DescentBB

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Power failure to turbines needed to cool rods down.

"japan informed the International Atomic Energy Agency the quake and tsunami cut the supply of off-site power to the plant and diesel generators intended to provide back-up electricity to the cooling system.

If power is lost, the options to cool the core are limited.

If the core overheats, then the fuel would become damaged and a molten mass could melt through the reactor vessel, releasing a large amount of radioactivity into the containment building surrounding the vessel.

It is unclear if the quake has undermined the containment building, which might allow radioactivity to leak out."

The link explains the process a bit more:

http://www.bbc.co.uk/news/world-12723092

following up on woody's interesting link -

(hey, these are wikipedia, so take it for what it's worth)

the BWR
http://en.wikipedia.org/wiki/Boiling_water_reactor

list of BWR's
http://en.wikipedia.org/wiki/List_of_BWRs

the Fukushima facility
http://en.wikipedia.org/wiki/Fukushima_ ... ower_Plant

http://en.wikipedia.org/wiki/Fukushima_ ... ower_Plant


something general on nuclear reactor types
http://hyperphysics.phy-astr.gsu.edu/hb ... actor.html
gives some interesting general operating parameters

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Bet51987 wrote:Ok, thanks. I see it's much more than just the rods shutting it down.

Yes, unfortunately these things don't have simple on/off switches. Given the circumstances I'm quite impressed how well they hold up so far, props to the japanese engineers ! (For their building engineering as well !) I sincerly hope there won't be more or further meltdowns.

Well, they do have on/off switches, but you can only shut off the fission power. At normal operation about 7% of the power comes from decaying fission products and although that reduces quickly in the first few hours after shutdown just 1% is still over 10 MW heat to deal with.

https://morgsatlarge.wordpress.com/2011 ... -reactors/

tl;dr:

This reactor has several levels of failsafes. Once the control rods dropped, the nuclear reaction stopped, and the main issue was dealing with residual heat and very short-term radiation. Radiation-wise, the stuff vented into the environment had decay on the order of "it's gone by the time you've typed the word radioactive", so you shouldn't worry about it unless you were sitting on the vent pipe. Heat-wise, the main issue is preventing "meltdown", which really just means "the reactor gets hot enough to breach the innermost of several layers of containment, and then spreads out in a huge concrete basin and cools down, still completely contained, but dang, we melted a bunch of expensive equipment." With multiple cooling systems down, the water-based cooling system needed to be refilled with seawater which will become slightly irradiated but is entirely contained within the reactor and will be treated there. It will likely take years to repair and inspect everything in the plant, but there's no significant environmental danger. The only people really harmed are the ones who have to pay for reactor repairs.

Not to mention Japan is short by about 9 gigawatts of electricity in the meantime.

"At the Fukushima I plant, radioactive cesium was discovered. Cesium is in the beta decay chain tellurium -> iodine -> xenon -> cesium. Its occurs roughly 16 hours after an unchecked uranium reaction and its presence indicates that one of the fuel rods may already have melted down.

Once one rod melts, it will be much more difficult to prevent the others from melting down as well."

Lothar, I don't think the Cesium will be "Gone" by the time you type "radioactive":

"The radioactive isotope caesium-137 has a half-life of about 30 years and is used in medical applications, industrial gauges, and hydrology. Although the element is only mildly toxic, it is a hazardous material as a metal and its radioisotopes present a high health risk in case of radiation leaks."

http://en.wikipedia.org/wiki/Caesium

woodchip wrote:"The radioactive isotope caesium-137 has a half-life of about 30 years and is used in medical applications, industrial gauges, and hydrology. Although the element is only mildly toxic, it is a hazardous material as a metal and its radioisotopes present a high health risk in case of radiation leaks."

http://en.wikipedia.org/wiki/Caesium

C-137 has a long half life. Many other cesium isotopes have half-lifes in the "few seconds" range. Which form of radioactive cesium was released, and in what quantities?

According to the article I linked, "The small amounts of Cesium that were measured told the operators that the first containment on one of the rods somewhere was about to give." This is what led to filling the cooling system with seawater -- preventing a breach of first containment (that is, the shell on an individual fuel rod). There was no direct threat of a breach of second or third containment.

Lothar wrote:

woodchip wrote:"The radioactive isotope caesium-137 has a half-life of about 30 years and is used in medical applications, industrial gauges, and hydrology. Although the element is only mildly toxic, it is a hazardous material as a metal and its radioisotopes present a high health risk in case of radiation leaks."

http://en.wikipedia.org/wiki/Caesium

C-137 has a long half life. Many other cesium isotopes have half-lifes in the "few seconds" range. Which form of radioactive cesium was released, and in what quantities?

According to the article I linked, "The small amounts of Cesium that were measured told the operators that the first containment on one of the rods somewhere was about to give." This is what led to filling the cooling system with seawater -- preventing a breach of first containment (that is, the shell on an individual fuel rod). There was no direct threat of a breach of second or third containment.

This is what I found:

"The Japanese nucle­ar reg­ulatory agency reported a jump in ra­diation near the main gate of the complex over the course of five hours Sat­urday. An­oth­er spike in ra­diation, including the ra­dioactive isotope cesium-137, was reported at a nearby observation post. Japanese au­thor­ities lat­er said the ra­diation had dropped again. But the cesium-137 was the equiva­lent of a flare, a dramat­ic sign of some­thing se­riously awry."

http://www.ongo.com/v/555500/-1/E9D7C55 ... r-hit-hard

Still want to take a stroll nearby?

here's some interesting points of perspective