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The Fukushima Daiichi nuclear disaster (福島第一原子力発電所事故) is a series of fires, equipment failures and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the 9.0 magnitude Tōhoku earthquake and tsunami on 11 March 2011. The plant comprises six separate boiling water reactors maintained by the Tokyo Electric Power Company (TEPCO). This accident is the largest of the 2011 Japanese nuclear accidents arising from the Tōhoku earthquake and tsunami, and experts consider it to be the second largest nuclear accident after the Chernobyl disaster, but more complex as multiple reactors are involved.

At the time of the quake, reactor 4 had been de-fueled while 5 and 6 were in cold shutdown for planned maintenance. The remaining reactors shut down automatically after the earthquake, with emergency generators starting up to run the control electronics and water pumps needed to cool reactors. The plant was protected by a seawall designed to withstand a 5.7 m (19 ft) tsunami but not the 14 m (46 ft) maximum wave which arrived 41–60 minutes after the earthquake. The entire plant was flooded, including low-lying generators and electrical switchgear in reactor basements and external pumps for supplying cooling seawater. The connection to the electrical grid was broken. All power for cooling was lost and reactors started to overheat, owing to natural decay of the fission products created before shutdown. The flooding and earthquake damage hindered external assistance.

Evidence soon arose of partial core meltdown in reactors 1, 2, and 3; hydrogen explosions destroyed the upper cladding of the buildings housing reactors 1, 3, and 4; an explosion damaged the containment inside reactor 2; multiple fires broke out at reactor 4. Despite being initially shutdown, reactors 5 and 6 began to overheat. Fuel rods stored in pools in each reactor building began to overheat as water levels in the pools dropped. Fears of radiation leaks led to a 20 km (12 mi) radius evacuation around the plant while workers suffered radiation exposure and were temporarily evacuated at various times. One generator at unit 6 was restarted on 17 March allowing some cooling at units 5 and 6 which were least damaged. Grid power was restored to parts of the plant on 20 March, but machinery for reactors 1 through 4, damaged by floods, fires and explosions, remained inoperable. Flooding with radioactive water continues to prevent access to basement areas where repairs are needed. However, on 5 May, workers were able to enter reactor buildings for the first time since the accident.

Measurements taken by the Japanese science ministry and education ministry in areas of northern Japan 30–50 km from the plant showed radioactive caesium levels high enough to cause concern. Food grown in the area was banned from sale. Based on worldwide measurements of iodine-131 and caesium-137, it was suggested that the releases of those isotopes from Fukushima are of the same order of magnitude as those from Chernobyl in 1986; Tokyo officials temporarily recommended that tap water should not be used to prepare food for infants. Plutonium contamination has been detected in the soil at two sites in the plant. Two workers hospitalized as a precaution on 25 March had been exposed to between 2000 and 6000 mSv of radiation at their ankles when standing in water in unit 3. Radiation levels varied widely over time and location, from well below 1 mSv/h to slightly over 1000 mSv/h. Normal background radiation varies from place to place but delivers a dose equivalent rate of about 0.3 µSv/h. For comparison, one chest x-ray is about 0.02 mSv and an abdominal CT scan is supposed to be less than 10 mSv.

Japanese officials initially assessed the accident as level 4 on the International Nuclear Event Scale (INES) despite the views of other international agencies that it should be higher. The level was successively raised to 5 and eventually to 7, the maximum scale value. The Japanese government and TEPCO have been criticized for poor communication with the public and improvised cleanup efforts. Experts have said that a workforce in the hundreds or even thousands would take years or decades to clean up the area. On 20 March, the Chief Cabinet Secretary Yukio Edano announced that the plant would be decommissioned once the crisis was over.

Fukushima I Nuclear Power Plant


The Fukushima I Nuclear Power Plant consists of six light water, boiling water reactors (BWR) designed by General Electric driving electrical generators with a combined power of 4.7 gigawatts, making Fukushima I one of the 25 largest nuclear power stations in the world. Fukushima I was the first nuclear plant to be constructed and run entirely by the Tokyo Electric Power Company (TEPCO).

Unit 1 is a 439 MWe type (BWR3) reactor constructed in July 1967. It commenced commercial electrical production on 26 March 1971. It was designed for a peak ground acceleration of 0.18 g (1.74 m/s2) and a response spectrum based on the 1952 Kern County earthquake. Units 2 and 3 are both 784 MWe type BWR-4 reactors, unit 2 commenced operating in July 1974 and unit 3 in March 1976. The design basis for all units ranged from 0.42 g (4.12 m/s2) to 0.46 g (4.52 m/s2). All units were inspected after the 1978 Miyagi earthquake when the ground acceleration was 0.125 g (1.22 m/s2) for 30 seconds, but no damage to the critical parts of the reactor was discovered.

Units 1–5 have a Mark 1 type (light bulb torus) containment structure, unit 6 has Mark 2 type (over/under) containment structure. From September 2010, unit 3 has been fueled by mixed-oxide (MOX) fuel.

At the time of the accident, the units and central storage facility contained the following numbers of fuel assemblies:
 * {| class="wikitable" border="1" style="text-align: center;"

! Location ! Unit 1 !! Unit 2 !! Unit 3 !! Unit 4 !! Unit 5 !! Unit 6 !! Central Storage ! Reactor Fuel Assemblies ! Spent Fuel Assemblies ! New Fuel Assemblies
 * 400 || 548 || 548 || 0 || 548 || 764 || 0
 * 292 || 587 || 514 || 1331 || 946 || 876 || 6375
 * 100 || 28 || 52 || 204 || 48 || 64 || N/A
 * }

Cooling requirements

 * See also: Decay heat – Power reactors in shutdown and Nuclear safety systems

A nuclear reactor generates heat by splitting atoms, typically uranium, in a chain reaction. The reactor continues to generate heat after the chain reaction is stopped because of the radioactive decay of unstable isotopes, fission products, created by this process. This decay of unstable isotopes, and the decay heat, cannot be stopped. Immediately after shutdown, this decay heat amounts to approximately 6% of full thermal heat production of the reactor. The decay heat in the reactor core decreases over several days before reaching cold shutdown levels. Nuclear fuel rods that have reached cold shutdown temperatures typically require another several years of water cooling in a spent fuel pool before decay heat production reduces to the point that they can be safely transferred to dry storage casks.

In order to safely remove this decay heat, reactor operators must continue to circulate cooling water over fuel rods in the reactor core and spent fuel pond. In the reactor core, circulation is accomplished by use of high pressure systems that pump water through the reactor pressure vessel and into heat exchangers. These systems transfer heat to a secondary heat exchanger via the essential service water system, taking away the heat which is pumped out to the sea or site cooling towers.

To circulate cooling water when the reactor is shut down and not producing electricity, cooling pumps can be powered by other units on-site, by other units off-site through the grid, or by diesel generators. In addition, boiling water reactors have steam-turbine driven emergency core cooling systems that can be directly operated by steam still being produced after a reactor shutdown, which can inject water directly into the reactor. Steam turbines results in less dependence on emergency generators, but steam turbines only operate so long as the reactor is producing steam. Some electrical power, provided by batteries, is needed to operate the valves and monitoring systems.

If the water in the unit 4 spent fuel pool had been heated to boiling temperature, the decay heat has the capacity to boil off about 70 tonnes of water per day (12 gallons per minute), which puts the requirement for cooling water in context. On 16 April 2011, TEPCO declared that reactors 1–4's cooling systems were beyond repair and would have to be replaced.

