A member international expert team assembled by the IAEA at the request of the Japanese government carried out a fact-finding mission in October on remediation strategies for contaminated land. Its report focused on the remediation of the affected areas outside of the 20 km restricted area.
The team said that it agreed with the prioritization and the general strategy being implemented, but advised the government to focus on actual dose reduction. They should "avoid over-conservatism" which "could not effectively contribute to the reduction of exposure doses" to people. It warned the government against being preoccupied with "contamination concentrations The four units at Fukushima Daini were shut down automatically due to the earthquake.
The tsunami — here only 9 m high — affected the generators and there was major interruption to cooling due to damaged heat exchangers, so the reactors were almost completely isolated from their ultimate heat sink.
Damage to the diesel generators was limited and also the earthquake left one of the external power lines intact, avoiding a station blackout as at Daiichi units Staff laid and energized 8.
Unit 3 was undamaged and continued to 'cold shutdown' status on the 12th, but the other units suffered flooding to pump rooms where the equipment transfers heat from the reactor heat removal circuit to the sea.
Pump motors were replaced in less than 30 hours. The almost complete loss of ultimate heat sink for a day proved a significant challenge, but the cores were kept fully covered. There was no technical reason for the Fukushima Daini plant not to restart. However, Tepco in October said it planned to transfer the fuel from the four reactors to used fuel ponds, and this was done.
In February the prime minister said that restarting the four units was essentially a matter for Tepco to decide. In July Tepco announced its decision to decommission the four reactors. The sequence of events relating to the fuel pond at unit 4 was rated INES level 3 — a serious incident. However, a month after the tsunami the NSC raised the rating to level 7 for units together, 'a major accident', saying that a re-evaluation of early radioactive releases suggested that some PBq of I equivalent had been discharged, mostly in the first week.
This then matched the criterion for level 7. Beyond whatever insurance Tepco might carry for its reactors is the question of third party liability for the accident. Japan was not party to any international liability convention but its law generally conforms to them, notably strict and exclusive liability for the operator. In mid-April , the first meeting was held of a panel to address compensation for nuclear-related damage.
The panel established guidelines for determining the scope of compensation for damage caused by the accident, and to act as an intermediary. On 11 May , Tepco accepted terms established by the Japanese government for state support to compensate those affected by the accident at the Fukushima Daiichi plant. The scheme included a new state-backed institution to expedite payments to those affected by the Fukushima Daiichi accident.
The body receives financial contributions from electric power companies with nuclear power plants in Japan, and from the government through special bonds that can be cashed whenever necessary. Tepco accepted the conditions imposed on the company as part of the package. That included not setting an upper limit on compensation payments to those affected, making maximum efforts to reduce costs, and an agreement to cooperate with an independent panel set up to investigate its management.
This Nuclear Damage Compensation Facilitation Corporation, established by government and nuclear plant operators, includes representatives from other nuclear generators and also operates as an insurer for the industry, being responsible to have plans in place for any future nuclear accidents. The provision for contributions from other nuclear operators is similar to that in the USA. The government estimates that Tepco will be able to complete its repayments in 10 to 13 years, after which it will revert to a fully private company with no government involvement.
Meanwhile it will pay an annual fee for the government support, maintain adequate power supplies and ensure plant safety. On 14 June , Japan's cabinet passed the Nuclear Disaster Compensation Bill, and a related budget to fund post-tsunami reconstruction was also passed subsequently. In September the Nuclear Damage Compensation Facilitation Corporation started by working with Tepco to compile a business plan for the next decade. In June shareholders voted to sell the Japanese government This was effected at the end of July, so that Tepco then became government-controlled, at least temporarily.
Tepco said it appreciated the chance to "transform to New Tepco". The government and 12 utilities are contributing funds into the new institution to pay compensation to individuals and businesses claiming damages caused by the accident.
Japan Nuclear Fuel Ltd. The utility companies also pay annual contributions to the body. Tepco is required to make extra contributions, with the specific amount to be decided later. This, it said, also resulted from "harmful rumours" about the possible health effects of consuming food products from the region near the damaged power plant. A provisional translation in English was published in February This focuses on a number of questions which remained unexplained in the National Diet Investigation Commission report.
