The March 11 Japanese earthquake and subsequent tsunami knocked out electricity and emergency generators required to cool reactors at the Fukushima Daiichi plant. The lack of cooling led to a partial nuclear meltdown in units 1, 2 and 3 at the crippled plant and the release of deadly radioactivity.
Tohoku Electric, responsible for operations at the Onagawa plant, reported it had restarted the system to cool its spent fuel pool, according to Reuters.
http://www.infowars.com/onagawa-nuclear-plant-loses-power-after-earthquake/