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megimoo
03-14-2011, 10:38 PM
MIT NSE Nuclear Information Hub (http://web.mit.edu/nse/)
SNIP
Construction of the Fukushima nuclear power plants

The plants at Fukushima are Boiling Water Reactors (BWR for short). A BWR produces electricity by boiling water, and spinning a a turbine with that steam. The nuclear fuel heats water, the water boils and creates steam, the steam then drives turbines that create the electricity, and the steam is then cooled and condensed back to water, and the water returns to be heated by the nuclear fuel. The reactor operates at about 285 °C.

The nuclear fuel is uranium oxide. Uranium oxide is a ceramic with a very high melting point of about 2800 °C. The fuel is manufactured in pellets (cylinders that are about 1 cm tall and 1 com in diameter). These pellets are then put into a long tube made of Zircaloy (an alloy of zirconium) with a failure temperature of 1200 °C (caused by the auto-catalytic oxidation of water), and sealed tight. This tube is called a fuel rod. These fuel rods are then put together to form assemblies, of which several hundred make up the reactor core.

The solid fuel pellet (a ceramic oxide matrix) is the first barrier that retains many of the radioactive fission products produced by the fission process. The Zircaloy casing is the second barrier to release that separates the radioactive fuel from the rest of the reactor.

The core is then placed in the pressure vessel. The pressure vessel is a thick steel vessel that operates at a pressure of about 7 MPa (~1000 psi), and is designed to withstand the high pressures that may occur during an accident. The pressure vessel is the third barrier to radioactive material release.

The entire primary loop of the nuclear reactor – the pressure vessel, pipes, and pumps that contain the coolant (water) – are housed in the containment structure. This structure is the fourth barrier to radioactive material release. The containment structure is a hermetically (air tight) sealed, very thick structure made of steel and concrete. This structure is designed, built and tested for one single purpose: To contain, indefinitely, a complete core meltdown. To aid in this purpose, a large, thick concrete structure is poured around the containment structure and is referred to as the secondary containment.

Both the main containment structure and the secondary containment structure are housed in the reactor building. The reactor building is an outer shell that is supposed to keep the weather out, but nothing in. (this is the part that was damaged in the explosions, but more to that later).

Fundamentals of nuclear reactions

The uranium fuel generates heat by neutron-induced nuclear fission. Uranium atoms are split into lighter atoms (aka fission products). This process generates heat and more neutrons (one of the particles that forms an atom). When one of these neutrons hits another uranium atom, that atom can split, generating more neutrons and so on. That is called the nuclear chain reaction. During normal, full-power operation, the neutron population in a core is stable (remains the same) and the reactor is in a critical state.

It is worth mentioning at this point that the nuclear fuel in a reactor can never cause a nuclear explosion like a nuclear bomb. At Chernobyl, the explosion was caused by excessive pressure buildup, hydrogen explosion and rupture of all structures, propelling molten core material into the environment. Note that Chernobyl did not have a containment structure as a barrier to the environment. Why that did not and will not happen in Japan, is discussed further below.

In order to control the nuclear chain reaction, the reactor operators use control rods. The control rods are made of boron which absorbs neutrons. During normal operation in a BWR, the control rods are used to maintain the chain reaction at a critical state. The control rods are also used to shut the reactor down from 100% power to about 7% power (residual or decay heat).

The residual heat is caused from the radioactive decay of fission products. Radioactive decay is the process by which the fission products stabilize themselves by emitting energy in the form of small particles (alpha, beta, gamma, neutron, etc.). There is a multitude of fission products that are produced in a reactor, including cesium and iodine. This residual heat decreases over time after the reactor is shutdown, and must be removed by cooling systems to prevent the fuel rod from overheating and failing as a barrier to radioactive release. Maintaining enough cooling to remove the decay heat in the reactor is the main challenge in the affected reactors in Japan right now.

It is important to note that many of these fission products decay (produce heat) extremely quickly, and become harmless by the time you spell “R-A-D-I-O-N-U-C-L-I-D-E.” Others decay more slowly, like some cesium, iodine, strontium, and argon.

