France

The circular economy is a decisive strategy for reconciling economic development and the environment. In France the CE was introduced into the law in 2015 with the objective of closing the loop. The legislation also delegates energy policy towards the French regions by granting them the jurisdiction to directly plan the energy–climate issues on their territory and to develop local energy resources. Thereby the SUD PACA region has redefined its objectives and targeted carbon neutrality and the transition to a CE by 2050. To study this transition we developed a TIMESPACA optimization model. The results show that following a CE perspective to develop a local energy system could contribute to reducing CO2 emissions by 50% in final energy consumption and reaching almost free electricity production. To obtain greater reductions the development of the regional energy systems should follow a careful policy design favoring the transition to low energy-consuming behavior and the strategical allocation of resources across the different sectors. Biomethane should be allocated to the buildings and industrial sector while hydrogen should be deployed for buses and freight transport vehicles.

The intermittent production of the renewable energy imposes the necessity to temporarily store it. Large amounts of exceeding electricity can be stored in geological strata in the form of hydrogen. The conversion of hydrogen to electricity and vice versa can be performed in electrolyzers and fuel elements by chemical methods. The nowadays technical solution accepted by the European industry consists of injecting small concentrations of hydrogen in the existing storages of natural gas. The progressive development of this technology will finally lead to the creation of underground storages of pure hydrogen. Due to the low viscosity and low density of hydrogen it is expected that the problem of an unstable displacement including viscous fingering and gravity overriding will be more pronounced. Additionally the injection of hydrogen in geological strata could encounter chemical reactivity induced by various species of microorganisms that consume hydrogen for their metabolism. One of the products of such reactions is methane produced from Sabatier reaction between H2 and CO2. Other hydrogenotrophic reactions could be caused by acetogenic archaea sulfate-reducing bacteria and iron-reducing bacteria. In the present paper a mathematical model is presented which is capable to reflect the coupled hydrodynamic and bio-chemical processes in UHS. The model has been numerically implemented by using the open source code DuMuX developed by the University of Stuttgart. The obtained bio-chemical version of DuMuX was used to model the evolution of a hypothetical underground storage of hydrogen. We have revealed that the behavior of an underground hydrogen storage is different than that of a natural gas storage. Both the hydrodynamic and the bio-chemical effects contribute to the different characteristics.

Alkaline electrolyzers are the most widespread technology due to their maturity low cost and large capacity in generating hydrogen. However compared to proton exchange membrane (PEM) electrolyzers they request the use of potassium hydroxide (KOH) or sodium hydroxide (NaOH) since the electrolyte relies on a liquid solution. For this reason the performances of alkaline electrolyzers are governed by the electrolyte concentration and operating temperature. Due to the growing development of the water electrolysis process based on alkaline electrolyzers to generate green hydrogen from renewable energy sources the main purpose of this paper is to carry out a comprehensive survey on alkaline electrolyzers and more specifically about their electrical domain and specific electrolytic conductivity. Besides this survey will allow emphasizing the remaining key issues from the modeling point of view.

Hydrogen storage is a key enabling technology for the extensive use of hydrogen as energy carrier. This is particularly true in the widespread introduction of hydrogen in car transportation. Indeed one of the greatest technological barriers for such development is an efficient and safe storage method. So in this tutorial review the existing hydrogen storage technologies are described with a special emphasis on hydrogen storage in hydrogen cars: the current and the ongoing solutions. A particular focus is given on solid storage and some of the recent advances on plasma hydrogen ion implantation which should allow not only the preparation of metal hydrides but also the imagination of a new refluing circuit. From hydrogen discovery to its use as an energy vector in cars this review wants to be as exhaustive as possible introducing the basics of hydrogen storage and discussing the experimental practicalities of car hydrogen fuel. It wants to serve as a guide for anyone wanting to undertake such a technology and to equip the reader with an advanced knowledge on hydrogen storage and hydrogen storage in hydrogen cars to stimulate further researches and yet more innovative applications for this highly interesting field.

