The use of hydrogen as an energy carrier on a large-scale commercial basis, while integral to the future hydrogen economy, is currently untested and underdeveloped. The development of an infrastructure for the future hydrogen economy will require the simultaneous development of a set of safety codes and standards to establish guidelines for building this structure. Such codes and standards are necessary to assure that related products and systems are safe and perform as designed.

As part of the U.S. Department of Energy' Hydrogen, Fuel Cells & Infrastructure Technologies Program, Sandia National Laboratories is developing the technical basis for assessing the safety of hydrogen-based systems for use in the development/modification of relevant codes and standards. The project impacts most areas of hydrogen utilization, including bulk transportation and distribution, storage, production and utilization. Sandia is developing benchmark experiments and a defensible analysis strategy for risk and consequence assessment of unintended releases from hydrogen systems. This work includes experimentation and modeling to understand the fluid mechanics and dispersion of hydrogen for different release scenarios, including investigations of hydrogen ignition, combustion, and subsequent heat transfer from hydrogen flames. The resulting technical information is incorporated into engineering models that are used for assessment of different hydrogen release scenarios and for input into quantitative risk assessments (QRA) of hydrogen facilities. The QRAs are used to identify and quantify scenarios for the unintended release of hydrogen and to identify the significant risk contributors at different types of hydrogen facilities. The results of the QRAs are envisioned as one input into a risk-informed codes and standards development process that also incorporates other important factors such as safety margins.

This paper describes the application of QRA methods to help establish one key code requirement: the minimum separation distances between a hydrogen refueling station and other facilities and the public at large. In this approach, the cumulative frequencies of all accidental releases of hydrogen resulting in a specified consequence are evaluated against the minimum distances required to protect people or equipment from harm. The accidental hydrogen releases can occur due to component failures or be induced by human errors. The availability of features to mitigate accidental releases (e.g., shutoff valves initiated by hydrogen or flame sensors) is included in the accident frequency evaluation. A selected accident frequency criterion or a frequency range is used to establish the risk-informed separation distances based on each selected consequence parameter. Accidents with frequencies below this criterion could potentially be eliminated from safety distance consideration. Uncertainties in the parameter inputs and model predictions are considered in this process.

Application of the risk-informed approach indicates that it can provide a defensible technical basis for specifying risk-informed separation distances. The results also demonstrate that separation distances for refueling stations can be significantly affected by facility design parameters such as the system operating pressure and available mitigating features, component leakage frequency data, and the selected consequence measures and risk criteria used in the evaluation.