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Accident at the Fukushima-1 nuclear power plant

Accident at the Fukushima-1 nuclear power plant


last update:

10 months ago

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  • Fukushima disaster

    The accident at Fukushima Daiichi occurred as a result of the magnitude 9.0 earthquake and subsequent tsunami that slammed into the coast of Japan on March 11, 2011. These natural disasters caused damage to the reactor buildings and reactor cooling systems at the Fukushima Daiichi nuclear power plant.
    When the earthquake struck, the reactors were automatically shut down, but because the tsunami damaged the cooling systems, the reactors overheated, causing explosions in the reactor buildings. In the process, radioactive substances were released into the atmosphere.
  • Causes of the accident

    The leading causes of the accident at Fukushima Daiichi were related to inadequate design decisions and technical equipment at the plant, as well as a lack of a strategy for managing crises such as earthquakes and tsunamis. In addition, insufficient training and simulations were conducted to train personnel to operate under extreme conditions.
    Problems were also identified with the long-term safety of the Fukushima Daiichi nuclear power plant, including a lack of flood and tsunami protection measures, and a lack of monitoring of plant operations and maintenance.
    More than 100,000 people were evacuated as a result of the accident, and radioactive releases led to significant environmental contamination and health hazards in the area around the plant. A total of three reactor units were lost, and the situation was recognized as the most serious nuclear accident since 1986 when the Chornobyl accident occurred.
  • Consequences of the Fukushima Daiichi disaster

    The consequences of the Fukushima Daiichi accident were catastrophic. About 160,000 residents evacuated the area around the plant to avoid radiation exposure. A large number of animals, including domestic and wild animals, died from radioactive contamination. Contamination of the air, water, and soil in the accident area was critical, and radioactive particles were found in the air, water, and soil in more remote areas of Japan and even beyond.
    One of the most serious consequences of the Fukushima Daiichi accident was an increase in cancer and other radiation-related illnesses among residents of the region. This forced the closure of many schools, hospitals, and other institutions, which hurt life and the economy of the region.
  • Long-term Risks

    Government and scientific organizations continue to assess the environmental and health effects of the Fukushima Daiichi accident. There is a risk of continued high levels of radiation in and around the accident area and the possibility that radioactive particles could be transmitted through the air, water, and soil to wider regions of Japan and even beyond.
    The international community is actively debating the consequences of the Fukushima Daiichi accident and considering measures to improve the safety of nuclear power plants and reduce the risks of nuclear accidents. This catastrophic incident was a reminder of how important it is to strictly adhere to safety measures in the design, construction, and operation of plants.



August 28

The opening of the floodgates at Fukushima has begun.
Discharging radioactive water from the damaged Fukushima Daiichi nuclear power plant is avoidable, risky and potentially illegal

The Japanese government moving forward with the discharge plan, disregarding its commitments to the global community and international efforts for environmental protection sets a precedent for how the global community responds to modern nuclear crises.

Approving this plan means approving a compromise on human and environmental health, inflicting a transboundary and transgenerational problem on peoples around the Pacific with no offsetting benefit or say in the decision, and a failure to engage state and non-state actors with stakes in the nuclear industry to question what’s acceptable.

The Japanese government must follow through on its commitments to the international community and critically consider alternatives for wastewater disposal. The discharge is planned to go on for 30-40 years and radioactive wastewater will continue to accumulate.

Even though it has already started, it can still be stopped and a better alternative implemented.

August 24

Japan said it will start releasing into the sea more than 1 million metric tons of treated radioactive water from the wrecked Fukushima nuclear power plant, going ahead with a plan heavily criticized by China.

The plan, approved two years ago by the Japanese government as crucial to decommissioning the plant operated by Tokyo Electric Power Company (Tepco) (9501.T), has also faced criticism from local fishing groups fearing reputational damage.

The water will initially be released in smaller portions and with extra checks, with the first discharge totalling 7,800 cubic metres over about 17 days starting Thursday, Tepco said.

That water will contain about 190 becquerels of tritium per litre, below the World Health Organisation drinking water limit of 10,000 becquerels per litre, according to Tepco. A becquerel is a unit of radioactivity.

Japan has said that the water release is safe. The International Atomic Energy Agency (IAEA), the U.N. nuclear watchdog, greenlighted the plan in July, saying that it met international standards and that the impact it would have on people and the environment was "negligible".

