Friday march the 11th at 2:46 pm an exceptionally powerful earthquake hit the pacific coast of Honshu the main island of Japan at 3:36 pm, less than an hour after the earthquake a tsunami swept over the coast; the waves went all the way up to 10 kilometers in land. Results over 20,000 people dead, or missing, destroyed towns, ports, and land devastated. Nuclear power plants were also affected and one in particular namely the Fukushima Daishi. Fukushima Daishi is 250 kilometers northeast of Tokyo; the Nuclear power plant has six rectors. Each reactors’ successively commissioned during the nineteen seventies. Unit 1, 2, and 3 were operating at full power, the core in unit 4 was unloaded. Unit 5 and 6 were in cold shutdown.

Fukushima reactors have a different technology than the pressurized water reactors built by the French operator EDF, they are boiling water reactors called BWR, and it is called a rector because the heat in the core is produced by fission reactions. Boiling water because the water that removes the heat from the core turns into steam and the steam goes directly to the turbine. The turbine drives a generator that produces electricity. Afterwards, the steam is condensed with the help of the seawater cooling system and returns to the core. A boiling water reactor has only one single system combining feed water and steam. The core is composed of fuel assemblies containing uranium; control rods introduced from the bottom that can stop the fission reactions in case of an emergency. Fission as uranium nuclei produces radioactive atoms that in turn produces heat and this continues to occur even after rector shutdown, this is called residual heat; keeping the fuel confined and cooled is a major safety issue. The fuel was isolated from the environment by different containment barriers jut like the famous Russian dolls. A first barrier the fuel cladding of zirconium alloy, a second barrier the steel reactor vessel in combination with steam and water cooling systems, finally the third barrier the containment building in concrete leait tight steel liner. The fuel is kept under water in the reactor as well as in the adjacent pool where the spent fuel isn’t loaded. The pool is located at the top of the reactor vessel to facilitate the transfer fuel underwater.

When the earthquake hit the coast, seismic sensors triggered the insertion of control rods, although fission reactions stooped; the residual heat had to be removed. The offsite power supply was lost and the emergency diesel generators took over automatically, they supply electricity to the backup system needed for poor cooling. In reactors 2 and 3, it is a turbo pump and the steam generated by the reactor operates the terrible palm, which feeds water into the reactor vessel. The steam in condensed in the wet well suppression pool within the containment. In reactor 1, there was no turbo pump, but a heat exchanger, which condense steam from the reactor vessel. The condensed water was reintroduced into the reactor vessel by gravity. This heat exchanger provide core cooling by natural convection for more the 10 hours; until then everything seemed under control. However, reactors 1 due to excessive cooling force the operators to temporarily isolate the heat exchanger in compliance with operating procedures.

The tsunami wave arrived less then an hour after the earthquake. The waves went over the seawall, flooding the lower part of buildings and disabled the emergency diesel generators. On reactor 1, the operator was unable to reactivate the heat exchanger, the core was no longer cooled, and it would be the first to melt. On unit 2 and 3, the batteries were still operational, they operated sum of the valves. The turbine driven pumps ran for nearly 24 hours and then stopped and the course were no longer cooled. The meltdown scenario is almost the same with all three reactors, only the dates were different. The water in the reactor vessel evaporated and the fuel become uncovered and heated up to a temperature of 2300 degree Celsius causing the fuel to melt and be mixed with the materials from the structure to from a magma called corium. The corium flowed down to the bottom of the reactor vessel. According to Japanese officials say, it pierced the reactor vessel before falling on the concrete basement inside the containment.

What quantity of corium fell? How deep did it a root the concrete? Did it pierce the steel liner? Even today it is not possible to learn more about the state of the corium in the three reactors. At the same time the reactor vessel’s steam was loaded with radioactive elements in hydrogen. As the steam pressure rose to a dangerous level in the reactor vessel the depressurizing vessel opened and the gas was forced into the web wall suppression pool by inventing line. The water acted as an phishing filter by trapping much of the radioactive element, but the water was no longer cold because the emergency diesel generator were out of order and it soon began to boil there by reducing its filtration capacity. The web well suppression pool in the communicating containment begin to enter into an over pressure situation to avoid containment rupture the operator decided to release the gas into the atmosphere, normally the venting line should have led all the gas outside the building, but hydrogen was escaping through uncontrolled leakage pathways and it was released into the reactor building. Hydrogen reacts violently with oxygen in the air; the explosion blew apart the frame and apparently without damaging the containment building; radioactive elements were released into the environment. Due to the absence of usable freshwater on the site, the operators decided to inject seawater into the reactor vessel. In addition, since salt is chemically active had at least the advantage of cooling in stabilizing the corian.

In the four days following the tsunami, explosions damaged the four reactors and three of them with core melt. Although is has kept its structure intact, reactor 2 is the current source of the most important radioactive releases into the soil as well as into the sea. The explosion took place inside the building; operators have probably encountered difficulties depressurizing the containment and the wet wall suppression pool broke. The explosion on reactor 4 was due to hydrogen even though the core was completely unloaded. The hydrogen came from reactor 3 via a joint pipe. The reactor’s storage pools were also in a great concern because they have lost their cooling system. In addition, they were not protected by any containment.

Gradually this situation began to stabilize, by the end of March 2011; freshwater had replaced seawater. In July the reactor cooling system was again in operation in closed circuit. Thereby, avoiding discharges of contaminated water into the environment. In December 2011, Japanese authorities officially declared that the nuclear power plant reached the cold shut down state; an expression used when cooling water does not evaporate anymore and remains liquid below 100 degree Celsius.

Men working under extremely difficult conditions managed this nuclear crisis. They were cut off from the rest of the world, with no news from their families after the tsunami, without any power supply, threatened by radiation; they fought with all their force to cool the reactors and trying to make in vain the backup system work again.

(This figure describes the six reactors in Fukushima from the inside)

Sources: 

– https://www.wikipedia.org

– http://www.naturalnews.com/Fukushima.html

– http://www.greenpeace.org/international/en/publications/Campaign-reports/Nuclear-reports/Lessons-from-Fukushima/

– http://news.nationalgeographic.com/news/energy/2013/08/130807-fukushima-radioactive-water-leak/b