An analysis of the amount of hydrogen taking part in the explosions that happened during the Fukushima-Daiichi (Unit 1) nuclear power plant accident is presented herein. Through a series of analytic approximations and numerical calculations of increasing complexity, it has been possible to estimate that 130 kg of H-2 was involved in the explosion. Also, the strength of the resulting explosion was examined determining that even with a significantly smaller amount of hydrogen taking part, a devastating explosion would have occurred regardless. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.