The Fukushima accident

On 11 March 2011, at 14.46 pm local time, an earthquake measuring 9.0 on the Richter scale struck the north of the main Japanese island of Honshu. A little later on, a tsunami reached the north-eastern coast of the island, with devastating consequences for the people living in the region.

Accident at the nuclear power plant Fukushima Daiichi

As a result of the earthquake, the Fukushima Daiichi nuclear power plant (also referred to as Fukushima-1 or Fukushima-I in the media) consisting of six boiling-water reactors lost its connection to the public grid. The nuclear chain reaction in reactor units 1 to 3 in operation at that time was stopped by reactor scram. So far it has not been possible to clarify what kind of damage to the facilities was directly caused by the earthquake.

Moreover, the emergency power supply in units 1 to 4 failed in the long range because of the tsunami following the earthquake. As a result, the energy supply to cool the fuel elements in the reactor cores and the fuel pool, which is also required after reactor scram, was lost.

In units 5 and 6 great parts of the emergency power supply also failed. One remaining operational emergency diesel was used mutually for unit 5 and unit 6. Thereby serious damage to the cores was prevented.

In addition to the emergency power supply, the auxiliary service water supply failed as well because of damages to pumps and switching boards resulting from the tsunami. Among others, the auxiliary service water system is required for residual heat removal.

Scheme of a Mark I-Containment: Units 1 to 5 in Fukushima Daiichi are designed with Mark I-Containment

Damages to the reactor core

The unavailability of the emergency power supply and the auxiliary service water supply led to the failure of the core cooling and of the cooling of the fuel pools in units 1, 2 and 3 of the Fukushima Daiichi nuclear power plant. The safety systems in unit 1 completely failed immediately after the tsunami. The operation of the emergency cooling systems of units 2 and 3 could be maintained for about 3 (unit 2) or, respectively, 2 days (unit 3); after that time these systems failed, too. Because some time was needed to start the feeding in of water (at first freshwater, later on salt water) for cooling purposes, the reactor cores superheated and nuclear material melted as a result.

According to analyses of the operator, TEPCO, it is possible that the melted nuclear material in unit 1 melted through the reactor pressure vessel, right through into the concrete at the bottom of the containment (inside the containment). It is unclear whether the emergency supply of cooling water by means of fire pumps in units 2 and 3 was sufficient to prevent the reactor pressure vessel from failing. It is assumed that at unit 2 the major part of the nuclear material is still inside the reactor pressure vessels and has solidified there, but that the reactor pressure vessel was damaged, too. In unit 3 it is assumed that parts of the melted core have leaked from the reactor pressure vessel into the containment.

It is not finally clarified to what extent the containments in unit 1 to 3 have been damaged.

Hydrogen explosions

Between 12 and 15 March 2011 several hydrogen explosions occurred and led to a massive destruction of the reactor buildings of units 1, 3 and 4. When the reactor core is damaged, hydrogen can be generated as a result of steam reacting with the cladding tubes of the fuel elements. This reaction starts at temperatures from around 900°C. In boiling water reactors the containment is made inert with nitrogen in normal operation, i.e. there is not enough oxygen available for hydrogen and oxygen to react explosively with one another. However, in the course of the accident in Fukushima, larger amounts of hydrogen got into the reactor buildings. Thus, explosive mixtures have been established there.

Another result of the explosions in the reactor buildings of units 1, 3 and 4 was that debris also fell into the fuel pools located in the upper part of the reactor buildings outside the containment. In July 2012, two unused fuel elements were experimentally recovered from the fuel pool in unit 4. They were analysed and no significant damages, changes in shape or damages by corrosion were detected.

Release of radioactivity into the environment

As a result of the accident large amounts of radioactive substances were released into the environment. This also led to the classification of the accident in Fukushima Daiichi as a Level 7 "Major Accident" on the International Nuclear and Radiological Event Scale - INES.

Releases into the atmosphere mainly occured:

1. Through the unfiltered pressure relief from the containments:
   Apart from the release of noble gases, which would also have occurred if the pressure had been relieved through filters, this led mainly to the release of volatile fission products such as iodine and caesium.

2. Through leakages in the containments:
   During the course of the accident, the pressure and temperature in the containments, which the containments had been designed for, of units 1 to 3 were clearly exceeded. Therefore, it is probable that leakages have occurred.

Apart from being released into the atmosphere, radioactivity was also released into water – mainly as contamination of the water fed in for emergency cooling. As no more closed cooling circuits do exist, large volumes of contaminated water have accumulated in the buildings over leakages in the containments. Until today obviously large amounts of water - mainly ground water - penetrate from outside into the buildings. At the beginning of April 2011 highly contaminated water was released into the sea. Until today, water (mainly ground water) is penetrating from outside into the buildings.

Meanwhile through various measures the inflow of ground water into the buildings could be substantially reduced. In addition, a cleaning system for the contaminated water is in operation. After treatment the decontaminated water, which is not re-used for reactor cooling, is stored on-site in different tanks. The storage capacities had to be continually expanded.

Part of the decontaminated water can now be discharged into the sea. This concerns in particular ground water, which has been passed around the power plant site. The concentration of radioactive substances in these waters is far below the legal limits.

Incident in August 2013

On 20 August 2013, TEPCO announced that 300 tons of medium-level radioactive contaminated water had dropped out of a storage tank on the site of the plant. The incident was rated on level 3 of the INES scale.

Measures to limit the consequences of the accident

On 17 April 2011, the operator, TEPCO, presented in a so-called roadmap its plans on how to achieve a controlled condition in the medium term for units 1 to 4 of the Fukushima Daiichi nuclear power plant. A number of measures are to be taken to achieve a stable plant situation regarding

• the cooling of the reactors and fuel pools,
  
• the limitation of release of radioactive materials and
  
• the monitoring of the radiological situation inside and outside the plant.
  

The main objective for units 1 to 3 was to ensure that the destroyed reactor cores be cooled in the long term at temperatures below boiling temperature of the coolant. According to a statement given by the Japanese government on 16 December 2011, this was achieved for all reactors.

Meanwhile contaminated water is treated in a cleaning facility and re-used to cool the reactors. Key activities aim to avoid the penetration of water into the buildings and the discharge of contaminated water. Some of those ongoing measures are

• the pumping of ground water in order to avoid its penetration into the units 1 to 4,
• the targeted removal and recycling of highly contaminated water,
• erection of a wall impermeable to water at the seaside and
• freezing the soil around the unit 1 to 4 for tightening purposes.

To reduce a further release of radioactive materials into the environment, it is also planned to cover the destroyed reactor buildings. Provisional structures consisting of a steel frame – covered with a plastic film that is impermeable to water and air and equipped with a ventilation system with filter systems – has meanwhile been constructed over units 1 and 4. The damage of the reactor building of unit 2 have been repaired. Housing of unit 3 is in progress.

Long-term plans

In the long term, the operator, TEPCO, plans to recover the reactor cores part of which have melted and of the fuel elements in the fuel pools and to dispose of them. Subsequently the plant is to be dismantled.

From 18 November 2013 to December 2014, the fuel elements in the fuel pools of unit 4 were recovered. In units 1 and 3 recovery of fuel elements is planned to start in 2017 after preparatory work like removal of debris and the erection of a housing have been completed.

Since the reactor cores melted according to analysis entirely or partially in blocks 1 to 3, conventional methods for the recovery of the nuclear fuel from the damaged reactors can be used only with difficulty. An exact timing has not yet been announced by TEPCO. According to current estimates the total decommissioning will last about 30 to 40 years.