highlight the need for a comprehensive framework that not only assesses individual risks but also connects various research methodologies into a cohesive process.

This paper aims to review the current state of hydrogen risk assessment research and proposes a structured ‘Hydrogen Risk Assessment Process’ to address the unique safety challenges associated with hydrogen energy systems. This process integrates theoretical studies, experimental validations, and simulation analyses, providing a systematic approach to evaluating hydrogen safety across different applications. For instance, Ng and Lee (2008) emphasized the role of turbulence in hydrogen combustion dynamics [7], while Pasman (2011) proposed Bayesian methodologies for scenario generation, demonstrating the need for advanced computational tools and empirical validations [8]. These contributions, alongside studies like Xiao et al. (2018) that focus on explosion simulations offer valuable insights into creating safer hydrogen infrastructure [9].

In this review, as follows in Figure 1, research trends are categorized by storage type—stationary (e.g., hydrogen refueling stations) and transportation (e.g., pipelines, fuel cell vehicles)—and by methodological approach, including theoretical, experimental, and simulation-based studies. For example, Molkov et al. (2015) provided experimental and predictive insights into blast wave decay following high-pressure tank ruptures [10]. Separately, Kashkarov et al. (2020) introduced nomograms to aid safety engineers in designing hydrogen systems by estimating explosion wave impact distances [11]. We also reviewed hydrogen control systems that ensure the safe and efficient operation of hydrogen storage and transportation facilities. By synthesizing these studies, this paper highlights gaps in current methodologies and identifies opportunities for integrating sustainability considerations into hydrogen risk assessment.

Ultimately, this review serves not only as a comprehensive summary of hydrogen risk assessment research but also as a proposal for advancing the field through the development of a structured research process. By emphasizing the interplay between theoretical, experimental, and simulation-based approaches, this paper seeks to provide academia and industry with a roadmap for fostering safer and more sustainable hydrogen energy systems. This focus on the ‘Hydrogen Risk Assessment Process’ aligns with the broader goal of achieving environmental sustainability through the safe and efficient use of hydrogen.

2. Main Body

2.1. Theory

Theoretical studies on the assessment of hydrogen energy risks have employed a range of risk assessment techniques commonly used in the field of chemical process safety. These techniques can be categorized into qualitative, quantitative, and semi-quantitative methods, each offering unique insights into hydrogen safety as shown in Figure 2.

Qualitative techniques, such as checklists, what-if analysis, relative risk ranking (e.g., Dow and Mond indices), Hazard and Operability Studies (HAZOPs), Failure Modes and Effects Analysis (FMEA), and Preliminary Hazard Analysis (PHA), are particularly effective for identifying preliminary risks. For example, HAZOP has been widely applied to hydrogen processing facilities, where its structured methodology for identifying potential deviation points in processes has proven effective, as demonstrated by Kikukawa et al. (2009) [12].

Quantitative techniques, including Fault Tree Analysis (FTA), Event Tree Analysis (ETA), and Cause–Consequence Analysis (CCA), are instrumental in mapping out failure scenarios and their probabilities. Dadashzadeh et al. (2018) extensively applied FTA and ETA to assess failure sequences and quantify risks in hydrogen storage and transportation settings, providing a deeper understanding of critical risk factors [13].

Semi-quantitative methods, such as bow-tie analysis and Layer of Protection Analysis (LOPA), bridge the gap between qualitative and quantitative techniques, enabling a more visual representation of risk pathways and mitigation measures.

However, traditional approaches often struggle to adapt to the dynamic nature of hydrogen applications. Recent advancements, such as the integration of dynamic Bayesian networks, allow for real-time data analysis, enhancing the accuracy of risk predictions. This capability is particularly valuable in urban hydrogen storage sites, where conditions can change rapidly, as highlighted in Zarei et al. (2021) [14].

Dynamic risk assessments leverage advanced technologies such as IoT-enabled sensors and Bayesian networks to address evolving conditions. For example, IoT sensors can detect real-time pressure or temperature changes in hydrogen systems, feeding data into Bayesian models for adaptive risk calculations. This allows for instantaneous adjustments to safety protocols, significantly enhancing the responsiveness of hydrogen infrastructure.

2.1.1. Theoretical Approach to Hydrogen-Transportation Risk Assessment

In the context of hydrogen transportation, theoretical studies have focused on evaluating risks associated with hydrogen movement and storage. Traditional techniques, such as ETA and FTA, have been widely applied to probabilistically analyze potential failure scenarios and assess their impacts.

Rodionov et al. (2011) utilized Event Tree Analysis (ETA) and Fault Tree Analysis (FTA) to construct accident scenarios related to hydrogen-powered vehicles. Their analysis included risks such as vehicle crashes, fires, and hydrogen leaks, culminating in a Quantitative Risk Assessment (QRA) through a Probability Safety Assessment (PSA). This study identified critical risk events across various environmental and operational conditions [15].

Similarly, Dadashzadeh et al. (2018) employed QRA techniques to evaluate the risk levels of hydrogen storage tanks in hydrogen-powered vehicles, considering parameters such as Fire Resistance Rating (FRR), TPRD failure frequency, and urban population density. Their findings provided insights into the societal and economic implications of engineering safety measures for hydrogen-powered transportation systems [13].

Crowl et al. (2007) further expanded the understanding of hydrogen risks by comparing its physical and chemical properties with those of conventional fuels, such as gasoline and methane. Their analysis highlighted hydrogen’s unique hazards, including a higher flammability range and detonation index, and quantitatively assessed its fire and explosion probabilities [16].

Additionally, Utgikar et al. (2005) investigated the thermal effects of hydrogen leakage in stationary vehicles, applying the Joule–Thomson effect to model risk scenarios. This study contributed significantly to understanding the safety of hydrogen storage in confined environments [17].

While these traditional studies provided foundational insights, they often relied on static models that lacked adaptability to dynamic environments. Static models, while effective for initial risk assessments, often fail to capture the dynamic nature of real-world scenarios. For example, a static model might predict the risk of hydrogen leakage under fixed pressure and temperature conditions, but it cannot account for fluctuating operational variables such as intermittent equipment failure or environmental changes. A notable incident occurred in a hydrogen storage facility where undetected pressure variations led to a delayed response, culminating in an explosion. By contrast, dynamic models incorporating real-time sensor data could have identified the abnormal conditions early, preventing the accident. This highlights the critical need for transitioning from static to dynamic risk assessment frameworks. Dynamic environments refer to conditions where variables such as temperature, pressure, wind speed, or human activity constantly change, influencing the likelihood and impact of safety incidents. Examples include urban areas with varying traffic densities, offshore hydrogen production sites exposed to fluctuating weather conditions, and industrial plants where operational conditions dynamically shift due to process changes. Recent efforts, such as those by Zarei et al. (2021), have introduced Bayesian networks capable of real-time probability adjustments. These advancements address the limitations of traditional techniques, particularly in high-risk scenarios like urban hydrogen applications [14]. Real-time probability adjustments involve recalibrating risk likelihoods instantaneously based on incoming data from monitoring systems or changing environmental conditions. This approach ensures that safety interventions can be implemented promptly as the situation evolves. For example, in a hydrogen storage facility, sensors monitoring pressure or hydrogen concentration can trigger immediate updates to risk models when abnormal readings are detected. This allows for automated responses, such as system shutdowns or alarms, to prevent escalation. Real-time adjustments are critical for addressing dynamic risks and improving overall system reliability.

Table 1. List of prior studies on hydrogen-transportation risk assessment.

No	Reference	Objective	Scenario
1	Andrei Rodionov et al. (2011) [15]	Identify and quantify additional risks associated with hydrogen explosions in vehicles using ETA and FTA	Vehicle crashes, vehicle fires, and ignition after hydrogen leaks in open (highway, country roads) and confined environments (garages, tunnels, gas stations)
2	Dadashzadeh et al. (2018) [13]	Evaluate the Fire Resistance Rating (FRR) impact on storage tanks and assess acceptability through QRA	Hydrogen-powered vehicles in urban settings (e.g., London) considering FRR, TPRD failure, and population density
3	Daniel A. Crowl et al. (2007) [16]	Compare the risks of hydrogen fuel with conventional fuels (petrol, natural gas) based on physical properties	Fire and explosion risks associated with different fuels in transportation and storage scenarios
4	Vivek P. Utgikar et al. (2005) [17]	Assess risks of high-pressure hydrogen tanks (5000–1000 psi) and evaluate leak impacts	Hydrogen leaks from onboard high-pressure tanks during vehicle operation

below is the list of prior studies on hydrogen-transportation risk assessment.

