One of Japan’s earthquake-damaged nuclear reactors exploded Saturday at 3:40 Japan time. There are conflicting reports on radiation released. Some reports maintain the background level of radiation is normal, and other reports indicate a radioactive isotope of the element cesium has been detected outside the plant. Cesium is a byproduct of the nuclear fission process. Finding it means there has been a breech of the fuel system.
“We’ve confirmed that the reactor container was not damaged. The explosion didn’t occur inside the reactor container. As such there was no large amount of radiation leakage outside,” Japan’s Chief Cabinet Secretary Yukio Edano told reporters Saturday night. “At this point, there has been no major change to the level of radiation leakage outside, so we’d like everyone to respond calmly.”
The explosion at the stricken Fukushima Daiichi Nuclear Power Station caused the release of radiation which prompted officials to create a 20 km (12 mile) evacuation zone.
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The plant was damaged by the earthquake, and the nuclear reaction was shut down normally. A nuclear generating station works by allowing the uranium fuel to get very hot, which creates steam for the generator turbines. When the process is shut down, there is a great deal of heat remaining, enough to melt the fuel and loose control of the process, a condition called a “melt-down.” It is mandatory to cool the fuel. When the tsunami hit the plant, the cooling system was damaged.
The residual heat caused water in the system to boil to steam, with a greater and greater pressure build-up. Some say that is what exploded, others say hydrogen gas exploded.
Tokyo Electric Power is going to take the drastic step to allow sea water into the reactor as a last-ditch attempt to keep the fuel from melting any further. If the fuel melts, it flows past the control rods and the nuclear chain reaction begins again, called “criticality.”
Mr. Edano said they were confident the sea water would “prevent criticality.”
Naoto Sekimura, from Tokyo University, told Japan public broadcaster NHK, that “only a small portion of the fuel has been melted. But the plant is shut down already, and being cooled down. Most of the fuel is contained in the plant case, so I would like to ask people to be calm.”
In 1987 the Ukranian nuclear plant at Chernobyl suffered a melt-down due to human error. The explosion caused radioactive smoke and thick soot that devastated the region and was detected all over Europe. That’s what is at stake with the Fukushima Daiichi plant.
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