Tokyo Electric Power Co (Tepco) said it was alerted by an alarm that the cooling system for the No 3 reactor had shut down at around 2.27pm (5.27am GMT) after a power board failed. Safety alert: Workers at Japan's Fukushima nuclear plant lost the ability to cool fuel rods in reactor 3 (seen here after the earthquake in 2011) after tripping the power supply while setting up a rat trap Crippled: The plant was the site of the worst nuclear disaster since Chernobyl in March 2011 when a magnitude-9 earthquake triggered a tsunami that destroyed back-up generators at Fukushima