Earthquake and tsunami
The 9.0 MW Tōhoku earthquake occurred at 14:46 JST on Friday, 11 March 2011 with epicenter near the island of Honshu. It resulted in maximum ground accelerations of 0.56, 0.52, 0.56 g (5.50, 5.07 and 5.48 m/s2) at units 2, 3 and 5 respectively, above their designed tolerances of 0.45, 0.45 and 0.46 g (4.38, 4.41 and 4.52 m/s2), but values within the design tolerances at units 1, 4 and 6. The Fukushima I facility had not initially been designed for a tsunami of the size that struck the plant, nor had the reactors been modified when later concerns were raised in Japan and by the IAEA. When the earthquake occurred, reactor units 1, 2, and 3 were operating, but units 4, 5, and 6 had already been shut down for periodic inspection. Units 1, 2 and 3 underwent an automatic shutdown (called SCRAM) when the earthquake struck.

When the reactors shut down, the plant stopped generating electricity, stopping the normal source of power for the plant. TEPCO reported that one of the two connections to off-site power for reactors 1–3 also failed so 13 on-site emergency diesel generators began powering the plant's cooling and control systems There are two emergency diesel generators for units 1 to 5 and three for unit 6).

The earthquake was followed by a 15 m (49 ft) tsunami arriving 41 minutes later which topped the plant's 5.7 m (19 ft) seawall,  flooding the basement of the Turbine Buildings and disabling the emergency diesel generators located there at approximately 15:41. At this point, TEPCO notified authorities, as required by law, of a "First Level Emergency." The Fukushima II plant, which was also struck by the tsunami, incorporated design changes which improved its resistance to flooding, and survived significantly better. Generators were located in the watertight reactor building rather than the turbine building which flooded. Seawater pumps for cooling were given protection from flooding, and although 3 of 4 failed in the tsunami, they were able to be restored to operation.

After the diesel generators failed, emergency power for control systems was supplied by batteries that were designed to last about eight hours. Further batteries and mobile generators were dispatched to the site, delayed by poor road conditions with the first not arriving until 21:00 JST 11 March, almost six hours after the tsunami struck.

Work to connect portable generating equipment to power water pumps was still continuing as of 15:04 JST on 12 March, because the normal connection point in a basement was flooded and because of difficulties finding suitable cables. TEPCO switched its efforts to provide power to the facility to re-stringing high voltage cables to the grid. One plant generator at unit 6 was restorted to operation on 17 March, and external power returned to units 5 and 6, on 20 March, allowing cooling equipment to be restarted.

Cooling problems and first radioactivity release


On 11 March at 14:46 JST, unit 1 scrammed successfully in response to the earthquake though evacuated workers reported violent shaking and burst pipes within the reactor building. All generated electrical power was lost following the tsunami leaving only emergency batteries, able to run some of the monitoring and control systems. 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.

After the loss of site power, unit 1 initially continued cooling using the isolation condenser system; 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 and that it was considering venting some of the mounting pressure into the atmosphere, which could result in the release of some radioactivity. Chief Cabinet Secretary Yukio Edano stated later in the morning the amount of potential radiation would be small and that the prevailing winds were blowing out to sea. At 02:00 JST, the pressure inside the reactor containment was reported to be 600 kPa (6 bar or 87 psi), 200 kPa higher than under normal conditions. At 05:30 JST, the pressure inside reactor 1 was reported to be 2.1 times the "design capacity", 820 kPa. Isolation cooling ceased to operate between midnight and 11:00 JST 12 March, at which point 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.

Rising heat within the containment area led to increasing pressure. Electricity was needed for both the cooling water pumps and ventilation fans used to drive gases through heat exchangers within the containment. Releasing gases from the reactor is necessary if pressure becomes too high and has the benefit of cooling the reactor as water boils off but this also means cooling water is being lost and must be replaced. If there was no damage to the fuel elements, water inside the reactor should be only slightly radioactive.

In a press release at 07:00 JST 12 March, TEPCO stated, "Measurement of radioactive material (iodine, 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 nGy/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, which indicated some of the core's fuel had been damaged. 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.

On 14 March, radiation levels had continued to increase on the premises, measuring at 02:20 an intensity of 0.751 mSv/h on one location and at 02:40 an intensity of 0.650 mSv/h at another location on the premises. On 16 March, the maximum readings peaked at 10.850 mSv/h.

Explosion
At 15:36 JST on 12 March, there was an explosion in the reactor building at unit 1. The side walls of the upper level were blown away, leaving in place only the vertical steel framed gridworks. The roof collapsed covering the floor and some machinery on the south side. The walls were relatively intact compared to later explosions at units 3 and 4. A video of the explosion shows that it was primarily directed sideways.

The roof of the building was designed to provide ordinary weather protection for the areas inside, not to withstand the high pressure of an explosion or to act as containment for the reactor. In the Fukushima I reactors the primary containment consists of "drywell" and "wetwell" concrete structures below the top level, immediately surrounding the reactor pressure vessel. The top floor has water filled pools for storing fuel either ready to be craned into the reactor or used fuel which is left to cool before being transferred elsewhere.

Experts soon agreed the cause was a hydrogen explosion. Almost certainly the hydrogen was formed inside the reactor vessel because of falling water levels, and this hydrogen then leaked into the containment building. Exposed zircaloy clad fuel rods became very hot and reacted with steam, oxidising the alloy, and releasing hydrogen. Safety devices normally burn the venting hydrogen before it reaches explosive concentrations. These systems failed, possibly owing to the shortage of electrical power.

Officials indicated reactor containment had remained intact and there had been no large leaks of radioactive material, although an increase in radiation levels was confirmed following the explosion. The Fukushima prefectural government reported radiation dose rates at the plant reaching 1.015 mSv/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 takes 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 performed by means of mobile fire trucks of the fire department. At 01:10 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 18 March, a new electrical distribution panel was installed in an office adjacent to unit 1 to supply power via unit 2 when it was reconnected to the transmission grid two days later. 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 to 18 m3/h (later reduced to 11m3/h), and on 24 March, lighting was restored to the central operating room.

As of 24 March, the spent fuel pool was "thought to be fully or partially exposed", according to CNN. Pressure in the reactor increased owing to the seawater injection, resulting in steam discharges, later alleviated by reducing the water flow. Temperature increases were also reportedly temporary. TEPCO attributed some of the steam to water in the spent fuel pool.

It was estimated that as much as 26 tonnes of sea salt may have accumulated in reactor unit 1, and twice that amount in units 2 and 3. As salt clogs cooling pipes and erodes zirconium oxide layer of the fuel rods, it has became a top priority to switch into freshwater cooling.

Reactor stabilization
On 25 March, fresh water became available again to be added to the reactor instead of salt water, while work continued to repair the unit's cooling systems. A volume of 1890 m3 (500,000 USgal) of fresh water was brought to the plant by a barge provided by US Navy. On 29 March, the fire trucks which had been used to inject water into the reactor were replaced by electrically-driven pumps.

On 28 March, pumping began to remove water contaminated with radioactive 137Cs and 131I from basement areas, storing it in the condenser system. By 29 March, pumping was halted because condensate reservoirs were almost full and plans were being considered to transfer water to the suppression pool surge tanks.

On 7 April, TEPCO began injecting nitrogen into the containment vessel, which was expected to reduce the likelihood of further hydrogen explosions, The injection was expected to take 6 days. If the injection was successful, TEPCO expected to repeat the operation on the other units at Fukushima. Later on 7 April, but prior to a large aftershock, temperatures in the reactor core unexpectedly "surged in temperature to 260 °C," the cause was unknown, but the temperature dropped to 246 °C by 8 April. On 27 April, TEPCO revised its estimate of damaged fuel in unit 1 from 70% to 55%.

