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Fukushima Daiichi units 4, 5 and 6
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; the source of the explosion is still unknown, although it is speculated to be due to hydrogen generation in the spent fuel pool. 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.
Reactors 5 and 6 were also shut down when the earthquake struck although, unlike reactor 4, they were still fueled. The reactors have been closely monitored, as cooling processes were not functioning well.
==Unit 4 Reactor==
At the time of the earthquake, Unit 4 had been shut down for shroud replacement and refueling since 29 November 2010.〔(【引用サイトリンク】title=NISA Seismic status report, pg 35. )〕〔(【引用サイトリンク】title=Core shroud replacement introduction )〕 All 548 fuel assemblies had been transferred in December 2010 from the reactor to the spent fuel pool on an upper floor of the reactor building〔(【引用サイトリンク】title=IAEA update on Japan earthquake )〕 where they were held in racks containing boron to damp down any nuclear reaction.〔 The pool is used to store rods for some time after removal from the reactor and contains 1,331 rods. Recently active fuel rods produce more decay heat than older ones. At 04:00 JST on Monday 14 March, water in the pool had reached a temperature of 84 °C compared to a normal value of 40–50 °C.〔 The IAEA was advised that the temperature value remained 84 °C at 19:00 JST on 15 March.〔(【引用サイトリンク】title=IAEA Update on Japan Earthquake )〕 The temperature in the pool remained relatively constant until 30 July, when a closed circuit cooling system was commissioned. On 11 April, a fire broke out at Unit 4.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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