Hydrogen is building up because, due to low levels of cooling water, fuel rods are partially exposed, causing them to overheat.

BBC: Japan quake: Nitrogen pumped into nuclear reactor

That was to prevent a repeat of the hydrogen explosions that blew radiation out of the plant soon after cooling systems failed in the wake of the tsunami.

ECONOMIST: A spreading cloud of economic and human costs

Cooling problems have resulted in damage to radioactive fuel rods, hydrogen gas explosions, fires and radiation releases.

WSJ: GE Steps Up Aid in Reactor Crisis

The nuke has had its share of scary moments, including a cooling tower collapse, and, more recently, discoveries of leaks of the radioactive hydrogen isotope tritium.

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