Safety assessment of onboard high-pressure hydrogen leakage in hydrogen-powered rail vehicles under tunnel conditions

To investigate the safety implications of onboard hydrogen leakage in hydrogen-powered rail vehicles under tunnel conditions, this study established a test platform incorporating a full-scale hydrogen storage module and tunnel section, together with high-pressure hydrogen release, concentration and overpressure measurement, and video recording. Six tests, comprising two leakage-only tests and four delayed-ignition tests, examined the blow-down of a single 140 L cylinder initially pressurised to 35 MPa through a manually actuated 5 mm TPRD. The tests were conducted under a fans-off condition and a preset ventilated condition, with the latter corresponding to a fan-controller setting of 50 km/h. A source-term analysis based on the Abel–Noble equation of state and the choked-flow relation gave an initial mass-flow rate of 0.32 kg/s and a choked-flow duration of approximately 56 s. Under the fans-off condition, the highest measured hydrogen concentration was 31.6 vol.% at the sensor above the release region. Clear ignition-related overpressure was recorded at only one of the five pressure sensors, with a maximum of approximately 5.0 kPa; the other sensors showed no response exceeding the approximately ±0.3 kPa baseline fluctuations. Sustained ignition occurred in three of the four delayed-ignition tests; each produced a jet fire, and no deflagration-type pressure signature was identified at the five instrumented pressure locations within the 30 s pressure-recording window. Under the preset ventilated condition, hydrogen was detected at the downstream sensor locations within the instrumented region, and the flame was progressively deflected as the release weakened. These measurements and the documented source-term procedure provide condition-specific engineering information that may inform future ventilation design and hazardous-zone assessment for comparable roof-mounted hydrogen-storage configurations.