Safety

Hydrogen plays an important role in the global transition towards Net-Zero emission. While pipelines are a viable option to transport large quantities of compressed hydrogen over long distances it is not always practical in many applications. In such situations a viable option is to transport and deliver large quantities of hydrogen as cryogenic liquid. The liquefaction process cools hydrogen to cryogenic temperatures below its boiling point of -259.2 0C. Such extreme low temperature implies specific hazards and risks which are different from those associated with the relatively well-known compressed gaseous hydrogen. Managing these specific issues brings new challenges for the stakeholders.<br/>Furthermore the transfer of liquid hydrogen (LH2) and its technical handling is relatively well known for industrial gas or space applications. Experience with LH2 in public and populated areas such as truck and aircraft refuelling stations or port bunkering stations for example is limited or non-existent. Safety requirements in these applications which involve or are in proximity of untrained public are different from rocket/aerospace industry.<br/>The manuscript reviews knowhow already gained by the international hydrogen safety community; and on such basis elucidate the gaps which are yet to be filled to meet industry needs to design and operate inherently safe LH2 operations including the implications for regulations codes and standards (RCS). Where relevant the associated gaps in some underpinning sciences will be mentioned; and the need to contextualise the information and safety practices from NASA1/ESA2/JAXA3 to inform risk adoption will be summarised.

In international regulations on explosion protection mechanical friction impact or abrasion is usually named as one of 13 ignition sources that must be avoided in hazardous zones with explosive atmospheres. In different studies it is even identified as one of the most frequent ignition sources in practice. The effectiveness of mechanical impacts as ignition source is dependent from several parameters including the minimum ignition energy of the explosive atmosphere the properties of the material pairing the kinetic impact energy or the impact velocity. By now there is no standard procedure to determine the effectiveness of mechanical impacts as ignition source. In some previous works test procedures with poor reproducibility or undefined kinetic impact energy were applied for this purpose. In other works only homogeneous material pairings were considered. In this work the effectiveness of mechanical impacts with defined and reproducible kinetic impact energy as ignition source for hydrogen containing atmospheres was studied systematically in dependence from the inhomogeneous material pairing considering materials with practical relevance like stainless steel low alloy steel concrete and non-iron-metals. It was found that ignition can be avoided if non-iron metals are used in combination with different metallic materials but in combination with concrete even the impact of non-iron-metals can be an effective ignition source if the kinetic impact energy is not further limited. Moreover the consequence of hydrogen admixture to natural gas on the effectiveness of mechanical impacts as ignition source was studied. In many cases ignition of atmospheres containing natural gas by mechanical impacts is rather unlikely. No influence could be observed for admixtures up to 25% hydrogen and even more. The results are mainly relevant in the context of repurposing the natural gas grid or adding hydrogen to the natural gas grid. Based on the test results it can be evaluated under which circumstances the use of tools made of non-iron-metals or other non-sparking materials can be an effective measure to avoid ignition sources in hazardous zones containing hydrogen for example during maintenance work.

Hydrogen safety is a critical issue during the construction and development of the hydrogen energy industry. Hydrogen refueling stations play a pivotal role in the hydrogen energy chain. In the event of an accidental hydrogen leak at a hydrogen refueling station the ability to quickly predict the leakage location is crucial for taking immediate and effective measures to prevent disastrous consequences. Therefore the development of precise and efficient technologies to predict leakage locations is vital for the safe and stable operation of hydrogen refueling stations. This paper studied the localization technology of high-risk leakage locations in the fuel cell system of a skid-mounted hydrogen refueling station. The hydrogen leakage and diffusion processes in the fuel cell system were predicted using CFD simulations and the hydrogen concentration data at various monitoring points were obtained. Then a multilayer feedforward neural network was developed to predict leakage locations using simulated concentration data as training samples. After multiple adjustments to the network structure and hyperparameters a final model with two hidden layers was selected. Each hidden layer consisted of 10 neurons. The hyperparameters included a learning rate of 0.0001 a batch size of 32 and 10-fold cross-validation. The Softmax classifier and Adam optimizer were used with a training set for 1500 epochs. The results show that the algorithm can predict leakage locations not included in the training set. The accuracy achieved by the model was 95%. This approach addresses the limitations of sensor detection in accurately locating leaks and mitigates the risks associated with manual inspections. This paper provides a feasible method for locating hydrogen leakage in hydrogen energy application scenarios.

