1. Introduction
Pipeline networks of pure hydrogen are necessary for the wide spread of combined heat and power fuel cell systems in residential areas. In order to secure the safety level of pure hydrogen pipeline network in future hydrogen society, the Japan Gas Association (JGA) has conducted experimental researches on hydrogen leak detection techniques.
In the experiments, distributions of leaked hydrogen in the soil were measured to understand the hydrogen diffusion behavior. Also, some hydrogen detection techniques such as practically used detectors, state-of-the-art detection devices and some gas odorants were exemplified in the experiments.

2. Experimental conditions
The large-scale experiments (figure 1, table 1) were designed to check the potential of the detectors in case of very small volume leakage, which is the most difficult-to-find situation in the actual pipeline operation.
The time changes of hydrogen and cyclohexene (cyclohexene was chosen as an promising odorant) concentration at each sampling point were measured by gas chromatography and some hydrogen analyzers.

3. Results
The contour of hydrogen concentration after 360 hours is shown in Figure 2.
The leaked hydrogen was detected on the overhead asphalt surface of the leak spot by some types of hydrogen analyzer. In addition, cyclohexene was detected by human sense of smell at the point of horizontal distance of 2.5 m from the leak spot at 0.5 m depth.

4. Conclusion
Some hydrogen analyzers were found to have enough potential to detect rather small hydrogen leak. The smell performance of some selected odorants was checked.