Theoretical approaches effectively evaluate the complex accident scenarios of hydrogen-transportation systems using probabilistic methods such as Quantitative Risk Assessment (QRA), addressing key risks like vehicle collisions, leaks, and fires. However, existing studies rely heavily on static models, which fail to reflect real-world dynamic conditions. To address this limitation, the integration of techniques like dynamic Bayesian networks to adjust probabilities in real time and develop comprehensive models that account for urban complexities is necessary.

2.1.2. Theoretical Approach to Hydrogen-Stationary Risk Assessment

Theoretical studies on hydrogen-stationary risk assessment have focused on evaluating risks associated with fixed hydrogen systems, such as refueling stations and storage facilities. Quantitative, qualitative, and semi-quantitative methods have been applied to understand and mitigate risks in these contexts.

Zarei et al. (2021) employed Fault Tree Analysis (FTA) and Event Tree Analysis (ETA) as quantitative techniques to identify potential failure scenarios in stationary hydrogen systems. These techniques were followed by semi-quantitative methods, such as the bow-tie approach and Bayesian Network models, to provide a comprehensive risk assessment framework. Their study demonstrated how integrating dynamic models with traditional techniques can enhance risk prediction for hydrogen leakage, particularly in high-pressure environments [14]. Rodionov et al. analyzed hydrogen vehicle leaks in traffic environments, while Zarei et al. focused on leakage risks in stationary systems. These comparisons illustrate the varying nature of risks and mitigation strategies across transportation and stationary applications.

Kikukawa et al. (2009) applied qualitative techniques, including Hazard and Operability Studies (HAZOPs) and Failure Modes and Effects Analysis (FMECA), to assess risks at liquefied hydrogen refueling stations. Their work emphasized the importance of identifying process deviations and potential equipment failures, which are critical for ensuring the safe operation of hydrogen infrastructure [12].

Huang and Ma (2018) further advanced stationary risk assessment by using a Bayesian Network model and a grid-based risk screening method to evaluate hydrogen refueling stations. This approach allowed for the spatial visualization of risk factors, enabling better resource allocation for risk mitigation [18]. Bang et al. (2016) predicted the impact on surrounding pipelines from hydrogen tank explosions. They used the Strong-Explosion theory to calculate parameters such as pipe bending amplitude, explosion distance, and pressure, providing critical insights into structural vulnerabilities [19]. Some researchers have also explored modeling techniques to analyze hydrogen explosion consequences. Lobat et al. (2006) studied Unconfined Vapor Cloud Explosions (UVCEs) using models like the TNT equivalency, TNO multi-energy, and Baker–Strehlow–Tang (BST) models, highlighting the variability in explosion impacts under different scenarios [20]. Mousavi and Parvini (2016) validated hydrogen dispersion models using leakage data from supply pipelines, providing empirical evidence for the accuracy of these models in predicting accident outcomes [21].

Additionally, efforts to establish standardized guidelines for hydrogen risk assessment have been made. LaChance et al. (2011) reviewed Quantitative Risk Assessment (QRA) criteria for hydrogen facilities under the IEA’s Hydrogen Implementing Agreement (HIA) Task 19. Their work emphasized the need for uniform standards to ensure safety and provided actionable recommendations for hydrogen facility risk evaluations [22].

Table 2. List of prior studies on hydrogen-stationary risk assessment.

No	Reference	Objective	Scenario
1	Zarei et al. (2021) [14]	Assess the risk of hydrogen leakage	Hydrogen leakage accidents in chemical plants, machinery, and storage by alkaline water analysis
2	Kikukawa et al. (2009) [12]	Perform risk assessment for liquefied hydrogen filling stations	Operational safety during refueling at liquefied hydrogen stations
3	Huang and Ma (2018) [18]	Evaluate risks at hydrogen charging stations	Hydrogen charging station environments using grid-based risk mapping
4	Bang et al. (2016) [19]	Predict pipeline impact from hydrogen tank explosions	Explosion impact on nearby pipelines, considering pipe thickness and distance
5	Lobat et al. (2006) [20]	Analyze hydrogen explosion consequences	Universal vapor cloud explosion (UVCE) scenarios using TNT, TNO, and BST models
6	Mousavi and Parvini (2016) [21]	Model hydrogen diffusion and perform consequence analysis	Hydrogen leakage from supply pipelines validated with diffusion data
7	LaChance et al. (2011) [22]	Conduct Quantitative Risk Assessment for hydrogen facilities	Hydrogen storage and operational risk evaluations

shows the list of prior studies on hydrogen-stationary risk assessment.

Risk assessments for hydrogen refueling stations and stationary systems effectively integrate quantitative and qualitative approaches, with dynamic models like Bayesian networks significantly improving prediction accuracy. However, these studies often lack experimental data or fail to adequately incorporate spatial factors. Future research should focus on integrating real-time data and developing sophisticated models that include spatial factors to enable more precise risk assessments.

2.2. Experiment

Experimental studies are foundational for assessing hydrogen risks, as they provide empirical data to validate theoretical models and simulations. While experimental methods offer high accuracy, they often face constraints such as cost, time, and safety concerns. Consequently, simulation studies are frequently employed to complement experiments, a topic that will be discussed later in this paper. Despite these challenges, experimental data remain indispensable for verifying hydrogen safety measures.

Hydrogen’s wide flammability range (4% to 75%) and low ignition energy make it especially hazardous. Even small leaks can result in ignition, fire, or explosion. To address these risks, various experiments have been conducted to evaluate hydrogen-related incidents, focusing on leaks, fires, and explosions.

One notable area of experimental research is the evaluation of hydrogen compression storage tanks, which are classified into four types based on material composition and strength.

Table 3. Comparison of hydrogen storage tanks [23].

Types	Materials	Characteristics	Usage
Type I	All metal	Heavy, internal corrosion, low hydrogen storage density	For hydrogen stations
Type II	Hoop-wrapped tank with inner container	Heavy, short life, serious hydrogen embrittlement problem	For industrial
Type III	Materials (liner, composite shell)	Lightweight, high burst pressure, no permeation	Vehicular use.
Type IV	Carbon fiber-reinforced polymer	Lightweight, lower burst pressure. Permeation through liner. High material cost	Vehicular use (longer life)

summarizes the characteristics of these storage tanks, along with their advantages and limitations.

Type I tanks made entirely of metals such as steel or aluminum, are highly durable under high pressures and resistant to internal corrosion. However, their heavy weight limits their storage efficiency, making them more suitable for stationary applications such as industrial storage or refueling stations.

Type II tanks incorporate a metal liner encased in composite materials, reducing weight while maintaining structural integrity. However, they are prone to hydrogen embrittlement and have a shorter lifespan, making them suitable for stationary applications with moderate safety demands.

Type III tanks, featuring a metal liner surrounded by carbon fiber-reinforced composites, provide a lightweight and durable option capable of withstanding high pressures. These tanks are ideal for mobile applications, such as hydrogen fuel cell vehicles, but are more expensive due to their use of advanced materials.

Type IV tanks, which use a plastic liner encased in carbon fiber, are the lightest and most efficient for vehicular use. However, their susceptibility to hydrogen permeation necessitates stringent safety monitoring, especially for high-pressure storage applications.

Comparative studies show that Type III and IV tanks are more suitable for hydrogen transportation due to their weight efficiency, whereas Type I and II tanks are often used in stationary applications where weight is less critical but cost considerations are paramount. This differentiation highlights the need for application-specific tank selection to optimize safety and efficiency.

These experimental findings contribute significantly to understanding the material properties and structural performance of hydrogen storage tanks. By examining these characteristics, researchers have laid the groundwork for establishing design standards that prioritize both efficiency and safety. Such insights are critical for the development of the Hydrogen Risk Assessment Process, offering guidance on selecting appropriate storage systems for specific applications.

2.2.1. Risk Assessment of Hydrogen Transportation Through Experimentation

As hydrogen fuel cell vehicles gain widespread adoption, experimental studies have been instrumental in assessing the risks associated with hydrogen transportation. One key focus is the evaluation of storage tanks equipped with Temperature-activated Pressure Relief Devices (TPRDs), which prevent tank rupture under high temperatures and pressures during fires. While studies such as Li et al. (2024) have explored the risks associated with Proton Exchange Membrane Fuel Cells (PEMFCs), including performance degradation and hydrogen accumulation over prolonged use, these topics, though significant, fall outside the primary scope of this review. Instead, we have chosen to focus on risk assessment targets that are most commonly addressed in both industrial applications and academic research, such as storage and transportation systems. This approach allows us to provide a comprehensive overview of widely applicable methodologies and trends [23].