This paper presents model predictions obtained with the CFD tool FLACS for hydrogen releases and vented deflagrations in containers and larger enclosures. The paper consists of two parts. The first part compares experimental results and model predictions for two test cases: experiments performed by Gexcon in 20-foot ISO containers (volume 33 m3 ) as part of the HySEA project and experiments conducted by SRI International and Sandia National Laboratories in a scaled warehouse geometry (volume 45.4 m3 ). The second part explores the use of the model system validated in the first part to accidental releases of hydrogen from forklift trucks inside a full-scale warehouse geometry (32 400 m3 ). The results demonstrate the importance of using realistic and reasonably accurate geometry models of the systems under consideration when performing CFD-based risk assessment studies. The discussion highlights the significant inherent uncertainty associated with quantitative risk assessments for vented hydrogen deflagrations in complex geometries. The suggestions for further work include a pragmatic approach for developing empirical correlations for pressure loads from vented hydrogen deflagrations in industrial warehouses with hydrogen-powered forklift trucks.

The influence of hydrogen-enriched natural gas (HENG) and CO2 on the mechanical property of X80 pipeline steel were investigated via fatigue crack growth rate (FCGR) tests and density functional theory (DFT) calculations. The results show that the FCGR in H2 was slightly faster than that in HENG while it was slower than that in the N2/CO2/H2 mixtures. The enhanced FCGR by CO2 further increased with the increasing CO2 content. DFT calculation results show that the adsorbed CO2 on the iron surface significantly increased the migration rate of H atoms from surface to subsurface. This promotes the entry of hydrogen into the steel.

The maritime industry is gaining momentum towards a more decarbonized and sustainable path. However most of the worldwide fleet still relies on fossil fuels for power producing harmful environmental emissions. Hydrogen as a clean fuel is a promising alternative but its unique properties pose significant safety challenges. For instance hydrogen has a wide flammability range inherently increasing the risk of ignition. Moreover its comparatively low volumetric energy density necessitates faster filling rates and larger volumes for bunkering and onboard storage leading to higher risk rates. Therefore the use of hydrogen for maritime applications requires the development of specialized riskbased approaches according to safety engineering principles and techniques. The key safety implications are discussed and reviewed with focus on onboard hydrogen storage handling and refueling while a priority-based Failure Mode and Effects Analysis (FMEA) method for risk assessment is proposed based on the revised guidelines of Automotive Industry Action Group (AIAG) and German Association of the Automotive Industry (VDA). The revised AIAG-VDA FMEA method replaces the conventional Risk Priority Number (RPN) with a new Action Priority (AP) rating enabling the prioritization of recommended actions for risk reduction. The paper aims to a more profound understanding of the safety risks associated with hydrogen as a maritime fuel and to provide an effective risk assessment method for hydrogen applications onboard maritime vessels.

To achieve the net zero carbon emissions goals by 2050 the transition to cleaner forms and carriers of energy should be accelerated without though jeopardizing the public safety. Although hydrogen has been deemed to play significant role in the energy transition for years now there are still concerns for its risks that hamper its widespread implementation in several applications like for instance automobile applications. Hydrogen-powered vehicles raise concerns about their safety especially inside confined spaces like tunnels and thus research on that topic has been intensified during the last years. In this context experiments have been conducted by UK HSE within the EU-funded project HyTunnel-CS to examine hydrogen dispersion and deflagration inside a scaled tunnel resulting from fuel cell car bus and train release.<br/>In this work that was also carried out within the HyTunnel-CS we present the Computational Fluid Dynamics (CFD) simulations of the HSE unignited experiments. Blowdown tests related to high-pressure hydrogen releases through Thermal Pressure Relief Device (TPRD) installed in car and in train were modeled using the ADREA-HF code. The scope of these simulations was two-fold: a) contribute to the design of the experiments (e.g. indicate sensor positioning ignition point etc.) and the interpretation of hydrogen behavior and b) validate the CFD code. For the former pre-test simulations preceded the experiments to provide design recommendations. When the experiments were conducted the measurements were used for the code validation. Overall the CFD results are in satisfactory agreement with the experiments. Finally simulations with different ventilation rates and with model vehicles inside the tunnel were conducted to examine their effect on mixture dispersion and tunnel safety.

