I believe the hydrogen that accumulates contains some radition (although mostly harmless) itself: http://mitnse.com/2011/03/15/explanation-of-hydrogen-explosion

I believe the hydrogen that accumulates contains some radition (although mostly harmless) itself: http://mitnse.com/2011/03/15/explanation-of-hydrogen-explosions-at-units-1-and-3/ http://mitnse.com/2011/03/15/explanation-of-hydrogen-explosi... (sixth paragraph)