What is more, the uranium atoms that have already split in two produce radioactive by-products that themselves give off a great deal of heat. So the reactor core continues to produce heat in the absence of fissioning. If the rest of the reactor is operating normally, pumps will continue to circulate coolant usually water to carry away the reactor core's heat.
In Japan the March 11 earthquake and tsunami caused blackouts that cut off the externally sourced AC power for the reactors' cooling system. According to published reports, backup diesel generators at the power plant failed shortly thereafter, leaving the reactors uncooled and in serious danger of overheating. Without a steady coolant supply, a hot reactor core will continuously boil off the water surrounding it until the fuel is no longer immersed.
If fuel rods remain uncovered, they may begin to melt, and hot, radioactive fuel can pool at the bottom of the vessel containing the reactor. In a worst-case meltdown scenario the puddle of hot fuel could melt through the steel containment vessel and through subsequent barriers meant to contain the nuclear material, exposing massive quantities of radioactivity to the outside world.
How can a meltdown be averted? The Japanese plant's operators have made a number of attempts to cool the reactors, including pumping seawater into the reactor core to replenish the dwindling cooling fluid. The Tokyo Electric Power Company has also injected boric acid , an absorber of neutrons, into the reactors. How does this incident compare with Chernobyl or Three Mile Island?
At present, three of the reactors at Fukushima Daiichi station are seriously crippled. Units 1 and 3 have experienced explosions that destroyed exterior walls, apparently from buildups of hydrogen gas produced by the zirconium in the fuel rods reacting with coolant water at extremely high temperatures—but the interior containment vessels there thus far seem to be intact.
A third explosion was reported March 15 at reactor No. Pressure in the suppression pool—a doughnut-shaped water vessel below the reactor—dropped after the explosion, indicating that the containment vessel had been compromised.
In reactor Nos. And a fire at a pool storing spent fuel rods at dormant reactor No. At that Pennsylvania nuclear station in a cooling malfunction combined with worker error led to a partial meltdown—about half of the reactor core melted and formed a radioactive puddle at the bottom of the steel pressure vessel.
The vessel remained intact, but some radiation did escape from the plant into the surrounding environment. The Chernobyl accident was far more devastating; it rates as a 7, or a "major accident," on the INES scale. In Ukraine, then part of the Soviet Union, a power surge caused an explosion in one of the plant's reactors, releasing huge doses of radioactive fallout into the air.
Image: European Bank for Reconstruction and Development. On 11 March , the strongest earthquake ever recorded in Japan triggered a massive tsunami along the Pacific Coast. The earthquake and the ensuing tsunami resulted in the death of 19, people with still missing and devastated communities up and down the country.
Reactors close to the earthquake, including those operating at Fukushima, shut down as designed. However, as a consequence of the flood caused by the tsunami, the backup generators at the Fukushima Daiichi plant, which were meant to pump cooling water through the reactor, were destroyed. As a result, three cores largely melted over the following three days and there were several hydrogen explosions, as well as the release of nuclear material into the environment. The accident at Fukushima Daiichi did not result in any radiation deaths or cases of radiation sickness, and the United Nations Committee on the Effects of Atomic Radiation UNSCEAR concluded that there will be no observable negative health effects for the public due to radiation.
However, the evacuation of residents resulted in death and suffering, mainly amongst elderly residents in Fukushima, as well as long-term psychosocial health effects and stigmatisation. Further information can be found in the information page on the Fukushima Daiichi Accident. Shortly after the Chernobyl accident it became evident that the main impacts of nuclear accidents are not radiological, but socio-economic and psychological, driven by misconceptions about the health effects of radiation.
A very similar situation is also seen following the Fukushima Daiichi accident. Stigmatisation of both exposed and evacuated populations following both accidents has strongly contributed to a significant rise in alcoholism, depression, anxiety, bullying and suicides. In addition, some doctors in Europe advised pregnant women to undergo unnecessary abortions on account of radiation exposure due to the Chernobyl accident, even though the radiation levels concerned were vastly below those likely to have any negative health effects.
Firefighters, doctors and nurses rushed to the plant not aware of the danger. As authorities realized the extent of the catastrophe, more than 16, policemen and military personnel where sent to the power plant to extinguish the fire, remove the radioactive debris and enclose the ruin in a protective shell made of steel and concrete.
Confirmed 31 people died from radiation sickness in the first days after the accident. The long-term effects on the 16, "liquidators" and evacuated , people from the towns of Chernobyl and Pripyat are still poorly understood, however, an increase of various cancer types was blamed on the released radioactivity.
The effects on the fauna and flora inside the evacuated region are still today studied by geneticists, ecologists, botanists and zoologists. Even areas thousands of kilometers away from Chernobyl are still today contaminated with radioactive particles, transported by the wind in a large plume over Europe. As the cooling system of the reactor was shut down and the insertion of control rods into the reactor core failed, the nuclear fission went out of control, releasing enough heat to melt the fuel rods, cases, core containment vessel and anything else nearby, including the concrete floor of the reactor building.
The fuel pellets inside the fuel rods are almost entirely made of uranium-oxide while the encasing in which the pellets are placed is made of zirconium alloys. Concrete doesn't melt, but decomposes and becomes brittle at high temperatures. Part of the concrete was incorporated in the lava flow, explaining its high content of silicates, minerals composed mostly of silicon, aluminum and magnesium. Due to its chemical composition and high temperature, the lava-like material has a very low viscosity.
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