One of the reactors involved in the Fukushima nuclear accident
The Fukushima Daiichi (Unit 1) 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. Unit 1 was immediately shut down automatically after the earthquake, and emergency generators came online to control electronics and coolant systems. However, the tsunami following the earthquake quickly flooded the low-lying rooms in which the emergency generators were housed. The flooded generators failed, cutting power to the critical pumps that must continuously circulate coolant water through the reactor core. While the government tried pumping fresh water into the core, it was already too late due to overheat. In the hours and days that followed, Unit 1 experienced a full meltdown.
In the intense heat and pressure of the melting Unit 1 reactor, a reaction between the nuclear fuel metal cladding and the remaining water surrounding it produced explosive hydrogen gas. As workers struggled to cool and shut down Unit 1, it exploded the following day (12 March). Eventually the reactor was stabilized by switching from freshwater to seawater which was pumped into the reactor. As a whole, the Japanese government estimated that the total amount of radioactivity released into the atmosphere was approximately one-tenth as much as was released during the Chernobyl disaster. The Japanese government and TEPCO were later criticized in the foreign press for poor communication with the public and improvised cleanup efforts. It's estimated that decommissioning the reactors as a whole will take 30–40 years to complete.
Background.
F. Tanabe has estimated that the core contained the following materials:
Disaster.
Cooling problems and radioactivity release.
On 11 March at 14:46 JST, in response to the earthquake, TEPCO successfully scrammed the reactor in Unit 1, shutting down all power-producing nuclear fission chain reactions. Evacuated workers reported violent shaking and burst pipes within the reactor building. At 15:37, the quake's tsunami inundated the plant and all electrical power to the facility was lost, leaving only emergency batteries. Some of the monitoring and control systems were still operational, though Unit 1's batteries had been damaged by the flood waters. At 15:42, TEPCO declared a "Nuclear Emergency Situation" for Units 1 and 2 because "reactor water coolant injection could not be confirmed for the emergency core cooling systems." The alert was temporarily cleared when water level monitoring was restored for Unit 1 but it was reinstated at 17:07 JST. Potentially radioactive steam was released from the primary circuit into the secondary containment area to reduce mounting pressure in the core.
To cool the reactor, operators resorted to the plant's Emergency core-cooling systems (ECCS), including the isolation condensers, and the High Pressure Coolant Injection systems (HPCI). According to NHK, the isolation condenser system had not been activated in the previous 40 years, and no one present had ever witnessed its operation. It was also later discovered that TEPCO had made changes to the system's original design, without approval or notification of NISA. During the crisis, operators couldn't tell if one of the system's valves was open or closed.
About 10 minutes after the earthquake, TEPCO operators removed both of Unit 1's isolation condensers from service, and instead activated the HPCI system. At 15:07, the core spray system was activated to cool the suppression pool. Both systems lost power after the tsunami struck the plant. The tsunami's arrival prevented operators from restarting the isolation condensers for more than 30 minutes. Afterwards, they were operated intermittently. Despite being designed to cool Unit 1 for at least 8 hours, the condensers' limited operation did not reduce the heat in the core and containment vessel.
By midnight, water levels in the reactor were falling and TEPCO gave warnings of the possibility of radioactive releases. In the early hours of 12 March, TEPCO reported that radiation levels were rising in the turbine building for Unit 1. Operators were considering venting some of the mounting pressure into the atmosphere, which could result in the release of some radioactivity. Later that morning, Chief Cabinet Secretary Yukio Edano stated that the amount of potential radiation would be small and that the prevailing winds were blowing out to sea. Pressure inside Unit 1 continued to rise. At 05:30 JST, it had reached 820 kPa, 2.1 times normal. After isolation cooling had ceased to operate, TEPCO started relieving pressure and injecting water. One employee working inside Unit 1 at this time received a radiation dose of 106 mSv and was later sent to a hospital to have his condition assessed.
Without electricity, needed for water pumps and ventilation fans, rising heat within the containment area led to increasing pressure. In a press release on 12 March at 07:00 JST, TEPCO stated, "Measurement of radioactive material (iodine-131, etc.) by monitoring car indicates increasing value compared to normal level. One of the monitoring posts is also indicating higher than normal level." Dose rates recorded on the main gate rose from 69 n Gy/h (for gamma radiation, equivalent to 69 nSv/h) at 04:00 JST, 12 March, to 866 nGy/h 40 minutes later, before hitting a peak of 0.3855 mSv/h at 10:30 JST. At 13:30 JST, workers detected radioactive caesium-137 and iodine-131 near Reactor 1, a sign that water levels in the coolant system had dropped so low that some of the core's fuel had melted, after being exposed to the air. Cooling water levels had fallen so much that parts of the nuclear fuel rods were exposed and partial melting might have occurred. Radiation levels at the site boundary exceeded the regulatory limits.
Meltdown.