Within integrated steelmaking industries significant research efforts are devoted to the efficient use of resources and the reduction of CO2 emissions. Integrated steelworks consume a considerable quantity of raw materials and produce a high amount of by-products such as off-gases currently used for the internal production of heat steam or electricity. These off-gases can be further valorized as feedstock for methane and methanol syntheses but their hydrogen content is often inadequate to reach high conversions in synthesis processes. The addition of hydrogen is fundamental and a suitable hydrogen production process must be selected to obtain advantages in process economy and sustainability. This paper presents a comparative analysis of different hydrogen production processes from renewable energy namely polymer electrolyte membrane electrolysis solid oxide electrolyze cell electrolysis and biomass gasification. Aspen Plus® V11-based models were developed and simulations were conducted for sensitivity analyses to acquire useful information related to the process behavior. Advantages and disadvantages for each considered process were highlighted. In addition the integration of the analyzed hydrogen production methods with methane and methanol syntheses is analyzed through further Aspen Plus®-based simulations. The pros and cons of the different hydrogen production options coupled with methane and methanol syntheses included in steelmaking industries are analyzed

In this study a techno-economic analysis tool for conducting detailed feasibility studies on the deployment of green hydrogen hubs for fuel cell bus fleets is developed. The study evaluates and compares five green hydrogen hub configurations’ operational and economic performance under a typical metropolitan bus fleet refuelling schedule. Each configuration differs based on its electricity sourcing characteristics such as the mix of energy sources capacity sizing financial structure and grid interaction. A detailed comparative analysis of distinct green hydrogen hub configurations for decarbonising a fleet of fuel-cell buses is conducted. Among the key findings is that a hybrid renewable electricity source and hydrogen storage are essential for cost-optimal operation across all configurations. Furthermore bi-directional grid-interactive configurations are the most costefficient and can benefit the electricity grid by flattening the duck curve. Lastly the paper highlights the potential for cost reduction when the fleet refuelling schedule is co-optimized with the green hydrogen hub electricity supply configuration.

Hydrogen is considered a clean and efficient energy carrier crucial for shapingthe net-zero future. Large-scale production transportation storage and use of greenhydrogen are expected to be undertaken in the coming decades. As the smallest element inthe universe however hydrogen can adsorb on diffuse into and interact with many metallicmaterials degrading their mechanical properties. This multifaceted phenomenon isgenerically categorized as hydrogen embrittlement (HE). HE is one of the most complexmaterial problems that arises as an outcome of the intricate interplay across specific spatialand temporal scales between the mechanical driving force and the material resistancefingerprinted by the microstructures and subsequently weakened by the presence of hydrogen. Based on recent developments in thefield as well as our collective understanding this Review is devoted to treating HE as a whole and providing a constructive andsystematic discussion on hydrogen entry diffusion trapping hydrogen−microstructure interaction mechanisms and consequencesof HE in steels nickel alloys and aluminum alloys used for energy transport and storage. HE in emerging material systems such ashigh entropy alloys and additively manufactured materials is also discussed. Priority has been particularly given to these lessunderstood aspects. Combining perspectives of materials chemistry materials science mechanics and artificial intelligence thisReview aspires to present a comprehensive and impartial viewpoint on the existing knowledge and conclude with our forecasts ofvarious paths forward meant to fuel the exploration of future research regarding hydrogen-induced material challenges.

The increasing demand for carbon-free energy in recent years has positioned hydrogen as a viable option. However its current production remains largely dependent on carbon-emitting sources. In this context natural hydrogen generated through geological processes in the Earth’s subsurface has emerged as a promising alternative. The present study provides the first national-scale assessment of natural dihydrogen (H2) potential in Uruguay by developing a catalog of potential H2-generating rocks identifying prospective exploration areas and proposing H2 systems there. The analysis includes a review of geological and geophysical data from basement rocks and onshore sedimentary basins. Uruguay stands out as a promising region for natural H2 exploration due to the significant presence of potential H2-generating rocks in its basement such as large iron formations (BIFs) radioactive rocks and basic and ultrabasic rocks. Additionally the Norte Basin exhibits potential efficient cap rocks including basalts and dolerites with geological analogies to the Mali field. Indirect evidence of H2 in a free gas phase has been observed in the western Norte Basin. This suggests the presence of a potential H2 system in this area linked to the Arapey Formation basalts (seal) and Mesozoic sandstones (reservoir). Furthermore the proposed H2 system could expand exploration opportunities in northeastern Argentina and southern Brazil given the potential presence of similar play/tramp.

