This paper describes a CFD study of a scenario involving the vertical downward release of hydrogen from a thermally-activated pressure relief device (TPRD) under a fuel cell car. The volumetric source model is applied to simulate hydrogen release dynamics during the tank blowdown process. Simulations are conducted for both unignited and ignited releases from onboard storage at 35 MPa and 70 MPa with TPRD orifice 4.2 mm. Results show that after TPRD opening the hazards associated with the release of hydrogen lasts less than two minutes, and the most hazardous timeframe occurs within ten seconds of the initiation of the release. The deterministic separation distances for unignited releases are longer than those for ignited releases, indicating that the separation distances are dominated by delayed ignition events rather than immediate ignition events. The deterministic separation distances for both unignited and ignited hydrogen downward releases under the car, are significantly shorter than those of free jets. To ensure the safety of people a deterministic separation distance of at least 10 m for 35 MPa releases is required. This distance should be increased to 12 m for the 70 MPa release case. To ensure that the concentration of hydrogen is always less than 4%2at the location of the air intake of buildings, the deterministic separation distance should be at least 11 m for 35 MPa releases and 13 m for 70 MPa releases.