Tamura et al. (2014) conducted fire experiments on hydrogen fuel cell vehicles equipped with TPRDs alongside adjacent gasoline vehicles. Their findings provided valuable insights into fire spread behavior under various conditions, including scenarios involving car-carrying ships with multiple hydrogen fuel cell vehicles [24].

Li et al. (2022) performed bonfire tests on Type III storage tanks, evaluating parameters such as failure pressure and Fire Resistance Ratings (FRRs). Their research underscored the need for standardized procedures to assess tank performance under extreme conditions [25].

Wang et al. (2023) investigated the explosion mechanisms of hydrogen storage tanks through bonfire and hydraulic burst pressure tests. They developed generalized procedures for evaluating explosion overpressure, offering a critical reference for tank safety assessments [26].

Hupp et al. (2019) conducted fire resistance tests on Type IV tanks, analyzing factors such as flame temperature, impact area, and initial charging pressure. Their work highlighted design considerations for improving tank resilience under fire conditions [27].

These experimental studies provide essential data for improving hydrogen-transportation safety. By examining the performance of hydrogen storage tanks under various stressors, these studies contribute to the refinement of safety protocols and inform the development of regulations that support the safe deployment of hydrogen fuel cell vehicles.

In addition, experiments have been conducted in confined environments to simulate hydrogen leakage scenarios. Dong Hao et al. (2007) replicated garage-like conditions to evaluate hydrogen leakage risks, providing recommendations for improved ventilation and equipment safety [28]. Merilo et al. (2011) conducted leakage and explosion experiments in similar settings, analyzing variables such as vehicle presence, leakage rates, and ventilation types [29].

Table 4. Research list of hydrogen-transportation risk assessment through experiments.

No	Reference	Objective	Scenario
1	Yohsuke Tamura et al. (2014) [24]	Examine flame diffusion behavior between gasoline and hydrogen fuel cell vehicles in transport conditions	Fire diffusion from hydrogen fuel cell vehicles with TPRD to adjacent vehicles, including petrol vehicles and transport vessel scenarios
2	Bei Li et al. (2022) [25]	Evaluate response behavior and impact of high-pressure Type 3 hydrogen tanks under fire conditions	Fire exposure tests on Type 3 hydrogen tanks (210 L, 35 MPa)
3	Xueying Wang et al. (2023) [26]	Analyze the explosion mechanism of hydrogen storage tanks and validate theoretical findings experimentally	Explosion tests on 6.8 L hydrogen storage tanks at 30 MPa
4	Nicola Hupp et al. (2019) [27]	Investigate factors affecting the fire resistance time of hydrogen pressure vessels	Bonfire tests on Type 4 hydrogen vessels with pressures ranging from 175 to 700 bar and varying flame impact areas
5	Dong Hao et al.(2007) [28]	Evaluate the safety of hydrogen fuel cell vehicles (FCVs) and propose safety standard improvements for confined spaces	Comparison of safety standards and hydrogen leak tests in confined spaces with/without ventilation
6	E.G.Merilo et al.(2011) [29]	Assess hydrogen leakage from fuel cell vehicles in garage environments	Leakage tests with 10 pressure gauge positions under natural and mechanical ventilation conditions

presents the research list of hydrogen-transportation risk assessment through experiments.

These studies highlight the importance of understanding hydrogen behavior in enclosed spaces, where leakage and explosion risks are amplified. The findings not only validate theoretical risk models but also provide actionable insights for improving infrastructure design and operational protocols. This is especially crucial for integrating hydrogen technologies into urban environments, where confined spaces are more common.

Experimental studies evaluating the fire resistance and leakage behavior of hydrogen storage tanks play a crucial role in improving the safety of hydrogen-transportation systems. However, current experimental data are limited to specific conditions, making it challenging to reflect large-scale accident scenarios. Expanding experimental evaluations to incorporate a broader range of conditions and integrating new materials and design technologies is essential.

2.2.2. Risk Assessment of Hydrogen Transportation Through Experimentation Research

Experimental research on stationary hydrogen systems, beyond hydrogen fuel cell transportation vehicles, has significantly advanced our understanding of explosion dynamics and mitigation strategies. Studies investigating unconfined hydrogen explosions have revealed critical insights into hydrogen behavior under various conditions. Jiang et al. (2022) explored flame instability in fan-stirred unconfined hydrogen, highlighting the influence of external turbulence on flame propagation and explosion severity. This work has practical implications for managing hydrogen-related risks in open environments, where turbulence often amplifies explosion hazards [30]. Zhou et al. (2023) examined the role of ignition height in explosion dynamics, providing actionable data for optimizing industrial safety measures [31]. Similarly, Kim et al. (2013) employed the soap bubble method to investigate hydrogen–air mixture explosions, generating data on flame propagation and overpressure intensity that informs the safer design of hydrogen systems [32].

High-pressure hydrogen systems have also been a focal point of experimentation. Proust et al. (2011) utilized advanced techniques like mass flow rate measurement and flame geometry imaging to characterize high-pressure hydrogen flames, offering a thermodynamic dataset crucial for predictive modeling [33]. Mogi et al. (2008) analyzed autoignition and explosion risks during high-pressure hydrogen releases, noting that pipe length plays a pivotal role in ignition probability. These findings emphasize the importance of system design in mitigating risks associated with high-pressure hydrogen pipelines [34]. Furthermore, Bauwens and Dorofeev (2014) investigated how initial turbulence affects vented explosion overpressure, identifying key factors such as flame velocity and hydrogen concentration that contribute to explosion dynamics. Their work provides foundational knowledge for enhancing venting systems in facilities handling hydrogen [35].

Research on explosion suppression and damage mitigation has also yielded critical advancements. Zheng et al. (2019) addressed hydrogen explosion risks in wet dust removal systems, demonstrating the effectiveness of sodium silicate in preventing hydrogen gas generation. This approach offers a practical solution for industries where hydrogen-producing reactions are prevalent [36]. Shang et al. (2023) evaluated explosion suppression devices, analyzing factors like spraying modes and operational timing to enhance suppression effectiveness. These insights contribute directly to the design of robust safety systems tailored to hydrogen environments [37].

In critical infrastructure, such as nuclear reactors, Schefer et al. (2011) studied the efficacy of barrier walls in mitigating explosion impacts. Their experiments showed that specific wall configurations, such as three-wall setups at varying angles, significantly reduce explosion damage, underscoring the importance of architectural designs in hydrogen safety systems [38]. Finally, Cao et al. (2017) investigated explosion venting in cylindrical vessels, analyzing how ignition position affects structural damage. Their findings provide actionable data for designing venting systems that minimize explosion impacts during high-risk scenarios [39]. These studies are listed in

Table 5. Research list of hydrogen-stationary risk assessment through experiments.

No	Reference	Objective and Scenario
1	Jiang et al. (2022) [30]	Unconfined fan-stirred hydrogen explosion experiment Study on the effect of flame instability and external turbulence
2	Zhou et al. (2023) [31]	Unconfined hydrogen explosion Study on the effect of ignition height on explosion
3	Kim et al. (2013) [32]	A study on the explosion in open space, flame propagation behavior, and blast wave intensity of hydrogen–air mixtures Soap bubble method
4	Proust et al. (2011) [33]	Experiments and measurements of high-pressure hydrogen fires, jet flames Flame: 900 bar down to 1 bar with orifices ranging from 1 to 3 mm
5	Mogi et al. (2008) [34]	A study on self-ignition and explosion in high-pressure hydrogen emissions Explosion pressure: 4–30 MPa Pipe length: 3–300 mm Nozzle diameter: 5, 10 mm
6	Bauwens and Dorofeev (2014) [35]	Experiments to investigate the effect of initial turbulence on ventilation explosion
7	Zheng et al. (2019) [36]	An experiment for the prevention of hydrogen explosive accidents in wet dust removal systems
8	Shang et al. (2023) [37]	- A study on explosion suppression equipment on hydrogen explosions - Main factor of failure of the equipment: actuation time, spraying mode of the suppressant
9	Schefer et al. (2011) [38]	A study on the occurrence, explosion, and mitigation of hydrogen accidents in nuclear reactor Vertical wall, 1-wall, 3-wall barrier, angle 135° and 90°
10	Cao et al. (2017) [39]	Explosion of stainless cylindrical explosion vessel Effect of ignition position on overall explosion ventilation Ignition position: rear, central, front

.