On 23 and 26 April, concerns that unit 1 fuel rods may be exposed to air caused TEPCO to consider filling the "containment vessel with water to cool the reactor" despite concerns for building integrity. However, efforts were slowed by unit 1 radiation measurements "as high as 1,120 mSv/hr." On 13 May, TEPCO announced it would proceed with a plan to fill the containment vessel despite the possibility of holes caused by melting fuel elements existing in the pressure vessel. TEPCO had expected to increase the amount of water pumped to unit 1 to compensate for any leakage from the holes, but decided on 15 May to abandon the plan after finding the unit 1 basement was already half flooded.

Possibility of criticality
Reports of 13 observations of neutron beams 1.5 km "southwest of the plant's No. 1 and 2 reactors" from 13 March to 16 March raised the possibility that nuclear fission could have occurred after the initial SCRAMing of the reactors at Fukushima Daiichi. 16 March reports that fuel rods in the spent fuel pool at unit 4 could have been exposed to air appeared to indicate that fission may have occurred in that fuel pool. Later reports of exceptionally high iodine-134 levels appeared to confirm this theory because very high levels of iodine-134 would be indicative of fission reactions. The same report also showed high measurements of chlorine-38, which some nuclear experts used to calculate that fission must be occurring in unit 1. Despite TEPCO suggesting the iodine-134 report was inaccurate, the IAEA appeared to accept the chlorine-based analysis as a valid theory suggesting fission when it stated at a press conference that "melted fuel in the No. 1 reactor building may be causing isolated, uncontrolled nuclear chain reactions". However, TEPCO confirmed its concern about the accuracy of the high iodine and chlorine report by formally retracting the report on 21 April, which eliminated both the exceptionally high iodine-134 and chlorine-38 levels as proof of recriticality. TEPCO did not appear to comment on the recriticality concern when withdrawing its report, but the IAEA has not withdrawn its comments, and some off-site experts find the currently-measured iodine-134 levels higher than expected.

Meltdown confirmed
On 12 May, TEPCO engineers confirmed that a meltdown occurred, with molten fuel having fallen to the bottom of the reactor's containment vessel. 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, and most of the fuel has probably melted. The nuclear fuel has possibly leaked into the containment vessel, which was damaged in an explosion during the crisis. This caused both the Japanese government and TEPCO to be criticized for consistently underestimating the severity of the situation. The operator found the basement flooded with 4.2 meters of water. Workers were unable to observe the flooding situation due to high levels of radiation from the water. TEPCO estimated 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 degrees Celsius 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.

Reactor unit 2
Unit two was operating at the time of the earthquake and experienced the same controlled initial shutdown as the other units. The diesel generators and other systems failed when the tsunami overran the plant. The reactor core isolation cooling (RCIC) system initially operated to cool the core, 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. The RCIC was reported by TEPCO to have shut down around 19:00 JST on 12 March, but reported to be operating again as of 09:00 JST 13 March. The pressure reduction of the reactor containment vessel commenced before midnight on 12 March although the IAEA reported that as of 13:15 JST 14 March, that according to information supplied to them, no venting had taken place at the plant. A report in The New York Times 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.

Cooling problems
On 14 March, TEPCO reported the failure of the RCIC system. 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.

At 22:29 JST, workers had 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 hoped that holes blown in the walls of reactor building 2 by the earlier blast from unit 3 would allow the escape of hydrogen vented from the reactor and prevent a similar explosion. At 21:37 JST, the measured dose rates at the gate of the plant reached a maximum of 3.13 mSv/h, which was enough to reach the annual limit for non-nuclear workers in twenty minutes, but had fallen back to 0.326 mSv/h by 22:35.

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 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. As of 04:11 JST on 15 March, water was being pumped into the reactor of unit 2 again.

Explosion
An explosion was heard after 06:14 JST on 15 March in unit 2, possibly damaging the pressure-suppression system, which is at the bottom part of the containment vessel. The radiation level was reported to exceed 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. 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%. The Japan's Nuclear and Industrial Safety Agency (NISA) stated 33% of the fuel rods were damaged, in news reports the morning of 16 March. On 30 March, NISA reiterated concerns about a possible unit 2 breach at either the suppression pool, or the reactor vessel. NHK World reported the NISA's concerns as "air may be leaking," very probably through "weakened valves, pipes and openings under the reactors where the control rods are inserted", but that "there is no indication of large cracks or holes in the reactor vessels."

Continuing radiation
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 20 March, 40 tons of seawater were added to the spent fuel pool. The temperature in the spent fuel pool was 53 °C as of 22 March 11:00 JST. The Japan Atomic Industrial Forum (JAIF) reported "high radiation readings" in the area.

Unit 2 was considered the most likely unit to have a damaged reactor containment vessel, as of 24 March. On 27 March, TEPCO reported measurements of very high radiation levels, over 1000 mSv/h, in the basement of the unit 2 turbine building, which officials reported was 10 million times higher than what would be found in the water of a normally functioning reactor. Hours into the media frenzy, the company retracted its report and stated that the figures were not credible. "because the level was so high the worker taking the reading had to evacuate before confirming it with a second reading." Shortly following the ensuing wave of media retractions that discredited the report worldwide, TEPCO clarified its initial retraction; the radiation from the pool surface in the basement of the unit 2 turbine building was found to be "more than 1,000 millisieverts per hour," as originally reported, but the concentration of radioactive substances was 100,000 times higher than usual, not 10 million. On 28 March, the Nuclear Safety Commission announced its suspicion that radioactive materials had leaked from unit 2 into water in trenches connecting unit 2's buildings, leading TEPCO to reduce the amount of water pumped into the reactor because of fears that the water could leak into the sea. The reduction in water pumping could have raised reactor temperatures.

On 27 March, the IAEA reported temperatures at the bottom of the Reactor Pressure Vessel (RPV) at unit 2 fell to 97 °C from 100 °C on Saturday. Operators attempted to pump water from the turbine hall basement to the condenser. However, "both condensers turned out to be full." Therefore, condenser water was first attempted to be pumped to storage tanks, freeing condenser storage for water currently in the basement of unit 2. The pumps now being used can move 10 to 25 tons of water per hour. On 19 April 2011, TEPCO began transferring excess, radioactive cooling water from the reactor's basement and maintenance tunnels to a waste processing facility.

On 29 March, Richard Lahey, former head of safety research for boiling-water reactors at General Electric, speculated that the reactor core could have possibly melted through the reactor containment vessel onto a concrete floor, raising concerns of a major release of radioactive material. On 27 April, TEPCO revised its estimate of damaged fuel in unit 2 from 30% to 35%.

Pressure vessel damage
On May 15, TEPCO revealed that the pressure vessel that holds nuclear fuel "is likely to be damaged and leaking water at units Nos. 2 and 3", which means most of the thousands of tons of water pumped into the reactors was leaked.

Reactor unit 3
Unlike the other five reactor units, reactor 3 runs on mixed uranium and plutonium oxide, or MOX fuel, making it potentially more dangerous in an incident owing to the neutronic effects of plutonium on the reactor, the long persistence of plutonium toxicity, and the plutonium carcinogenic effects in the event of release to the environment. Units 3 and 4 have a shared control room.

Cooling problems
Early on 13 March an official of the Japan Nuclear and Industrial Safety Agency (NISA) told at a news conference that the emergency cooling system of unit 3 had failed, spurring an urgent search for a means to supply cooling water to the reactor vessel in order to prevent a meltdown of its reactor core. At 05:38 there was no means of adding coolant to the reactor, owing to loss of power. Work to restore power and to vent excessive pressure continued. At one point, the top three meters of mixed oxide (MOX) fuel rods were not covered by coolant.

