The Fukushima Daiichi (Unit 2) reactor, was 1 out of 4 reactors seriously affected during the on 11 March 2011. Overall, the plant had 6 separate boiling water reactors originally designed by General Electric (GE), and maintained by the Tokyo Electric Power Company (TEPCO). At the time of the earthquake, Reactor 4 had been de-fueled while 5 and 6 were in cold shutdown for planned maintenance.

Background The plant comprises six separate boiling water reactors originally designed by General Electric (GE), and maintained by the Tokyo Electric Power Company (TEPCO). At the time of the quake, Unit 4 had been de-fueled while 5 and 6 were in cold shutdown for planned maintenance. Unit 2 was operating at the time of the earthquake and experienced the same controlled initial shutdown as the other units. As with unit 1, the reactor scrammed following the earthquake. The two diesel generators came online and initially all cooling systems were available. Initially the high pressure coolant injection (HPCI) system was primary cooling the core and at 15:00 operators activated the residual heat removal system main pump and the containment vessel spray pump at 15:07 to cool the suppression pool; all these systems failed following both AC power and DC power loss after the tsunami as the diesel generators and other systems failed when the tsunami overran the plant. The reactor core isolation cooling (RCIC) system was manually activated by operators at 15:39 following power loss, but by midnight the status of the reactor was unclear; some monitoring equipment was still operating on temporary power. The coolant level was stable and preparations were underway to reduce pressure in the reactor containment vessel should it become necessary, though TEPCO did not state in press releases what these preparations were, and the government had been advised that this might happen.

Disaster

Cooling problems TEPCO reported that the RCIC shut down around 19:00 JST on 12 March, and that the reactor containment vessel started reducing pressure before midnight. The IAEA later revealed that no venting had taken place at the plant. The following day at 09:00 JST, the RCIC was reported to be operating again. A New York Times report suggested that plant officials initially concentrated efforts on a damaged fuel storage pool at Unit 2, diverting attention from problems arising at the other reactors, but that incident was not reported in official press releases. The IAEA reported that on 14 March at 09:30, the RCIC was still operating and that power was being provided by a mobile generator. At some point during the day TEPCO reported the shutdown of the RCIC system presumably due to low reactor pressure. Operators had for days taken measures to prevent the reactor pressure from dropping below the level at which the RCIC can operate to keep it running as long as possible. The system though, was never designed to be used for an extended period. Fuel rods had been fully exposed for 140 minutes and there was a risk of a core meltdown. Reactor water level indicators were reported to be showing minimum-possible values at 19:30 JST on 14 March. Meanwhile, radiation dose rates measured at the gate of the plant (at 21:37 JST) reached a maximum of 3.13 m Sv/h, which was enough to reach the annual limit for non-nuclear workers in twenty minutes. These levels later subsided as another measurement taken at 22:35 showed a lower reading of 0.326 mSv/h. Workers concurrently succeeded in refilling half the reactor with water but parts of the rods were still exposed, and technicians could not rule out the possibility that some had melted. It was believed that around 23:00 JST, the 4 m long fuel rods in the reactor were fully exposed for the second time.

At 00:30 JST on 15 March, NHK ran a live press conference with TEPCO stating that the water level had sunk under the rods once again and pressure in the vessel was raised. The utility said that the hydrogen explosion at Unit 3 might have caused a glitch in the cooling system of Unit 2: Four out of five water pumps being used to cool the Unit 2 reactor had failed after the explosion at Unit 3. In addition, the last pump had briefly stopped working when its fuel ran out. To replenish the water, the contained pressure would have to be lowered first by opening a valve of the vessel. The unit's air flow gauge was accidentally turned off and, with the gauge turned off, flow of water into the reactor was blocked leading to full exposure of the rods. Water was later pumped into Unit 2 again by 04:11 JST. It was later revealed that workers were minutes from restoring power to the standby liquid control (SLC) system pumps in unit 2 as a way to inject borated water once the RCIC shut down and had spent hours laying cable from a generator truck to the unit 2 power center when the unit 1 explosion occurred. This damaged the cable preventing this method from being used. It is possible this system could have prevented a complete meltdown as it took hours after the explosion until injection using fire trucks could be started.TEPCO (10 August 2011) Chronology of Events at Fukushima Daiichi nuclear power station.

Hydrogen explosion

After Unit 3 exploded from built up hydrogen gas, the debris blew holes through the walls of Unit 2. It was initially hoped that these holes would vent any hydrogen gas coming from Unit 2 and prevent a similar explosion. This unfortunately did not hold true as an explosion was heard after 06:14 JST on 15 March in Unit 2, possibly damaging the pressure-suppression system at the bottom part of the containment vessel. The radiation level was reported as exceeding the legal limit, and the plant's operator started to evacuate all non-essential workers from the plant. Only a minimum crew of 50 men, also referred to as the Fukushima 50, was left at the site. Soon after, radiation equivalent dose rates had risen to 8.2 mSv/h around two hours after the explosion and again down to 2.4 mSv/h, shortly after. Three hours after the explosion, the rates had risen to 11.9 mSv/h.