Despite assurances, some neighbouring countries have also expressed scepticism over the safety of the plan, with Beijing the biggest critic.


March 10

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 the nuclear waste, fuel rods, and more than one million tons of radioactive water still stored at the site.

But some residents have decided never to return because they are afraid of radiation, have built new lives elsewhere, or don't want to return to where disaster struck.
Some scientists believe that a huge ocean would dilute the water and pose a low risk to human and animal health. However, the environmental group Greenpeace said that the water contains substances that can potentially damage human DNA.


June 01

Following the devastating Tohoku earthquake and tsunami that struck Japan in March 2011, some reactors at the Fukushima Dai-ichi nuclear power plant began releasing radioactive materials into the environment. This study draws lessons from these experiences regarding technological countermeasures for dealing with radioactive contamination to improve the response to future radiological or nuclear emergencies. In particular, it focuses on how technology has been used to measure contamination in space and time, to limit the spread of radioactive materials in the environment, to decontaminate areas or items, and to store radioactive materials for long periods.
The authors gathered data through an extensive literature review and dozens of interviews with experts in both Japan and the United States. The report analyzes the successful use of technology and identifies capability gaps that can be addressed with new technologies or improved use of existing technologies. Also included in the report is an abbreviated bibliography for further reading.
Rapidly deployable sensors capable of rapidly surveying large areas are critical for the initial characterization and ongoing monitoring of a radioactive dispersal event.
In addition, thinner local sensors suitable for supporting the creation and maintenance of safe corridors and waiting areas will be needed, as well as reinforced unmanned systems carrying sensors.
Gaps in knowledge of the extent of contamination in the early stages of the Fukushima accident prevented officials from taking a fully effective response. Thus, information technology to quickly and accurately share and display real-time radiological sensor measurements is needed to support the response to the disaster.


August 11

With the closure of all nuclear power plants, Japan relies on imported fossil fuels for energy, a huge expense. The government has said that nuclear power must be revived to reduce import costs and rising CO2 emissions.

After passing stringent new safety tests, Kyushu Electric Power restarted the number one reactor at its Sendai plant. Experts were concerned because such a mass restart of inactive reactors has never been seen before.
The Japan Nuclear Regulatory Authority approved the restart of two reactors at the Sendai plant in September 2014 under stricter safety regulations. 


January 01

Caesium-137 of Fukushima origin has been detected in the waters of the Canadian continental shelf at concentrations of 0.5 Bq/m³, which is below the global concentration of radiocaesium in the ocean of 1 Bq/m³.
As a result of the accident, the population of Japan was subjected to additional exposure. The average effective dose of the evacuated population, depending on the time spent in the exclusion zone, was 6...10 mSv for the first year after the accident. Residents of Fukushima Prefecture received doses on average below 4 mSv, and the exposure of most of the population of Japan was comparable to exposure from natural background or much lower.


December 16

The accident stage was officially completed when the destroyed reactors were transferred to the "equivalent cold" state, in which the temperature of the environment inside the containment stabilized below 100 °C. According to the International Nuclear Event Scale (INES), the accident was assigned the maximum, 7th level - "Major accident", which was previously assigned only once during the accident at the Chornobyl nuclear power plant.

June 27

The cooling of the reactors began to be carried out in a closed circuit: the water flowing from the reactors entered the turbine buildings of the power units, from where it was taken by pumps, cleaned on filters, and sent back to the reactors. In addition, the shells of units 1–3 were filled with an inert gas, nitrogen.

April 17

TEPCO has published a program of measures aimed at stabilizing the situation at nuclear power plants. As a result of the program's implementation was supposed to achieve a stable reduction in the radiation dose rate and control the discharge of radioactive substances.

April 04

To ensure the pumping of highly radioactive water from the underground structures of power units No. 1, 2, and 3, TEPCO announced the forced discharge into the sea of approximately 10 thousand tons of low-radioactive water from the station's storage of radioactive waste. This measure was necessary to release the volume of highly active water, and the Japanese government permitted the operation. According to TEPCO, the release of water could add only 0.6 mSv to the radiation dose for a person who would live near the station.

March 26

The power supply of the 3rd and 4th units was established.

March 20

Only after the delivery of mobile switchgear and transformers, as well as the laying of temporary cables, the external power supply of the 1st and 2nd power units were restored.

