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. Radioactive releases to air 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.
Reactor covers 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.
Radiation exposure on the plant site By the end of , Tepco had checked the radiation exposure of 19, people who had worked on the site since 11 March.
Radiation exposure and fallout beyond the plant site On 4 April , radiation levels of 0. Public health and return of evacuees Permanent return remains a high priority, and the evacuation zone is being decontaminated where required and possible, so that evacuees can return.
Managing contaminated water Removing contaminated water from the reactor and turbine buildings had become the main challenge by week 3, along with contaminated water in trenches carrying cabling and pipework. Remediation on site and decommissioning units Tepco published a six- to nine-month plan in April for dealing with the disabled Fukushima reactors, and updated this several times subsequently.
Fukushima Daini plant The four units at Fukushima Daini were shut down automatically due to the earthquake. Radiation monitoring figures remained at low levels, little above background. Accident liability and compensation Beyond whatever insurance Tepco might carry for its reactors is the question of third party liability for the accident. Inquiries and reports: radiation effects 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 earthquake measured 8. The remaining two units 5 and 6 were also shut down for regular inspection. The emergency onsite generation had failed to provide the necessary backup power needed to support the critical instruments and control systems. Even the special cooling system known as the reactor core isolation cooling system that uses waste heat to run the critical systems could not provide the power needed to operate the control systems.
Unit 1 exploded on 12 March knocking down the external concrete building. However, the reactor and the steel containment structure remained intact. The radiation levels rose to microsievert, which is equivalent to the maximum permissible level for a year in a single day. This scale measures from deviation-no safety significance — major accident and is used to communicate the safety significance of events associated with radiation sources.
Unit 3 exploded due to hydrogen ignition on 14 March Pressure in the reactor was built-up to kiloPascals kPa even while sea water was being injected into the reactor to control the radiation.
In addition, the power distribution buses that would have allowed an external power source to be connected to the plant were also swamped and extensively damaged. They were flooded and completely destroyed. Thus, even if electricity had been available to drive the emergency cooling systems, there would have been no way of dissipating the heat.
Over the next three days, one by one, the three reactors that had been operating when the earthquake struck lost core cooling capability, resulting in a loss of coolant accident: without cooling, the water in the reactor pressure vessels boiled, uncovering the fuel, which subsequently melted. A large quantity of radioactivity from the damaged fuel escaped into the environment.
As cooling water evaporated and turned into steam, pressure inside the primary containment grew, creating leaks that allowed radiation to escape. Yet more radiation was released by a series of explosions that occurred in the reactor buildings of units 1, 3, and 4 in the four days following the tsunami.
It built up in the reactor buildings of units 1 and 3 before eventually exploding. Hydrogen may also have caused an explosion in unit 4 after it migrated there from unit 3 along their common venting system. In its June report to the IAEA explaining the accident, the Japanese government estimated that the quantity of radiation released into the atmosphere by the accident was about 15 percent of the radiation released from Chernobyl.
In any case, the quantity of radiation released by the Fukushima accident has proved controversial and estimates may change as more information becomes available. A much smaller quantity of radiation was released into the Pacific Ocean, most of it in the form of overflow of contaminated water that had been used to cool the reactors.
Nonetheless, complete remediation of the site is likely to take three or four decades, and the biggest challenge will probably be removing all the melted fuel. The road to complete recovery will be an extremely long and expensive one. There is still much to be learned about the accident sequence, including the actions of the plant operators to mitigate it. In contrast to the report by an IAEA fact-finding mission which was highly complimentary of the plant operators , an interim report by a commission appointed by the Japanese government to investigate the accident expressed direct and significant criticism of plant operators in units 1 and 3 for delays in implementing emergency cooling procedures.
The actions of the operators will undoubtedly come under considerable scrutiny in the months ahead. In assessing these actions, it is necessary to keep two points in mind. First, the accident progressed extremely quickly. The table below shows estimates, by both the Japanese regulator, the Nuclear and Industrial Safety Agency NISA , and TEPCO of the length of time that passed after the earthquake until i fuel became exposed, ii fuel started to melt, and iii molten fuel started to damage the reactor pressure vessels.
At unit 1, it appears that the emergency cooling system became inoperative immediately after the tsunami and fuel damage began two or three hours later that is, three or four hours after the earthquake. All instrumentation in the main control room of units 1 and 2 was lost following the tsunami, and it was almost three hours before some instrumentation had been restored and the operators had reason to suppose that the emergency cooling system had failed.
