When the Fukushima Daiichi nuclear disaster began on 11 March 2011, reactor units 4, 5 and 6 were all shut down. An explosion damaged unit 4 four days after the 2011 Tōhoku earthquake and tsunami. Damages from the earthquake and tsunami on units 5 and 6 are relatively minor.

The 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. 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 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 4pm 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 7pm JST on 15 March. 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.

Explosion At approximately 6am JST on 15 March, an explosion damaged the 4th floor rooftop area of the Unit 4 reactor as well as part of the adjacent Unit 3. The explosion is thought to be caused by the ignition of hydrogen that had accumulated near the spent fuel pond, the hydrogen was initially thought to have come from the stored fuel rods, but later, TEPCO believed the hydrogen came from Unit 3. A hand-written summary of a conference call, taken by an aide to then-Nuclear Regulatory Commission chairman Gregory Jazcko, stated that fuel could have been ejected from the Unit 4 spent fuel pond during this explosion; this was later found to be in error. Later reports that the Unit 4 spent fuel pool had caught fire were found to be erroneous; later reports clarified that the fire did not involve the fuel pool. Current versions of the IAEA report do not mention a fire (as of 6/2014, the logs of the fuel transfer process from the Unit 4 spent fuel pool to the common fuel pool indicate steady progress with no fuel damage reported). As radiation levels rose, some of the employees still at the plant were evacuated. On the morning of 15 March, Secretary Edano announced that according to the TEPCO, radiation dose equivalent rates measured from the Unit 4 reached 100 mSv/h. Edano said there was no continued release of "high radiation".

Japan's nuclear safety agency NISA reported two holes, each 8 meters square, or 64 m2 (690 sq ft), in a wall of the outer building of Unit 4 after the explosion. At 5:48pm it was reported that water in the spent fuel pool might be boiling. By 9:13pm on 15 March, radiation inside the Unit 4 control room prevented workers from staying there permanently. Seventy staff remained at the plant, while 800 had been evacuated. By 10:30pm TEPCO was reportedly unable to pour water into the spent fuel pool. By 10:50pm the company was considering using helicopters to drop water, but this was postponed because of concerns over safety and effectiveness, and the use of high-pressure fire hoses was considered instead.

A fire was discovered at 5:45am JST on 16 March in the northwest corner of the reactor building by a worker taking batteries to the central control room of Unit 4. This was reported to the authorities, but on further inspection at 6:15am no fire was found. Other reports stated that the fire was under control. At 11:57am, TEPCO released a photograph showing "a large portion of the building's outer wall has collapsed". Technicians considered spraying boric acid on the building from a helicopter.

Structural damage of reactor building no. 4

On 24 June 2012 TEPCO revealed that some exterior walls were slightly out of vertical in reactor building no.4, due to the hydrogen explosion in March 2011. Further investigations showed damage in various parts of the building structures at the west and south side. At the third floor a wall was found to be leaning 4.6 centimeters out of vertical. All deviations were within legal limits, and the walls were strong enough to withstand an earthquake. TEPCO noted that the building was safe, because these variations were all found in the outer walls, and the spent fuel pool was supported by pillars and other structures.JAIF (26 June 2012) Earthequake report 452: Tilted walls found at Fukushima No.4 reactor

Spent fuel pool On 16 March, the chairman of United States Nuclear Regulatory Commission (NRC), Gregory Jaczko, said in Congressional testimony that the NRC believed all of the water in the spent fuel pool had boiled dry. Japanese nuclear authorities and TEPCO contradicted this report, but later in the day Jaczko stood by his claim, saying it had been confirmed by sources in Japan. At 1pm TEPCO said that helicopter observation indicated that the pool had not boiled off. The French Institut de Radioprotection et de Sûreté Nucléaire (IRSN) agreed, stating that helicopter crews diverted planned water dumps to Unit 3 on the basis of their visual inspection of Unit 4.Jonathan Weisman. "Did NRC's Jaczko misspeak?". The Wall Street Journal. 17 March 2011.

At approximately 2:30pm on 16 March, TEPCO announced that the storage pool, located outside the Unit 4 containment area, might be boiling. Around 8pm JST, it was then planned to use a police water cannon to spray water on Unit 4.