Through these experimental studies, key aspects of hydrogen safety, including explosion dynamics, high-pressure behavior, and suppression methods, have been systematically explored. The findings from this body of research not only validate theoretical models but also inform the development of practical safety measures. This integration of empirical data into the Hydrogen Risk Assessment Process strengthens its applicability across diverse industrial settings, contributing to a more comprehensive framework for hydrogen safety.

Experimental evaluations of explosion behaviors in high-pressure hydrogen systems highlight the critical role of structural designs in mitigating explosion risks. However, the limited scope of external environmental conditions and variable settings in current studies constrains their applicability. Developing experimental designs that reflect complex environmental conditions and optimizing structural mitigation designs are necessary for future advancements.

2.3. Study on Hydrogen Risk Assessment Using Simulation

While experiments are often considered the gold standard for hydrogen risk assessment, practical limitations such as high costs and logistical challenges have led researchers to increasingly rely on simulation-based methods. Simulations enable the modeling of accident scenarios, providing detailed insights into hydrogen safety risks without the constraints of physical experimentation. This section reviews key simulation tools and methodologies, demonstrating their contribution to advancing the Hydrogen Risk Assessment Process.

For 2D simulations, commonly used software includes PHAST, SAFETI, RISKCURVES, and EFFECTS, along with HyRAM, which has emerged as the most comprehensive toolkit for quantitative hydrogen risk assessment. Groth and Hecht (2017) introduced HyRAM for evaluating hydrogen safety, conducting QRAs, and analyzing potential consequences [40]. These tools provide valuable outputs, such as F-N curves, which depict societal risks by correlating accident frequency (F) with the number of fatalities (N). Such visualizations help stakeholders prioritize safety measures by identifying the highest-risk scenarios. Figure 3 shows the simulation tools for hydrogen risk assessment.

Additionally, 3D simulation studies utilizing Computational Fluid Dynamics (CFD) have gained traction for their ability to model complex phenomena, including hydrogen dispersion, jet fires, and explosions. These capabilities make CFD an essential component of the Hydrogen Risk Assessment Process, as it allows for the accurate prediction of accident impacts under diverse environmental and operational conditions. FLACS excels in modeling explosion scenarios in complex environments, while HyRAM offers specialized capabilities for hydrogen-specific risk assessment. PHAST, on the other hand, provides flexibility for multi-chemical scenarios but lacks detailed CFD capabilities. Figure 4 shows the simulation process of CFD.

The risk assessment using these simulations is based on the following models.

First, the TNT Equivalent Model is used to calculate the explosion hazard, which quantifies the physical effects of an explosion by converting the energy generated by a hydrogen explosion to the equivalent amount of TNT. This model is specifically used to assess the impact on surrounding structures in the event of a high-pressure hydrogen storage tank explosion.

$$E=W·\eta$$

where E represents the explosion energy, $W$ represents the amount of hydrogen released, $H_2$ represents he calorific value of hydrogen, and $\eta$ represents the emission efficiency.

A typical model used to model leaks and diffusion is the Gaussian Plume Model. This model is used to predict the concentration distribution of hydrogen released into the atmosphere. It can be used to define hazardous areas by establishing areas above the lower and upper explosive limits (LFL) in the event of a hydrogen leak.

$$C\left(x,y,z\right)={\displaystyle \frac{Q}{2\pi {\sigma }_y{\sigma }_{z}u}}\exp \left(-{\displaystyle \frac{y^2}{2{{\sigma }_y}^2}}\right)\exp \left(-{\displaystyle \frac{z^2}{2{{\sigma }_z}^2}}\right)$$

Here, $C$ represents the concentration, $Q$ represents the emission rate, ${\sigma }_y{\sigma }_z$ represents the variance coefficient, and $u$ represents the wind speed.

2.3.1. Utilizing Simulation for Hydrogen-Transportation Risk Assessment

Simulation-based risk assessment studies for transported hydrogen can be categorized into two main targets: the transportation systems for produced hydrogen and the storage systems installed in hydrogen fuel cell transportation vehicles. Each of these systems presents unique safety challenges, necessitating tailored approaches to risk assessment.

Hydrogen transportation typically involves pipelines and tube trailers. Pipelines are primarily used for short-distance transportation, connecting production and usage facilities. However, hydrogen embrittlement can cause permeation and leakage, leading to significant risks. Tube trailers, on the other hand, are employed for medium-to-long distances and store high-pressure hydrogen in long, tube-shaped containers. These systems face risks such as leakage from containers during transportation. Froeling et al. (2021) utilized SAFETI Ver 8.21 to compare jet fire risks between hydrogen and methane pipelines under varying conditions, such as pipe diameter and wind speed, providing critical data for understanding fire propagation risks [41]. Similarly, Gerboni et al. (2009) used PHAST to conduct quantitative risk assessments for fires and explosions in pipelines and tube trailers, assessing the economic feasibility of hydrogen-transportation safety measures [42].

Storage systems in hydrogen fuel cell vehicles present additional safety concerns due to their high-pressure conditions. Ma et al. (2024) constructed a Bayesian Network model to evaluate the risks of fires and explosions in high-pressure hydrogen storage systems, offering actionable insights for vehicle design and safety standards [43]. Ahmad Al-douri et al. (2023) integrated FMEA with HyRAM to identify the causes of hydrogen leaks in forklift truck systems, conducting a comprehensive QRA to bridge fault analysis with quantitative risk evaluation [44]. Xu et al. (2023) applied HyRAM to assess accident scenarios for hydrogen fuel cell ships, quantifying jet fire impacts and flame propagation risks, thus contributing to maritime hydrogen safety protocols [45].

Furthermore, Schiaroli et al. (2023) investigated the damage impact distances caused by hydrogen leaks from storage tanks onboard hydrogen buses. By employing PHAST and constructing Event Trees (ETs) to model accident scenarios, they highlighted the importance of robust containment designs to mitigate potential accidents in urban transit systems [46].

Through these studies, the integration of advanced simulation tools such as SAFETI, PHAST, and HyRAM has significantly enhanced the Hydrogen Risk Assessment Process for transportation systems. By modeling specific scenarios and quantifying risks, these tools provide critical insights into mitigating risks associated with hydrogen leaks, fires, and explosions. These contributions are vital for ensuring the safety and sustainability of hydrogen as a transportation fuel, further supporting the global transition to cleaner energy systems.

2.3.2. Utilizing Simulation for Hydrogen-Transportation Risk Assessment

In research utilizing Computational Fluid Dynamics (CFD), the actual accident terrain and its implications can be thoroughly evaluated. These simulations are particularly valuable for hydrogen fuel cell transportation vehicles, where assessing accidents in enclosed spaces such as tunnels or underground parking lots is crucial. By incorporating fluid dynamics, CFD-based studies offer realistic insights into accident impacts, enabling the development of more accurate and practical hydrogen risk management strategies.

Accidents caused by hydrogen leakage are influenced by the presence and timing of ignition. As shown in Figure 5, such accidents can be classified into three main types: dispersion (if ignition does not occur immediately), jet fire (if ignition occurs instantly), and delayed ignition (such as flash fires or explosions if leaked hydrogen accumulates before ignition) [47].

CFD studies have been extensively conducted to analyze the dispersion behavior of hydrogen leaks, focusing on the specific impacts of each accident type and the characteristics of surrounding buildings or terrain. For example, Dadashzadeh et al. (2016) used FDS to evaluate hydrogen dispersion in enclosed areas with hydrogen fuel cell vehicles under various ventilation conditions and proposed improved ventilation strategies based on the results [48]. Salva et al. (2012) analyzed the diffusion of hydrogen inside vehicles equipped with hydrogen storage tanks during leakage scenarios, employing FLUENT to simulate various inlet velocities and positions as parameters, ultimately suggesting mitigation measures [49]. Choi et al. (2013) employed STAR-CCM to assess hydrogen leaks in underground parking lots, focusing on diffusion simulations with different hydrogen leakage rates and ventilation fan flow rates to evaluate the volume of flammable areas over time [50]. Hajji et al. (2022) utilized FLUENT to simulate gas cloud risks in garages containing hydrogen fuel cell vehicles, accounting for parameters such as roof shapes and leak durations to understand risk profiles [51].

Figure 5. Event sequence diagram for H2 releases [49].

Figure 5. Event sequence diagram for H2 releases [49].