Hydrogen as an energy carrier plays an important role in carbon neutrality and energy transition. Hydrogen is the lightest element with a density of only 0.08375 kg/m3 in gaseous form at standard temperature and pressure (STP); as a result hydrogen is usually stored and transported in a highly compressed form. It is prone to leakage and has a very low ignition energy of 0.017 mJ. Safety remains a challenge in the use of hydrogen as an energy source. This paper examines approximately 20 hydrogen-related accidents in China over a 20-year period focusing on the root causes consequences of the accidents and responses to them. These accidents occurred in the production storage transport and application of hydrogen with different causes in different locations and resulting in losses at different scales. Some statistical evaluations were conducted to learn lessons from the accidents. The main objective of this paper is (i) to retrieve a set of hydrogen related incidents from a region which is under-represented in incident repositories (ii) to contribute to a generalised lesson learned from them and (iii) to assist the definition of realistic scenarios for commonly occurring hydrogen accidents.

During a severe accident in a nuclear power plant one of the potential threats to the containment is the occurrence of energetic combustion events. In modern plants Severe Accident Management Guidelines (SAMG) as well as dedicated mitigation hardware are in place to minimize/mitigate this combustion risk and thus avoid the release of radioactive material into the environment. Advancements in SAMGs are in the focus of AMHYCO an EU-funded Horizon 2020 project officially launched on October 1st 2020. The project consortium consists of 12 organizations (from six European countries and one from Canada) and is coordinated by the Universidad Politécnica de Madrid (UPM). The progress made in the first two years of the AMHYCO project is here presented. A comprehensive bibliographic review has been conducted providing a common foundation to build the knowledge gained during the project. After an extensive set of accident transients simulated both for phases occurring inside and outside the reactor pressure vessel a set of challenging sequences from the combustion risk perspective for different power plant types were identified. At the same time three generic containment models for the three considered reactor designs have been created to provide the full containment analysis simulations with lumped parameter models 3-dimensional containment codes and CFD codes. In order to further consolidate the model base combustion experiments and performance tests on passive auto-catalytic recombiners under explosion prone H2/CO atmospheres were performed at CNRS (France) and FZJ (Germany). Finally it is worth saying that the experimental data and engineering models generated from the AMHYCO project are useful for other industries outside the nuclear one.

After the Paris Agreement was signed in 2015 many countries worldwide focused on the hydrogen economy aiming for eco-friendly and renewable energy by moving away from the existing carbon economy which has been the primary source of global warming. Hydrogen is the most common element on Earth. As a light substance hydrogen can diffuse quickly; however it also has a small risk of explosion. Representative explosion accidents have included the Muskingum River Power Plant Vapor Cloud Explosion accident in 2007 and the Silver Eagle Refinery Vapor Cloud Explosion accident in 2009. In addition there was an explosion in a hydrogen tank in Gangneung Korea in May 2019 and a hydrogen refueling station (HRS) in Norway exploded in 2018. Despite this risk Korea is promoting the establishment of HRSs in major urban centers including downtown areas and public buildings by using the Regulatory Sandbox to install HRSs. This paper employed the Hydrogen Risk Assessment Model (HyRAM) of Sandia National Laboratories (SNL) a quantitative risk assessment (QRA) program specialized in hydrogen energy for HRSs installed in major urban hubs. A feasibility evaluation of the site conditions of an HRS was conducted using the French land use planning method based on the results obtained through evaluation using the HyRAM and the overpressure results of PHAST 8.0. After a risk assessment we confirmed that an HRS would be considered safe even if it was installed in the city center within a radius of influence of jet fires and overpressure.