March 15

At 3:00 a.m. on March 15, Prime Minister Kan was informed of the possible evacuation of the plant, and he immediately rejected the suggestion as totally unacceptable. Even before this request, Kan had a persistent mistrust of TEPCO and doubted the adequacy of the measures taken to eliminate the accident. At 05:30 the Prime Minister arrived at the TEPCO crisis center in Tokyo and officially announced the creation of a joint (government and TEPCO) accident control center. According to officials, this allowed the government to take control of the situation.
Meanwhile, at the nuclear power plant, after the personnel of the next work shift arrived at the third power unit on March 15, even through protective masks the employees at 06:10 heard the sound of a powerful explosion. They were soon ordered to return to the safe control room. Once outside, the staff saw the destruction of the Unit 4 reactor building and a lot of debris that made it difficult to get around. The staff had to walk and did not manage to transmit information about the destruction to the crisis center until 8 a.m. As the investigation established, the cause of the explosion in Unit 4 was hydrogen, which entered through the ventilation system from Unit 3 when it was venting from its containment. There was no source of hydrogen at Unit 4 itself, the fuel had been unloaded from the reactor, and there was plenty of water in the spent fuel pool.
Masao Yoshida learned of the explosion shortly after 6 a.m., but he was not yet aware of the destruction of Unit 4. At the same time, the pressure gauge reading in the condensation chamber of Unit 2 dropped to zero, and Yoshida decided that the explosion had occurred inside the containment of Unit 2. This prompted him to give instructions to shelter employees in places with the lowest possible radiation background near the Fukushima Daiichi NPP until the situation stabilizes. At 7 a.m., however, 650 people went to Fukushima Daiichi instead.
There was no explosion at Unit 2 of the plant. Although the fuel was damaged and there was a steam-zirconium reaction, the hydrogen produced escaped into the atmosphere through a knocked-out panel of the reactor building. The panel was torn from its place and fell onto the roof of an adjacent building after an explosion in one of the adjacent units. However, Unit 2 could have been the source of much of the release to the environment when its containment was presumably depressurized between 0700 and 1100.

March 14

By 6 a.m. on March 14, Masao Yoshida became increasingly concerned about the possibility of a hydrogen explosion at Unit 3 of the nuclear power plant. There were enough reasons for this: a probable core drain, increased radiation levels near the reactor building, the appearance of steam outside the reactor doors, and increased pressure in the containment - everything was the same at Unit 1 as before.
At 6:30, Yoshida ordered all workers off the site, but the seawater cooling situation called for a major operation. The water supply in the Unit 3 switching chamber, from which water was also taken to cool Reactor 1, was running low. Already at 07:30, Yoshida was forced to resume operations. Several fire trucks arrived and used the water directly from the ocean, raising it to a height of over 10 meters.
Work to organize an uninterrupted supply of seawater to the reactors was actively continuing when at 11:01 a.m. a hydrogen explosion occurred at Unit 3. The explosion injured four TEPCO employees and three Nanmei employees, as well as four members of the Japan Self-Defense Forces, fire brigades who came to the aid of the plant personnel. Reactor cooling was again interrupted, and because of the difficult radiation situation and continuing aftershocks, it was only possible to restore it by the evening of the same day (at 16:30 of Unit 3, at 19:57 of Unit 2, at 20:30 of Unit 1).
Surprisingly, the RCIC system of Unit 2 had been operating without power up to that point, but its performance was declining. Earlier, on March 12 at 04:00, the water intake of the system was switched to the Mark-I containment vessel (tank shape - torus) due to the depletion of condensate pumped into the reactor. The circulation of coolant through the reactor began to run on a closed loop, and the entire system gradually heated up. Around 1:25 p.m. on March 14, the coolant level in the Unit 2 reactor dropped, and there were all signs that the RCIC system was shut down.
Personnel constantly had problems maintaining low pressure in the Unit 2 reactor, the supply from the fire engines was intermittently interrupted, and Yoshida began seriously considering evacuating most of the personnel from the plant because of the risk of breaching the containment. On the night of March 14-15, TEPCO President Shimizu discussed the issue with Minister Kayeda, who took it as a request to evacuate the plant altogether.