Hydrogen internal combustion engines (ICEs) have gained significant attention as a promising solution for achieving zero-carbon emissions in the transportation sector. This study investigates the conversion of a 2 L Diesel ICE into a lean hydrogen-powered ICE focusing on key challenges such as hydrogen mixing pre-ignition combustion flame development and NOx emissions. The novelty of this research lies in the specific modifications made to optimize engine performance and reduce emissions while utilizing the existing Diesel engine infrastructure. The study identifies several important design changes for the successful conversion of a Diesel engine to hydrogen including the following: Intake port design: transitioning from a swirl to a tumble design to enhance hydrogen mixing; Injection and spark plug configuration: using a lateral injection system combined with a central spark plug to improve combustion; Piston design: employing a lenticular piston shape with adaptable depth to enhance mixing; Mitigating Coanda effect: preventing hydrogen issues at the spark plug using deflectors or caps; and Head design: maintaining a flat head design for efficient mixing while ensuring adequate cooling to avoid pre-ignition. These findings highlight the importance of specific modifications for converting Diesel engines to hydrogen providing a solid foundation for further research in hydrogen-powered ICEs which could contribute to carbon emission reduction and a more sustainable energy transition.

Adopting machine learning (ML) in hydrogen systems is a promising approach that enhances the efficiency reliability and sustainability of hydrogen power systems and revolutionizes the hydrogen energy sector to optimize energy usage/management and promote sustainability. This study explores hydrogen energy systems including production storage and applications while establishing a connection between machine learning solutions and the challenges these systems face. The paper provides an in-depth review of the literature examining not only ML techniques but also optimization algorithms evaluation methods explainability techniques and emerging technologies. By addressing these aspects we highlight the key factors of new technologies and their potential benefits across the three stages of the hydrogen value chain. We also present the advantages and limitations of applying ML models in this field offering recommendations for their optimal use. This comprehensive and precise work serves as the most current and complete examination of ML applications within the hydrogen value chain providing a solid foundation for future research across all stages of the hydrogen industry.

Today hydrogen is already widely regarded as up-and-coming source of energy. It is essential to meet energy needs while reducing environmental pollution since it has a high energy capacity and does not emit carbon oxide when burned. However for the widespread application of hydrogen energy it is necessary to search new technical solutions for both its production and storage. A promising effective and cost-efficient method of hydrogen generation and storage can be the use of solid materials including nanomaterials in which chemical or physical adsorption of hydrogen occurs. Focusing on the recommendations of the DOE the search is underway for materials with high gravimetric capacity more than 6.5% wt% and in which sorption and release of hydrogen occurs at temperatures from −20 to +100 ◦C and normal pressure. This review aims to summarize research on hydrogen generation and storage using silicon nanostructures and silicon composites. Hydrogen generation has been observed in Si nanoparticles porous Si and Si nanowires. Regardless of their size and surface chemistry the silicon nanocrystals interact with water/alcohol solutions resulting in their complete oxidation the hydrolysis of water and the generation of hydrogen. In addition porous Si nanostructures exhibit a large internal specific surface area covered by SiHx bonds. A key advantage of porous Si nanostructures is their ability to release molecular hydrogen through the thermal decomposition of SiHx groups or in interaction with water/alkali. The review also covers simulations and theoretical modeling of H2 generation and storage in silicon nanostructures. Using hydrogen with fuel cells could replace Li-ion batteries in drones and mobile gadgets as more efficient. Finally some recent applications including the potential use of Si-based agents as hydrogen sources to address issues associated with new approaches for antioxidative therapy. Hydrogen acts as a powerful antioxidant specifically targeting harmful ROS such as hydroxyl radicals. Antioxidant therapy using hydrogen (often termed hydrogen medicine) has shown promise in alleviating the pathology of various diseases including brain ischemia–reperfusion injury Parkinson’s disease and hepatitis.

The production of iron and steel plays a significant role in the anthropogenic carbon footprint accounting for 7% of global GHG emissions. In the context of CO2 mitigation the steelmaking industry is looking to potentially replace traditional carbon-based ironmaking processes with hydrogen-based direct reduction of iron ore in shaft furnaces. Before industrialization detailed modeling and parametric studies were needed to determine the proper operating parameters of this promising technology. The modeling approach selected here was to complement REDUCTOR a detailed finite-volume model of the shaft furnace which can simulate the gas and solid flows heat transfers and reaction kinetics throughout the reactor with an extension that describes the whole gas circuit of the direct reduction plant including the top gas recycling set up and the fresh hydrogen production. Innovative strategies (such as the redirection of part of the bustle gas to a cooling inlet the use of high nitrogen content in the gas and the introduction of a hot solid burden) were investigated and their effects on furnace operation (gas utilization degree and total energy consumption) were studied with a constant metallization target of 94%. It has also been demonstrated that complete metallization can be achieved at little expense. These strategies can improve the thermochemical state of the furnace and lead to different energy requirements.