What happened at Fukushima (as of March 12, 2011)


The following is a summary of the main facts. The earthquake that hit Japan was several times more powerful than the worst earthquake the nuclear power plant was built for (the Richter scale works logarithmically; for example the difference between an 8.2 and the 8.9 that happened is 5 times, not 0.7).

When the earthquake hit, the nuclear reactors all automatically shutdown. Within seconds after the earthquake started, the control rods had been inserted into the core and the nuclear chain reaction stopped. At this point, the cooling system has to carry away the residual heat, about 7% of the full power heat load under normal operating conditions.

The earthquake destroyed the external power supply of the nuclear reactor. This is a challenging accident for a nuclear power plant, and is referred to as a “loss of offsite power.” The reactor and its backup systems are designed to handle this type of accident by including backup power systems to keep the coolant pumps working. Furthermore, since the power plant had been shut down, it cannot produce any electricity by itself.

For the first hour, the first set of multiple emergency diesel power generators started and provided the electricity that was needed. However, when the tsunami arrived (a very rare and larger than anticipated tsunami) it flooded the diesel generators, causing them to fail.



http://mitnse.com/

Rockntractor
03-14-2011, 11:08 PM
The most serous damage will be done by environmentalists and liberals who will use this to further handcuff our energy sector.

noonwitch
03-15-2011, 07:50 AM
The most serous damage will be done by environmentalists and liberals who will use this to further handcuff our energy sector.



It is a good time to review safety standards on our nuclear plants that are placed near fault lines, like those in California, Oregon and Washington state. Although, that just makes me think of the plot of the first Superman movie, when Lex Luthor planned to blow up all the nuclear plants in California, causing it to slide into the ocean, because he had bought all the land in Nevada which would now be oceanfront.

FlaGator
03-15-2011, 08:32 AM
Ever since global warming became a trendy pseudo science I have lost a lot of faith in experts actually knowing what they are talking about.

Gingersnap
03-15-2011, 08:58 AM
The Japanese are not exactly short in the I.Q. department. They will do whatever it takes to minimize the damage here and learn from it. This was a 300 year storm - it would be insane to reject nuclear power over a situation that may not happen again for centuries and would certainly happen in an era benefiting from a greater understanding of materials science and more advanced technology.

I doubt the impact on the rest of the planet will be significant. There's a big difference between an explosion that injects radioactive particulates into the stratosphere and ground-level soil/water contamination from a point source. I'm not a big fan of radioactivity but I'm slightly less alarmed than someone who gets their nuclear physics from "Panic in Year Zero".

Starbuck
03-15-2011, 10:30 AM
The most serous damage will be done by environmentalists and liberals who will use this to further handcuff our energy sector.

And not just in America, either.

BERLIN -- Chancellor Angela Merkel says Germany will take seven of its 17 reactors offline for three months while the country reconsiders plans to extend the life of its nuclear power plants.

Merkel said Tuesday that Germany will temporarily shut down reactors that went into operation before the end of 1980, affecting seven reactors. The decision comes amid fears sparked by the crisis under way at Japan's tsunami-stricken nuclear power plant.

Merkel spoke after meeting with the governors of states that have nuclear power plants.

A previous government decided a decade ago to shut all 17 German nuclear plants by 2021, but Merkel's administration last year moved to extend their lives by an average 12 years. That decision was suspended for three months on Monday.

Read more: http://www.foxnews.com/world/2011/03/15/germany-shuts-seven-nuclear-reactors/#ixzz1GgNOPVQ6

Rockntractor
03-15-2011, 10:39 AM
And not just in America, either.

When the people can no longer afford to pay for the intermittent power they will scramble to get them back on line again.

fettpett
03-15-2011, 10:43 AM
And not just in America, either.

yep...cuz the Germans are soo affected by Earthquakes....and quite regularlly too :rolleyes::rolleyes:

malloc
03-15-2011, 07:31 PM
From what I can tell from the article, it looks like the worst case scenario is a safe, but useless really expensive reactor.

The facility experienced multiple catastrophic failures of a magnitude it was not specifically designed to handle, and things look as if they are going to remain safe. Even if cooling can't be maintained, the author seems to think the plant won't be a threat to public safety.

I call that a win.