And at PM , eight minutes before the JMA released its final tsunami estimate—more than 10 meters—the waters began to surge. Crisis-management protocols had been serving Masuda and his workers well. But suddenly the second step on their checklist—cooldown—was in jeopardy.
A tsunami higher than 5. He estimated that to knock out the lights, the waters must have been 17 meters high.
Part of the cooling system sat only four meters above sea level; the reactors themselves were only eight meters higher than that. If those systems were damaged or their power supply was compromised, cooldown would be next to impossible.
When the waters began to recede, a few hours later, Masuda learned that three of the four reactors had indeed lost their cooling functions. Despite the pre-tsunami shutdown, the fuel rods inside each core continued to generate heat that would normally have been removed by the cooling system and absorbed by the sea. Workers could still inject each reactor core with cold water, but Masuda worried that even if the cores retained their integrity, a buildup of steam pressure might compromise full function of their containment vessels.
Then, if conditions worsened and a meltdown did occur, it would be difficult to prevent a radioactive breach. With so many aftershocks and an ongoing tsunami alert, this was a possibility he could not afford to ignore. Until he could persuade his team to venture out on the ground and examine the damage up close, he would not know exactly what had to be done to stabilize the plant.
The workers Masuda needed to persuade were frantic for news of their families most lived near the plant and were operating beyond their training and experience. The tsunami waters had penetrated more deeply than anyone had expected, and frequent aftershocks, some greater than 7.
He had to move right away. And yet he held back. Commandeering a whiteboard in a corner of the crowded ERC, Masuda began writing down numbers: the frequency and magnitude of the aftershocks. Marking a line for each new quake, he created a simple chart depicting—he hoped—the decreasing danger. He waited and he wrote. Leaders often view themselves as intermediaries who digest the raw materials of the world—the unpredictable, the contingent—and transmute them into a more refined, reliable end product for their organizations.
Even if Masuda had been so inclined, he could not have persuaded the Daini workers that way. So he offered data, giving the workers an opportunity to confront and process the uncertainty for themselves. He prompted them to do their own sensemaking: to reflect on how their emerging reality fit their assessment of risk.
At 10 PM, standing face-to-face with his team leaders, Masuda finally asked them to pick four groups of 10 workers, one for each reactor unit, to go out and survey the damage. To his deep relief, no one refused. Masuda gave each group detailed instructions about where to go and what to do. Having worked at the plant on and off for decades, he knew every nook. What he had on hand was insufficient. With assistance from TEPCO headquarters and the Japan Self-Defense Forces, more supplies and cables would be brought in from off-site so that spoiled pump motors could be replaced and damaged parts of the cooling system could be connected to a building that was still drawing power.
The cables came in meter sections that weighed a ton each. Once the additional materials arrived, on the morning of March 13, they would have about 24 hours to complete the task. Under normal circumstances, a job like that would take 20 people using heavy machinery more than a month to finish.
Masuda returned to his whiteboard. He ordered a subordinate to write up the overall picture of the plant and an outline of the recovery strategy. He was determined to share information with his workers as it became available, slowly replacing uncertainty with meaning. But displaying those plans so openly had an unintended consequence: Workers interpreted that as a public commitment to seeing the plans through, which would temporarily, anyway reduce their ability to adapt to further surprises.
Masuda had initially chosen to use the radioactive-waste building as his power source, because its interior provided the least complicated pathway for the cables and he was loath to risk disabling his only operative generator.
Each segment of cable would require people to move it, so hooking everything up to the radioactive-waste building would take too long; they would need a supplementary, more convenient power source to hasten the process.
Reluctantly he decided to use the generator. This was not the last time Masuda had to revise something on his whiteboard. He said he was concerned about a lack of transparency and the use of subsidies to sweeten local opinion to get the necessary restart approval. A giant earthquake off the northeast coast of Japan in March spawned a tsunami that killed more than 15, people and knocked out cooling at the Fukushima Daiichi nuclear station. Explosions rocked the site as reactors melted down, causing huge clouds of radioactive matter to scatter over land and sea.
Lax oversight allowing Tokyo Electric to downplay tsunami risks was one of the failings highlighted in the crisis. Bureaucrats from the Ministry of Economy, Trade and Industry METI , which supports nuclear energy to power Japan's industrial economy, went to Fukui prefecture times over a two-year period until early this year.