In the context of renewable energy systems microgrids (MG) are a solution to enhance the reliability of power systems. In the last few years there has been a growing use of energy storage systems (ESSs) such as hydrogen and battery storage systems because of their environmentally-friendly nature as power converter devices. However their short lifespan represents a major challenge to their commercialization on a large scale. To address this issue the control strategy proposed in this paper includes cost functions that consider the degradation of both hydrogen devices and batteries. Moreover the proposed controller uses scenarios to reflect the stochastic nature of renewable energy resources (RESs) and load demand. The objective of this paper is to integrate a stochastic model predictive control (SMPC) strategy for an economical/environmental MG coupled with hydrogen and battery ESSs which interacts with the main grid and external consumers. The system's participation in the electricity market is also managed. Numerical analyses are conducted using RESs profiles and spot prices of solar panels and wind farms in Abu Dhabi UAE to demonstrate the effectiveness of the proposed controller in the presence of uncertainties. Based on the results the developed control has been proven to effectively manage the integrated system by meeting overall constraints and energy demands while also reducing the operational cost of hydrogen devices and extending battery lifetime.

To meet the new regulation for the deployment of alternative fuels infrastructure which sets targets for electric recharging and hydrogen refuelling infrastructure by 2025 or 2030 a large infrastructure comprising trucksuitable hydrogen refuelling stations will soon be required. However further standardisation is required to support the uptake of hydrogen for heavy-duty transport for Europe’s green energy future. Hydrogen-powered vehicles require pure hydrogen as some contaminants can reduce the performance of the fuel cell even at very low levels. Even if previous projects have paved the way for the development of the European quality infrastructure for hydrogen conformity assessment sampling systems and methods have yet to be developed for heavy-duty hydrogen refuelling stations (HD-HRS). This study reviews different aspects of the sampling of hydrogen at heavy-duty hydrogen refuelling stations for purity assessment with a focus on the current and future specifications and operations at HD-HRS. This study describes the state-of-the art of sampling systems currently under development for use at HD-HRS and highlights a number of aspects which must be taken into consideration to ensure safe and accurate sampling: risk assessment for the whole sampling exercise selection of cylinders methods to prepare cylinders before the sampling filling pressure and venting of the sampling systems.

A sudden release of compressed gases and the formation of a jet flow can occur in nature and various engineering applications. In particular high-pressure hydrogen jets can spontaneously ignite when released into an environment that contains oxygen. For some scenarios these high-pressure hydrogen jets can be released into a mixture containing hydrogen and oxygen. This scenario can possibly lead to a wide range of combustion regimes such as jet flames slow or fast deflagrations or even hazardous detonations. Each combustion regime is characterized by typical pressures and temperatures however fast transition between regimes is also possible.<br/>A common project between Tel Aviv University (TAU) Nuclear Research Center Negev (NRCN) and Commissariat à l’Energie Atomique et aux énergies alternatives (CEA) has been recently launched in order to understand these phenomena from experimental modelling and numerical points of view. The main goal is to investigate the dynamics and combustion regimes that arise once a pressurized hydrogen jet is released into a reactive environment that contains inhomogeneous concentrations of hydrogen steam and air.<br/>In this paper we present the first numerical results describing high-pressure hydrogen release obtained using a massively parallel compressible structured-grid flow solver. The experimental arrangements devoted to this phenomenon will also be described.

In the past few years CEA has been fully involved at both experimental and modeling levels in projects related to hydrogen safety in nuclear and chemical industries and has carried out a test program using the experimental bench SSEXHY (Structure Submitted to an EXplosion of HYdrogen) in order to build a database of the deformations of simple structures following an internal hydrogen explosion. Different propagation regimes of explosions were studied varying from detonations to slow deflagrations.<br/>During the experimental campaign it was found that high-speed deflagrations corresponding to relatively poor hydrogen-air mixtures resulted in higher specimen deformation compared to those related to detonations of nearly stoichiometric mixtures. This paper explains this counter-intuitive result from qualitative and quantitative points of view. It is shown that the overpressure and impulse from head-on reflections of hydrogen flames corresponding to poor mixtures of specific concentrations could have very high values at the tube end.