Since hydrogen is predominantly stored or used as a high-pressure gas, leakage often results in jet fires. Unlike other flammable material jet fires, hydrogen jet fires exhibit unique characteristics such as longer flame lengths, faster speeds, and temperatures reaching approximately 1500 °C. Additionally, hydrogen jet flames can be nearly invisible in daylight, posing a heightened risk of undetected ignition. These distinct properties have driven significant research, as highlighted in studies like those conducted by Middha et al. (2009), emphasizing the importance of tailored risk mitigation strategies [52].

CFD studies on hydrogen jet fires include the work of Gu et al. (2020), who analyzed the impacts and risks of hydrogen jet fires in tunnels under various leakage conditions (e.g., tunnel size, ventilation rate, and leak orifice) using FDS, subsequently developing guidelines for secondary hazard prevention [53]. Yuan et al. (2021) investigated the effect of fine water mist on jet fires caused by leaks from hydrogen storage tanks onboard fuel cell ships, considering factors like mist size, spray velocity, and wind conditions, demonstrating the potential of water mist for risk mitigation [54]. Li et al. (2019) compared the risk distances and durations of jet flames resulting from the Thermal Pressure Relief Device (TPRD) in hydrogen and Compressed Natural Gas (CNG) vehicles, conducting CFD-based simulations under similar driving conditions [55].

Further studies using CFD have explored multiple fire interactions resulting from hydrogen leaks. Shibani et al. (2022, 2023) investigated the effects of factors such as ventilation rates, tunnel slopes, and storage capacities on hydrogen flames in tunnels. Using FDS, they examined flame interactions and provided design recommendations for safer tunnel configurations [56,57].

Research on explosions caused by hydrogen leaks has also benefited significantly from CFD simulations. Middha et al. (2009) conducted a study using FLACS to assess explosion risks in tunnels, comparing hydrogen vehicle incidents with those involving natural gas under varying conditions such as tunnel geometry, storage pressure, and leak size [52]. Hansen et al. (2008) evaluated ignition impacts using FLACS and proposed a three-step methodology for hydrogen risk assessment. This approach integrates comprehensive evaluations of ventilation, dispersion, and explosion scenarios to achieve realistic and effective safety measures [58]. Cui et al. (2023) employed FLACS to analyze the effects of factors such as leak direction, location, wind speed, and ignition timing on explosions within tunnels, offering actionable insights for hydrogen safety in confined spaces [59].

Lastly, Lim et al. (2024) investigated liquefied hydrogen (LH2) spills and subsequent pool fires, utilizing FDS to analyze the effects of parameters like droplet size, spray discharge velocity, and the number of sprinklers on fire suppression. Their study identified optimal sprinkler parameters for mitigating LH2 spill fires, contributing to advancements in fire suppression technologies [60].

Table 6. List of prior studies on hydrogen-transportation risk assessment using simulation.

No	Reference	Objective	Scenario
1	H.A.J. Froeling et al. (2021) [41]	Compare risks of hydrogen and methane pipelines based on jet fire behavior	Jet fire in pipelines under varying pipe diameters, wind speeds, and material conditions
2	R. Gerboni et al. (2009) [42]	Conduct QRA for hydrogen transportation and establish economic evaluation foundations	Fire and explosion risks in pipelines and tube trailers during hydrogen transport
3	Qiuju Ma et al. (2024) [43]	Develop QRA model for high-pressure hydrogen storage and supply systems in FCVs	Fire and explosion risks in high-pressure hydrogen systems using Bayesian Network modeling
4	Ahmad Al-douri et al. (2023) [44]	Assess leakage causes and impacts in hydrogen fuel cell forklifts	Leakage analysis using FMEA, ETA, and FT for arm shaft-mounted hydrogen storage systems
5	Xiaojina Xu et al. (2023) [45]	Risk assessment of hydrogen fuel cell vessels during navigation	Fire and explosion scenarios in fuel cell ships, calculating flame size, radiation range, and personal risk
6	Alice Schiaroli et al. (2023) [46]	Evaluate damage distances and LOC thresholds for compressed and cryo-compressed hydrogen	Hydrogen bus storage tanks under varying pressure and storage types (compressed, liquefied, cryo-compressed)
7	Mohammad Dashzadeh et al. (2016) [47]	Propose ventilation improvements for hydrogen diffusion in enclosed areas	Diffusion behavior under varying ventilation conditions (no ventilation, single/double natural ventilation)
8	Jose Antonio Salva et al. (2012) [48]	Analyze hydrogen diffusion in fuel cell vehicles and propose risk mitigation measures	Hydrogen storage tank leakage in vehicles under varying ventilation rates and positions
9	Choi Jongrak et al. (2013) [49]	Assess safety of hydrogen leakage in underground parking lots	Leakage scenarios with varying rates and ventilation fan flow rates
10	Yassine Hajji et al. (2022) [50]	Assess risks of gas clouds in enclosed vehicle garages	Hydrogen leakage risks under varying roof types, leakage times, and ventilation conditions
11	Xiaochen Gu et al. (2020) [51]	Develop fire prevention guidelines for hydrogen jet fires in tunnels	Jet fires in tunnels under varying cross-sectional areas, ventilation speeds, and leakage distances
12	Yupeng Yuan et al. (2021) [54]	Investigate effects of fine water mist on hydrogen jet fires	Hydrogen storage tank jet fires under varying droplet sizes, spray velocities, and wind conditions
13	Zhiyong Li et al. (2019) [55]	Compare hazardous distances for hydrogen and CNG vehicle TPRD releases	TPRD releases from hydrogen (35 MPa) and CNG (25 MPa) under varying storage masses
14	Shibani et al. (2022) [56,57]	Analyze interaction of multiple hydrogen flames in tunnels	Tunnel fires under varying ventilation speeds, leak areas, slopes, and sealing ratios
15	Prankul Middha et al. (2009) [52]	Conduct explosion risk analysis for hydrogen vehicles in tunnels	Hydrogen vehicle explosion risks under varying tunnel shapes, leak conditions, and pressures
16	Olav R. Hansen et al. (2008) [58]	Propose a three-step methodology for hydrogen risk assessment using CFD	Hydrogen gas cloud explosions with stoichiometric concentrations and modified conditions
17	Shaoqi Cui et al. (2023) [59]	Combine theoretical analysis and simulation for fire risk in hydrogen fuel cell vehicles	Fire scenarios under varying leak directions, wind speeds, and leak locations
18	Sang-Jin Lim et al. (2024) [60]	Optimize sprinkler parameters for extinguishing pool fires in liquefied hydrogen systems	Pool fires from liquefied hydrogen under varying ventilation speeds and sealing ratios

shows the list of prior studies on hydrogen-transportation risk assessment using simulation.

Simulation-based approaches provide valuable data for evaluating accident scenarios and establishing design and operational standards for hydrogen-transportation systems. However, there is limited comparison between different simulation tools, and variations in modeling details hinder reliability. Addressing these issues requires benchmarking multiple tools under consistent conditions to ensure reliability and developing dynamic simulation tools capable of leveraging real-time data.

2.3.3. Hydrogen-Stationary Hazard Assessment Utilizing Simulation

Hydrogen-stationary hazard assessments have increasingly relied on advanced simulation tools to analyze risks and enhance safety protocols. Gye et al. (2019) developed a risk assessment model for hydrogen refueling stations in Korean cities using SAFETI and HyRAM, combined with qualitative hazard assessment techniques such as HAZOP. Their study demonstrated the practicality of integrating multiple assessment methods to predict risks associated with urban hydrogen infrastructure [61].

Liu et al. (2023) utilized FLACS to examine hydrogen leakage, dispersion, delayed ignition, explosions, and the influence of wind speed, providing critical insights into the interplay between environmental variables and hydrogen safety [62]. Similarly, Liang et al. (2019) employed FLACS to calculate parameters such as harmful and lethal areas, farthest harmful distance, and longest lethal distance during hydrogen leakage accidents at refueling stations, illustrating the spatial implications of hydrogen incidents [63]. Kim et al. (2013) investigated the behavior of pressurized hydrogen tanks and pipes at Korean hydrogen refueling stations through detailed leakage simulations using FLACS, contributing to the design of safer refueling systems [64].

The safety of refueling stations serving both hydrogen and gasoline vehicles has also been explored. Sakamoto et al. (2016) evaluated the risks associated with hydrogen storage tanks exposed to gasoline pool fires using TRACE and ANSYS. Their study highlighted the unique challenges of hybrid refueling stations and proposed measures to prevent catastrophic failures [65].

