It is well-known that fuel cells using hydrogen should be one of the most promising energy resources in the 21th century because they can have high energy efficiency and supply clean and stable energy without the production of CO₂, NOx, SOx, PM etc in the generation processes of electric power by using hydrogen produced from various fuels such as petroleum, natural gas, LP gas, kerosene. On the other hand, it should be essential for hydrogen energy society to ensure safety in the hydrogen utilization processes. Therefore, we should estimate all the risks in the hydrogen utilization processes by obtaining the knowledge on the basic properties of hydrogen and its explosion and fire properties and should establish the technologies to ensure safety. A risk management method has been discussed in the hydrogen utilization processes and some subjects have been described on the risk management and the safe countermeasures in the paper.

Self-ignition and explosion of hydrogen in a closed vessel can be explained on the basis of detailed chemical kinetics. Firstly, a chain branching chemical kinetics of hydrogen combustion is outlined. Explosion limits of hydrogen are derived from the chain branching kinetic mechanism. Next, an updated detailed chemical mechanism is presented. Results of flame speed and ignition delay time calculations are compared with experimental results. It has been shown that there is some discrepancy between simulated and measured mass burning velocities of hydrogen flames at high-pressure (p>10atm). Possible reason of this discrepancy is discussed briefly. Lastly, detonation characteristics of hydrogen is described and recent studies on the detonation limit are reviewed.

Fuel cell vehicle is expected as a next-generation car. However, hydrogen is recognized to be dangerous gas. Therefore it is necessary to investigate the hydrogen safety. As a result of the studied on the hydrogen safety, we got the following conclusions. 1) Hydrogen diffuses in the air very fast, and becomes safety mixture soon. 2) Diluted hydrogen mixture is not dangerous as it was said. 3) Hydrogen is not more dangerous than existing conventional gasoline or natural gas fuel.

CFD simulation was carried out on diffusion of hydrogen leakage in tunnels. In this study, two typical shapes of tunnel which is fifty meters long and existing tunnel (Kanetsu tunnel) were selected. For the amount of hydrogen leaked, 60 m³ was selected, which corresponds to the amount necessary for fuel cell vehicles to achieve their desired driving range. We investigated the influence of tunnel shape and the effect of tunnel ventilation. As a result, we found that leaked hydrogen is immediately carried away from point of leakage under existing ventilation conditions. The basic data on behavior of leaked hydrogen was obtained.

In order to achieve the public acceptance of hydrogen stations as social infrastructure in the early commercialization stage of fuel cell vehicles, it is quite important never to bring about severe accident which leads to casualties or property damage outside of the hydrogen station. In this paper the analysis of accident scenarios of hydrogen station which focuses the consequence of hydrogen release is performed, and safety measures for the risk reduction of severe accidents are discussed.

Thermodynamic simulation on the transient of pressure and temperature of an in-vehicle hydrogen container with the hydrogen leak from a crack was performed. It is assumed that the volume of the container is 40 L, which is filled with hydrogen at 70 MPa and 50 °C as an initial condition, and the crack area is 1 mm² or 10 mm². Hydrogen at 70 MPa and -40 °C is supplied from a refueling station with the flow rate of 2 kg/min or 6 kg/min. Conservation equations of mass and energy were solved using a thermophysical property database compiling an accurate equation of state. Moreover, the results were compared with those calculated by the ideal gas equation of state. It was found that the pressure of the real gas decreases more rapidly than that of the ideal gas.

Today, the total producing capacity of liquid hydrogen is about 17 T/day (metric) in Japan. This chapter describes the development trace of the liquid hydrogen.