The impact of the HyResponder project on the training of responders in 10 European countries is described. An overview is presented of training activities undertaken within the project in Austria Belgium Czech Republic France Germany Italy Norway Spain Switzerland and the United Kingdom. National leads with training expertise are given and the longer-term plans in each region are mentioned. Responders from each region took part in a specially tailored “train the trainer” programme and then delivered training within their regions. A flexible approach to training within the HyResponder network has enabled fit for purpose region appropriate activities to be delivered impacting over 1250 individuals during the project and many more beyond. Teaching and learning materials in hydrogen safety for responders have been made available in 8 languages: English Czech Dutch French German Italian Norwegian Spanish. They are being used to inform training within each of the partner countries. Dedicated national working groups focused on hydrogen safety training for responders have been established in Belgium the Czech Republic Italy and Switzerland.

This work aims to study the technical and economical feasibility of a new hydrogen transport network by 2035 in France. The goal is to furnish charging stations for fuel cell electrical vehicles with hydrogen produced by electrolysis of water using low-carbon energy. Contrary to previous research works on hydrogen transport for road transport we assume a more realistic assumption of the demand side: we assume that only drivers driving more than 20000 km per year will switch to fuel cell electrical vehicles. This corresponds to a total demand of 100 TWh of electricity for the production of hydrogen by electrolysis. To meet this demand we primarily use surplus electricity production from wind power. This surplus will satisfy approximately 10% of the demand. We assume that the rest of the demand will be produced using surplus from nuclear power plants disseminated in regions. We also assume a decentralized production namely that 100 MW electrolyzers will be placed near electricity production plants. Using an optimization model we define the hydrogen transport network by considering decentralized production. Then we compare it with more centralized production. Our main conclusion is that decentralized production makes it possible to significantly reduce distribution costs particularly due to significantly shorter transport distances.

This paper presents an innovative Fuel Cell Combined Heat and Power (FC–CHP) system designed to enhance energy efficiency in hospital settings. The system primarily utilizes solar energy captured through photovoltaic (PV) panels for electricity generation. Excess electricity is directed to an electrolyzer for water electrolysis producing hydrogen which is stored in high-pressure tanks. This hydrogen serves a dual purpose: it fuels a boiler for heating and hot water needs and powers a fuel cell for additional electricity when solar production is low. The system also features an intelligent energy management system that dynamically allocates electrical energy between immediate consumption hydrogen production and storage while also managing hydrogen release for energy production. This study focuses on optimization using genetic algorithms to optimize key components including the peak power of photovoltaic panels the nominal power of the electrolyzer fuel cell and storage tank sizes. The objective function minimizes the sum of investment and electricity costs from the grid considering a penalty coefficient. This approach ensures optimal use of renewable energy sources contributing to energy efficiency and sustainability in healthcare facilities.

Heat flux on walls from under-expanded H2/AIR jet flames have been numerically investigated. The thermal behaviour of a plate close to different under-expanded jet flames has been compared with rear-face plate temperature measurements. In this study two straight nozzles with millimetric diameter were selected with H2 reservoir pressure in a range from 2 to 10 bar. The CFD study of these two quite different horizontal jet flames employs the Large Eddy Simulation (LES) formalism to capture the turbulent flame-wall interaction. The results demonstrated a good agreement with experimental wall heat fluxes computed from plate temperature measurements. The present study assesses the prediction capability of LES for flame-wall heat transfer.

In the current context of the ban on fossil fuel vehicles (diesel and petrol) adopted by several European cities the question arises of the development of the infrastructure for the distribution of alternative energies namely hydrogen (for fuel cell electric vehicles) and electricity (for battery electric vehicles). First we compare the main advantages/constraints of the two alternative propulsion modes for the user. The main advantages of hydrogen vehicles are autonomy and fast recharging. The main advantages of battery-powered vehicles are the lower price and the wide availability of the electricity grid. We then review the existing studies on the deployment of new hydrogen distribution networks and compare the deployment costs of hydrogen and electricity distribution networks. Finally we conclude with some personal conclusions on the benefits of developing both modes and ideas for future studies on the subject.