The visits to Fukui by officials including the head of the powerful natural resources agency were raised at a recent hearing of the parliamentary committee. A subsidy of 2. Kansai was the most reliant on nuclear power before Fukushima, getting about half its electricity supplies from atomic energy. Five workers died at Mihama power station in after a pipe that had not been inspected for nearly a decade burst, releasing high pressure steam and hot water.
In , Kansai Electric executives admitted to receiving cash and gifts worth million yen from an official from a town hosting one of its other nuclear plants. Later analysis suggested that a leak of the primary containment developed on Tuesday Most of the radioactive releases from the site appeared to come from unit 2.
In unit 3 , the main backup water injection system failed at about am on Saturday 12, and early on Sunday 13 water injection using the high pressure system failed also and water levels dropped dramatically. RPV pressure was reduced by venting steam into the wetwell, allowing injection of seawater using a fire pump from just before noon.
Early on Sunday venting the suppression chamber and containment was successfully undertaken. It is now understood that core damage started about am and much or all of the fuel melted on the morning of Sunday 13 and fell into the bottom of the RPV, with some probably going through the bottom of the reactor pressure vessel and onto the concrete below.
Early on Monday 14 PCV venting was repeated, and this evidently backflowed to the service floor of the building, so that at am a very large hydrogen explosion here above unit 3 reactor containment blew off much of the roof and walls and demolished the top part of the building.
This explosion created a lot of debris, and some of that on the ground near unit 3 was very radioactive. In defuelled unit 4 , at about am on Tuesday 15 March, there was an explosion which destroyed the top of the building and damaged unit 3's superstructure further. This was apparently from hydrogen arising in unit 3 and reaching unit 4 by backflow in shared ducts when vented from unit 3.
Units Water had been injected into each of the three reactor units more or less continuously, and in the absence of normal heat removal via external heat exchanger this water was boiling off for some months. In June this was adding to the contaminated water onsite by about m 3 per day.
In January 4. There was a peak of radioactive release on Tuesday 15, apparently mostly from unit 2, but the precise source remains uncertain. Due to volatile and easily-airborne fission products being carried with the hydrogen and steam, the venting and hydrogen explosions discharged a lot of radioactive material into the atmosphere, notably iodine and caesium.
NISA said in June that it estimated that kg of hydrogen had been produced in each of the units. Nitrogen was being injected into the PCVs of all three reactors to remove concerns about further hydrogen explosions, and in December this was started also for the pressure vessels. Gas control systems which extract and clean the gas from the PCV to avoid leakage of caesium were commissioned for all three units.
RPV pressures ranged from atmospheric to slightly above kPa in January, due to water and nitrogen injection. However, since they were leaking, the normal definition of 'cold shutdown' did not apply, and Tepco waited to bring radioactive releases under control before declaring 'cold shutdown condition' in mid-December, with NISA's approval. This, with the prime minister's announcement of it, formally brought to a close the 'accident' phase of events. The AC electricity supply from external source was connected to all units by 22 March.
Power was restored to instrumentation in all units except unit 3 by 25 March. However, radiation levels inside the plant were so high that normal access was impossible until June. Results of muon measurements in unit 2 in indicate that most of the fuel debris in unit 2 is in the bottom of the reactor vessel.
Summary : Major fuel melting occurred early on in all three units, though the fuel remained essentially contained except for some volatile fission products vented early on, or released from unit 2 in mid-March, and some soluble ones which were leaking with the water, especially from unit 2, where the containment is evidently breached.
Cooling is provided from external sources, using treated recycled water, with a stable heat removal path from the actual reactors to external heat sinks. Access has been gained to all three reactor buildings, but dose rates remain high inside. Tepco declared 'cold shutdown condition' in mid-December when radioactive releases had reduced to minimal levels. See also background on nuclear reactors at Fukushima Daiichi. Used fuel needs to be cooled and shielded. This is initially by water, in ponds.
After about three years underwater, used fuel can be transferred to dry storage, with air ventilation simply by convection. Used fuel generates heat, so the water in ponds is circulated by electric pumps through external heat exchangers, so that the heat is dumped and a low temperature maintained.
There are fuel ponds near the top of all six reactor buildings at the Daiichi plant, adjacent to the top of each reactor so that the fuel can be unloaded underwater when the top is off the reactor pressure vessel and it is flooded. There is some dry storage onsite to extend the plant's capacity. At the time of the accident, in addition to a large number of used fuel assemblies, unit 4's pond also held a full core load of fuel assemblies while the reactor was undergoing maintenance, these having been removed at the end of November, and were to be replaced in the core.
