Nuclear reactors at the Fukushima Daiichi station in Japan are critically endangered but have not reached full meltdown status. Our nuclear primer explains what that means and how the situation compares with past nuclear accidents

By John Matson | March 15, 2011

How does a nuclear reactor work?
Most nuclear reactors, including those at Japan's Fukushima Daiichi generating station, are essentially high-tech kettles that efficiently boil water to produce electricity. They rely on harnessing nuclear fission—the splitting of an atom into two smaller atoms, which also yields heat and sends neutrons flying. If another atom absorbs one of those neutrons, the atom becomes unstable and undergoes fission itself, releasing more heat and more neutrons. The chain reaction becomes self-sustaining, producing a steady supply of heat to boil water, drive steam turbines and thereby generate electricity.

How much electricity does nuclear power provide in Japan and elsewhere?
With 54 nuclear reactors generating 280 billion kilowatt-hours annually, Japan is the world's third-largest producer of nuclear power, after the U.S. and France, according to data from the International Atomic Energy Agency. The Fukushima Daiichi station, which has been hit hard by the March 11 earthquake, houses six of those reactors, all of which came online in the 1970s.

Worldwide, nuclear energy accounts for about 15 percent of electricity generation; Japan gets nearly 30 percent of its electricity from its nuclear plants. The U.S. produces more nuclear power overall, but nuclear constitutes a smaller share of its energy portfolio. About 20 percent of U.S. electricity comes from nuclear power plants, making it the third-largest source of electricity in the country after coal (45 percent) and natural gas (23 percent).

What fuels a nuclear reactor?
Most nuclear reactors use uranium fuel that has been "enriched" in uranium 235, an isotope of uranium that fissions readily. (Isotopes are variants of elements with different atomic masses.) Uranium 238 is much more common in nature than uranium 235 but does not fission well, so fuel manufacturers boost the uranium 235 content to a few percent, which is enough to maintain a continuous fission reaction and generate electricity. Enriched uranium is manufactured into fuel rods that are encased in metal cladding made of alloys such as zirconium.

Reactor No. 3 at the Fukushima Daiichi station runs on so-called mixed oxide (MOX) fuel, in which uranium is mixed with other fissile materials such as plutonium from spent reactor fuel or from decommissioned nuclear weapons.

How do you turn off a nuclear reaction?
Sustained nuclear fission reactions rely on the passing of neutrons from one atom to another—the neutrons released in one atom's fissioning trigger the fissioning of the next atom. The way to cut off a fission chain reaction, then, is to intercept the neutrons. Nuclear reactors utilize control rods made from elements such as cadmium, boron or hafnium, all of which are efficient neutron absorbers. When the reactor malfunctions or when operators need to shut off the reactor for any other reason technicians can remotely plunge control rods into the reactor core to soak up neutrons and shut down the nuclear reaction.

Can a reactor melt down once the nuclear reaction is stopped?
Even after the control rods have done their job and arrested the fission reaction the fuel rods retain a great deal of heat. What is more, the uranium atoms that have already split in two produce radioactive by-products that themselves give off a great deal of heat. So the reactor core continues to produce heat in the absence of fissioning.

If the rest of the reactor is operating normally, pumps will continue to circulate coolant (usually water) to carry away the reactor core's heat. In Japan the March 11 earthquake and tsunami caused blackouts that cut off the externally sourced AC power for the reactors' cooling system. According to published reports, backup diesel generators at the power plant failed shortly thereafter, leaving the reactors uncooled and in serious danger of overheating.


Without a steady coolant supply, a hot reactor core will continuously boil off the water surrounding it until the fuel is no longer immersed. If fuel rods remain uncovered, they may begin to melt, and hot, radioactive fuel can pool at the bottom of the vessel containing the reactor. In a worst-case meltdown scenario the puddle of hot fuel could melt through the steel containment vessel and through subsequent barriers meant to contain the nuclear material, exposing massive quantities of radioactivity to the outside world.

How can a meltdown be averted?
The Japanese plant's operators have made a number of attempts to cool the reactors, including pumping seawater into the reactor core to replenish the dwindling cooling fluid. The Tokyo Electric Power Company has also injected boric acid, an absorber of neutrons, into the reactors.

How does this incident compare with Chernobyl or Three Mile Island?
At present, three of the reactors at Fukushima Daiichi station are seriously crippled. Units 1 and 3 have experienced explosions that destroyed exterior walls, apparently from buildups of hydrogen gas produced by the zirconium in the fuel rods reacting with coolant water at extremely high temperatures—but the interior containment vessels there thus far seem to be intact. A third explosion was reported March 15 at reactor No. 2, and the situation there appears direr. Pressure in the suppression pool—a doughnut-shaped water vessel below the reactor—dropped after the explosion, indicating that the containment vessel had been compromised.