Hydrogen safety concerns extend beyond refueling stations to include pipelines and production facilities. Jang et al. (2015) analyzed jet fires in hydrogen pipelines using CFD simulations with KFX, emphasizing the importance of accounting for pipe rack configurations in fire risk assessments [66]. Lin et al. (2023) investigated fire scenarios caused by hydrogen leaks on large-scale hydrogen production platforms using FLUENT, considering various leakage points, flow directions, velocities, and rates [67]. Rigas and Sklavounos (2005) evaluated hazards in hydrogen storage and distribution systems, offering probabilistic insights into potential accidents using CFX [68].

Research into hydrogen fires and explosions has revealed critical factors influencing accident dynamics. Li et al. (2019) developed a theoretical methodology for predicting explosion pressure in confined chambers using fractal theory. By validating their CFD-based predictions with experimental data, they enhanced the reliability of explosion modeling for confined hydrogen environments [69].

Additionally, nuclear power plant safety has been a focal point in hydrogen hazard assessments. Xiong et al. (2009) evaluated the effectiveness of Passive Autocatalytic Recombiners (PARs) in mitigating hydrogen risks during large break loss-of-coolant accidents (LBLOCAs) using GASFLOW [70]. The Fukushima Daiichi accident highlighted the severe consequences of hydrogen explosions in nuclear power plants, as analyzed by Yanez et al. (2015) [71]. Heitsch et al. (2010) conducted a Level 2 Probabilistic Safety Assessment (PSA) at the Paks Nuclear Power Plant, analyzing hydrogen flow dynamics during severe accidents using GASFLOW, FLUENT, and CFX. Their findings contributed to the development of advanced containment strategies for extreme scenarios [72].

To optimize computational efficiency, Skjold et al. (2017) combined HyRAM and FLACS with finite element analysis (FEA) and reduced-order models. They evaluated hydrogen dispersion, fire, and explosion scenarios resulting from loss of containment (LOC) at hydrogen refueling stations, providing a comprehensive risk assessment framework [73].

Table 7. List of prior studies on hydrogen-stationary risk assessment using simulation.

No	Reference	Objective	Scenario
1	Gye et al. (2019) [61]	Conduct risk assessment for urban hydrogen charging stations in Korea	Catastrophic rupture in tube trailers (100 bar) and dispenser leaks (700 bar, hole sizes 0.11, 1.11, 11.11 mm)
2	Liu et al. (2023) [62]	Analyze hydrogen leakage, diffusion, delayed ignition, and explosion effects at refueling stations	Qingdao hydrogen refueling station: wind speeds (2–8 m/s), station size (50 × 50 × 30 m), tank pressure (39 MPa)
3	Liang et al. (2019) [63]	Calculate harmful areas and lethal distances at hydrogen refueling stations	Dalian station: considered wind speed, leak direction, and tube trailer scenarios
4	Kim et al. (2013) [64]	Study explosive flame propagation in open hydrogen–air mixtures and intensity of blast waves	Soap bubble experiments in open spaces
5	Sakamoto et al. (2016) [65]	Assess risks of hydrogen storage tanks exposed to gasoline pooling at hydrogen–gasoline stations	Thermal radiation damaging cold evaporators, causing catastrophic hydrogen leakage
6	Jang et al. (2015) [66]	Simulate jet fire in hydrogen pipelines, considering pipe rack influences	Pipeline leaks: discharge rate (20.2 kg/s), leak direction, wind direction (70°), wind speed (3 m/s)
7	Lin et al. (2023) [67]	Simulate potential fires due to hydrogen leaks on large hydrogen production platforms	Different leak points, directions, and flow rates
8	Rigas and Sklavounos (2005) [68]	Evaluate risks from hydrogen storage and distribution systems	General risks from hydrogen storage and potential leak accidents
9	Li et al. (2019) [69]	Predict explosive pressure in confined chambers and develop fractal theory-based methods	Confined chamber explosions: dynamic frame interlocking factor limit value calculation
10	Xiong et al. (2009) [70]	Assess hydrogen risks in nuclear power plants during LBLOCA	GASFLOW simulations of coolant loss accidents
11	Heitsch et al. (2010) [72]	Use CFD codes to model gas flow in containers during extreme accidents	Dominant Plant Damage States (PDSs) from Level 2 PSA studies at Paks NPP
12	Skjold et al. (2017) [73]	Conduct risk assessments for hydrogen charging stations	Diffusion, fire, explosion scenarios due to loss of containment (LOC) of hydrogen gas

shows the list of prior studies on hydrogen-stationary risk assessment using simulation.

These studies collectively underscore the indispensable role of simulations in advancing hydrogen safety at stationary facilities. By systematically modeling the physical and chemical dynamics of hydrogen, they contribute to the ongoing refinement of the Hydrogen Risk Assessment Process. These simulation-driven insights are pivotal for designing safer hydrogen systems and aligning them with the broader goals of environmental sustainability.

CFD tools like FLACS effectively visualize spatial risks in hydrogen refueling stations and stationary infrastructure, providing valuable insights for design improvements and safety enhancements. However, these studies are sensitive to environmental conditions and design parameters, which can significantly affect results. Developing simulation models that accurately reflect real-world environmental conditions and enhancing their applicability to policymaking are necessary steps forward.

2.4. Experimental and Simulation Approaches

As mentioned earlier, experiments provide highly accurate results but are often limited by their significant costs and time requirements. In contrast, simulations offer an efficient alternative, requiring less time and cost, but they may lack the precision of experimental methods. To overcome these limitations, researchers increasingly employ a hybrid approach. By conducting minimal experiments to obtain validation data and then performing simulations under similar conditions, they can compare the results to validate the simulation models. This strategy ensures that discrepancies between experimental and simulation outcomes are minimized, allowing researchers to extrapolate findings to different scenarios with confidence.

Such an integrated approach enables a balance between the accuracy of experiments and the efficiency of simulations, optimizing resources while generating meaningful and reliable insights. This methodology has proven particularly valuable in advancing the Hydrogen Risk Assessment Process by bridging experimental reliability with simulation scalability.

2.4.1. Hydrogen-Transportation Hazard Assessment Using Experimental and Simulation Approaches

Hybrid experimental and simulation methods have been extensively utilized in assessing hydrogen-transportation hazards. Cirrone et al. (2023) developed a CFD model using FLACS to measure the maximum pressure generated after the rupture of a liquefied hydrogen storage tank due to fire. By considering variables such as pressure, temperature, and capacity, they validated their model’s accuracy through comparison with experimental data, demonstrating its applicability for hydrogen safety assessments [74].

Zheng et al. (2013) conducted localized fire tests for Type III hydrogen tanks to ensure the practicality and validity of fire safety standards. They complemented these tests by simulating the flame impingement process using FLUENT, analyzing critical parameters such as filling pressure and localized fire exposure time on the activation of Thermal Pressure Relief Devices (TPRDs), thereby enhancing our understanding of fire behavior in hydrogen-transportation scenarios [75].

Halm et al. (2017) conducted fire simulations for Type IV hydrogen storage containers, using bonfire pool tests and Finite Element (FE) modeling to evaluate their behavior under fire exposure. The thermo-mechanical model developed in their study was validated through experimental data, providing reliable insights for optimizing hydrogen container designs [76].

Houf et al. (2012) simulated various ventilation conditions inside tunnels using the FEUGO/FLACS program. By modeling hydrogen leakage scenarios through the TPRD from hydrogen tanks in fuel cell vehicles, they calculated the flammable hydrogen gas volume and associated explosion overpressure. These simulations were further validated through controlled tunnel experiments, enabling improved safety strategies for enclosed hydrogen-transportation environments [77].

Table 8. List of prior studies on hydrogen-transportation risk assessment using both experiments and simulations.

No	Reference	Objective	Scenario
1	Donatella Cirrone et al. (2023) [74]	Develop and validate CFD models for measuring maximum explosion pressure from liquefied hydrogen storage tank bursts due to fire	Experimental: single-wall tank (120 L, 2–15 bar abs, 1.5–5.4 kg) with sudden rupture within 0.2 m/s; CFD (FLACS): minimum (1.8 kg) and maximum (5.4 kg) hydrogen mass
2	Jinyang Zheng et al. (2013) [75]	Conduct local fire tests and simulate flame impact to validate the practicality of fire tests	Experimental: local fire tests for Type 3 tanks; CFD (FLUENT): analyze charging pressure and fire exposure time effects on TPRD activation
3	Damien Halm et al. (2017) [76]	Understand pressurized composite containers’ behavior in fire and validate thermo-mechanical models	Experimental: bonfire pool test for Type 4 vessel (36 L); FE simulation: thermo-mechanical model validation
4	William G. Houf et al. (2012) [77]	Investigate hydrogen leakage behavior in tunnels from hydrogen fuel cell cars	Experimental: tunnel test (SRI Corral Hollow) with TPRD releases from three hydrogen tanks; ventilation rates: 10, 15, 30/hr; FEUGO, FLACS: flammable hydrogen cloud (4–75%)

shows the list of prior studies on hydrogen-transportation risk assessment using both experiments and simulations.