A separate set of problems arose as the fuel ponds, holding fresh and used fuel in the upper part of the reactor structures, were found to be depleted in water. The primary cause of the low water levels was loss of cooling circulation to external heat exchangers, leading to elevated temperatures and probably boiling, especially in the heavily-loaded unit 4 fuel pond. Here the fuel would have been uncovered in about 7 days due to water boiling off. However, the fact that unit 4 was unloaded meant that there was a large inventory of water at the top of the structure, and enough of this replenished the fuel pond to prevent the fuel becoming uncovered — the minimum level reached was about 1.
After the hydrogen explosion in unit 4 early on Tuesday 15 March, Tepco was told to implement injection of water to unit 4 pond which had a particularly high heat load 3 MW from used fuel assemblies in it, so it was the main focus of concern. Initially this was attempted with fire pumps but from 22 March a concrete pump with metre boom enabled more precise targeting of water through the damaged walls of the service floors.
There was some use of built-in plumbing for unit 2. Analysis of radionuclides in water from the used fuel ponds suggested that some of the fuel assemblies might have been damaged, but the majority were intact.
There was concern about the structural strength of unit 4 building, so support for the pond was reinforced by the end of July. Each has a primary circuit within the reactor and waste treatment buildings and a secondary circuit dumping heat through a small dry cooling tower outside the building. The next task was to remove the salt from those ponds which had seawater added, to reduce the potential for corrosion. In July two of the fresh fuel assemblies were removed from the unit 4 pool and transferred to the central spent fuel pool for detailed inspection to check damage, particularly corrosion.
They were found to have no deformation or corrosion. Unloading the spent fuel assemblies in pond 4 and transferring them to the central spent fuel pool commenced in mid-November and was completed 13 months later. These comprised spent fuel plus the full fuel load of The next focus of attention was the unit 3 pool. In the damaged fuel handling equipment and other wreckage was removed from the destroyed upper level of the reactor building. Toshiba built a tonne fuel handling machine for transferring the fuel assemblies into casks and to remove debris in the pool, and a crane for lifting the fuel transfer casks.
Installation of a cover over the fuel handling machine was completed in February Removal and transferral of the fuel to the central spent fuel pool began in mid-April and was completed at the end of February In June , Tepco announced it would transfer some of the fuel assemblies stored in the central spent fuel pool to an onsite temporary dry storage facility to clear sufficient space for the fuel assemblies from unit 3's pool.
The dry storage facility has a capacity of at least assemblies in 65 casks — each dry cask holds 50 fuel assemblies. Summary: The spent fuel storage pools survived the earthquake, tsunami and hydrogen explosions without significant damage to the fuel, significant radiological release, or threat to public safety.
The new cooling circuits with external heat exchangers for the four ponds are working well and temperatures are normal. Analysis of water has confirmed that most fuel rods are intact.
See also background on Fukushima Fuel Ponds and Decommissioning section below. Regarding releases to air and also water leakage from Fukushima Daiichi, the main radionuclide from among the many kinds of fission products in the fuel was volatile iodine, which has a half-life of 8 days. The other main radionuclide is caesium, which has a year half-life, is easily carried in a plume, and when it lands it may contaminate land for some time.
It is a strong gamma-emitter in its decay. Cs is also produced and dispersed; it has a two-year half-life. Caesium is soluble and can be taken into the body, but does not concentrate in any particular organs, and has a biological half-life of about 70 days. In assessing the significance of atmospheric releases, the Cs figure is multiplied by 40 and added to the I number to give an 'iodine equivalent' figure. As cooling failed on the first day, evacuations were progressively ordered, due to uncertainty about what was happening inside the reactors and the possible effects.
By the evening of Saturday 12 March the evacuation zone had been extended to 20 km from the plant. See later section on Public health and return of evacuees.
A significant problem in tracking radioactive release was that 23 out of the 24 radiation monitoring stations on the plant site were disabled by the tsunami. There is some uncertainty about the amount and exact sources of radioactive releases to air see also background on Radiation Exposure. Most of the release was by the end of March Tepco sprayed a dust-suppressing polymer resin around the plant to ensure that fallout from mid-March was not mobilized by wind or rain.
