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Sunday, March 13, 2011

FACTS: The chain of events at Daiichi

-The chain of events at the seriously affected Fukushima Daiichi plants has not clearly been explained to the layman in many outlets. Here are the briefest details we can provide that anyone, even young people, can understand.

What happened at the nuclear plants?
When the earthquake happened, equipment at the nuclear plants detected it and automatically sent shutdown signals to the nuclear reactors and to the steam turbines that provide electric power to towns and cities. At the same time, emergency diesel generators are automatically started to provide power for the nuclear plants.

Why do nuclear plants need power if the reactor is "shut down?"
A nuclear reactor is shut down by inserting control rods into its core that stop the nuclear reaction, and that knocks the heat output down very quickly. However, decay of fission products in the core continues and develops heat which still has to be removed. To do this requires pumps for continuing to circulate water through the reactor; also, the electrically powered control and remote indication equipment used by plant operators in the control room (safely away from, and shielded from, the reactor) to operate the power plant must remain operating.

What happens if the reactor is shut down but there is no electric power? In this case, the reactor will continue to heat up increasing temperature and pressure in the primary coolant system.

Didn't the operators vent pressure from these systems? Why didn't that cool them down so they were safe? This is done so that the 'decay heat' does not overpressurize the coolant system which would result in a serious failure of the piping that would release all the coolant to the inside of the containment building. However, if this is done without any way to get new, cooler water in to replace the steam that was let out the result will be less cooling water in the system every time venting happens. If this continues, the water level may drop far enough in the reactor core that some of the upper part of the core is no longer covered by the water and will then overheat and could melt.

Why didn't they put seawater in earlier? Someone at some point has to make a decision that the seriously bad effects seawater has on a nuclear reactor's metal components are outweighed by public risk and give the order to inject seawater. Once this happens, the reactor will likely later (once cooled off both temperature wise and radiation wise) have to be dismantled and scrapped. This may have happened too late at Daiichi No. 1 but was done early enough, apparently on Daiichi No. 3.

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