At 07:30 JST, TEPCO prepared to release radioactive steam, indicating that "the amount of radiation to be released would be small and not of a level that would affect human health" and manual venting took place at 08:41 and 09:20. At 09:25 JST on 13 March, operators began injecting water containing boric acid into the primary containment vessel (PCV) via the pump of a fire truck. When water levels continued to fall and pressure to rise, the injected water was switched to seawater at 13:12. By 15:00 it was noted that despite adding water the level in the reactor did not rise and radiation had increased. A rise was eventually recorded but the level stuck at 2 m below the top of reactor core. Other readings suggested that this could not be the case and the gauge was malfunctioning.

Injection of seawater into the primary containment vessel (PCV) was discontinued at 01:10 on 14 March because all the water in the reserve pool had been used up. Supplies were restored by 03:20 and injection of water resumed. On the morning of 15 March, Secretary Edano announced that according to TEPCO, at one location near reactor units 3 and 4, radiation at an equivalent dose rate of 400 mSv/h was detected. This might have been due to debris from the explosion in unit 4.

Explosion
At 12:33 JST on 13 March, the chief spokesman of the Japanese government, Yukio Edano said hydrogen gas was building up inside the outer building of unit 3 just as had occurred in unit 1, threatening the same kind of explosion. At 11:15 JST on 14 March, the envisaged explosion of the building surrounding reactor 3 of Fukushima 1 occurred, owing to the ignition of built up hydrogen gas. The Nuclear and Industrial Safety Agency of Japan (NISA) reported, as with unit 1, the top section of the reactor building was blown apart, but the inner containment vessel was not breached. The explosion was larger than that in unit 1 and felt 40 kilometers away. Pressure readings within the reactor remained steady at around 380 kPa at 11:13 and 360 kPa at 11:55 compared to nominal levels of 400 kPa and a maximum recorded of 840 kPa. Water injection continued. Dose rates of 0.05 mSv/h were recorded in the service hall and of 0.02 mSv/h at the plant entrance. It was reported that day that eleven people were injured in the blast. The Telegraph reported that six soldiers from the Japanese Central Nuclear Biological Chemical Weapon Defence Unit had been killed in the explosion. , but only injuries were reported in Japan. TEPCO and NISA announced that four TEPCO employees, three subcontractor employees, and four Self-Defence-Force soldiers were injured. American nuclear engineer Arnold Gundersen, noting the much greater power and vertical debris ejection compared to the unit 1 hydrogen blast, has theorized that the unit 3 explosion involved a prompt criticality in the spent fuel pool material, triggered by the mechanical disruption of an initial, smaller hydrogen gas explosion in the building.

Spent fuel pool
Around 10:00 JST on 16 March, NHK helicopters flying 30 km away videotaped white fumes rising from the Fukushima I facility. Officials suggested that the reactor 3 building was the most likely source, and said that its containment systems may have been breached. The control room for reactors 3 and 4 was evacuated at 10:45 JST but staff were cleared to return and resume water injection into the reactor at 11:30 JST. At 16:12 JST, Self Defence Force (SDF) Chinook helicopters were preparing to pour water on unit 3, where white fumes rising from the building was believed to be water boiling away from the fuel rod cooling pond on the top floor of the reactor building, and on unit 4 where the cooling pool was also short of water. The mission was cancelled when helicopter measurements reported radiation levels of 50 mSv. At 21:06 pm JST, the government reported that major damage to reactor 3 was unlikely but that it nonetheless remained their highest priority.

Early on 17 March, TEPCO requested another attempt by the military to put water on the reactor using a helicopter and four helicopter drops of seawater took place around 10:00 JST. The riot police used a water cannon to spray water onto the top of the reactor building and then were replaced by members of the SDF with spray vehicles. On 18 March a crew of firemen took over the task with six fire engines each spraying 6 tons of water in 40 minutes. 30 further hyper rescue vehicles were involved in spraying operations. Spraying continued each day to 23 March because of concerns the explosion in unit 3 may have damaged the pool (total 3,742 tonnes of water sprayed up to 22 March) with changing crews to minimise radiation exposure. Lighting in the control room was restored on 22 March after a connection was made to a new grid power supply and by 24 March it was possible to add 35 tonnes of seawater to the spent fuel pool using the cooling and purification system. Grey smoke was reported to be rising from the southeast corner of unit 3 on 21 March. The spent fuel pool is located at this part of the building. Workers were evacuated from the area. TEPCO claimed no significant change in radiation levels and the smoke subsided later the same day.

On 23 March, black smoke billowed from unit 3, prompting another evacuation of workers from the plant, though Tokyo Electric Power Co. officials said there had been no corresponding spike in radiation at the plant. "We don't know the reason for the smoke", Hidehiko Nishiyama of the Nuclear Safety Agency said. On 27 April, TEPCO revised its estimate of damaged fuel in unit 3 from 25% to 30%. Radiation measurements of the water in the unit 3 spent fuel pool were reported at 140 kBq of radioactive cesium-134 per cubic centimeter, 150 kBq of cesium-137 per cubic centimeter, and 11 kBq per cubic centimeter of iodine-131 on 10 May.

Possibility of criticality in the spent fuel pool
TEPCO claimed that there was a small but non-zero probability that the exposed fuel assemblies could reach criticality. The BBC commented that criticality would never mean a nuclear explosion, but could cause a sustained release of radioactive materials. Criticality is usually considered highly unlikely, owing to the low enrichment level used in light water reactors. There was, however, speculation on Russia Today by low-dose radiation researcher and anti-nuclear activist Christopher Busby that the explosion that destroyed the reactor 3 building was a "nuclear explosion" of some kind in the spent fuel pool. Similarly, as noted above, Arnie Gundersen surmised a prompt criticality for the 13 Mar 2011 explosion at the spent fuel pool located on top of the reactor 3 building.

On 11 May, TEPCO released underwater robotic video from the spent fuel pool. The video appears to show large amounts of debris contaminating the pool. Based on water samples, unnamed experts and TEPCO opined that the fuel rods were left "largely undamaged".

Nuclear core damaged
On 25 March, officials announced the reactor vessel might be breached and leaking radioactive material. High radiation levels from contaminated water prevented work. Japan Nuclear and Industrial Safety Agency (NISA) reiterated concerns about a unit 3 breach on 30 March. NHK World reported the NISA's concerns as "air may be leaking," very probably through "weakened valves, pipes and openings under the reactors where the control rods are inserted," but that "there is no indication of large cracks or holes in the reactor vessels." As with the other reactors, water was transferred from condenser reservoirs to the suppression pool surge tanks so that condensers could be used to hold radioactive water pumped from the basement. On May 15, TEPCO revealed that the pressure vessel that holds nuclear fuel "is likely to be damaged and leaking water at units Nos. 2 and 3", which means most of the thousands of tons of water pumped into the reactors was leaked.

Reactor unit 4
At the time of the earthquake unit 4 had been shut down for shroud replacement and refueling since 29 November 2010. All 548 fuel rods 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 now contains 1,479 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, but as of 18 March, no further information was reported. On 11 April, a fire broke out at unit 4.

Explosion
At approximately 06:00 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. Later reports from the US Nuclear Regulatory Commission speculated that fuel could have been ejected from the unit 4 spent fuel pond during this explosion. Later on the morning of 15 March, at 09:40, the unit 4 spent fuel pool caught fire, likely releasing radioactive contamination from the fuel stored there. TEPCO said workers extinguished the fire by 12:00. 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 17:48 it was reported that water in the spent fuel pool might be boiling. By 21:13 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 22:30, TEPCO was reportedly unable to pour water into the spent fuel pool. By 22:50, 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 05:45 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 06:15 no fire was found. Other reports stated that the fire was under control. At 11:57, 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.