March 13

While the accident at Unit 1 was being dealt with, the situation at Units 2 and 3 remained relatively stable. These units used a cooling system consisting of a steam turbine and a pump connected to it (Reactor Core Isolation Cooling - RCIC). The turbine was driven by steam from the reactor and the pump supplied cooling water from the condensate storage tanks to the reactor plant. DC power was required for control and regulation, but at first, even when Unit 2 was completely de-energized, the system was able to perform its functions because it was activated manually only a few minutes before the blackout.

Already on March 12, the RCIC system at Unit 3 shut down spontaneously, despite the presence of DC power. When the reactor coolant level dropped, the High-Pressure Coolant Injection (HPCI) system was automatically activated. The HPCI system, although designed similarly to RCIC, has a much higher capacity and was not designed for long-term reactor cooling. Because of the large amount of cooling water supplied, the reactor pressure dropped to 0.8 MPa, and the HPCI turbine operated at a reduced speed. Because out-of-range system operation was unreliable, Unit 3 personnel decided to supply water to the reactor from a stationary diesel-driven fire pump. To do this, the plan was to maintain reduced reactor pressure by opening its safety valves. These intentions were not properly communicated to Superintendent Yoshida.

On March 13, the operators of Unit 3 began implementing their strategy. At 02:42, the HPCI system was manually shut down at a reactor pressure of 0.580 MPa, but attempts to open the safety valve were unsuccessful. Most likely by this time the batteries could no longer provide the necessary current to actuate the valve. The pressure in the reactor began to rise and by 03:44 reached 4.1 MPa, which greatly exceeded the capacity of the fire extinguishing pump. The backup batteries used at Fukushima to control systems such as HPCI cannot be transported manually. It is unlikely that even by finding such a battery, personnel would have been able to transport it to the installation site.

March 12

According to Masao Yoshida, general director of the Tokyo Electric Power Company's Nuclear Asset Management Department and director of the Fukushima Daiichi NPP, none of the methods described in the emergency instructions for feeding water to the reactors could be applied in the current situation. Most of the emergency measures required a power supply, and the use of a stationary diesel fire pump was questionable because the tanks from which it drew water were outside and had most likely been damaged by the disaster. The method proposed by Yoshida was to use conventional fire trucks whose hoses could be connected to the extinguishing system outlets located outside the turbine buildings.

The possibility of supplying water to the reactor from a fixed fire extinguishing system was not foreseen in the original plant design and was implemented in 2002 by installing jumpers between the respective pipelines. Additional extinguishing system outlets on the outer walls of the turbine buildings were installed in 2010, just 9 months before the accident. These outlets were intended only for water replenishment, and the use of fire trucks to supply water to the reactor was not considered in the instructions because it was believed that a diesel-powered fire pump was not dependent on power sources and was available in any situation.
Thus, Yoshida's decision was improvised, the procedure had not been established in advance, and personnel responsibilities were not assigned, which ultimately led to a significant delay in the reactor water supply.
Shortly before midnight on March 11-12, plant personnel was able to restore some instrument readings using a small mobile generator found by a contractor. The pressure in the containment of Unit 1 was 0.6 MPa (abs.), which exceeded the maximum allowable value of 0.528 MPa (abs.).

March 11

At 14:46 local time, the main shock of the Great East Japan Earthquake of magnitude 9 occurred, with the epicenter located 180 km from Fukushima-1 NPP. At that time power units, 1-3 were operating at nominal capacity, and power units 4-6 were shut down for refueling and maintenance. The earthquake caused the operating reactors to shut down immediately and automatically.
Earthquake-induced damage to high-voltage equipment and transmission towers resulted in the loss of off-site power, after which backup diesel generators automatically started. Analysis of plant operations before the tsunami showed that the plant withstood the effects of the seismic shocks.

As a result of rock displacement, the seafloor was deformed, raising it by 7-10 meters, which caused several tsunami waves. The first wave, 4 meters high, reached the plant 40 minutes after the shock, and at 3:35 pm the second wave, 14-15 meters high, exceeded the height of the protective dam, designed for a wave of 5.5 meters, and the level of the nuclear power plant itself.
The tsunami washed away heavy tanks, equipment, and vehicles in the street and reached buildings far from the shore, leaving behind much debris. The flooding also caused a loss of life: two TEPCO employees in the turbine building of Unit 4 were slammed by the surging water and died.



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