Fukushima disaster

March's magnitude 9.3-earthquake was as strong as hundreds of thousands of the explosions caused by the atomic bomb dropped on Hiroshima in 1945. It triggered a tsunami that covered 470 square kilometers (181 square miles) with rubble and water, while more than 19,000 people died in the flooding or are still missing. Water levels were as high as 16 meters (52 feet) – as high as a four story building.

The Fukushima Daiichi nuclear power plant, one of the biggest nuclear power plants in Japan and only 240 km northeast of Tokyo, was not built for this kind of catastrophe. The reactor's walls were only designed to sustain tsunami waves of 5.7 meters. When the 13-meter wave hit the plant, it was completely flooded.

Nuclear fission after the plant's cooling system went down

Even a late scramble could not stop the catastrophe. Since emergency power supplies were flooded and seawater pumps were destroyed, the plant's cooling technology failed. In the aftermath of the earthquake, plant operator Tokyo Electric Power Company (Tepco) was not able to reactivate the cooling systems. To compound matters, the surrounding streets were impassable and the necessary equipment could not reach Fukushima fast enough.

Of the plant's six reactors, reactors 1 to 4 were hit the hardest. They were covered in water as high as five meters, whereas reactor blocks 5 and 6 only had to cope with a flooding of one meter. This is where the only generator that survived the tsunami was located and later served as a power supply.

After the cooling system went offline, the reactor cores overheated in the first three blocks. Plant operator Tepco had to reduce the pressure on the valves by opening them, which is how the first radioactive plumes were discharged. Within the first four days after the tsunami hit, meltdowns in reactors as well as hydrogen explosions destroyed the reactor buildings.

Block 4 was not running when the earthquake hit, but nuclear fuel was stored in a cooling pond. After the water had evaporated due to the plant's lack of cooling, this fuel was partly exposed, and the heat caused a fire and further hydrogen explosions.

Technicians measuring the neutron radiation were worried critical fission reactions had started to occur in some reactors. That's why they decided to add boric acid to the reactor cores. The 10-bor isotope is able to capture neutrons and prevent further meltdowns. South Korea and France delivered 150 tonnes of boric to Fukushima.

Favorable winds and contaminated water

Luckily most of the radioactive plumes were blown out to the sea. Wind coming from the north-west would have endangered the densely populated area around Tokyo. Even though the wind direction was favorable, vast areas around Fukushima were contaminated.

In the following weeks and months, radioactivity continued to emerge from the plant, in the form of water and steam. That's why the plant operator thought it necessary to cool down the reactor cores with water from the sea. They used water cannon and concrete pumps on cranes to get close to the cores.

The radioactive waste water accumulated in the plant's turbine building. Until April, some 60,000 tonnes of highly contaminated water accumulated, forcing Tepco to discharge more than 10,000 tonnes into the sea. In June, engineers were able to start up several water decontamination plants which can clean up to 1,000 tonnes of water per day. The resulting radioactive sludge is currently stored on site.

In November, Tepco allowed the first journalists to visit the power plant in order to demonstrate the now "stable" situation. Working conditions were said to have drastically improved. The cooling system is up and running. Still, in one area a reporter's dosimeter recorded radiation at 50 microsieverts per hour – as much as an x-ray. Japan's threshold value for evacuating people from their homes is 20 millisieverts (20,000 microsievert) per year.

200,000 people directly affected

Because of the radioactive contamination, Japanese authorities evacuated an area of 30 kilometers around the nuclear power plant. Initially, some 200,000 people were affected. In the fall, the evacuation zone around the damaged plant was reduced to 20 kilometers, still leaving about 100,000 people who cannot return to their homes.

Authorities issued a ban on vegetables, meat and diary products from five prefectures of the power plant's region. Since the summer, the ban is limited to products that have been produced within 20 kilometers.

Even though some workers have been exposed to very high radiation, no one has died from it so far. The cleanup on site will still take at least 30 years.

The Japanese nuclear safety agency estimates the amount of radioactivity that was released in all four affected reactors to be one fifth to one tenth the amount exposed in the 1986-Chernobyl disaster. Yet the International Atomic Energy Agency classified the Fukushima meltdown as a level-7 incident – the most serious level in its ranking – and therefore described it as a "major accident" with a "major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures."

Author: Fabian Schmidt / sst
Editor: Ben Knight