This report summarizes the investigation of the release of approximately 300kg of hydrogen at the AC Transit Facility in Emeryville, CA. The hydrogen release was avoidable in both the root cause and contributing factors. The report summarizes the findings of the incident investigation and metallurgical analysis of the failed valve. Hydrogen embrittlement, manufacturing quality as well as miscommunication played significant roles in the event. No injuries or fatalities resulted from the incident.

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