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The earthquake in Japan
In-depth on the earthquake in Japan
Japan is one of the countries in which earthquakes occur most frequently. The reason of this phenomenon is simple to explain: the Japanese archipelago. in fact. is inside what is known as the “ring of fire”, an area that stretches along the edge of the Pacific Ocean, where 75% of the active volcanoes in the world are found (the term ring of fire is, in fact, due to the intense volcanic activity that is present in this area), and where 90% of the seismic activity of the planet takes place. This horseshoe shaped belt extends approximately 40 thousand kilometres : North-west of New Zealand to the Philippines, north-east of Japan , east of Alaska, south of Oregon, California and Mexico, right up to the Andes mountains in South America. This area is characterized by particular geodynamic phenomena, due to the complex and tumultuous geology of this portion of the planet. In particular, Japan is located on a point where four lithosphere plates meet: the Pacific plate, that coincides approximately with the Pacific Ocean, the Philippine Sea plate, the Eurasian plate and the North American plate. The plates are not stationary, they “float” like “rafts” on the partially molten astenosphere below. The earthquake that hit Japan on 11th March, took place in the area of contact between the Pacific plate and the North American plate. In this area, the Pacific plate is subducted, i.e. it slides under the North American plate, and it moves about 10 centimetres per year. Along the entire subduction plane, there are numerous earthquakes every day, most of which are recorded only by the seismographs, while others, like the one on the 11th March, are extremely violent. Furthermore, often these earthquakes are followed by exceptionally high anomalous waves, known as tsunami. On the 11th of March, the worst took place, an 8.9 magnitude earthquake followed by a tsunami, with waves reaching up to 10 metres high, causing numerous victims and extremely serious damages to structures and towns on the eastern coast of the Japanese archipelago.
Japan is a country in which there have always been big earthquakes. Up to two hundred years ago, the Japanese were convinced that earthquakes were caused by the contortions of large cat fish, and in the prints of the past, we often see illustrations of these animals that were killed because they were considered the cause of devastating earthquakes. Today, modern science has cast a light on the causes of the earthquakes and Japan has learnt to live with these phenomena, developing a widespread anti-seismic culture and using avant-garde techniques to face these natural phenomena, minimizing the number of victims and damages to structures. In fact, the buildings in Japan are built to resist the effect of very violent earthquakes and the Japanese population is frequently made to carry out drills on how to behave in the case of an earthquake. Furthermore, in all workplaces and in private homes, it is compulsory to keep a survival-bag containing water, lyophilized food, a helmet, an electric torch and radio, to be used in case of emergency. When there is an earthquake, besides the images, the radio and television network broadcasts an acoustic alarm that can also be heard when the radio is turned off. After two minutes the citizens are informed of the intensity and the epicentre of the earthquake, and the possible danger of an anomalous wave, together with the estimated time the tsunami wave will reach the coast. This efficient earthquake management system has made it possible to limit the damages and loss of human lives that an earthquake of such an intensity could potentially have caused. In fact, what increased the magnitude of the disaster was the effect of the tsunami, with waves that were more than 10 metres (about 30 feet) high, that swept away entire towns, destroyed many kilometres of the coast.
The earthquake and nuclear risk
And after the earthquake, the nuclear alarm set off. The large earthquake of the 11th in fact damaged the power plant in Fukushima, in the north of Japan, and led to the explosions. The nuclear risk at the Fukushima power plant has once again brought up the discussion about the safety of nuclear power plants. The theoretical risk, for a nuclear power plant, is always present, but it must be said that this earthquake was the most disruptive and devastating one ever recorded in the history of Japan. Considering the above, every man made structure, every building could be damaged or affected by malfunctioning parts, due to the effect of such intense stress. It is true that at the power plant in Fukushima, an explosion took place, but its origin was not nuclear, it was a conventional type of chemical explosion. The deflagration shattered the external structure of the reactor leaving, so it seems, the inner part practically intact. To avoid greater damages, the steam that had accumulated inside the structure was released in a controlled manner. This steam, even though it was not extrinsically radioactive, transported a certain dose of radiation, and for this reason the order was issued to evacuate the surrounding area. Currently, contamination that is however present, is limited and circumscribed in the area.
How does a nuclear power plant work?
A simple example can help us understand how a nuclear power plant works. A nuclear power plant is not very different from the boiler at home, where a burner, usually fed by gas, produces heat and heats the water. In the case of a nuclear reactor instead, the core, following nuclear fission reactions of the atoms in the nuclear combustible, produces heat. The reactions take place in a vessel in which enriched uranium rods are alternated with reaction moderating rods. The water is pumped in pipes, heated by the energy developed in the fission reactions, and transformed into steam that turns the turbines for the production of electric energy. The vessel containing the fission reactions is surrounded by another shield, that is the primary shield, that contains a reserve of water to prevent overheating. Since the fission reaction is a chain reaction, in order to stop it, reaction moderator bars must be used to interrupt the connection between the uranium components.
What happened in Fukushima?
Due to natural causes, the Fukushima power plant incurred significant damages, the cooling system was no longer efficient, and there was overheating. Just like in a car radiator, when it starts to lose the cooling liquid, the temperature increases and there is a risk that the gaskets do not hold, and consequently clouds of steam can escape. In this specific case, the Japanese Authorities, that were aware of the danger that was at hand, and the possibility of other explosions, deliberately released the steam, even if it was partially contaminated, to prevent greater risks.
Taken and re-elaborated from an interview with Valerio Rossi Albertini, Researcher at CNR Consiglio Nazionale delle Ricerche, the Italian National Research Council, broadcast on the Italian TV network Rai News, on 13 March 2011
In the case of Japan, the worst hypothesis is a core meltdown, and explosion of the reactor. In this case the leakage of radioactive material would be inevitable and massive. This is a theoretical hypothesis, that has never occurred in the history of civil nuclear power. In the accident at Three Mile Island, in 1979, in fact, the core meltdown stage was reached, but there was no damage because the reactor remained perfectly intact. And therefore nothing escaped from the power plant and all the radioactive material was sealed by dumping in cement. In Chernobyl instead, the core meltdown was never reached, but there was an explosion, and some radioactive combustible escaped, with much more severe consequences. In the case of Fukushima, a core meltdown is a remote hypothesis at present.
written by Tiziana Bosco and Benedetta Palazzo