These hybrid approaches highlight the significance of combining experiments and simulations in the Hydrogen Risk Assessment Process. By leveraging the accuracy of experimental methods and the scalability of simulations, researchers can refine risk prediction models and enhance the safety of hydrogen-transportation systems. Such methodologies are pivotal in addressing the inherent complexities of hydrogen as a fuel, supporting its integration into sustainable energy infrastructures. The combination of experimental data and simulation significantly improves the accuracy and reliability of models for hydrogen-transportation system safety. However, the limited conditions of experiments and inconsistencies with simulation results pose challenges. To overcome these, more comprehensive experiments under diverse conditions and integrated simulation studies are required to build reliable risk models for complex environments.

2.4.2. Hydrogen-Stationary Hazard Assessment Using Experimental and Simulation Approaches

Research on hydrogen leakage and fire in stationary settings has increasingly utilized a hybrid methodology of experiments and simulations, yielding significant insights into hazard assessments. By integrating these approaches, researchers can address both practical constraints and the need for detailed hazard analysis, leading to advancements in safety protocols and the Hydrogen Risk Assessment Process.

Brennan et al. (2009) employed Large Eddy Simulation (LES) models to study high-pressure hydrogen jet fires. Their quasi-steady-state simulation results, validated against experimental data from Schefer et al. (2007), demonstrated the effectiveness of LES in capturing flame dynamics during hydrogen leakage incidents. This integration of simulation and experiment enabled a more accurate representation of real-world scenarios, particularly under high-pressure conditions [78,79]. Similarly, Swain et al. (1999) compared experimental data on hydrogen and helium leakage with FLUENT simulations, revealing critical insights into spatial and temporal distribution patterns during leakage events. Their work emphasized the importance of accurate simulation models in replicating complex leakage behaviors [80].

Bragin and Molkov (2011) examined the phenomenon of spontaneous ignition in high-pressure hydrogen storage systems, combining LES with experimental observations. Their findings illuminated the intricate conditions that lead to ignition, highlighting the necessity of dynamic simulations for predicting and mitigating such risks [81].

These studies collectively demonstrate how combining experimental validation with simulations contributes to refining hydrogen safety measures. This approach ensures that hydrogen leakage and ignition phenomena are not only understood but also effectively addressed in safety protocols, paving the way for more robust risk mitigation strategies.

Further studies have investigated hydrogen explosions, focusing on their rapid escalation and extensive impact. Makarov et al. (2009) conducted experiments and CFD simulations using FLACS, COM3D, and FLUENT to analyze explosion scenarios at hydrogen refueling stations. This work validated CFD models’ predictive capabilities, providing essential data for developing effective explosion mitigation measures [82].

Pang et al. (2019) explored the effects of explosion-venting surface opening times on hydrogen explosion behavior in industrial sites. Using AutoReaGas, they simulated gas explosion flows and confirmed their findings through experimental validation, offering insights into strategies for minimizing explosion impacts [83]. Middhal et al. (2007) focused on detonation deflagration transition (DDT) phenomena in confined tubes. Their study, supported by FLACS simulations, revealed the influence of obstacles on flame acceleration and detonation risks, offering practical guidance for improving safety in confined spaces [84].

Table 9. List of prior studies on hydrogen-stationary risk assessment using both experiments and simulations.

No	Reference	Objective	Scenario
1	Brennan et al. (2009) [78,79]	Model high-pressure jet fire behavior and validate using simulations and experiments	LES: quasi-steady-state jet fire simulation 5 s after the start of leakage
2	Swain et al. (2023) [80]	Assess hydrogen risks through helium leak simulations and compare CFD and experimental results	FLUENT: spatial and temporal distribution of hydrogen and helium leakage
3	Bragin and Molkov (2011) [81]	Study the physical phenomenon of spontaneous ignition of hydrogen and transition to jet fires	LES: high-pressure hydrogen leakage from reservoirs and comparison with experimental data
4	Makarov et al. (2009) [82]	Model high-pressure jet fires and validate using experimental values	LES: quasi-steady-state jet fire 5 s after leakage start
5	Pang et al. (2019) [83]	Analyze hydrogen gas explosive flow and identify the effect of venting surface opening time	CFD and experiments: industrial site hydrogen explosions with venting time intervals (0–0.1 s)
6	Middhal et al. (2007) [84]	Compare experiments and simulations in confined tubes and study smoke exposure transitions (DDT)	Confined tube tests: 8 experiments with varying obstacle numbers and distances

shows the list of prior studies on hydrogen-stationary risk assessment using both experiments and simulations.

Explosion mitigation strategies vary significantly based on structural designs and operational parameters. Schefer et al. demonstrated that incorporating structural barriers effectively reduces blast overpressure by up to 40%, providing critical protection for nearby equipment and personnel. Mogi et al. analyzed the influence of vent size and placement, showing that larger vents positioned strategically can reduce the risk of secondary explosions. Pang et al., using CFD tools, optimized vent designs to maximize the dispersion of explosive gases, reducing peak overpressure by over 50% compared to conventional designs. These findings collectively underscore the importance of tailoring explosion mitigation designs to specific environmental and operational contexts, emphasizing the need for integrating experimental insights with advanced simulation techniques.

This comprehensive review of hydrogen-related hazard assessments highlights the critical role of merging experimental data with advanced simulation techniques. Such integration not only enhances the predictive accuracy of safety models but also broadens the scope of hazard mapping across varying scenarios. These insights are crucial for informing the development of global safety standards and advancing the Hydrogen Risk Assessment Process. By reflecting on these studies, the necessity of a balanced approach—where experimental and simulation methods complement each other—becomes evident, ensuring both efficiency and reliability in hydrogen safety research.

The integration of experimental and simulation approaches is essential for improving safety and mitigating explosion risks in stationary hydrogen systems, particularly in high-pressure scenarios. While studies have provided critical insights into spontaneous ignition and explosion mitigation designs, they are often limited by insufficient experimental data to account for complex environmental conditions. Future research should focus on collecting robust datasets and developing integrated approaches to address these gaps.

2.5. Study on Hydrogen Control System

Hydrogen control systems are essential to ensure the safe and efficient operation of hydrogen storage and handling facilities. These systems combine advanced technologies such as real-time monitoring, nanomaterial applications, and predictive control strategies to address the inherent risks of hydrogen, such as its high flammability and storage challenges. More recently, technologies such as artificial intelligence (AI), big data analytics, and the Internet of Things (IoT) are being integrated to further enhance hydrogen storage technology and safety management.

IoT-based real-time monitoring continuously tracks pressure, temperature, and hydrogen concentration in storage tanks and pipelines and provides a framework for early leak detection. Patil et al. (2024) demonstrated that IoT technologies, combined with artificial intelligence, significantly enhance the safety and efficiency of hydrogen storage and management systems, emphasizing their critical role in proactive hazard prevention and the transition to a hydrogen-based economy [85].

Nanomaterials contribute to increased storage capacity and stability while reducing the risks associated with high-pressure environments. They are particularly effective at optimizing storage capacity while maintaining stability under high temperature and pressure conditions, which is critical for industrial-scale hydrogen storage systems. Züttel’s (2004) research focused on how metal hydrides and porous carbon structures can improve the safety of hydrogen storage systems [86]. Metal hydrides are thermally stable, storing hydrogen reliably in high-temperature environments, and operate safely at low pressures, reducing the risk of accidents. Porous carbon structures provide high surface area and thermal management properties that mitigate overheating and pressure instability in storage systems and contribute to reducing the likelihood of explosions. The study emphasized that these materials play an important role in increasing the stability and safety of hydrogen storage.

