It’s a little more complicated, but you’re basically right. The hydrogen buildup happened when they tried to prevent a meltdown by continually pumping water thr

It’s a little more complicated, but you’re basically right. The hydrogen buildup happened when they tried to prevent a meltdown by continually pumping water through the reactors, because the earthquake could have cracked the secondary concrete containment, allowing molten core materials to “China syndrome” their way into the groundwater. As far as I know several cores melted down anyway, and there’s no conclusive evidence that the molten cores escaped secondary containment. It would be a stretch to say that trying to prevent meltdowns was a bad decision in this case. Ironically though, pumping so much seawater through the reactors probably led to a much greater release of radiation into the spent fuel pools, which as I recall have been leaking that heavy (and impure) water into the ground and ocean.