In addition it removed a lot of rubble with remote control front-end loaders, and this further reduced ambient radiation levels, halving them near unit 1. In mid-May work started towards constructing a cover over unit 1 to reduce airborne radioactive releases from the site, to keep out the rain, and to enable measurement of radioactive releases within the structure through its ventilation system.
The frame was assembled over the reactor, enclosing an area 42 x 47 m, and 54 m high. The sections of the steel frame fitted together remotely without the use of screws and bolts.
All the wall panels had a flameproof coating, and the structure had a filtered ventilation system capable of handling 40, cubic metres of air per hour through six lines, including two backup lines. The cover structure was fitted with internal monitoring cameras, radiation and hydrogen detectors, thermometers and a pipe for water injection. The cover was completed with ventilation systems working by the end of October It was expected to be needed for two years.
In May Tepco announced its more permanent replacement, to be built over four years. It started demolishing the cover in and finished in In December it decided to install the replacement cover before removing debris from the top floor of the building. A crane and other equipment for fuel removal will be installed under the cover, similar to that over unit 4.
A cantilevered structure was built over unit 4 from April to July to enable recovery of the contents of the spent fuel pond. This is a 69 x 31 m cover 53 m high and it was fully equipped by the end of to enable unloading of used fuel from the storage pond into casks, each holding 22 fuel assemblies, and removal of the casks.
This operation was accomplished under water, using the new fuel handling machine replacing the one destroyed by the hydrogen explosion so that the used fuel could be transferred to the central storage onsite. Transfer was completed in December A video of the process is available on Tepco's website. A different design of cover was built over unit 3, and foundation work began in Large rubble removal took place from to , including the damaged fuel handling machine.
An arched cover was prefabricated, 57 m long and 19 m wide, and supported by the turbine building on one side and the ground on the other. A crane removed the fuel assemblies from the pool and some remaining rubble. Spent fuel removal from unit 3 pool began in April and was completed in February Maps from the Ministry of Education, Culture, Sports, Science and Tehcnology MEXT aerial surveys carried out approximately one year apart show the reduction in contamination from late to late Areas with colour changes in showed approximately half the contamination as surveyed in , the difference coming from decay of caesium two-year half-life and natural processes like wind and rain.
Tests on radioactivity in rice have been made and caesium was found in a few of them. Summary : Major releases of radionuclides, including long-lived caesium, occurred to air, mainly in mid-March. The population within a 20km radius had been evacuated three days earlier. Considerable work was done to reduce the amount of radioactive debris onsite and to stabilize dust. The main source of radioactive releases was the apparent hydrogen explosion in the suppression chamber of unit 2 on 15 March.
A cover building for unit 1 reactor was built and the unit is now being dismantled, a more substantial one for unit 4 was built to enable fuel removal during By the end of , Tepco had checked the radiation exposure of 19, people who had worked on the site since 11 March. For many of these both external dose and internal doses measured with whole-body counters were considered.
It reported that workers had received doses over mSv. Of these had received to mSv, twenty-three mSv, three more mSv, and six had received over mSv to mSv apparently due to inhaling iodine fumes early on. There were up to workers onsite each day. Recovery workers wear personal monitors, with breathing apparatus and protective clothing which protect against alpha and beta radiation.
The level of mSv was the allowable maximum short-term dose for Fukushima Daiichi accident clean-up workers through to December , mSv is the international allowable short-term dose "for emergency workers taking life-saving actions". No radiation casualties acute radiation syndrome occurred, and few other injuries, though higher than normal doses, were being accumulated by several hundred workers onsite.
High radiation levels in the three reactor buildings hindered access there. Monitoring of seawater, soil and atmosphere is at 25 locations on the plant site, 12 locations on the boundary, and others further afield. Government and IAEA monitoring of air and seawater is ongoing.
Some high but not health-threatening levels of iodine were found in March, but with an eight-day half-life, most I had gone by the end of April A radiation survey map of the site made in March revealed substantial progress: the highest dose rate anywhere on the site was 0. The majority of the power plant area was at less than 0.
These reduced levels are reflected in worker doses: during January , the workers at the site received an average of 0. Media reports have referred to 'nuclear gypsies' — casual workers employed by subcontractors on a short-term basis, and allegedly prone to receiving higher and unsupervised radiation doses.
