High Quality Content by WIKIPEDIA articles! When the Fukushima Daiichi nuclear disaster began on 11 March 2011, reactor unit 4 was shut down and all fuel rods had been transferred to the spent fuel pool on an upper floor of the reactor building. On 15 March, an explosion damaged the fourth floor rooftop area of the unit 4 reactor. Japan`s nuclear safety agency NISA reported two large holes in a wall of the outer building of unit 4 after the explosion. It was reported that water in the spent fuel pool might be boiling. Radiation inside the unit 4 control room prevented workers from staying there permanently. Visual inspection of the spent fuel pool of reactor 4 on 30 April showed that there was no significant visible damage to the fuel rods in the pool.