At the IAEA General Conference in the Director General promised a comprehensive report which would be "an authoritative, factual and balanced assessment, addressing the causes and consequences of the accident as well as the lessons learned. In May its final report was delivered to member states, and was published in September. It was broadly positive regarding progress since , but said that some challenging issues remain.
It contains advisory points on topics such as long-term radioactive waste management, measures concerning contaminated water, and issues related to the removal of used fuel and fuel debris. This assumption was accepted by nuclear power plant operators and was not challenged by regulators or by the government.
As a result, Japan was not sufficiently prepared for a severe nuclear accident in March It also said there were certain weaknesses "in plant design, in emergency preparedness and response arrangement and in planning for the management of a severe accident". The Director General said: "I am confident that the legacy of the Fukushima Daiichi accident will be a sharper focus on nuclear safety everywhere. I have seen improvements in safety measures and procedures in every nuclear power plant that I have visited.
There is widespread recognition that everything humanly possible must be done to ensure that no such accident ever happens again. Some of the factors that contributed to the Fukushima Daiichi accident were not unique to Japan. Continuous questioning and openness to learning from experience are key to safety culture and are essential for everyone involved in nuclear power.
The Executive Summary includes recommendations, but the following paragraphs indicate some salient points from the actual investigation. Before the accident, there was a basic assumption in Japan that the design of nuclear power plants and the safety measures that had been put in place were sufficiently robust to withstand external events of low probability and high consequences. Because of the basic assumption that nuclear power plants in Japan were safe, there was a tendency for organizations and their staff not to challenge the level of safety.
The reinforced basic assumption among the stakeholders about the robustness of the technical design of nuclear power plants resulted in a situation where safety improvements were not introduced promptly. Before the accident, the operator had conducted some reassessments of extreme tsunami flood levels, using a consensus based methodology developed in Japan in , which had resulted in values higher than the original design basis estimates.
Based on the results, some compensatory measures were taken, but they proved to be insufficient at the time of the accident. There were no indications that the main safety features of the plant were affected by the vibratory ground motions generated by the earthquake on 11 March This was due to the conservative approach to earthquake design and construction of nuclear power plants in Japan, resulting in a plant that was provided with sufficient safety margins.
However, the original design considerations did not provide comparable safety margins for extreme external flooding events, such as tsunamis. Despite the efforts of the operators at the Fukushima Daiichi nuclear power plant to maintain control, the reactor cores in units overheated, the nuclear fuel melted and the three containment vessels were breached. Radionuclides were released from the plant to the atmosphere and were deposited on land and on the ocean.
There were also direct releases into the sea. Venting of the containment was necessary to relieve pressure and prevent its failure. The operators were able to vent units 1 and 3 to reduce the pressure in the primary containment vessels. However, this resulted in radioactive releases to the environment.
Even though the containment vents for units 1 and 3 were opened, the primary containment vessels for units 1 and 3 eventually failed. Containment venting for unit 2 was not successful, and the containment failed, resulting in radioactive releases. People within a radius of 20 km of the site and in other designated areas were evacuated, and those within a radius of km were instructed to shelter before later being advised to voluntarily evacuate.
Restrictions were placed on the distribution and consumption of food and the consumption of drinking water. Many people are still living outside the areas from which they were evacuated. No early radiation induced health effects were observed among workers or members of the public that could be attributed to the accident. It had two technological advisers. An initial report was published in December and a final report in July The panel set up four teams to undertake investigations on the causes of the accident and ensuing damage and on measures to prevent the further spread of damage caused by the accident, but not to pursue the question of responsibility for the accident.
The report called for fundamental changes across the industry, including the government and regulators, to increase openness, trustworthiness and focus on protecting public health and safety. In recognizing that fact, each of us every Japanese citizen should reflect on our responsibility as individuals in a democratic society.
The NAIIC reported that Tepco had been aware since that Fukushima Daiichi could face a station blackout if flooded, as well as the potential loss of ultimate heat sink in the event of a major tsunami. However, the regulator, NISA, gave no instruction to the company to prepare for severe flooding, and even told all nuclear operators that it was not necessary to plan for station blackout. During the initial response to the tsunami, this lack of readiness for station blackout was compounded by a lack of planning and training for severe accident mitigation.
