As we all know after the earthquake occurred giant scale of tsunami wiped all cities near the sea. As I mentioned before it was the worst tragedy I ever saw. It happened in just 6 minutes then it ended. However another disaster happened when Fukushima Daiichi (means No 1) Nuclear Plant that was hit by tsunami had major damages. At that time, eleven nuclear reactors at the four nearest power plants automatically shut down upon sensing ground accelerations, stopping the nuclear fission of uranium in their cores. Nuclear fuel requires continued cooling even after a plant is shut down, though, because residual fission products continue to decay and produce a huge amount of heat.
TThe Japanese plants use continually-pumped water, which absorbs a great deal of heat, to cool their nuclear reactors. The earthquake knocked out the electricity at the Fukushima Daiichi plant run by Tokyo Electric Power Company (TEPCO). Emergency diesel generators were used to pump water to cool Reactor Units 1, 2 and 3, which had been operating at the time of the quake, but an hour later, the back-up generators were knocked out by tsunami flooding.
Insufficient power means that water could not be pumped through the nuclear cores quickly enough. As the water inside the reactors heated up too high and started boiling, the water level dropped inside the cores, and the pressure rised from the steam. TEPCO declared a state of emergency. Japanese authorities ordered the evacuation of residents within a 20-kilometer radius of Fukushima Daiichi, and told people within a 20 to 30-kilometre radius to remain indoors. There had been release of radiation from the nuclear power plant.
Reactor Units 5, 6 thought to be in a cold and safe shutdown and the cooling water temperature was found to be dropping. Japanese Government and TEPCO still struggle to solve Unit 1,2,3,4 cooling problems.
Over the following days there was evidence of partial nuclear meltdowns in reactors 1, 2 and 3; hydrogen explosions destroyed the upper cladding of the building housing reactors 1 and 3; an explosion damaged reactor 2's containment; and severe fires broke out at reactor 4. The Japanese authorities rated the events at reactors 1, 2 and 3 as a level 5 (Accident With Wider Consequences) on the International Nuclear Event Scale, while the events at reactor 4 were placed at level 3 (Serious incident).
The problem that scares me is about Nuclear Radiation leakage. I personally think the accidents are different from Chernobyl nuclear plant because the type of reactor used is very different. The Chernobyl plant used graphite as a neutron moderator to produce nuclear fission of the fuel rods. The reactor had no core containment vessel. After the explosion, the graphite, which is made of carbon, burned along with the fuel rods in an exposed state. The possibility is low that a large fire will break out at the Fukushima plant.
On the other hand, the six reactors at the Fukushima plant have a much higher combined power output, and the volume of radioactive materials contained in the reactors are several times greater than at Chernobyl. The outputs of the reactors at the Fukushima No. 1 plant range from 460,000 kilowatts at the No. 1 reactor to 784,000 kilowatts for the No. 2, No. 3 and No. 4 reactors. The combined power output is about three times than 1 million-kilowatt output at Chernobyl. There is a high possibility of large amounts of toxic radioactive materials in the fuel rods, but the levels depend on how long the fuel rods have been used.
[For Detail Information]
- http://en.wikipedia.org/wiki/Fukushima_I_Nuclear_Power_Plant
- Timeline of Events at Japan's Fukushima Nuclear Reactors By Natalie Wolchover, Life's Little Mysteries Staff Writer]
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