A magnitude 9 earthquake disabled the external power supply for the Fukushima Daiichi power plant site. As a result, the emergency diesel generators of the individual units started up. They ensured the supply of important safety systems, especially the residual heat removal system.

The earthquake caused a tsunami that reached the nuclear power plant about three quarters of an hour later. The waves at the site of the power plant measured up to 15.5 metres – more than twice the height of the site design basis tsunami (5.7 metres).

Source: picture alliance / dpa | Aflo / Mainichi Newspaper

Water entered the buildings and caused the failure of the running emergency diesel generators, the associated electrical switchgear and the cooling water systems. The uninterruptible DC power supply was also affected. This meant that both the external power supply and the emergency power supply failed in units 1-4 - this is referred to as a station blackout.

Large parts of the emergency power supply in units 5 and 6 also failed. One emergency diesel generator continued to function and was used alternately for units 5 and 6. Severe core damage in these units was thus avoided.

Overview of the Fukushima Daiichi plant Source: BASE

As a result of the station blackout, the residual heat removal systems were no longer supplied with power. Only a number of passive systems (pressure limit, emergency condenser in unit 1, turbine-powered feed pumps) continued to function for a while longer, at least to a certain extent.

These systems, which operate without an external power supply, slowed down the course of the accident but were unable stop it.

Without functioning feed-in systems pumping water into the reactor pressure vessels and without heat removal from the containment vessels, it was not possible to keep the plant in safe operation permanently. The water level in the reactor pressure vessels subsequently dropped and exposed the reactor cores.

This caused the reactor cores in units 1-3 to overheat and finally melt down.

Due to the absence of heat removal from the containment vessel, the pressure inside the containments in units 1-3 increased. The emergency countermeasure provided for such cases is the so-called venting to relieve containment pressure. Several valves are used to release the pressure in the containment vessels to the atmosphere through an exhaust stack. In the event of an accident, this should significantly reduce the release of radioactivity into the atmosphere.

During a nuclear meltdown, the fuel rod cladding material reacts with water at high temperatures, and produces hydrogen. In Units 1, 3 and 4, insufficient venting caused hydrogen explosions that severely damaged the reactor buildings. The explosion in unit 4 - where no meltdown occurred- apparently resulted from hydrogen arising in unit 3 and reaching unit 4 by backflow through shared ducts.

The explosions hampered and delayed the implementation of emergency countermeasures such as pumping water into the reactor pressure vessels.

View of the site of the destroyed Fukushima Daiichi nuclear power plant Source: (c) dpa

The original 1966 tsunami design had defined the maximum wave height at +3.122 metres above sea level. Until 2009, this design had been re-evaluated several times. Based on these re-evaluations, retrofitting measures were carried out to increase the maximum wave height to 5.7 metres at the time of the accident.

Starting in 2009, the operator had carried out a series of voluntary analyses. These showed possible tsunami heights of up to 9.3 metres for units 1-4. With regard to locations near the northern and southern boundaries of the plant site, the analysis identified possible tsunami heights of up to 15 metres (cf. the tsunami height of 13.1 metres observed at the plant site on 11 March 2011).

Yet, no changes were made to the plant as a result of these analyses. IAEA investigations also showed that the emergency power supply had not been adequately designed to withstand flooding.

Inadequate containment pressure relief or venting following the tsunami also played a decisive role in the sequence of events. In this process, several valves are used to release the pressure in the containment vessels to the atmosphere through an exhaust stack. According to the IAEA, timely and successful venting in time would have facilitated more effective emergency measures for core cooling and could have prevented the hydrogen explosions of the reactor buildings. The explosion in Unit 4 (which was not affected by a core meltdown) - caused by the entry of hydrogen from Unit 3 - also demonstrates the inadequate venting.

Cross-section of the plant and height of the tsunami Source: BASE

Human and cultural factors Source: BASE/Michael Meier

Japanese and international teams of experts concluded that Fukushima could have been prevented with appropriate precautions. Human and cultural factors played a decisive role in the catastrophic accident.

Why were there no adequate precautions? Why was the accident not prevented or at least mitigated by comprehensive risk management?
The technical weaknesses of the nuclear facilities were largely known and avoidable. Furthermore, there was no comprehensive safety culture in the cooperation between operating companies, the Japanese supervisory authority and the government. It was believed that a severe accident was not possible, and that the Japanese nuclear system was sufficiently safe and efficient. In addition, the inquiries launched after the accident claimed that the Japanese national culture, which is very much group-oriented and authority-centred, was one of the reasons for the poorly developed safety culture. The failure to learn from other serious accidents, such as those at the Three Mile Island (USA) or Chernobyl (Ukraine) nuclear power plants, was also cited.

After the catastrophic accident in Fukushima, government organisations and operators worldwide reviewed their understanding of the concept of safety culture. Topics such as the independence of oversight authorities, the monitoring of operators’ safety culture, as well as the reflection on and promotion of individual safety culture concepts at the respective oversight authorities were put on the agenda.

A more detailed account of the impact and significance of human, organisational and cultural factors can be found in the technical report (in German).

The release of radioactivity into the atmosphere was mainly caused by:

1. Unfiltered containment venting:
   In addition to the release of noble gases, which would also have occurred with filtered venting, this led to the release of mainly highly volatile fission products such as iodine and caesium.
2. Containment leakage:
   In the course of the accident, the design pressure and temperature of the containments were (in part significantly) exceeded in units 1-3. Leakage probably occurred during this process.