Plans and procedures for venting and manual operation of emergency cooling were incomplete and their implementation in emergency circumstances proved very difficult as a result. NISA was also criticized for its "negligence and failure over the years" to prepare for a nuclear accident in terms of public information and evacuation, with previous governments equally culpable.
Earlier, on 7 June the government submitted a page report to the IAEA compiled by the nuclear emergency task force, acknowledging reactor design inadequacies and systemic shortcomings. It said: "In light of the lessons learned from the accident, Japan has recognized that a fundamental revision of its nuclear safety preparedness and response is inevitable. On 11 September a second report was issued by the government and submitted to the IAEA, summarising both onsite work and progress and offsite responses.
It contained further analysis of the earthquake and tsunami, the initial responses to manage and cool the reactors, the state of spent fuel ponds and the state of reactor pressure vessels. It also summarised radioactive releases and their effects. Its Appendix has some constructive comment on radiation exposure and balancing the costs of dose avoidance in circumstances of environmental contamination. This page report gives a detailed account of events. Also in November the Japan Nuclear Technology Institute published a page report on the accident, with proposals to be addressed in the future.
On 2 December Tepco released its interim investigation report on the accident in Japanese. A follow-up white paper was published in An analysis by the Carnegie Endowment in March said that if best practice from other countries had been adopted by Tepco and NISA at Fukushima, the serious accident would not have happened, underlining the need for greater international regulatory collaboration.
In April the US Electric Power Research Institute EPRI published Fukushima Daiichi Accident — Technical Causal Factor Analysis , which identified the root cause beyond the flooding and its effects as a failure to consider the possibility of the rupture of combinations of geological fault segments in the vicinity of the plant. A preliminary report from the World Health Organization WHO in May estimated the radiation doses that residents of Japan outside the evacuated areas received in the year following the accident.
The report's headline conclusion is that most people in Fukushima prefecture would have received a radiation dose of between 1 and 10 mSv during the first year after the accident. This compares with levels of about 2. In two places the doses were higher — between 10 and 50 mSv, still below any harmful level.
UNSCEAR also surveyed Fukushima prefecture to compare its data with Japanese measurements of exposures of some 2 million people living there at the time of the accident. People were promptly evacuated from the vicinity of the nuclear power plant, and later from a neighbouring area where radionuclides had accumulated. This action reduced their radiation exposure by a factor of ten, to levels that were "low or very low. In October a member IAEA mission visited at government request and reported on remediation and decontamination in particular.
The government ordered nuclear risk and safety reassessments — so-called 'stress tests' — based on those in the EU for all Japan's nuclear reactors except Fukushima's before they restart following any shutdown, including for routine checks. These were in two stages, and are described in the Japan Nuclear Power information page. Fukushima Daiichi Accident Updated April Following a major earthquake, a metre tsunami disabled the power supply and cooling of three Fukushima Daiichi reactors, causing a nuclear accident beginning on 11 March All three cores largely melted in the first three days.
The accident was rated level 7 on the International Nuclear and Radiological Event Scale, due to high radioactive releases over days 4 to 6, eventually a total of some PBq I eq. All four Fukushima Daiichi reactors were written off due to damage in the accident — MWe net. After two weeks, the three reactors units were stable with water addition and by July they were being cooled with recycled water from the new treatment plant.
Official 'cold shutdown condition' was announced in mid-December. Apart from cooling, the basic ongoing task was to prevent release of radioactive materials, particularly in contaminated water leaked from the three units. This task became newsworthy in August There have been no deaths or cases of radiation sickness from the nuclear accident, but over , people were evacuated from their homes as a preventative measure.
Government nervousness has delayed the return of many. Official figures show that there have been disaster-related deaths among evacuees from Fukushima prefecture. Disaster-related deaths are in addition to the about 19, that were killed by the earthquake or tsunami. The two Fukushima plants and their siting The Daiichi first and Daini second Fukushima plants are sited about 11 km apart on the coast, Daini to the south. Tepco has written off the four reactors damaged by the accident, and is decommissioning them.
