Risk and Consequences of Rapid Phase Transition for Liquid Hydrogen

Safe handling of liquid hydrogen (LH2) has gained extra attention over the last years due to an increase in usage to mitigate climate changes. Rapid phase transition (RPT) is a potential safety concern when cryogens, like LH2 and liquefied natural gas (LNG), are accidentally spilled onto water. A theoretical assessment of the risk and consequences of LH2 RPT has been conducted. The assessment is based on the RPT theory established from LNG research, as well as published reports on actual LH2 spills. We give a review of the established theory on LNG RPT, examine the probability of an LH2 RPT event, and give estimates on the theoretical consequence in terms of the peak pressure and the explosive energy yield. There are two main findings of this study. Firstly, the known theoretical pathways to LNG RPT are impossible or very unlikely when applied to LH2 spills. Secondly, the theoretical consequences of an explosive LH2 RPT event are low compared to LNG RPT. The expected peak pressure is about 25% of an LNG RPT, while the expected explosive energy yield is only about 10% of an LNG RPT, given the same volume of participating cryogen. Combined with the knowledge that LNG RPT events are only moderately dangerous, the hypothetical LH2 RPT event is possibly characterized by a low destructive potential. Keywords: Liquid hydrogen, Safe fuel handling, Cryogens, Rapid phase transition, Spill accidents, Risk and consequence analysis