Predictive control technology is also becoming increasingly important in the field of hydrogen safety management. In particular, technologies such as Model Predictive Control (MPC) utilize historical and real-time data to proactively detect and respond to potential risks in the system, and these technologies are increasingly being used to manage hydrogen-transportation vehicles and pipelines as well as large-scale storage facilities. Morin et al. (2018) demonstrated that Model Predictive Control (MPC) is effective in optimizing performance and reducing the operational risk of integrated hydrogen storage in renewable energy systems [87]. The study showed that MPC can simultaneously improve the safety and efficiency of hydrogen storage systems operating in real time based on a variety of data. The MPC continuously monitors the pressure and temperature conditions in the storage tank and uses predictive algorithms to proactively detect potential risks. MPCs are also designed to balance energy supply and demand, making them effective in mitigating the volatility of renewable energy, especially intermittent energy sources such as solar and wind. The results of the study showed that systems with MPCs were able to significantly reduce safety accidents caused by hydrogen leakage or overpressurization and contributed to increasing energy conversion efficiency while reducing operating costs. It concluded that these technologies will play an important role in the future integration of renewable energy and hydrogen storage systems.

As such, advances in hydrogen control systems are focused on increasing sustainability. System design is evolving to maximize energy efficiency, minimize environmental impact, and reduce operating costs. In this regard, Schrotenboer et al. (2022) highlighted the importance of optimal control strategies for integrated hydrogen storage and power generation systems, specifically emphasizing the balance between renewable energy supply and demand to ensure operational reliability and sustainability in hydrogen-based energy systems [88].

Table 10. List of prior studies on hydrogen control system.

No	Reference	Objective and Scenario
1	Hossain et al. (2023) [85]	- IoT technology improves emergency response times and operational reliability in hydrogen-renewable energy systems. - Real-time monitoring of hydrogen leaks and system performance through IoT sensors.
2	Züttel (2004) [86]	- Demonstrated that nanomaterials such as metal hydrides and porous carbon structures improve hydrogen adsorption efficiency. - Enhanced thermal stability and safety in hydrogen storage systems.
3	Morin et al. (2018) [87]	- Proved that Model Predictive Control (MPC) optimizes performance and reduces operational risks in integrated hydrogen storage systems. - Effective in maintaining system stability and preventing accidents in renewable energy contexts.
4	Schrotenboer et al. (2022) [88]	- Highlighted the importance of advanced control strategies for balancing energy demand and supply in hybrid hydrogen-renewable systems. - Ensures seamless integration of hydrogen storage with intermittent energy sources like wind and solar.

shows the list of prior studies on hydrogen control system.

3. Conclusions

This study comprehensively reviewed the state of hydrogen risk assessment research, categorizing studies based on storage methods (stationary and transportation) and hazard assessment approaches (theoretical models, experimental data, and simulations). The review highlights both advancements in understanding hydrogen risks and ongoing challenges requiring further attention.

A key takeaway is the critical role of Hydrogen Risk Assessment Processes in enabling a hydrogen-based energy system, particularly as the global transition to cleaner energy accelerates. Hydrogen’s unique physical and chemical properties, such as its wide flammability range and low ignition energy, demand risk assessment models that go beyond traditional chemical hazard approaches. This is especially important in urban environments, where hydrogen infrastructure is increasingly integrated.

While traditional techniques like Fault Tree Analysis (FTA) and Hazard and Operability Studies (HAZOPs) remain foundational, they often lack the flexibility to address dynamic hydrogen environments. Advances in simulation-based methods, such as Computational Fluid Dynamics (CFD) and tools like HyRAM, have improved hazard prediction accuracy, offering reliable insights into hydrogen dispersion, combustion, and explosion behaviors. However, the static nature of many models highlights the need for adaptive, real-time systems capable of responding to evolving hydrogen infrastructure and predicting risks in real time, particularly in urban storage and transportation scenarios.

This review uniquely contributes to the field by synthesizing diverse methodologies—ranging from theoretical to experimental and simulation-based approaches—into an integrated ‘Hydrogen Risk Assessment Process.’ Unlike existing reviews that focus on isolated aspects of hydrogen safety, this paper provides a comprehensive framework that emphasizes the interplay between these methods. By identifying key trends, research gaps, and practical applications, the proposed process serves as a foundational guide for advancing hydrogen safety research and establishing robust standards in both academic and industrial contexts.

The integration of theoretical, experimental, and simulation-based approaches offers significant potential for policymaking and industrial practice. For instance, simulation tools like FLACS have been utilized to establish safety zones for hydrogen refueling stations, influencing urban planning policies. Experimental findings on the fire resistance of hydrogen tanks have guided industrial standards for storage system design. Furthermore, theoretical models, including Bayesian networks, have informed risk management protocols by identifying critical control points in hydrogen production and transportation. These examples underscore the importance of bridging academic research with practical applications to enhance hydrogen safety frameworks globally.

4. Future Research Direction

Through this review, we can propose several directions for the future advancement of hydrogen-related risk assessment research. In the future, hydrogen will become a major new energy source, replacing fossil fuels, with its utilization expected to increase significantly. Therefore, rather than conducting fragmented risk assessments for ‘individual stages’ as is currently done, there is a growing need for comprehensive risk assessment research that integrates the ‘entire life cycle’ of hydrogen. Current studies primarily focus on individual stage-specific risk assessments. While such fragmented approaches to ‘incident response and mitigation’ may hold some value, they are becoming insufficient to contribute to the ‘fundamental prevention of accidents’, which is the ultimate goal of safety efforts. Fragmented approaches often fail to address the root causes of accidents. For instance, focusing solely on response mechanisms such as fire suppression systems does not mitigate the underlying risk of hydrogen leaks caused by inadequate maintenance protocols. Similarly, implementing isolated monitoring systems without integrating real-time data can result in delayed hazard detection, as seen in incidents where undetected leaks escalated into catastrophic explosions. Such limitations highlight the necessity for a holistic framework that prioritizes fundamental accident prevention. It is now imperative for research methodologies in the field of safety to advance. Consequently, it is essential to develop a ‘risk assessment process’ capable of actively addressing the entire life cycle of this ‘new energy’.

Furthermore, to establish a new era of ‘safety’ research that integrates the entire life cycle, it is essential to embrace ‘convergence with emerging industrial technologies’. The era of relying solely on static data for risk assessment is over. While the applications and scope of hydrogen use are expanding, current research trends still tend to rely on limited datasets. A closer look at recent technological advancements related to hydrogen reveals the development of various measurement technologies, such as leak detection sensors and high-purity analyzers. By integrating these cutting-edge technologies, research must evolve toward ‘dynamic risk assessments’ that leverage real-time data for more accurate and responsive safety evaluations. Dynamic risk models are frameworks designed to continuously update risk assessments by incorporating real-time input variables such as sensor data, environmental conditions, and operational changes. These models adapt to fluctuating scenarios, enabling a more accurate prediction of safety risks compared to static models. For instance, in hydrogen-transportation systems, a dynamic risk model can account for variables like traffic density, ambient temperature, and equipment conditions to adjust the likelihood of incidents dynamically. By leveraging machine learning algorithms and IoT data, these models enhance decision-making processes and support proactive risk mitigation strategies.

Moreover, as the global hydrogen economy continues to grow, the frequency of hydrogen-related incidents is also on the rise. To address these increasing risks, ongoing research into hydrogen risk assessment is urgently needed. However, the ‘incident frequency data’ currently used in risk assessments remains outdated. It is imperative to develop ‘incident frequency data’ that reflects the unique characteristics of hydrogen-using processes and the specific physical and chemical properties of hydrogen under new environmental conditions.

This review also addresses the importance of creating hybrid models that combine theoretical, experimental, and simulation-based approaches to hydrogen risk assessment. These integrated models could serve as the foundation for more accurate safety protocols and regulatory frameworks. Such frameworks would not only guide the safe design and operation of hydrogen infrastructure but would also support the ongoing development of hydrogen safety standards globally. This Hydrogen Risk Assessment Process, when refined, will be crucial in ensuring that hydrogen can be safely integrated into existing energy systems, ultimately facilitating the transition to a hydrogen-based economy.

In conclusion, this review presents an opportunity for both academia and industry to establish a clear Hydrogen Risk Assessment Process that incorporates the best aspects of traditional and modern techniques. The integration of theoretical models, experimental research, and simulation-based methods offers a comprehensive, adaptive approach to hydrogen safety. The development of such a unified process will be instrumental in shaping a safer, more sustainable hydrogen economy. It is crucial for future research to continue to refine these methods, ensuring that hydrogen energy systems can meet both safety standards and environmental objectives. By doing so, we can ensure the successful, safe, and sustainable deployment of hydrogen as a key energy source in the years to come.