Fuel ponds: developing problems Used fuel needs to be cooled and shielded. Because radiation levels have fallen significantly following decontamination, full protection gear is only needed in a few places in the plant, including in and around the melted reactor buildings. On a recent visit, AP journalists donned partial protective gear to tour a low-radiation area: a helmet, double socks, cotton gloves, surgical masks, goggles and a vest with a personal dosimeter.
Full protection gear, which means hazmat coveralls, a full-face mask, a head cover, triple socks and double rubber gloves, was required at a shared storage pool where fuel relocation from the No. TEPCO says it needs to get rid of the water storage tanks to free up space at the plant so workers can build facilities that will be used to study and store melted fuel and other debris.
There are about , tons of solid radioactive waste, including contaminated debris and soil, sludge from water treatment, scrapped tanks and other waste. Local officials and residents say they expect the complex to one day be open space where they can walk freely.
Tokyo correspondent Mari Yamaguchi has visited the Fukushima nuclear plant nine times, starting in Sections U. The storage pools at Fukushima Dai-1 are reportedly about 14 meters deep; the 4-meter tall spent fuel assemblies sit at the bottom of the pool. When the 9. The first step went fine: To stop the nuclear fission chain reaction, control rods with neutron-absorbing properties were inserted among the fuel rods.
But even though the fission reaction came to a halt, the danger wasn't over. Radioactive byproducts of past fission reactions continued to generate heat inside the pressure vessel even though the reactor was no longer active, so cooling systems were supposed to kick in to circulate cold water and remove steam. But the tsunami that swiftly followed the earthquake swamped the coastal facility and damaged the generators and power systems that ran Fukushima Dai-1's cooling mechanisms.
That's when things started to go bad. The plant's operators rushed in new generators and turned on battery-powered backup systems, but apparently this gear couldn't keep the recently active reactors from heating up. It happened first in reactor No.
As the water level dropped, the zirconium alloy fuel rods reacted with the steam and other gases to produce hydrogen gas. As pressures in the inner chamber reached dangerously high levels, operators decided to vent steam containing some radioactive elements first into the primary containment vessel, and then into the secondary containment building. But the volatile hydrogen gas appears to have reacted with oxygen in the secondary containment structure, causing an explosion that ripped the roof off the building.
This explosion released some radioactive material; it's not clear whether it damaged the primary containment vessel. Extremely high temperatures inside the reactor in those first few days are believed to have melted parts of the zirconium alloy fuel rods and some of the uranium pellets themselves. That's a serious concern, because melted uranium can drip down and pool at the bottom of the pressure chamber.
If enough of it gathers there, it can eat through the chamber's steel floor and drip down into the primary containment vessel. Over time, it can even eat through the thick concrete floor of the containment structure.
That worst-case scenario is commonly referred to as a "meltdown. Following the hydrogen explosion in reactor building No. The corrosive salt water has rendered the reactors unfit for future use. On March 25 TEPCO officials began to switch the pumping system back over to fresh water, due to concerns that the salt water would corrode the pumping equipment and cause it to malfunction.
Scroll down for more information on that threat. By April 5, water temperatures inside reactor No. To prevent another accident, TEPCO workers began injecting nitrogen gas into the containment vessel to thin the concentrations of hydrogen. Periodic nitrogen injections have continued through April and May. At the beginning of May, workers entered the reactor building for the first time since the earthquake to install air-filtering equipment that could remove radioactive elements and make working conditions safer.
Despite this procedure, radiation levels have remained very high in some parts of the No. Workers have repeatedly entered the building since then to assess conditions and install monitoring equipment. The announcement was based on the company's analysis of water gauge readings, which showed that the water level in the pressure chamber was below the normal location of the fuel rods, suggesting the the fuel rods had been partially or completely exposed.
This led a TEPCO official to announce that the fuel rods had probably melted, at least partially, and the molten fuel had "fallen to the bottom of the reactor. So it can be said the No. The TEPCO official went on to say that the melted fuel may have damaged the bottom of the pressure chamber, allowing radioactive water to leak out into the primary containment vessel.
It's possible that leaks in the primary containment vessel have allowed that contaminated water to reach other parts of the building. The good news is that the melted fuel is thought to have been cooled by water at the bottom of the pressure chamber, as temperatures in the chamber have been relatively low and stable.