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 13:00 TEPCO claimed 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.

At approximately 14:30 on 16 March, TEPCO announced that the storage pool, located outside the unit 4 containment area, might be boiling. Around 20:00 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 21:05 UTC 24 March to 01:20 25 March. White smoke was still being observed coming from the reactor building as of 23:00 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." 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 SFP covered in water. 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." 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. 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.

Possibility of criticality in the spent fuel pool
Visual inspection of the spent fuel rod pool on reactor 4 on April 30 has shown that that there is no significant visible damage to the fuel rods in the pool. This observation is inconsistent with speculation of prompt criticality.

Reactor units 5 and 6


Both reactors were offline at the time the earthquake struck (reactor 5 had been shut down on 3 January 2011 and reactor 6 on 14 August 2010), although they were still fueled, unlike reactor 4 where the fuel rods had been removed prior to the earthquake.

Government spokesman Edano stated on 15 March that reactors 5 and 6 were being closely monitored, as cooling processes were not functioning well. At 09:16 JST, the removal of roof panels from reactor buildings 5 and 6 was being considered in order to allow any hydrogen build-up to escape. At 21:00 on 15 March, water levels in unit 5 were reported to be 2 m above fuel rods, but had fallen 40 cm in 5 hours. Published water temperatures on 18 March showed 182 °C inside reactor 5 and 161 °C in reactor 6.

On 17 March, unit 6 was reported to have operational diesel-generated power and this was to be used to power pumps in unit 5 to run the Make-up Water Condensate System (MUWC) to supply more water. Preparations were made to inject water into the reactor pressure vessel once external power could be restored to the plant, as water levels in the reactors were considered to be declining. NISA reported that connections from the grid to all units was complete 20 March through new cables and transformers.

Information provided to the IAEA indicated that storage pool temperatures at both units 5 and 6 remained steady around 60–68 °C between 19:00 JST 14 March and 21:00 JST 18 March, though rising slowly. On 18 March reactor water levels remained around 2 m above the top of fuel rods. It was confirmed that panels had been removed from the roofs of units 5 and 6 to allow any hydrogen gas to escape. At 04:22 on 19 March, the second unit of emergency generator A for unit 6 was restarted which allowed operation of pump C of the residual heat removal system (RHR) in unit 5 to cool the spent fuel storage pool. Later in the day, pump B in unit 6 was also restarted to allow cooling of the spent fuel pool there. Temperature at unit 5 pool decreased to 48 °C on 19 March 18:00 JST, and 37 °C on 20 March when unit 6 pool temperature had fallen to 41 °C. On 20 March, NISA announced that both reactors had been returned to a condition of cold shutdown.

On 23 March, it was reported that the cooling pump at reactor No 5 stopped working when it was transferred from backup power to the grid supply. This was repaired and the cooling restarted approximately 24 hours later. RHR cooling in unit 6 was switched to the permanent power supply on 25 March.

Radiation levels and radioactive contamination


Radioactive material has been released from the Fukushima containment vessels as the result of deliberate venting to reduce gaseous pressure, deliberate discharge of coolant water into the sea, and accidental or uncontrolled events. Junichi Matsumoto, acting head of TEPCO's Nuclear Power & Plant Siting Division, acknowledged the seriousness of the Fukushima accident at a [12 April] press conference stating, "although the details of the [Chernobyl and Fukushima] accidents are different, from the standpoint of how much radiation has been released, [Fukushima] is equal to or more serious than Chernobyl."

Using Japanese Nuclear Safety Commission numbers, Asahi Shimbun reported that by 24 March the accident might have emitted 30,000 to 110,000 TBq of iodine-131. The highest reported radiation dose rate outside was 1000 mSv/h on 16 March. On 29 March, at times near unit 2, radiation monitoring was hampered by a belief that some radiation levels may be higher than 1000 mSv/h, but that "1,000 millisieverts is the upper limit of their measuring devices." The maximum permissible dose for Japanese nuclear workers was increased to 250 mSv/year, for emergency situations after the accidents. TEPCO has been criticized in providing insufficient safety equipment for its workers, including accusations of a lack of monitoring and decontamination equipment, and for giving the most dangerous work to subcontractors.

The Japanese Ministry of Health, Labour and Welfare announced that levels of radioactivity exceeding legal limits had been detected in milk produced in the Fukushima area and in certain vegetables in Ibaraki. On 23 March, Tokyo drinking water exceeded the safe level for infants, prompting the government to distribute bottled water to families with infants. Seawater near the discharge of the plant elevated levels of iodine-131 were found on 22 March, which had increased to 3,355 times the legal limit on 29 March. Also concentrations far beyond the legal limit were measured for caesium-134 and caesium-137 were more than 100 times above the limit.

Contamination of basements, wiring trenches, and pipe tunnels
As illustrated in the diagram to the right, the Fukushima I nuclear plant has a number of trenches and pipe tunnels that stretch from each unit's reactor (diagram #1), to the unit's turbine building (diagram #2), to the sea (to the right of diagram #6). In some locations these connections are open trenches, while in other locations the connections are pipe tunnels.

During work to restore power to unit 2 on 27 March, TEPCO reported very high levels of radiation in water in the basement of the unit 2 turbine building. While first reported radiation levels of more than 10 million times usual appeared later to be erroneous, the radiation measurements were more than 100,000 times higher than usual. On 28 March, the Nuclear Safety Commission announced its suspicion that "radioactive substances from temporarily melted fuel rods at the No. 2 reactor had made their way into water in the reactor containment vessel and then leaked out through an unknown route". Highly radioactive water was later found in trenches at three of the units. These trenches stretch toward, but do not directly connect to, the sea (see diagram #6). On 30 March, the units 2 and 3 trenches were 1 m below the level at which they would overflow into the sea. In comparison, the unit 1 trench was 10 cm from overflowing.

The high levels of water in the trenches combined with their potential to overflow to the sea complicated the cooling efforts because the water required to cool the reactor was believed to also be filling the trenches. Hence, cooling unit 2 with large quantities of fresh water was expected to cause the trenches, leading to the sea, to fill and overflow—worsening the radioactivity release. Consequently, TEPCO reduced the amount of water injected into unit 2 from 16 to 7 ton per hour. TEPCO used two approaches to prevent the highly radioactive water from leaking into the sea.

Pumping the water from the basement
The first approach to prevent tunnel water from leaking into the sea was to pump the tunnels dry. Beginning on 27 March, operators attempted to pump water from the turbine hall basement (see the tunnel below diagram #2) to the condenser (the large black vessel). By pumping water out of the basement, TEPCO expected to lower the trench water level, and reduce the likelihood of overspill to the sea. However, "both condensers turned out to be full," which prevented pumping. Therefore, pumps able to shift 10 to 25 tons of water per hour were used to move condenser water to storage tanks, freeing condenser storage for water that was in the basement of unit 2. However, since both the storage tanks and the condensers were nearly full, TEPCO also considered using tankers or a "mega float" as a temporary storage location for the radioactive water. Regardless of the availability of offshore storage for radioactive-contaminated water, TEPCO decided to pump its least contaminated water, approximately 100 times the legal limit, from a wastewater treatment plant, out to sea on 5 April to free storage space. ). At the same time, on 5 April, TEPCO began pumping water from the units condensers of units 1–3 to their respective condensation storage tanks to free room for the trench water.

Plugging the source of the water
The second approach used by TEPCO to limit overflow into the sea was to plug leaks into pits that were connected to the trenches. Eventually, leaks would be discovered in pits in unit 2 (discovered 1 April) and unit 3 (discovered 11 May). While the later found leak in unit 3 was reported to be plugged with one day, the unit 2 pit-leak took much longer to stop.

