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Two plants near Tokyo, each with multiple reactors are on the verge of meltdown after emergency backup cooling was shut down by loss of power due to flooding.  (Source: CNN)

An explosion damaged the roof of one plant, releasing radiation on Saturday.  (Source: Reuters)

The plants lie within the Tokyo metropolis. People are being evacuated from within a 20 km radius.  (Source: CNN)
Japanese nuclear disaster is cause for pause, reflection

The Sendai Earthquake struck Japan early Friday morning with unrelenting fury.  Measuring 8.9 to 9.1 Mw-megathrust the quake was among the five most severe in recorded history and the worst quake to hit Japan.  In the aftermath of this severe disaster, as the nation searches for survivors and contemplates rebuilding, an intriguing and alarming storyline has emerged -- the crisis at the Fukushima Daiichi nuclear plant.

You may recall that a few years back Japan was struck by another quake which cracked the concrete foundation of a nuclear plant, but yielded virtually no damage.

By contrast, this time the damage was far worse, creating what could legitimately be called a nuclear disaster.

I. Fukushima Daiichi - a Veteran Installation

The Tokyo district of Fukushima is home to two major nuclear power installations.  

In the north there is the Fukushima "Daini" II plant, which features four reactors -- the first of which went online in 1982.  These units produce a maximum of 4.4 GW of power and are operated by the Tokyo Electric Power Company.

To the south lies the Fukushima "Daiichi" I plant, a larger and older installation featuring six reactors, the first of which went online in 1970.  Operated by Tepco, the installation offers a combined 4.7 GW of power.  It was here that disaster struck.

II. Disaster at Fukushima Daiichi 1

While the southern installation is over four decades old, Japan has been responsible in retrofitting the plant with modern safeguards.  Among those is an automatic switch which shuts off the reactor when an earthquake struck.

The switch performed perfectly when the quake hit Friday morning, shutting of the three reactors that were active at the time.  Control rods lowered and the reaction stopped.

The next step was the cooling the power rods, composed of uranium-235, to prevent them from melting.

Cooling water was pumped over the rods for about an hour, but before the rods could be fully cooled, stopping the reaction, the pumps failed.  According to the International Atomic Energy Agency, and multinational oversight group, the failure was due to failure in the backup generators due to the tsunami flooding.

On Saturday Japanese authorities and power officials tried to use sea-water injections to complete the cooling process, but those plans were stalled when another tsunami warning arrived.

An explosion occurred inside at least one of the reactor buildings.  It is believed to be due to the build-up of pressure after the pumps failed creating hydrogen and oxygen gases, which subsequently combusted from the heat.

Malcolm Grimston, Associate Fellow for Energy, Environment and Development at London's Chatham House told CNN:

Because they lost power to the water cooling system, they needed to vent the pressure that's building up inside.My suspicion is that as the temperature inside the reactor was rising, some of the metal cans that surround the fuel may have burst and at high temperature, that fuel cladding can react with water to produce zirconium oxide and hydrogen.

That hydrogen then will be part of the gases that need to be vented. That hydrogen then mixes with the surrounding air. Hydrogen and oxygen can then recombine explosively. So it seems while the explosion wasn't directly connected with the nuclear processes, it was indirectly connected, because the hydrogen was only present because of what was going on in the reactor core.

The explosion damaged the roof of the plant and sent billows of smoke up into the air.  According to officials some radioactive material was released into the atmosphere.  Outside the plant perimeter, levels of radiation measured 8 times higher than normal.

Meanwhile reactors at the newer Fukushima II are also beginning to heat up after their own cooling systems failed.

Japanese officials have evacuated people from an expanding radius around the plants as a precaution.  Currently the evacuation zone is at almost 20 km.  They hope to try to continue cooling, but have to work around tsunami alarms from earthquake aftershocks that have continued into Saturday.  U.S. Secretary of State Hillary Clinton has announced that the U.S. is sending high-tech coolant to the plants, in and attempt to avert disaster.

III. Can a Meltdown be Avoided?

Without proper cooling, the rods will continue to heat and proceed towards meltdown, releasing clouds of radioactive gas.  The first question is thus whether meltdown can be avoided.

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.