And there is no sign that the melted fuel has reignited in a nuclear chain reaction. A chain of events similar to those that caused the explosion in building No. In that building, operators had already resorted to pumping seawater into the pressure chamber to cool it, but they weren't able to prevent the rise in temperature, the pressure buildup, or the hydrogen explosion pictured below.
But on March 16 white steam began to rise from building No. If the steam was leaking from the primary containment vessel, it was likely to be contaminated with radiation. As of yet there has been no firm answer on the status of the primary containment vessel in reactor No. On the morning of March 17, new problems arose at the No.
Word came that the pool had heated up, causing some of its water to evaporate away and possibly exposing the spent fuel rods to the air. That exposure would cause the rods to heat further and could cause the nuclear fuel within to begin melting, which would increase the amount of radiation emitted. Since the water circulation system intended to keep the storage pool cool wasn't working, TEPCO called in the big water guns.
On March 17, two helicopters flew over the building and dumped water on building No. Later in the day police trucks used water cannons to send jets of water into the building, with limited success. Finally the Japanese military sent in its own water-spraying trucks, which reportedly blasted 30 tons of water into building No.
On March 18 the military trucks repeated the water-spraying operation, blasting 45 tons of water into building No. TEPCO said that the white steam that billowed up during the military's mission indicated that the water reached the spent fuel pool.
Throughout the crisis, spikes of radiation made the situation dangerous for workers in the plant's shielded control rooms, and made it difficult for outside personnel to approach the site. Water-spraying trucks have continued to hose down building No. In the second week of the crisis TEPCO got to work reconnecting the plant to the electricity grid, which would allow its cooling systems to go back into operation.
On March 22 the company announced that it was ready to restore power to building No. In the midst of this promising news, however, a terrible incident occurred in building No. On March 24, three subcontractors were laying electrical cables in the turbine building behind the No.
It soon became clear that they had waded into highly radioactive water, which seeped through the protective clothing of two of the men. The workers were taken to the hospital to be treated for radiation exposure pictured , and were discharged on March Officials from Japan's Nuclear and Industrial Safety Agency said that the radioactive water indicates a problem with the No.
The officials said the primarily containment vessel may be leaking, but other experts said it's more likely that a pipe or a valve in the reactor's water circulating system is cracked. Following the accident with the workers, more radioactive water has been found outside many of the reactor buildings.
Scroll down for more information on that development. This "mixed oxide fuel" can produce dangerously radioactive materials. On March 28 TEPCO officials announced that plutonium had been detected in five soil samples taken around the plant, but declared that the levels were very low and didn't pose a threat to human health.
The composition of the plutonium suggested that it came from a nuclear reactor. However, the plutonium didn't necessarily come from reactor No. The explosion in the No. The incident occurred while operators were trying, with limited success, to pump seawater into the pressure chamber. Even so, they remain in the water, some scientists say, raises a concern about their impact on marine life from a long-term release. TEPCO has managed to cut the amount of contaminated water to one-third of what it used to be through a series of measures.
In another building, plant workers — about 4, per day now — go through automated security checkpoints and radiation measurements. Because radiation levels have fallen significantly following decontamination, full protection gear is only needed in a few places in the plant, including in and around the melted reactor buildings. On a recent visit, AP journalists donned partial protective gear to tour a low-radiation area: a helmet, double socks, cotton gloves, surgical masks, goggles and a vest with a personal dosimeter.
Full protection gear, which means hazmat coveralls, a full-face mask, a head cover, triple socks and double rubber gloves, was required at a shared storage pool where fuel relocation from the No.
A decade after the accident, Japan doesn't yet have a plan to dispose of the highly radioactive melted fuel, debris and waste at the plant. Technology also isn't advanced enough yet to manage the waste by reducing its toxicity.
TEPCO says it needs to get rid of the water storage tanks to free up space at the plant so workers can build facilities that will be used to study and store melted fuel and other debris. There are about , tons of solid radioactive waste, including contaminated debris and soil, sludge from water treatment, scrapped tanks and other waste.
It's unclear what the plant will look like when the work there is done. Local officials and residents say they expect the complex to one day be open space where they can walk freely.
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