On 18 March, it was reported that water sprayed into the spent fuel pool was disappearing faster than evaporation could explain, suggesting leakage. SDF military trucks sprayed water onto the building to try to replenish the pool on 20 March. On 22 March, the Australian military flew in Bechtel-owned robotic equipment for remote spraying and viewing of the pool. The Australian reported this would give the first clear view of the pool in the "most dangerous" of the reactor buildings.

The IAEA reported, "From 22 March to 25 March 130 to 150 tonnes of seawater were poured into the spent fuel pool each day using a concrete pump equipped with a long articulated arm. Seawater was also poured in through spent fuel cooling system from 9:05pm UTC 24 March to 01:20 25 March. White smoke was still being observed coming from the reactor building as of 11pm UTC 25 March." On 29 March, the seawater was changed to fresh water.

Analysis of spent fuel pool water collected on 12 April suggests that while some of the 1535 fuel assemblies stored there may have been damaged, the majority of the stored fuel assemblies are intact based on measured radiation levels. TEPCO further stated that "the fuel rods in the Unit 4 pool had released caesium-134 and −137 in the process of being damaged", and that TEPCO would "need to continue monitoring it".Radiation surges above 4's fuel pool|The Japan Times Online. Search.japantimes.co.jp (13 April 2011). Retrieved on 30 April 2011. On 13 April, TEPCO reported that the temperature of the spent fuel pool had increased to 90 °C, and that the radiation level 6 meters above the pool had reached 84 mSv/h. The spike was later attributed to a failure to properly keep the spent fuel pool covered in water.NTI: Global Security Newswire – Japan plant emits more radiation after cooling lapse . Global Security Newswire (14 April 2011). Retrieved on 30 April 2011. As of 25 April, TEPCO was still pumping between 70 and 210 tons of water into the pool, varying the amounts depending on the temperature in the pool. TEPCO also reported that it was attempting to minimize the amount of water added to the pool for fear "the weight of the water could weaken the reactor building".Monitoring rising temperatures, NHK, 25 April 2011 On 28 April, TEPCO announced it believed that water was not leaking from the pool but only evaporating. TEPCO based its belief on calculations that the heat generated by the spent fuel stored in the pool would be expected to evaporate 140 to 210 tons of water daily, in line with the amount of replacement water it adds.TEPCO: Water isn't leaking from No. 4 reactor pool, NHK, 28 April 2011 On 9 May, TEPCO began work to install a supporting structure for the Unit 4 spent fuel pool, due to the concerns that explosions could have weakened the structure.Seismic Damage Information (the 132nd Release).

On 11 June, it was discovered that the water level in the spent fuel pool was only one third of normal, and only part of the fuel rods were covered with water. This was probably the cause of the high radiation levels measured. This pool has also been used to dump equipment. On Sunday 19 June, the pool was refilled, to minimize the radiation and making it possible to work again at this place.NHK-world (20 June 2011)Spent Fuel Pool refilled On 21 June, the first stage of strengthening at the second floor in the building under the pool was finished: 32 steel columns 8 meters long with a weight of 40 tons each were placed at the second floor.NHK-world (21 June 2011) First stage strengthening of floor under spent fuel pool Reactor 4 finished By 30 July, the concreting of the supporting columns was completed.

From 16 June, water injection to the spent fuel pool was switched from using the concrete pumping vehicle to an alternative water injection line.

On 31 July, the spent fuel pool was switched from the water-injection cooling system, to a circulatory cooling system.

On 31 January 2012, six liters of radioactive water (35.500 becquerels per liter) leaked from the spent fuel pool of reactor 4 onto the floor in the building from a broken pipe. The leakage was stopped after a valve was closed, and was thought to have been caused by the cold weather.The Mainichi Daily News (1 February 2012)Radioactive water leaking from inside Fukushima No. 4 reactor The next day (1 February 2012), TEPCO released an even higher figure: 8500 liters were leaked after a pipe was dropped off after the water inside had turned into ice. The leakage appeared to have started at around 5 p.m. on 31 January 2012. This water was contaminated with radioactive isotopes, because it was mixed with water that was in contact with the fuel rods from the spent-fuel pool. TEPCO made plans to check whether there were similar cases in the other damaged reactor buildings.The Mainichi Daily News (2 February 2012) TEPCO says 8.5 tons of water leaked from Fukushima No. 4 reactor