According to reports, the coolant temperatures inside the reactor have exceeded 100 degrees Celsius.  If they reach 540 degrees Celsius the fuel rods will fully melt down.

The question now becomes what to do.  

According to reports by Nippon Hoso Kyokai (Japan Broadcasting Corporation), three individuals have already been exposed been the victims of radiation poisoning (likely plant workers) and that radioactivity levels at the plant have risen to 1,000 times the normal levels at the plant control room.

One option on the table is to vent the reactors, allowing them to blow off the steam and prevent a greater buildup of pressure and heat.  However, doing so could release significant levels of radioactivity into the surrounding area.

The alternative is to try last ditch cooling and hope that if the rods do melt, that the secondary containment will hold.  The release of radioactive gases from venting would pale to that if the secondary containment was breached.  Such a scenario would likely result in the modern day equivalent of Chernobyl.

Whichever course of action is selected, there's a great deal of risk of radiation exposure to those who inhabit the area in the near future.  States James Acton, international physicist, in an interview with CNN, "There's a possibility of cancer in the long term -- that's the main hazard here."

IV. Grim Lessons From the Disaster

At Three Mile Island, the U.S. learned the hard way not to put vital controls in the hands of plant operators.  Operators almost created a meltdown, when they accidentally disabled necessary cooling.  That was due to the poor quality of indicators. 

As the result, the nuclear community learned to automate shutdown processes.

Ultimately the Fukushima disaster illustrates the need for sealed backup generators.  The containment procedures in all their modern glory are useless if the backup power goes out.  And, if possible, it shows that it is desirable to build new nuclear plants farther from the sea and from fault lines (though this could cause costs to increase).

As the fight to avert meltdown plays out, the final damage won't be known for weeks to come.  But the international community is already reacting.

At this time it's vital not to overreact to this worse case scenario.  

The disaster does illustrate that nuclear fission power is far from failsafe, particularly older reactors -- even if retrofitted with modern controls.  Ultimately the international community needs to work towards fusion power, which should be much safer and cheaper.

At the same time, it's important to consider that there's a great deal of background radiation released from the burning of fossilized coal and that mining fossil fuels has led to many a great loss of life and resources as illustrated by recent coal and oil disasters.

And nuclear power is far less expensive than solar or wind power in base costs, and generally less expensive even after all the red tape that increases plant creation costs by an order of magnitude in the U.S.

There's no easy answers here.  Oil and coal power emit dangerous nitrogen and sulfur-containing gases and carbon dioxide into the ozone.  And their fuel is dangerous to obtain.  But they're cheap.  Solar and wind power are relatively safe, but they're expensive and offer inconsistent power.  Nuclear power is cheap and produces no emissions normally, but it can be a danger in the case of natural disaster or malicious attack.

It's important not to turn a blind eye to this disaster, but it's equally important not to overreact.

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Control rods
By drycrust3 on 3/12/2011 5:05:26 PM , Rating: 2
The next step was to halt the reaction by cooling the power rods, composed of uranium-235.

I thought that a nuclear reactor used carbon control rods to "turn on" and "turn off" the reaction in the pile, and that in an emergency these rods would be lowered (automatically or manually) to kill the reaction; but the way everyone has been talking (and the above quote is just one example) is as though either the control rods were damaged and couldn't kill the reaction, or there weren't any control rods.

RE: Control rods
By JonB on 3/12/2011 5:39:11 PM , Rating: 4
Control rods are not carbon (never have been). For BWR and PWR designs, and this is a BWR, the control rods are made of either Halfnium or Silver/Gadolinium. They are neutron absorbers. Carbon is not a neutron absorber.

The Chernobyl design plant (RBMK) used large blocks of carbon as a Moderator to slow neutrons, not absorb them.

This reactor is NOT still running. The control rods did fully insert. But - since it was running at full power and had been for weeks or months, there is a lot of residual heat from nuclear decay that remains for hours and days. That is the heat that must be removed to avoid melting the fuel pellets.

BWR - Boiling Water Reactors - are relatively simple in their core design and emergency cooling methods. But they need electrical power to work and the earthquake and tsunami disabled their diesel generators used for emergency power.

RE: Control rods
By mianmian on 3/12/2011 6:02:10 PM , Rating: 2
I wounder why there is no secondary emergency power generator. Or they do have them but all get wipped out by tsunami?