This transient workforce has been part of the nuclear scene for at least four decades, and at Fukushima their doses are very rigorously monitored. If they reach certain levels, e. Tepco figures submitted to the NRA for the period to end January showed workers had received more than mSv six more than two years earlier and had received 50 to mSv.
Early in there were about onsite each weekday. Summary : Six workers received radiation doses apparently over the mSv level set by NISA, but at levels below those which would cause radiation sickness. On 4 April , radiation levels of 0. Monitoring beyond the 20 km evacuation radius to 13 April showed one location — around Iitate — with up to 0.
At the end of July the highest level measured within 30km radius was 0. The safety limit set by the central government in mid-April for public recreation areas was 3. In June , analysis from Japan's Nuclear Regulation Authority NRA showed that the most contaminated areas in the Fukushima evacuation zone had reduced in size by three-quarters over the previous two years. In August The Act on Special Measures Concerning the Handling of Radioactive Pollution was enacted and it took full effect from January as the main legal instrument to deal with all remediation activities in the affected areas, as well as the management of materials removed as a result of those activities.
It specified two categories of land: Special Decontamination Areas consisting of the 'restricted areas' located within a 20 km radius from the Fukushima Daiichi plant, and 'deliberate evacuation areas' where the annual cumulative dose for individuals was anticipated to exceed 20 mSv.
The national government promotes decontamination in these areas. Intensive Contamination Survey Areas including the so-called Decontamination Implementation Areas, where an additional annual cumulative dose between 1 mSv and 20 mSv was estimated for individuals. Municipalities implement decontamination activities in these areas. The doses to the general public, both those incurred during the first year and estimated for their lifetimes, are generally low or very low.
No discernible increased incidence of radiation-related health effects are expected among exposed members of the public or their descendants. However, the report noted: "More than additional workers received effective doses currently estimated to be over mSv, predominantly from external exposures. Among this group, an increased risk of cancer would be expected in the future. However, any increased incidence of cancer in this group is expected to be indiscernible because of the difficulty of confirming such a small incidence against the normal statistical fluctuations in cancer incidence.
These workers are individually monitored annually for potential late radiation-related health effects. By contrast, the public was exposed to times less radiation. Most Japanese people were exposed to additional radiation amounting to less than the typical natural background level of 2.
The Report states: "No adverse health effects among Fukushima residents have been documented that are directly attributable to radiation exposure from the Fukushima Daiichi nuclear plant accident. People living in Fukushima prefecture are expected to be exposed to around 10 mSv over their entire lifetimes, while for those living further away the dose would be 0. The UNSCEAR conclusion reinforces the findings of several international reports to date, including one from the World Health Organization WHO that considered the health risk to the most exposed people possible: a postulated girl under one year of age living in Iitate or Namie that did not evacuate and continued life as normal for four months after the accident.
Such a child's theoretical risk of developing any cancer would be increased only marginally, according to the WHO's analysis. The man had been diagnosed with lung cancer in February Eleven municipalities in the former restricted zone or planned evacuation area, within 20 km of the plant or where annual cumulative radiation dose is greater than 20 mSv, are designated 'special decontamination areas', where decontamination work is being implemented by the government.
A further municipalities in eight prefectures, where dose rates are equivalent to over 1 mSv per year are classed as 'intensive decontamination survey areas', where decontamination is being implemented by each municipality with funding and technical support from the national government.
Decontamination of all 11 special decontamination areas has been completed. In October a member IAEA mission reported on remediation and decontamination in the special decontamination areas.
Its preliminary report said that decontamination efforts were commendable but driven by unrealistic targets. Also, there is potential to produce more food safely in contaminated areas. The total area under consideration for attention is 13, km 2.
Summary : There have been no harmful effects from radiation on local people, nor any doses approaching harmful levels.
However, some , people were evacuated from their homes and only from were allowed limited return. As of July over 41, remained displaced due to government concern about radiological effects from the accident.
Permanent return remains a high priority, and the evacuation zone is being decontaminated where required and possible, so that evacuees can return. There are many cases of evacuation stress including transfer trauma among evacuees, and once the situation had stabilized at the plant these outweighed the radiological hazards of returning, with deaths reported see below. The government said it would consider purchasing land and houses from residents of these areas if the evacuees wish to sell them.
In November the NRA decided to change the way radiation exposure was estimated. Instead of airborne surveys being the basis, personal dosimeters would be used, giving very much more accurate figures, often much less than airborne estimates.
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