the fukushima nuclear disaster began on march 11 2011, just hours after the initial wave. the connection to the electrical grid was broken. all power for cooling was lost and reactors started to overheat. there was a partial core meltdown in reactors 1, 2, and 3; hydrogen explosions destroyed the upper part of the buildings housing reactors 1, 3, and 4; an explosion damaged the containment inside reactor 2; fires broke out at reactor 4. despite being initially shutdown, reactors 5 and 6 began to overheat. spent nuclear fuel rods stored in pools in each reactor building overheated as water levels in the pools dropped. the accident is the second biggest nuclear accident after the chernobyl disaster, but more complex as all reactors are involved.