Computational Fluid Dynamics codes are increasingly being considered for safety assessment demonstrations in many industrial fields as tools to model accidental phenomena and to design mitigation (risk reducing) systems. Thus, they naturally complement experimental programmes which may be expensive to run or difficult to set up. However, to trust numerical simulations, the validity of the codes must be firmly established, and a certain number of error sources (user effect, modelling errors, discretization errors, etc) reduced to the minimum. Code validation and establishment of best practice guidelines in the application of simulation tools to hydrogen safety assessment are some of the objectives pursued by the HYSAFE Network of Excellence. This paper will contribute to these goals by describing some of the validation efforts that CEA is making in the areas of release, dispersion, combustion and mitigation, thereby proposing the outline of a validation matrix for hydrogen safety problems.

The release of gases heavier than air like propane, at ground level, or lighter than air like hydrogen, close to a ceiling, can both lead to fire and explosion hazards that must be carefully considered in safety analyses.  Even if the simulation of accident scenarios in complex installations and long transients often appears feasible only using lumped parameter computer codes, the phenomenon of denser or lighter gas dispersion is not implicitly accounted by these kind of tools.
In the aim to set up an ad hoc model to be used in the computer code ECART, fluid-dynamic simulations by the commercial FLUENT 6.0 CFD code are used. The reference geometry is related to cavities having variable depth (2 to 4 m) inside long tunnels, filled with a gas heavier or lighter than air (propane or hydrogen). Three different geometrical configurations with a cavity width of 3, 6 and 9 m are considered, imposing different horizontal air stream velocities, ranging from 1 to 5 m/s.
A stably-stratified flow region is observed inside the cavity during gas shearing. In particular, it is found that the density gradient tends to inhibit turbulent mixing, thus reducing the dispersion rate.
The obtained data are correlated in terms of main dimensionless groups by means of a least squares method. In particular, the Sherwood number is correlated as a function of Reynolds, a density ratio modified Froude numbers and in terms of the geometrical parameter obtained as a ratio between the depth of the air-dense gas interface and the length of the cavity.
This correlation is implemented in the ECART code to add the possibility to simulate large installations during complex transients, lasting many hours, with reasonable computation time. An example of application to a typical case is presented.

The current work is devoted to problems connected with application of CFD tools for safety analysis of large-scale industrial structures. With the aim to preserve conservatism of overall process of multi-stage procedure of such analysis special efforts are required. A strategy which has to lead to obtaining of reliable results in CFD analysis is discussed. Different aspects of proposed strategy, including: adequate choice of physical and numerical models, procedure of validation simulations, and problem of 'under-resolved' simulations are considered. For physical phenomena which could cause significant uncertainties in the course of scenario simulation, an approach, which complements CFD simulations by application of auxiliary criteria, is presented. Physical basis and applicability of strong flame acceleration and detonation-to-deflagration transition criteria are discussed. In concluding part two examples of application of presented approach for nuclear power plant and workshop cell for hydrogen driven vehicles are presented.

The development and promulgation of codes and standards are essential to establish a market-receptive environment for commercial, hydrogen-based products and systems. The focus of the U.S. Department of Energy (DOE) is to conduct the research and development (R&D) needed to strengthen the scientific basis for technical requirements incorporated in national and international standards, codes and regulations. In the U.S., the DOE and its industry partners have formed a Codes and Standards Tech Team (CSTT) to help guide the R&D. The CSTT has adopted an R&D Roadmap to achieve a substantial and verified database of the properties and behavior of hydrogen and the performance characteristics of emerging hydrogen technology applications sufficient to enable the development of effective codes and standards for these applications. However, to develop a more structured approach to the R&D described above, the CSTT conducted a workshop on Risk Assessment for Hydrogen Codes and Standards in March 2005. The purpose of the workshop was to attain a consensus among invited experts on the protocols and data needed to address the development of risk-informed standards, codes, and regulations for hydrogen used as an energy carrier by consumers. Participants at the workshop identified and assessed requirements, methodologies, and applicability of risk assessment (RA) tools to develop a framework to conduct RA activities to address, for example, hydrogen fuel distribution, delivery, on-site storage, and dispensing, and hydrogen vehicle servicing and parking. The CSTT was particularly interested in obtaining the advice of RA experts and representatives of standards and model code developing organizations and industry on how data generated by R&D can be turned into information that is suitable for hydrogen codes and standards development. The paper reports on the results of the workshop and the RA activities that the DOE s program on hydrogen safety, codes and standards will undertake. These RA activities will help structure a comprehensive R&D effort that the DOE and its industry partners are undertaking to obtain the data and conduct the analysis and testing needed to establish a scientific and technical basis for hydrogen standards, codes, and regulations.

A quantitative risk analysis to a central pressurized storage tank for gaseous hydrogen has been performed to attend requirements of licensing procedures established by the State Environment Agency of S o Paulo State, Brazil. Gaseous hydrogen is used to feed the reactor to promote hydrogenation at the surfactant unit. HAZOP was the hazard identification technique selected. System components failures were defined by event and fault tree analysis. Quantitative risk analysis was complied to define the acceptability concepts on societal and individual risks required by the State Environmental Agency to approve the installation operation license. Acceptable levels to public society from the analysis were reached. Safety recommendations to the gaseous hydrogen central were proposed to assure minimization of risk to the near-by community, operators, environment and property.