On 12 May, TEPCO engineers confirmed that a meltdown occurred, with molten fuel having fallen to the bottom of the reactor's pressure vessel, or RPV. The utility said that fuel rods of the No. 1 reactor are fully exposed, with the water level 1 meter (3.3 feet) below the base of the fuel assembly. According to a Japanese press report, there are holes in the base of the pressure vessel – these holes were meant for the control-rods. After the fuel had melted, it produced holes in the bottom of the RPV, and then escaped into the containment vessel. In November 2011 TEPCO did not know the shape or porosity of the fuel mass, which is at the bottom of the containment vessel. As a result, it is impossible to know exactly how far the fuel mass would have eroded the concrete floor, but TEPCO estimate that no more than 70 cm of a 7.6 meter concrete slab was eroded away by the hot fuel. The production of heat and steam in unit 1 has decreased, as suggested by both radioactive decay calculations and photographic evidence (same source from TEPCO).
TEPCO estimates the nuclear fuel was exposed to the air less than five hours after the earthquake struck. Fuel rods melted away rapidly as the temperature inside the core reached 2,800 °C within six hours. In less than 16 hours, the reactor core melted and dropped to the bottom of the pressure vessel, burning a hole through the vessel. By that time, water was pumped into the reactor in an effort to prevent the worst-case scenario – overheating fuel melting its way through the containment and discharging large amounts of radionuclides in the environment. In June the Japanese government confirmed that Unit 1 reactor vessel containment was breached, and pumped cooling water continues to leak months after the disaster.
On 11 October 2012, TEPCO released results of the first direct inspections (by remotely operated camera) of conditions in the interior of the Reactor 1 PCV. These suggest that the initial assumptions concerning the behaviour of the fuel mass during the accident may have been incorrect. In particular, the distribution of radiation levels within the PCV, with peak levels being around the bottom head of the RPV, suggest that the majority of the fuel has in fact been retained within the RPV. Radiation levels are also notably lower around the lower parts of the "Drywell", suggesting that fuel had not reached the bottom of the containment vessel, or damaged the concrete floor slab. There is a further issue in that radiation levels within the water inside the containment are markedly higher than those in the reactor basements, suggesting that either there is limited flow from the PCV to the basement, or that substantial dilution is taking place - raising the issue of what is the flow path for the water.
Hydrogen explosion.
At 07:00 JST on 12 March, Prime Minister Naoto Kan asked Daiichi director Masao Yoshida why his workers were not opening the valves to release rising steam pressure within the reactor. Yoshida answered that they couldn't open the electrical valves because of the power failure and the radiation was too high to send workers in to manually do the job. Nevertheless, with the pressure and temperatures continuing to rise, at 09:15, TEPCO sent workers to begin manually opening the valves. The high radiation slowed the work and the valves were not opened until 14:30.
At 15:36 JST on 12 March, there was an explosion in the reactor building at Unit 1 that was primarily directed sideways. The side walls of the upper level were blown away, leaving in place only the vertical steel framed gridworks. Despite the damage, the remaining walls were relatively intact compared to later explosions at Units 3 and 4. The roof of the building was not designed to withstand the high pressure of an explosion, and collapsed filling the reactor with debris. The primary containment consists of "drywell" and "wetwell" concrete structures below the top level, immediately surrounding the reactor pressure vessel. The secondary containment includes the top floor with water-filled pools for storing fresh or irradiated fuel and for storage of irradiated tools and structures.
Experts soon agreed that the cause was a hydrogen explosion. Almost certainly the hydrogen was formed inside the reactor vessel because of falling water levels exposing zircaloy structures/fuel assembly cladding, which then reacted with steam and produced hydrogen, with the hydrogen subsequently vented into the containment building. When the hydrogen reached ignition concentration in the air of the secondary containment building, an ignition source such as a spark triggered a hydrogen-oxygen explosion, blowing out the walls of this building from the inside.
Officials indicated that reactor primary containment had remained intact and that there had been no large leaks of radioactive material, although an increase in radiation levels was confirmed following the explosion. The report of the fact-finding commission states that "There is a possibility that the bottom of the RPV [reactor pressure vessel] was damaged and some of the fuel might have dropped and accumulated on the D/W [dry well] floor (lower pedestal)." The Fukushima prefectural government reported radiation dose rates at the plant reaching 1.015 m Sv/h. The IAEA stated on 13 March that four workers had been injured by the explosion at the Unit 1 reactor, and that three injuries were reported in other incidents at the site. They also reported one worker was exposed to higher-than-normal radiation levels but the level fell below their guidance for emergency situations.
Seawater used for cooling.
At 20:05 JST on 12 March, the Japanese government ordered seawater to be injected into Unit 1 in a new effort to cool the reactor core. The treatment had been held as a last resort since it ruins the reactor. TEPCO started seawater cooling at 20:20, adding boric acid as a neutron absorber to prevent a criticality accident. The water would take five to ten hours to fill the reactor core, after which the reactor would cool down in around ten days. The injection of seawater into the reactor pressure vessel was done by fire department trucks. 
At 01:10 JST on 14 March, injection of seawater was halted for two hours because all available water in the plant pools had run out (similarly, feed to Unit 3 was halted). NISA news reports stated 70% of the fuel rods had been damaged when uncovered. On 21 March, injection of seawater continued, as did repairs to the control instrumentation. On 23 March, it became possible to inject water into the reactor using the feed water system rather than the fire trucks, raising the flow rate from 2