Discovered on 1 April, the leak in the unit 2 pit was located near the unit 2 reactor basement and above the trench system. The crack in the pit was reported at the time to be the primary source of water to the trench system; however, at that time, the unit 3 leak was unknown. TEPCO reported the unit 2 leak was from a crack 20 cm in size, and that it may have been leaking since the magnitude 9 earthquake shook the plant on 11 March until finally patched on 6 April. However, radiation levels above the pit exceeded 1000 mSv/h (1 Sv/h, 100 Rem/h), hampering technicians to safely work. Regardless, TEPCO attempted to use sandbags and concrete to plug the leak. However, by 2 April, TEPCO acknowledged the water was still leaking into the trenches and to the sea. On 2 April, TEPCO said that it had again attempted to plug the hole, now using 2,000 liters of a synthetic resin. TEPCO attempted to inject a polymeric water absorbent, used for diapers, into pipes leading to the pit; this absorbent was also coupled with sawdust and shredded newspapers. However, on 3 April and 4 April, this approach appeared to have failed to slow the leak, leading TEPCO to use a colored dye to confirm the location and size of the leak. The dye indicated the leak was from a cracked pipe and seeping through gravel into the pit. On 5 April, TEPCO began using liquid glass to attempt to stop the leak. Finally, on 6 April, TEPCO drilled a hole into the pit near unit 2 and injected water glass (sodium silicate) into the pit. The residual heat carried by the water used for cooling the damaged reactors accelerated the setting of the injected mixture. Shortly afterward, TEPCO announced that water had stopped leaking from the pit.

In an attempt to prevent future leaks, TEPCO installed seven steel plates at unit 2 that would prevent water from flowing out the plant's water intakes (see diagram #6). Additional plates were expected to be added at the other Fukushima units. However, these plates were later suspected of "stirring up" radioactive debris, and to have significantly increased radiation measured in the sea. Long term, TEPCO is "also considering pouring adhesive concrete into the suppression chamber of reactor 2 to patch the hole that is believed to be causing radioactive water to leak into the turbine building and the trench." On 21 April, TEPCO estimated that 520 tons radioactive water leaked into the sea before leaks were plugged, releasing 4,700 TBq (20,000 times facility's annual limit). TEPCO did not estimate the amount of water that escaped from the unit 3 leak, but did say the leaked water was contaminated with iodine-131, caesium-137 and caesium-134 far beyond regulatory limits, and that the leak was patched the same day it was discovered.

Ongoing efforts
With the leak plugged, at least temporarily, on 10 April TEPCO returned to the work began on 27 March, removing water from the tunnel system so repairs could be made to the plant's original cooling system. Removal is considered essential because the water is so radioactive, in excess to the 1000 mSv/h measuring equipment's range, that repair work cannot be safely conducted without removing the water

By 13 April, TEPCO had pumped approximately "250 tonnes of highly radioactive water from the trench into the unit's turbine condenser," lowering the trench water-level by 4 cm. The water was approximately 99 cm deep originally. TEPCO estimated that pumping would take "about 40 hours to move some 700 tonnes of water from the trench." Water would eventually have to be removed from the unit 2 basement, as well as from the trenches and basements of units 1 and 3. By 15 April, TEPCO estimated that 660 tons of 60,000 tons of the highly radioactive water had been pumped from the trenches. The water level was believed to have fallen by 8 cm. and TEPCO announced it expected to start storing some of the trench water in storage spaces freed up by dumping 9,100 tons of slightly contaminated water from a wastewater treatment plant from 4 April to 10 April. However, shortly after announcing the reduction in level, the water level began increasing again, a 2.5 cm increase on 16 April, and a 3 cm increase on 17 April were believed to have been caused by the earlier efforts to patch leaks to the ocean. On 19 April, TEPCO estimated that the unit 2 turbine basement contained 25,000 cubic meters of contaminated water, it would later estimate this water contained 400 PBq of radioactivity. Then, on 20 April, TEPCO began pumping the basement water to the wastewater treatment facility. By 27 April, TEPCO had pumped 1.89 million liters of the highly contaminated water to the processing plant, and announced plans to add more pumping capability. While progress was being made on pumping the unit 2 basement, on 14 May, TEPCO announced that it appeared that the unit 1 basement is also "half full" of radioactive water that was expected to delay cleanup efforts. On 15 May, TEPCO announced plans to pump approximately 4,000 tons of 22,000 tons contaminated water from the unit 3 turbine building basement and trench system; the water was 1.4 m high in the basement.

Central fuel storage areas
Used fuel assemblies taken from reactors are initially stored for at least 18 months in the pools adjacent to their reactors. They can then be transferred to the central fuel storage pond. This contains 6375 fuel assemblies and was reported "secured" with a temperature of 55 °C. After further cooling, fuel can be transferred to dry cask storage, which has shown no signs of abnormalities. On 21 March temperatures in the fuel pond had risen a little to 61 °C and water was sprayed over the pool. Power was restored to cooling systems on 24 March and by 28 March temperatures were reported down to 35 °C.

Accident rating


The severity of the nuclear accident is provisionally rated 7 on the International Nuclear Event Scale (INES). This scale runs from 0, indicating an abnormal situation with no safety consequences, to 7, indicating an accident causing widespread contamination with serious health and environmental effects. Prior to Fukushima, the Chernobyl disaster was the only level 7 accident on record, while the Three Mile Island accident was a level 5 accident.

The Japan Atomic Energy Agency initially rated the situation at unit 1 below both of these previous accidents; on 13 March it announced it was classifying the event at level 4, an "accident with local consequences". On 18 March it raised its rating on units 1, 2 and 3 to level 5, an "accident with wider consequences". It classified the situation at unit 4 as a level 3 "serious incident".

Several parties disputed the Japanese classifications, arguing that the situation was more severe than they were admitting at the time. On 14 March, three Russian experts stated that the nuclear accident should be classified at Level 5, perhaps even Level 6. One day later, the French nuclear safety authority ASN said that the Fukushima plant could be classified as a Level 6. , the French nuclear authority—and as of 15 March, the Finnish nuclear safety authority—estimated the accidents at Fukushima to be at Level 6 on the INES. On 24 March, a scientific consultant for noted anti-nuclear environmental group Greenpeace, working with data from the Austrian ZAMG and French IRSN, prepared an analysis in which he rated the total Fukushima accident at INES level 7.



The Asahi Shimbun newspaper reported on 26 March that the accident might warrant level 6, based on its calculations. The Wall Street Journal stated that Japan's NISA would make any decision on raising the level. INES level 6, or "serious accident," had only been applied to the Kyshtym disaster (Soviet Union, 1957), while the only level 7 was Chernobyl (Soviet Union, 1986). Previous level 5 accidents included the Windscale fire (United Kingdom, 1957); the Lucens reactor (Switzerland, 1969); Three Mile Island (United States, 1979); and the Goiânia accident (Brazil, 1987).

Assessing "seriousness" as partial or full meltdown at a civilian plant, The New York Times reported on 3 April that based on remote sensing, computer "simulations suggest that the number of serious accidents has suddenly doubled, with three of the reactors at the Fukushima Daiichi complex in some stage of meltdown." The Times counted three previous civilian meltdowns, from World Nuclear Association information: Three Mile Island; Saint-Laurent Nuclear Power Plant (France, 1980, INES level 4); and Chernobyl.