In this paper, CFD techniques were applied to the simulations of hydrogen release from a 400-bar tank to ambient through a Pressure Relieve Device (PRD) 6 mm ( ) opening. The numerical simulations using the TOPAZ software developed by Sandia National Laboratory addressed the changes of pressure, density and flow rate variations at the leak orifice during release while the PHOENICS software package predicted extents of various hydrogen concentration envelopes as well as the velocities of gas mixture for the dispersion in the domain. The Abel-Noble equation of state (AN-EOS) was incorporated into the CFD model, implemented through the TOPAZ and PHOENICS software, to accurately predict the real gas properties for hydrogen release and dispersion under high pressures. The numerical results were compared with those obtained from using the ideal gas law and it was found that the ideal gas law overestimates the hydrogen mass release rates by up to 35%2during the first 25 seconds of release. Based on the findings, the authors recommend that a real gas equation of state be used for CFD predictions of high-pressure PRD releases.

Air pollution and traffic congestion are two of the major issues affecting public authorities, policy makers and citizens not only in Italy and European Union but worldwide; this is nowadays witnessed by always more frequent limitations to the traffic in most of Italian cities, for instance.

Hydrogen use in automotive appears to offer a viable solution in medium-long term; this new perspective involves the need to carry out adequate infrastructures for distribution and refuelling, and consequently the need to improve knowledge on hydrogen technologies from a safety point of view.In the present work possible different configurations for gaseous hydrogen refuelling station has beencompared: "stand-alone" and "multi-fuel".

These two alternative scenarios has been taken into consideration, each of one with specific hypotheses: "stand-alone" configuration, based on the hypothesis of a potential model consisting of a hydrogen refuelling station composed by on-site hydrogen production via electrolysis, a trailer of compressed gas for back-up, compressor unit, intermediate storage unit and dispenser. In this model it is assumed that no other refuelling equipment and/or dispenser of traditional fuel is present in the same site.

"Multi-Fuel" configuration, where it is assumed that the same components for hydrogen refuelling station are placed in the same site beside one or more refuelling equipment and/or dispenser of traditional fuel. Comparisons have been carried out from the point of view of specific risk assessment, which have been conducted on both the two alternative scenarios.

To make "Hydrogen vehicles and refueling station systems" fit for public use, research on hydrogen safety and designing mitigative measures are significant. For compact storage, it is planned to store under high pressure (40MPa) at the refueling stations, so that the safety for the handling of high-pressurized hydrogen is essential. This paper describes the experimental investigation on the hypothetical dispersion and explosion of high-pressurized hydrogen gas which leaks through a large scale break in piping and blows down to atmosphere. At first, we investigated time history of distribution of gas concentration in order to comprehend the behavior of the dispersion of high-pressurized hydrogen gas before explosion experiments. The explosion experiments were carried out with changing the time of ignition after the start of dispersion. Hydrogen gas with the initial pressure of 40MPa was released through a nozzle of 10mm diameter. Through these experiments, it was clarified that the explosion power depends not only on the concentration and volume of hydrogen/air pre-mixture, but also on the turbulence characteristics before ignition. To clarify the explosion mechanism, the numerical computer simulation about the same experimental conditions was performed. The initial conditions such as hydrogen distribution and turbulent characteristics were given by the results of the atmospheric diffusion simulation. By the verification of these experiments, the results of CFD were fully improved.

Today it is not known - neither qualitatively not quantitatively - how large the impact can be of the promoters on sensitivity to hydrogen-air detonation in hypothetical accidents at hydrogen-containing installations, transport or storage facilities. Report goal is to estimate theoretically an effect of hydrogen-peroxide (as representative promoter) on sensitivity to detonation of the stoichiometric hydrogen-oxygen gas mixtures. The classical H2-O2-Ar (2:1:7) gas mixture was chosen as reference system with the well established and unambiguously interpreted experimental data. In kineticsimulations it was found that the ignition delay time is sensitive to H2O2 addition for small initial H2O2concentrations and is nearly constant for the large ones. Parametric reactive CFD studies of two-dimensional cellular structure of 2H2-O2-7Ar-H2O2 detonations with variable hydrogen peroxide concentration (up to 10 vol.%2 were also performed. Two un-expected results were obtained. First result: detonation cell size is practically independent upon variation of initial hydrogen peroxideconcentration. For practical applications it means, that presence of hydrogen-peroxide did not change drastically sensitivity of the stoichiometric hydrogen-oxygen gas mixtures. These theoreticalspeculations require an experimental verification. Second result: for large enough initial H2O2concentrations (> 1 vol.%2at least), a new element of cellular structure of steady detonation wave was revealed. It is a system of multiple secondary longitudinal shock waves (SLSW), which propagates inthe direction opposite to that of the leading shock wave. Detailed mechanism of SLSW formation is proposed.

A safety study of gaseous hydrogen supply stations with 40MPa storage system is undertaken through a risk based approach. Accident scenarios are identified based on a generic model of hydrogen station. And risks of identified accident scenarios are estimated and evaluated comparing with risk acceptance criteria. Also, safety measures for risk reduction are discussed. Especially for clearance distance, it is proposed that the distance from high-pressurized equipment to site borders should be at least 6 meters. As a result of the study, it is concluded that risks of accidental scenarios can be mitigated to acceptable level under the proposed safety measures, with several exceptions. These exceptional scenarios are very unlikely to occur, but expected to have extremely severe consequence once occurred.