This was just plain hydrogen gas. But, hydrogen gas is what has been building up in nuclear waste containers at Hanford in Washington State, and appears responsible for explosions in multiple nuclear waste dumps. This appears one of the biggest if not the biggest challenge of permanent nuclear waste disposal! Why are countries still promoting nuclear anything? It is a problem at the stage of mining, at the stage of power production, at the disposal level. Uranium itself can spontaneously ignite posing a transportation risk.