While admitting that the suppression pool at the bottom of the containment vessel had been damaged in the explosion, causing a drop of pressure there, Japanese nuclear authorities emphasized that the containment had not been breached as a result of the explosion and contained no obvious holes.15 March 2011, Plant operator says reactor seal apparently not holed , Channel News Asia. Agence France-Presse. In a news conference on 15 March the director general of the IAEA, Yukiya Amano, said that there was a "possibility of core damage" at Unit 2 of less than 5%. Japan's Nuclear and Industrial Safety Agency (NISA) stated 33% of the fuel rods were damaged, in news reports the morning of 16 March. In the week that followed concerns were raised over a possible breach from the explosion at either the suppression pool, or the reactor vessel. Images taken by a robotic inspection in 2012 later revealed superficial surface damage to pipework, but show the outer surface of the torus to be in normal condition. The potential break points of the manhole covers were also found to be intact.

Meltdown and seawater used for cooling

In their attempt to prevent a meltdown, the government ordered that seawater be used to cool the reactors as no alternatives were available at the time. At 20:05 JST on 14 March, the Japanese government ordered seawater to be injected into Unit 2 in a new effort to cool the reactor core. This process was later changed to fresh water and then back to sea water. It was initially thought that Unit 2 had been spared a meltdown, but this notion soon came into question. On 29 March, Richard Lahey, former head of safety research for boiling-water reactors at General Electric, speculated that the reactor core may have melted through the reactor containment vessel onto a concrete floor, raising concerns of a major release of radioactive material, while failing to divulge the report by Dale G. Bridenbaugh which condemned the design as "unsafe". On 27 April, TEPCO revised its estimate of damaged fuel in Unit 2 from 30% to 35%. TEPCO finally confirmed on 23 May that Unit 2 suffered a meltdown about 100 hours after the earthquake.

Tepco-workers were able to enter the building of reactor 2 a month later where they installed and replaced a provisional gauge for measuring the water level inside the reactor. The results were disappointing, as 2 days later Tepco reported that it was still not possible to obtain accurate data on the water level and pressure from Unit 2. The temperature near the containment vessel was reported to be very high which caused the new gauge to not function properly. One of the key issues mentioned was the evaporated water inside the tubes of the gauge.NHK-World (25 June 2011) TEPCO unable to gauge No.2 reactor water level

Reactor stabilization By midday on 19 March grid power had been connected to the existing transformer at Unit 2 and work continued to connect the transformer to the new distribution panel installed in a nearby building. Outside electricity became available at 15:46 JST on 20 March, but equipment still had to be repaired and reconnected. On 24 March, electrical power (initially from temporary sources, but off-site power used from 3 April) was restored to parts of the unit which included the Main Control Room. The IAEA reported temperatures on 27 March at the bottom of Unit 2's Reactor Pressure Vessel (RPV) fell from 100 °C (212 °F) to 97 °C (206.6 °F). While operators attempted to pump water from the turbine hall basement to the condenser, both of condensers turned out to be full. Pumps were installed which moved up to 25 tons of condensed water per hour into on site storage tanks. This in turn freed up the needed storage condenser space for water in Unit 2's basement. Ventilation systems were installed in Unit 2 on 11 June to clean the highly radioactive air encapsulated within the Reactor Building.

On 28 June, TEPCO began injecting nitrogen into the containment vessel, which was expected to reduce the likelihood of further hydrogen explosions.

Since 2 July, the Reactor has been cooled using fresh water treated by the on site water treatment plant.

On 14 September at 11AM (JST) TEPCO began injecting water into the No. 2 reactor using the core spray system piping in addition to the feed water piping already being used as this method seemed to be effective in reducing the temperature in the No. 3 reactor. At that time the temperature at the bottom of the No. 2 reactor was still 114.4 degrees Celsius (237.92 °F), compared to the 84.9 °C degrees(184.82 °F) in the No.1 reactor and the 101.3 °C (214.34 °F) in the No. 3 reactor. The new method has led to some temperature decrease, but not as significant as the decrease that occurred in the No.3 reactor.JAIF (15 September 2011) Earthquake report 205: TEPCO spraying water directly into No.2 reactor

After some positive effect was noticed using both the core spray system and feed water piping, TEPCO decided on 16 September to increase the amount of water pumped into the No. 2 reactor by one ton, in an attempt to further lower the temperature in the core, to a total of 7 tons per hour. The same was done for reactor No. 3, where 5 tons were added, bring the total to 12 tons per hour. TEPCO also added that the volume of cooling water into the No. 1 reactor would be increased as necessary.JAIF (16 September 2011) Earthquake report 207: TEPCO injects more water in two reactors

On 18 April 2011, remote control robot was used to enter the Reactor Building and performed a series of inspections.

On 19 April 2011, TEPCO began transferring excess radioactive cooling water from the reactor's basement and maintenance tunnels to a waste processing facility.NHK, "Transfer of highly contaminated water begins", 19 April 2011.