The accident progressed somewhat more slowly in units 2 and 3. The emergency cooling systems in those units failed after about seventy and thirty-five hours, respectively, and in each case fuel damage began about seven or eight hours later that is, about seventy-seven and forty-three hours, respectively, after the earthquake.
Second, the conditions at the plant site confronting plant operators were truly appalling. The IAEA report notes:.
Work at night was conducted in the dark. There were many obstacles blocking access to the road such as debris from the tsunami and rubble that was produced by the explosions that occurred in Units 1, 3 and 4.
All work was conducted with respirators and protective clothing and mostly in high radiation fields. The off-site nuclear emergency response headquarters had to be evacuated because it was so underprepared.
For instance, a cable and a hose that had been laid to supply power and water to unit 2 were destroyed by fragments from the explosion in unit 1. Indeed, during the early stages of the accident, many of them would not have known whether their families had survived the disasters. These two observations have important implications for assessing the Fukushima Daiichi accident. Given the short time that might be available for operators to take action in the event of a station blackout and the extraordinary stress under which they are likely to be working, actions to be taken after an extreme external event and measures to prevent fuel damage must be prepared in advance, must have been practiced extensively, and must rely only on local resources if they are to have a realistic chance of success.
None of these criteria was met at Fukushima Daiichi. As a result, we believe it would be unfair to apportion significant blame for the accident on the actions the operators took or failed to take after the tsunami, as the official investigation committee has done.
Furthermore, given the potential challenges of a complete loss of AC power, it is clear that prevention is the best form of management.
To this end, the key questions raised by the accident are why was the tsunami hazard at Fukushima Daiichi so dramatically underestimated? And could changes in plant design resulting from effective safety reviews have prevented a severe accident in the event that a tsunami struck the plant? The answers to these questions help shed light on whether the accident could have been prevented. The Fukushima Daiichi Nuclear Power Station was not designed to withstand a tsunami even half the size of the one that ultimately struck the Japanese coast in March Given that the revised design-basis tsunami was now 1.
The maximum height of the tsunami that actually hit the plant is not known exactly since the sea-level gauge at the plant was destroyed. However, TEPCO and the Japan Society of Civil Engineers, using computer modeling to re-create the observed pattern of flooding at the plant, have estimated that just before it made landfall, the tsunami had a height of The size of the tsunami at Fukushima Daiichi was the result of a number of factors conspiring together.
A tsunami actually consists of a series of waves. In this case, more than about 10 kilometers from the coast, the largest of these had a height of only about 6 meters. For comparison, at Fukushima Daiini Nuclear Power Station, about 12 kilometers south of Fukushima Daiichi, the tsunami height was 9 meters. The earthquake that preceded the tsunami exceeded the seismic design basis of the plant at units 2, 3, and 5. Because the underlying geophysical phenomena are extremely complicated, accurate hazard assessment for earthquakes and tsunamis is exceedingly challenging.
But it is becoming increasingly evident that there were significant flaws in the methodology used to assess hazards to the Fukushima Daiichi plant. An earthquake offshore of the Miyagi region, where the epicenter of the March 11 earthquake was located, had been long anticipated.
Indeed, even within the last fifteen years, there are a number of other examples of beyond-design-basis earthquakes and floods at nuclear plants. In December , for example, a storm surge caused flooding at two reactors at the Blayais Nuclear Power Plant in France. Just five and a half months after the Fukushima accident, on August 23, , an earthquake on the East Coast of the United States marginally exceeded the design basis of the North Anna Nuclear Generating Station in Virginia.
However, the fact that all operating units were brought successfully into cold shutdown suggests that, for most beyond-design-basis events, plant safety margins are probably sufficient to compensate for this difficulty. Notwithstanding the intrinsic difficulties of hazard prediction, the approach to hazard prediction for Fukushima Daiichi appears to have been at variance, in three important areas, with both international best practices and, in some cases, with Japanese best practices.
Best practice, as promulgated by the IAEA, requires the collection of data on prehistorical and historical earthquakes and tsunamis in the region of a nuclear power plant in order to protect the plant against rare extreme seismic events that may occur only once every ten thousand years.
The original design-basis tsunami for Fukushima Daiichi of 3. Over the last decade or so, evidence of much larger tsunamis in and around Miyagi has emerged. Japanese researchers have discovered layers of sediment that appear to have been deposited by tsunamis and have concluded that the region had been inundated by massive tsunamis about once every one thousand years.