On 11 April, the Japanese Nuclear and Industrial Safety Agency (NISA) temporarily raised the disaster at Fukushima Daiichi to Level 7 on the INES scale, by considering the whole event and not considering each reactor as an individual event per se (rated between 3 and 5). This would make Fukushima the second Level 7 "major accident" in the history of the nuclear industry; having said that, radiation released as a result of the events at Fukushima was, as of April 12, only approximately 10% of that released as a result of the accident at Chernobyl (1986), also rated as INES Level 7.

Radiation in other countries
The Fukushima accident has led to "trace" amounts of radiation, including iodine-131 and caesium-134/137, being observed around the world (New York State, Alaska, Hawaii, Oregon, California, Montreal, and Austria). A widely cited Austrian Meteorological Service report estimated the total amount of I-131 radiation released as of 19 March based on extrapolating data from several days of ideal observation at a handful of worldwide CTBTO radionuclide measuring facilities (Freiburg, Germany; Stockholm, Sweden; Takasaki, Japan and Sacramento, USA) during the first 10 days of the accident. The report's estimates of total I-131 emissions based on these worldwide measuring stations ranged from 10 PBq to 700 PBq. This estimate was 1% to 40% of the 1760 PBq of I-131 estimated to be release at Chernobyl. This report may not have been updated, but for comparison, a 12 April NISA report estimated the total I-131 release (based upon Japanese measurement equipment) at 130 to 150 PBq total release for the longer period of time. This would be approximately 7% to 9% of the I-131 Chernobyl release. A UC Berkeley professor of nuclear engineering who is measuring radionuclide detected in California, but not estimating the total release, asserted "that the fallout poses no significant health threat." The expert who prepared the Austrian Meteorological Service report asserted that the "Chernobyl accident emitted much more radioactivity and a wider diversity of radioactive elements than Fukushima Daiichi has so far, but it was iodine and caesium that caused most of the health risk – especially outside the immediate area of the Chernobyl plant." As of 28 April, the Washington State Department of Health, one of the U.S. states nearest Japan, reported that levels of radioactive material from the Fukushima plant had dropped significantly, and were often below levels that could be detected with standard tests.

Radiation from direct fallout
The levels detected by air filters in countries outside Japan are extremely low. Health Canada stated that the increase measured in Canada was less than the natural day-to-day variation in the existing background levels; the presence of fallout could only be detected by analysing the isotopes present, and there was no significant increase in the total level of radiation.

Health Canada put the extra radiation in Canada's air due to Fukushima for 18 March at 0.5 nSv, compared to background levels from 20 to 1200 nSv depending on the region.

In the US, monitoring was carried out by government agencies – the EPA, the Department of Energy and the Department of Health – as well as independent university teams. Both found low levels of radiation. The government bodies were criticised for their slower, less detailed release of information; Robert Alvarez, a nuclear policy scholar, noted that "the 'lack of transparency' fueled mistrust.".

Radioactivity in rainwater and food
Radioisotopes can be concentrated by precipitation or by bioaccumulation (where plants/animals, including ultimately humans, selectively take up and concentrate particular elements). The caesium radioisotopes are potentially more dangerous than iodine-131 in the long term, because they have longer half-lives (two years for Cs-134, 30 years for Cs-137) than I-131 (half-life of 8 days), so the risk of persistence in the environment and of long-term accumulation in organisms is greater. Iodine-131 can be concentrated by leafy vegetables and in milk/cheese. CRIIRAD, a French NGO, warned on 7 April that children and pregnant women in Europe should limit consumption of these, in addition to avoiding rainwater as a primary drinking source, as a precautionary measure, although it put the risk as "quite low". CRIIRAD concluded that the risk radioactive particles that remain outside the body or so called "direct fallout" was trivial.

Low levels of caesium radioisotopes were detected in China, and in CA, USA. Detectable levels of radioactive isotopes in milk were present in 6 of the cities tested by the EPA in the USA, with the maximum levels reported in the city of Hilo, HI; the levels were 24 pCi/l (0.89 Bq/l), 19 pCi/l (0.70 Bq/l) and 18 pCi/l (0.670 Bq/l) for caesium-134, caesium-137 and iodine-131 respectively.

A university of Berkeley team observed a peak I-131 level of 540 picocuries per liter (20 Bq/l) in rainwater. This greatly exceeded the EPA's 3 picocurie per liter (0.1 Bq/l) standard for radioactive iodine in drinking water, although that is based on consumption of the water every day for 70 years.

It is important to note that the allowed level set by regulatory agencies can vary; the levels mandated by the FDA for milk are thousands of times higher than those mandated by the EPA for water. This is partly due to different assumptions about how long the product will be consumed, and partly due to different thresholds of risk.

As of 12 April 2011, no serious contamination had been observed of food and water produced outside Japan.

Radioactive debris
A marine oceanographer at the International Pacific Research Center expressed concern that current models are not adequate to predict how contaminated debris swept out of the reactor will behave.

Reaction in Japan and evacuation measures


A nuclear emergency was declared by the Government at 19:03 on 11 March. Initially a 2 km, then 10 km evacuation zone was ordered. Later Prime Minister Naoto Kan issued instructions that people within a 20 km (12 mile) zone around the plant must leave, and urged that those living between 20 km and 30 km from the site to stay indoors. Those in the zone between 20 km and 30 km from the facility were subject to voluntary evacuation. The 20 km evacuation zone was not strictly enforced, and residents were reported to have returned to their homes to recover valuables. In an apparent change in policy, on 21 April, the Japanese government formally announced that the 20 km evacuation zone would be more strictly enforced, and that only one person per residence could return for a maximum of two hours. Then on 22 April, the Japanese government announced that the evacuation zone would be extended from the 20 km "circular" zone to an irregular zone extending northwest of the Fukushima site. On 16 May, the Japanese government began evacuating people from outside the official exclusion zone, including the village of Iitate, where high levels of radiation had been repeatedly measured.

The Prime Minister visited the plant for a briefing on 12 March. He called for calm and against exaggerating the danger. TEPCO established a "base camp" at J-Village, a sport training centre located in Naraha and Hirono, some 20 km South of the plant.

On 30 March, the IAEA announced that 20 MBq/m2 of iodine-131 were found in samples taken from 18 to 26 March in Iitate, Fukushima, 40 km northwest of the Fukushima I reactor. The IAEA recommended expanding the evacuation area, based on its criteria of 10 MBq/m2. Secretary Edano stated the government would wait to see if the high radiation continued. On 31 March, the IAEA announced a new value of 7 MBq/m2, in samples taken from 19 to 29 March in Iitate. The material decays at 8% to 9% each day.

Six weeks after the crisis began, plans were announced for a large-scale study of the environmental and health effects of radioactive contamination from the nuclear plant. Academics and researchers from across Japan will work with the Fukushima Prefectural Government starting in May.

International reaction


The international reaction to the nuclear accidents has been a humanitarian response to the 2011 Tōhoku earthquake and tsunami, also to those people affected by the events at Fukushima I. The response has also included the expression of concern over the developments at the reactors and the risk of escalation. The accidents have furthermore prompted re-evaluation of existing and planned national nuclear energy programs, with some commentators questioning the future of the nuclear renaissance.

USA, Australia and Sweden instructed their citizens to evacuate a radius of minimum 80 km. South Korea advised to leave farther than 80 km and to have plans to evacuate by all possible means. Spain has advised their citizens to leave an area of 120 km. Embassies of France, UK, Germany, Switzerland, Austria, Italy, Australia, New Zealand, Finland, Kenya, Israel advised their citizens to leave even the metropolitan area of Tokyo.

Travel to Japan is very low, but additional flights have been chartered by some countries to assist those who wish to leave. In mid-March, several nations had begun official efforts to evacuate their citizens from Japan.

