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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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RE: A bad placement decision on Japan's part....
By PrinceGaz on 3/13/2011 8:22:28 PM , Rating: 1
It's actually 3 of the 7 are in trouble now, and the authorities seem to have decided to scupper them by flooding them with seawater, rather than risk a major radioactive release. Given the cost of building a nuclear power-station, that is a pretty expensive choice, which suggests it was/is either right them off or something much worse could happen given the cooling system problems. The much worse being a total core meltdown and breach of the inner containment chamber resulting in a release of large amounts of highly radioactive material on a Chernobyl scale.


RE: A bad placement decision on Japan's part....
By Solandri on 3/14/2011 4:20:44 AM , Rating: 5
The plant was actually scheduled for decommission in a couple weeks. I'm hearing now that the government granted an extension on its use last month. But still, it's nearing the end of its expected useful lifespan (various internal structures tend to become more brittle over decades of irradiation). So it's not exactly a huge loss to kill it this way like the media is making it out to be. I've stopped counting the number of times I've had to roll my eyes every time they try to over-dramatize what's going on.

Normally you cool these things with deionized water to stave off corrosion (heat + ionized water = really bad corrosion). Because the cooling system isn't functioning, they've been having to vent steam to cool it, which means they're losing water. They probably used up their entire supply of deionized water, forcing them to resort to seawater.
quote:
The much worse being a total core meltdown and breach of the inner containment chamber resulting in a release of large amounts of highly radioactive material on a Chernobyl scale.

Sigh. This type of reactor can't release radioactivity on a Chernobyl scale. No western reactor can. Chernobyl's reactor was an inherently unstable design. You had to actively work to keep its power from spiking out of control. That's why it exploded instead of simply melting. Many of the radioactive isotopes produced in a fission reactor are normally too heavy to become airborne, but the explosion and graphite fire provided a vehicle for those particles to become airborne.

The uranium in this reactor is already sub-critical. It is not fissioning anymore. The heat they're trying to get rid of is being generated by they decay of short-lived radio-isotopes created by the fissioning process prior to shutdown. The primary concern with this reactor are isotopes which can become airborne (iodine) or are water-soluble (cesium). Every time they vent steam to cool the reactor, it allows these airborne and water-soluble isotopes to escape. There is no graphite inside, nothing which can catch fire or cause an explosion*, and the containment vessels are specifically designed to contain a worst-case meltdown. So the worst-case would involve dumping lots of water and neutron absorbers like boron onto a completely molten core. None of this stuff can catch fire, so the only vehicle for radioactivity to escape is still airborne or water(steam)-borne.

The reason they are so concerned about melting (as opposed to a meltdown) is because the fuel is surrounded by a zirconium cladding which keeps the fuel and isotopes created by fission on the inside, while the water stays on the outside. This is what lets you cool the core with water without picking up any radioactive iodine or cesium. If part of the fuel melts, the cladding melts with it, and the containment it provides is breached. That allows the water to directly contact the fuel, pick up water-soluble isotopes and mixing with airborne isotopes.

No melting = no radioactive iodine and cesium released when you vent steam.
Melting = radioactive iodine and cesium released when you vent steam.

*The hydrogen comes from a reaction between the water and zirconium. If the temperature becomes high enough, the zirconium disassociates the water into hydrogen and oxygen. Hydrogen molecules are really small so can leak through things which are otherwise water- and air-tight. So it leaked out and collected in the roof of the building. Some time later, a spark caused it to react with airborne oxygen, causing the explosions. Otherwise, it should be released into the atmosphere via the regular venting they're doing. As long as you're venting, it doesn't threaten the containment vessel.


By 3DoubleD on 3/14/2011 11:03:10 AM , Rating: 2
That is the best explanation I have yet read. THANK YOU!

I've studied different reactor designs (Candu heavy water) in the past and so I was unfamiliar with the design of the Japan reactor. Also, I kept thinking there was a contradiction in the news reports where they would say the reaction was stopped, but heat was still being produced. I had forgotten about isotope decay!

So it would appear the best case scenario is then to keep cooling the fuel for several half-lifes of the fission by-products while keeping the Zr tubes cool enough to prevent excess hydrogen build-up. By minimizing high temperatures they minimize the venting requirements and the less radiation released.

So is it safe to say that one or more of the Zr fuel tubes have melted if the release of steam causes an increase in environmental radiation levels?


By cpeter38 on 3/14/2011 12:28:33 PM , Rating: 2
Excellent description!!

Check out http://bravenewclimate.com/2011/03/13/fukushima-si... for a more detailed explanation of the situation.

Solandri,
You appear to either work in the industry or do good research.

Thank you!!


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