The Mainichi Daily News now reports that Tokyo Electric Power Company (Tepco) is asserting that hydrogen gas from the Unit 3 meltdown(s), rather than being vented out the stack shared with Unit 4, flowed instead into the Unit 4 secondary containment reactor building, blowing it up. So much for the "hardened vent" retrofits constituting a "safety improvement" on the General Electric Boiling Water Reactor Mark 1 containment system! If this is truly what caused the "mysterious" Unit 4 explosion, then not only did the "hardened vents" fail to prevent meltdowns and containment failures at Units 1, 2, and 3, but they also caused a large explosion in Unit 4 -- a reactor that had been de-fueled, and was not operating -- which now risks the release of large-scale amounts of hazardous radioactivity directly into the environment if its high-level radioactive waste storage pool boils dry, allowing the irradiated nuclear fuel within to catch on fire.