For instance, one compilation of historical tsunamis in and around Japan lists twelve events since having a maximum amplitude of more than 10 meters, six of which had a maximum amplitude of over 20 meters. The challenge of sifting through and evaluating the stream of potentially relevant geophysical studies to extract data important to nuclear power plant safety should not be underestimated.
Perhaps not surprisingly, there has been a fairly bitter debate within Japan about whether academia did not provide suitable warnings or whether it did and industry and regulators ignored them. Nonetheless, Japan has a historical legacy of severe tsunamis; it does appear that heeding this record, especially as it relates to the area around the plant, would have led to an upward revision of the design basis for Fukushima Daiichi Nuclear Power Station and perhaps consequently to infrastructural improvements to better defend the installation.
Second, there appear to have been deficiencies in tsunami modeling procedures, resulting in an insufficient margin of safety at Fukushima Daiichi.
A nuclear power plant built on a slope by the sea must be designed so that it is not damaged as a tsunami runs up the slope. In , the Japan Society of Civil Engineers developed a detailed methodology for determining the maximum run-up of a tsunami. However, in at least one important respect, TEPCO does not appear to have implemented the relevant procedures in full.
In keeping with international best practices, the Japan Society of Civil Engineers methodology requires computer modeling based on detailed site-specific data. However, preliminary results from a study by TEPCO that was not reported to the IAEA and is discussed further below reportedly indicated that a 9 meter tsunami could have a run-up of over 15 meters. Given that such a tsunami might have run up higher than anticipated, it is possible it could have damaged vulnerable low-lying components such as the seawater pumps.
Improved modeling of tsunami run-up—had it been heeded—might have provided information that could have prompted TEPCO to take mitigating action in advance of the accident on March 11, even if that modeling had assumed a smaller tsunami than the one that actually inundated the plant. Enhanced defenses would have widened safety margins at the plant and might have mitigated the consequences of a tsunami that was larger than the plant was designed to withstand. International best practices, as promulgated by the IAEA, requires such phenomena to be considered, as does the U.
Nuclear Regulatory Commission. To be fair, it appears that there were no suitable tools available in Japan for TEPCO to analyze the full range of effects of a tsunami. But given the prevalence of tsunamis in Japan, NISA should have encouraged the development of such instruments in keeping with international standards.
Since the IAEA mission it has emerged that, in , TEPCO did in fact perform some preliminary computer modeling that tentatively suggested the tsunami hazard to the plant had been severely underestimated. These simulations assumed a repeat of the AD earthquake.
Given the new simulations were based on an actual historical earthquake, they should have been followed up on immediately. Had the results been verified, TEPCO may have been able to take corrective action in time to avert the disaster of March 11, The World Health Organization WHO released a report in that said the disaster will not cause any observable increase in cancer rates in the region.
Scientists both inside and outside Japan believe that aside from the region immediately around the plant, the risks of radiation remain relatively low.
On 9 March , ahead of the year anniversary, a UN report said there had been "no adverse health effects" documented among Fukushima residents directly related to the radiation from the disaster. Any future radiation-related health effects were "unlikely to be discernible", it said. But many believe the dangers are far greater, and residents remain wary.
Though officials have lifted restrictions in many areas most people have not returned to their homes. In , the Japanese government announced that one worker had died after exposure to radiation and agreed his family should be compensated. A number of people are however confirmed to have died in the evacuation, including dozens of hospital patients who had to be moved due to fears of radiation.
The Fukushima Disaster is classified as a level seven event by the International Atomic Energy Agency, the highest such event and only the second disaster to meet this classification after Chernobyl. Critics blamed the lack of preparedness for the event, as well as a muddled response from both the plant operator Tokyo Electric Power Tepco and the government.
An independent investigation set up by Japan's parliament concluded that Fukushima was "a profoundly man-made disaster", blaming the energy company for failing to meet safety requirements or to plan for such an event. However, in a Japanese court cleared three former Tepco executives of negligence in what was the only criminal case to come out of the disaster.
In , Japan's then prime minister Yoshihiko Noda said the state shared the blame for the disaster. A court ruled in that the government bore partial responsibility and should pay compensation to evacuees.
Ten years later, several towns in north-eastern Japan remain off limits. Authorities are working to clean up the area so residents can return. Major challenges remain. Tens of thousands of workers will be needed over the next 30 to 40 years to safely remove nuclear waste, fuel rods and more than one million tons of radioactive water still kept at the site.
But some residents have decided never to return because they fear radiation, have built new lives elsewhere or don't want to go back to where the disaster hit.
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