In addition to being released into the atmosphere, radioactive substances were also released into water – especially the water injected for emergency cooling. As there were no more closed cooling circuits, large quantities of contaminated water escaped through leaks in the containment vessels and accumulated in the buildings. In early April 2011, heavily contaminated water leaked into the sea. In addition, water - mainly groundwater - entered the buildings from the outside.

Various measures, including the sealing of leaks on buildings, were taken to successfully reduce the inflow of groundwater into the buildings. These include:

• Commissioning of groundwater drainage wells and drainage wells.
• Sealing leakages on buildings and building ducts
• Construction of a waterproof structural groundwater barrier directly in front of the quay wall
• Freezing the soil around the reactor buildings of units 1-4
• Sealing off a large part of the plant site and the harbour basin seabed

In addition, a purification plant for the contaminated water is in operation. Water that is not fed back into the reactors for cooling after treatment is temporarily stored in various tanks on the plant site. A constant expansion of the storage capacities has been necessary so far. Parts of the treated water are to be discharged into the sea from the end of 2023 on. This involves, in particular, groundwater that has been diverted around the power plant site. The concentration of radioactive substances still present in these waters is far below the legal limits.

Tanks with radioactively contaminated water on the site of the Fukushima Daiichi nuclear power plant Source: picture alliance / ASSOCIATED PRESS | Yasushi Kanno

250 km

... from Fukushima - in Tokyo - the iodine-131 contamination of the drinking water temporarily exceeded the safe level for young children. Numerous food items such as vegetables, milk or herbs from the affected regions were banned for consumption. At the end of March/beginning of April, high concentrations of caesium-137 and iodine-131 in particular were detected in the sea near the nuclear power plant, but dropped to low levels by the end of April. Fishing had to be suspended in part, because the radioactivity in several types of fish caught in the Fukushima area was above the legal limits.

20,000

... People have died or are still reported missing as a result of the quake and tsunami. The tsunami flooded more than 560 km² of the Japanese mainland, over 470,000 buildings were severely damaged or destroyed, about 4,000,000 households had no electricity, and 2,300,000 households had no drinking water.

470,000

... is the total number of evacuees in all prefectures, according to the Japanese government. The total number of persons evacuated due to the radiological situation in Fukushima Prefecture was approximately 110,000, and the total number of evacuees in Fukushima Prefecture was approximately 165,000. Of these, 37,000 still had not returned at the end of 2020.

Prevailing winds carried the released radionuclides, spreading them locally, regionally and globally, and successively dispersing them over land and sea. Which radioactive substance ended up where depended largely on the time of its release and the prevailing weather conditions at the time, i.e. wind and precipitation.

For about a month after the Fukushima reactor accident, an increased concentration of iodine-131 and caesium-134/137 was measured in the air in Germany. However, the measured concentrations were low enough not to pose a health risk to people and the environment in Germany and Europe. By the end of May 2011, the measured values had returned to a pre-accident level.

The reactor buildings were badly damaged during the accident. The first step was to ensure the stability and functioning of the buildings throughout the entire decommissioning process.

Enclosures were erected to prevent the release of radioactive substances into the environment. These enclosures also facilitate the installation of equipment to retrieve the fuel assemblies from the fuel pools and the nuclear material from the reactors.

Radioactively contaminated water is a major problem. This mainly refers to water that was injected and subsequently contaminated during emergency cooling after the catastrophic accident.

Since there were no more closed cooling circuits, large quantities of contaminated water accumulated in the buildings via leakages from the containment vessels. Through various measures, including sealing leaks on buildings, the inflow of groundwater into the buildings has since been significantly reduced (see "Release of radioactivity"). The operator TEPCO works to purify this contaminated water. Tritium, however, will remain. TEPCO plans to discharge the purified water into the sea in diluted form.

Tepco plans to retrieve the destroyed reactor cores and the fuel elements from the fuel pools and to dispose of them. The fuel assemblies in the storage pools of Unit 4 were recovered between November 2013 and December 2014. Unloading for Unit 3 began in April 2019. Preparatory work, such as the removal of debris, is underway in Units 1 and 2. Investigations regarding the salvage of molten and subsequently solidified nuclear material from the reactors of units 1-3 are in progress.

Two storage facilities for fuel elements are available on site: A central wet storage facility and a temporary dry storage facility.

An overview of the condition of the inner areas of the reactor buildings is needed to retrieve the nuclear material from the reactors. Radiation levels in the buildings are high, and the condition and distribution of the nuclear material are unclear.

Therefore, the retrieval will be carried out mainly by remotely-controlled robots. Initial trials using a newly developed robotic arm and gripping tools are to begin in unit 2.

Following several years of decontamination work in the vicinity of the Fukushima nuclear power plant, some of the evacuated areas were declared decontaminated. Important infrastructure facilities are also back in operation. To achieve this, roofs and gutters in the vicinity of the nuclear power plant were cleaned, soil surface layers were removed and organic material was collected.

The large quantities of low-level radioactive waste were initially stored in many temporary storage facilities in the Fukushima region. Today, most of the waste is stored in a newly constructed interim storage facility, and the old storage sites have been re-cultivated.