RE: Control rods
By Solandri on 3/12/2011 8:01:53 PM , Rating: 2
The primary backup power source was diesel generators which were knocked out by the tsunami (unclear if they're damaged or contaminated with water or both).

The secondary backup was batteries. They ran off those, but they only had enough electrical capacity for a few hours.

The little water they are getting into the reactor is probably being pumped in using power from one of the emergency vehicles at the scene.

The irony in all this is that a nuclear reactor used to generate electricity is suffering a serious accident because it lacks electricity...

RE: Control rods
By Tuor on 3/12/2011 11:21:49 PM , Rating: 2
From what I read, they were submerged in salt water due to the tsunami. That will end their usefulness.

As for your last line, here is a grim story:

There was a US Navy submarine (the USS Thresher, IIRC) that was lost because they scrammed the reactor and did an emergency blow. As they ascended towards the surface, the drop in water pressure caused a drop in temperature, freezing certain valves. When the sub surfaced violently, the air used for ballast was lost and the sub began to sink again. The frozen valves prevented them from putting more air into the ballast tanks. The scrammed reactor could not be restarted anywhere near quickly enough, so they had no propeller. They just sank down until they imploded. All hands were lost.

RE: Control rods
By johnsonx on 3/13/2011 1:28:08 AM , Rating: 2
well, sort of. you got the sub's name right, but most of the details wrong.

the wikipedia account seems accurate:

not sure what any of it has to do with an article on nuclear power plant disasters though. all else being equal, the sub's fate would have been the same if the (presumed) broken water pipe had shorted out electrical systems controlling diesel engines.

RE: Control rods
By Tuor on 3/13/2011 10:39:02 PM , Rating: 2
I was commenting on the last line of the previous post: the irony of not being able to fix a problem due to lack of power, when a nuclear plant produces tons of power. The same was true for the Thresher.

RE: Control rods
By Iaiken on 3/12/2011 6:44:09 PM , Rating: 2
This reactor is NOT still running. The control rods did fully insert. But - since it was running at full power and had been for weeks or months, there is a lot of residual heat from nuclear decay that remains for hours and days. That is the heat that must be removed to avoid melting the fuel pellets.

JonB forgot to mention that cooling a BWR is a very delicate procedure as cooling it even a fraction too quickly can cause the fuel rods to shatter into thousands of tiny fragments due to thermostatic shock. If this happens, the fuel rods will not cool evenly and you can end up with a situation where the external fragments cool while the center-most fragments melt together and slowly consume the rest of the rod from the inside out.

BWR's are basically like walking a tight rope, a little too far to one side or the other and you've got no choice but to rely on the net, if the net fails... Well, then that's all she wrote.

RE: Control rods
By drycrust3 on 3/12/2011 8:27:04 PM , Rating: 2
My thanks to both you and JonB for your clear and considerate explanations, none of the broadcasts I had heard mentioned any of this, and especially the way the fuel rods are like glass in that they need to be cooled carefully otherwise they will shatter.

RE: Control rods
By JonB on 3/13/2011 3:16:13 PM , Rating: 2
The fuel rods are really long metal tubes. Inside the tubes are ceramic cylinders of uranium. The melting temperature of uranium is very high, so a true "meltdown" is hard to achieve. The more likely scenario is that the hot metal tubes could become cooled too quickly and cracks could form. The salt water won't help here because chlorides will make metals crack even faster when there is heat and stress.

IF the tubes crack, then they could possibly spill their pellets into a pile at the bottom of the reactor vessel. That is potentially good and bad. It gets them directly cooled by water rather than being inside a tube, but any fission product gases that were trapped in the ceramic matrix will escape into the water, bubble to the surface and then possibly released to the atmosphere. Radioactive Iodine is the predominate gas; that is why Iodine tablets are given to people to saturate your thyroid gland with non-radioactive iodine.

RE: Control rods
By rika13 on 3/12/2011 8:34:32 PM , Rating: 2
The RMBK design had carbon-tipped control rods, which screwed with the speed of the reaction as they SCRAMed the reactor. Most SCRAM systems now are based on dead-man switches, that is they are forced to be off and will snap on given half a chance.

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