The World Health Organization announced its intention to conduct continuing public health studies over the next 20 years.

Casualties
Major news source reporting at least 2 TEPCO employees confirmed dead from "disaster conditions" following the earthquake. "The two workers, aged 21 and 24, sustained multiple external injuries and were believed to have died from blood loss, TEPCO said. Their bodies were decontaminated as radiation has been spewing from the plant for three weeks."

45 patients were reported dead after the evacuation of a hospital in Futaba. Some of them "were suffering from dehydration because they had not eaten anything for three days".

Cleanup
Initially, TEPCO did not put forward a strategy to regain control of the situation in the reactors. Helmut Hirsch, a German physicist and nuclear expert, says "they are improvising with tools that were not intended for this type of situation". However, on 17 April, TEPCO appeared to put forward the broad basis of a plan which includes: (1) reaching "cold shutdown in about six to nine months;" (2) "restoring stable cooling to the reactors and spent fuel pools in about three months;" (3) putting "special covers" on units 1, 3, and 4 starting in June; (4) installing "additional storage containers for the radioactive water that has been pooling in the turbine basements and outside trenches;" (5) using radio-controlled equipment to clean up the site; and (6) using silt fences to limit ocean contamination. Previously, TEPCO publicly committed to installing new emergency generators 20m above sea level, twice the height of the generators destroyed by the 11 March tsunami. Toshiba and Hitachi had both proposed plans for shuttering the facility.

Critics were "not fully convinced TEPCO could meet the timetable it has set for itself to achieve a cold shutdown" because the "scale and complexity of the challenge is unprecedented." Long term plans for units 5 and 6 have not been announced, "but they too may need to be decommissioned."

On 5 May, workers were able to enter reactor buildings for the first time since the accident. The workers began to install air filtration systems to clean air of radioactive materials to allow additional workers to install water cooling systems.

Boron
Officials have considered insertion or targeted aerial dropping of boric acid, boronated plastic beads or boron carbide pellets into the spent fuel pools to absorb neutrons. France flew 95 tonnes of boron to Japan on 17 March 2011 and the US has provided 9 tons. Neutron absorbing boric acid has been injected into the reactor cores, but is unclear if boron was included with the spraying of spent fuel pools (SFP)s.

Sarcophagus
On 18 March, Reuters reported that Hidehiko Nishiyama, Japan's nuclear agency spokesman when asked about burying the reactors in sand and concrete, said: "That solution is in the back of our minds, but we are focused on cooling the reactors down." Considered a last-ditch effort since it would not provide cooling, such a plan would require massive reinforcement under the floor, as for the Chernobyl Nuclear Power Plant sarcophagus.

Fabric cover
An effort has been undertaken to fit the three damaged reactor buildings with fabric covers and filters to limit radiation release. The cost of building structures around units 1 – 4 and wrapping them with the sheets is estimated to reach 80 billion yen. On 6 April, sources told Kyodo News that a major construction firm was studying the idea, and that construction wouldn't "start until June." The plan has been criticized for potential only having "limited effects in blocking the release of radioactive substances into the environment." On 14 May, TEPCO announced that it had begun to clear debris to create a space to install the cover over reactor building number 1.

Scope of cleanup
International experts have said that a workforce in the hundreds or even thousands would take years or decades to clean up the area. John Price, a former member of the Safety Policy Unit at the UK's National Nuclear Corporation, has said that it "might be 100 years before melting fuel rods can be safely removed from Japan's Fukushima nuclear plant". Edward Morse, a professor of nuclear engineering at the University of California, Berkeley, has said: ... there would be at least six months of emergency stabilisation, about two years of temporary remediation and up to 30 years of full-scale clean-up. Furthermore, the high levels of ground contamination at the site are raising concerns about the viability of individuals to work at the site in coming decades.

However, according to BBC News, Japanese reactor maker Toshiba said it could decommission the earthquake-damaged Fukushima nuclear power plant in about 10 years, a third quicker than the American Three Mile Island plant. As a comparison, it took 11 years after the accident before the vessel for the partially melted core at Three Mile Island was first opened, with cleanup taking several more years.

TEPCO announced on 17 April 2011 that it expected to have the automated cooling systems restored in the damaged reactors in about three months and have the reactors put into cold shutdown status in six months.

Debris
On 10 April 2011, TEPCO began using remote-controlled, unmanned heavy equipment to remove debris from around reactors 1–4. The debris and rubble, caused by hydrogen explosions at reactors 1 and 3, was impeding recovery operations both by being in the way and emitting high radioactivity. The debris will be placed into containers and kept at the plant.

Water
The Japanese government has requested that Russia send the floating water decontamination plant Landysh to assist in processing radioactive water from the damaged reactors. Landysh was built by Russia with funding from Japan to process liquid wastes produced during the decommissioning of nuclear submarines.

Safety record
The Fukushima Daiichi nuclear power complex was central to a falsified-records scandal that led to the departure of a number of senior executives of TEPCO. It also led to disclosures of previously unreported problems at the plant. In 2002, TEPCO admitted it had falsified safety records at the No. 1 reactor at Fukushima Daiichi. As a result of the scandal and a fuel leak at Fukushima, the company had to shut down all of its 17 nuclear reactors to take responsibility. A power board distributing electricity to a reactor's temperature control valves was not examined for 11 years. Inspections did not cover devices related to cooling systems, such as water pump motors and diesel generators.

In addition to concerns from within Japan, the International Atomic Energy Agency (IAEA) has also expressed concern about the ability of Japan's nuclear plants to withstand seismic activity. At a meeting of the G8's Nuclear Safety and Security Group, held in Tokyo in 2008, an IAEA expert warned that a strong earthquake with a magnitude above 7.0 could pose a "serious problem" for Japan's nuclear power stations.

In March 2006 the Japanese government opposed a court order to close a nuclear plant in the west part of the country over doubts about its ability to withstand an earthquake. Japan's Nuclear and Industrial Safety Agency believed it was "safe" and that "all safety analyses were appropriately conducted."

Regulatory relationship with nuclear industry


In 2010, Toru Ishida, the former director general of the Ministry of Economy, Trade and Industry (METI), which has responsibilities that include regulating nuclear industry, left the agency and joined TEPCO a few months later to become a senior adviser. He followed Susumu Shirakawa, another METI veteran who was a board member and executive vice president at TEPCO until retiring in June 2010.

Regulatory capture may have contributed to the cascade of failures which were revealed after the tsunami receded. Regulatory capture may have also contributed to the current situation. Critics argue that the government shares blame with regulatory agency for not heeding warnings, for not ensuring the independence of the nuclear industry's oversight while encouraging the expansion of nuclear energy domestically and internationally. World media has argued that the Japanese nuclear regulatory system tends to side with and promote the nuclear industry because of amakudari (roughly translated as descent from heaven), in which senior regulators accept high paying jobs at the companies they once oversaw. To protect their potential future position in the industry, regulators seek to avoid taking positions that upset or embarrass the utilities they regulate. TEPCO's position as a the largest electrical utility in Japan led it to be the most desirable position for retiring regulators, typically the "most senior officials went to work at Tepco, while those of lower ranks ended up at smaller utilities" according to the New York Times.

Insurance
According to Munich Re, a major reinsurer, the private insurance industry will not be significantly affected by the accidents at the Fukushima nuclear power plant. Swiss Re similarly states "Coverage for nuclear facilities in Japan excludes earthquake shock, fire following earthquake and tsunami, for both physical damage and liability. Swiss Re believes that the incident at the Fukushima nuclear power plant is unlikely to result in a significant direct loss for the Property & Casualty insurance industry."