In this work real scale experiments involving hydrogen dispersion inside a road tunnel have been modelled using the Computational Fluid Dynamics (CFD) methodology. The aim is to assess the performance of the ADREA-HF CFD tool against full-scale tunnel dispersion data resulting from high-pressure hydrogen leakage through Thermal Pressure Relief Device (TPRD) of a vehicle. The assessment was performed with the help of experiments conducted by the French Alternative Energies and Atomic Energy Commission (CEA) in a real inclined tunnel in France. In the experiments helium as hydrogen surrogate has been released from 200 bar storage pressure. Several tests were carried out examining different TPRD sizes and release directions (upwards and downwards). For the CFD evaluation two tests were considered: one with downwards and one with upwards release both through a TPRD with a diameter of 2 mm. The comparison between the CFD results and the experiments shows the good predictive capabilities of the ADREA-HF code that can be used as a safety tool in hydrogen dispersion studies. The comparison reveals some of the strengths and weaknesses of both the CFD and the experiments. It is made clear that CFD can contribute to the design of the experiments and to the interpretation of the experimental results.

HyCARE project supported by the Clean Hydrogen Partnership of the European Union deals with a prototype of hydrogen storage tank using a solid-state hydrogen carrier. Up to 40 kilograms of hydrogen are stored in twelve tanks at less than 50 barg and less than 100 °C. The innovative design is based on a standard twenty-foot container including twelve TiFe-based metal hydride (MH) hydrogen storage tanks coupled with a thermal energy storage in phase change materials (PCM). This article aims at showing the main risks related to hydrogen storage in a MH system and the safety barriers considered based on HyCARE’s specific risk analysis.<br/>Regarding the TiFe MH material used to store hydrogen experimental tests showed that the exposure of the MH to air or water did not cause spontaneous ignition. Furthermore an explosion within the solid MH cannot propagate due to internal pore size. Additionally in case of leakage the speed of hydrogen desorption from the MH is self-limited which is an important safety characteristic since it reduces the potential consequences from the hydrogen release scenario.<br/>Regarding the integrated system the critical scenarios identified during the risk analysis were: explosion due to release of hydrogen inside or outside the container internal explosion inside MH tanks due to accidental mix of hydrogen and air and asphyxiation due to inert gas accumulation in the container. This identification phase of the risk analysis allowed to pinpoint the most relevant safety barriers already in place and recommend additional ones if needed to further reduce the risk that were later implemented.<br/>The main safety barriers identified were: material and component selection (including the MH selected) safety interlocks safety valves ventilation gas detection and safety distances.<br/>The risk management process based on risk identification and assessment contributed to coherently integrate inherently safe design features and safety barriers.

In recent years the use and development of hydrogen as a carbon-free energy carrier have grown. However as hydrogen is flammable with air safety issues are raised. In the case of ignition especially in confined space the flame can accelerate and reach the detonation regime causing severe structural damage [1].<br/>To assess these safety issues it is required to understand the fluid-structure interaction in the case of a detonation impacting a deformable structure and to quantify and model this interaction [2]. At the CEA (Commissariat à l’énergie atomique et aux energies alternatives) a combustion tube experimental facility [3] for studying the fluid-structure interaction in the case of hydrogen combustion has been developed. Several Photomultipliers and Pressure sensors are placed along the tube to monitor the flame acceleration and the detonation location. A fluid-structure interaction (FSI) module or a non-deformable flange can be placed at the end of the tube. Post-processing of the sensor’s signal will provide insight into the occurring phenomena inside the tube.<br/>Several experimental campaigns have been conducted with various initial conditions and configurations at the end of the tube. In this contribution the experiments resulting in a detonation are presented. First the recorded pressure and velocities will be compared to theoretical values coming from combustion models [4] [5]. Secondly the impulse before and after reflection for thin plate and non-deformable flange will be compared to quantify the energy transmitted to the plate and the influence of the fluid-structure interaction on the reflected shock.

Hydrogen embrittlement is a phenomenon that affects the mechanical properties of steels intended for hydrogen transportation. One affected by this embrittlement is the Ductile to Brittle Transition Temperature (DBTT) which characterizes the change in the failure mode of the steel from ductile to brittle. This temperature is conventionally defined and compared to the operating temperature as an acceptability criterion for codes. Transition temperature does not depend only on the material but also on specimen geometry particularly the thickness. Generally the transition temperature is defined for the conservative reason by Charpy impact test. Standard Charpy specimens are straight beams with a thickness of 10 mm. For thin pipes it is impossible to extract these standard specimens. One uses in this case Mini-Charpy specimens with a reduced thickness due to pipe curvature. This paper aims to quantify the effect of hydrogen embrittlement on the transition temperature of pipe steel (API 5L X65) using two types of Charpy specimens.