In reactor Nos. 1, 2 and 3 water levels dropped enough to leave the fuel assemblies temporarily uncovered; those fuel rods are presumed to have suffered damage. And a fire at a pool storing spent fuel rods at dormant reactor No. 4 is posing additional hazards to the few workers remaining at the site.

Japanese officials initially rated the incident a level 4, an "accident with local consequences," on the seven-tier International Nuclear and Radiological Event Scale (INES), but Princeton University physicist Frank von Hippel told The New York Times that the Fukushima Daiichi situation is "way past Three Mile Island already." Three Mile Island, the highest-profile U.S. nuclear accident, was classified level 5—an "accident with wider consequences".

At that Pennsylvania nuclear station in 1979 a cooling malfunction combined with worker error led to a partial meltdown—about half of the reactor core melted and formed a radioactive puddle at the bottom of the steel pressure vessel. The vessel remained intact, but some radiation did escape from the plant into the surrounding environment.

The 1986 Chernobyl accident was far more devastating; it rates as a 7, or a "major accident," on the INES scale. In Ukraine, then part of the Soviet Union, a power surge caused an explosion in one of the plant's reactors, releasing huge doses of radioactive fallout into the air. Two plant workers died within hours, according to the U.S. Nuclear Regulatory Commission; 28 more died in the following months from radiation poisoning. The fallout from Chernobyl was widespread, and the health effects of the disaster are difficult to quantify. A report from the United Nations Scientific Committee on the Effects of Atomic Radiation found that 6,000 individuals who were under the age of 18 in Ukraine, Belarus or Russia at the time of the disaster had by 2006 contracted thyroid cancer, "a substantial fraction" of whom likely contracted the disease due to radiation exposure.

Koeberg can withstand quake, tsunami

2011-03-15 14:36

Cape Town - Eskom on Tuesday assured MPs that its Koeberg nuclear power plant near Cape Town was designed to withstand both earthquakes and tsunamis.

The electricity utility's operations and planning division MD, Kannan Lakmeeharam, said the design of Koeberg's two reactors was more advanced than those at Japan's Fukushima plant.

A major nuclear crisis is brewing at Fukushima in the wake of the Richter scale 9.0 earthquake that hit Japan last week. Cooling systems have failed at three of the plant's reactors; three massive explosions have rocked the complex; and there are reports of major radiation leaks.

Tens of thousands of people living within a 20km radius of the plant have been evacuated.

Briefing a joint meeting of Parliament's energy and public enterprises portfolio committees, Lakmeeharam said while the reactors at Fukushima were "generation I" design, those at Koeberg were "generation II".

"There are several elements to what is going on in Japan that we have to compare to South Africa. The first is... design of the plant.

"The Fukushima plant is generation I technology; Koeberg is generation II technology, which is a pressurised water reactor. So it's different technology."

'Koeberg can cope with a tsunami'

On Koeberg's ability to withstand a major earthquake, he said the plant was built to withstand a Richter scale 7 seismic shock.

"The Koeberg plant is built on a raft to withstand a Richter scale 7 earthquake. So you've got to look at what is the history of seismic activity in South Africa, and whether that design is sufficient.

"Every nuclear power station is built to what is known as a safety case... based on looking at environmental issues and the past history of seismic activity, and so on."

He told MPs that the emergency power supply for Koeberg's reactors was designed to cope with a tsunami.

"A nuclear power station needs electricity to make sure the cooling water system works. The cooling system for the reactor core is one of the most critical elements of a nuclear power station.

"At Koeberg, the primary source of electricity for this cooling system is the national grid. [Over and above this], we have a dedicated source for Koeberg supplied by gas turbines.

"We [also] have diesel generators on site, and further generators at a higher elevation.

"So we have four layers of security to provide power for the cooling water system. We test them [all] regularly... All of this is overseen by the National Nuclear Regulator.

"So there are some differences; obviously what's happening in Japan is of concern, and we have to keep reviewing our own practices in the light of what we learn," he said.

The largest recorded earthquake in South Africa measured 6.1 on the Richter scale. It struck the Western Cape towns of Tulbagh and Ceres in 1969.

- SAPA

Another Setback at Reactor Creates Race Against Time

Michio Kaku on March 27, 2011, 3:14 PM



The reactor situation in Japan suffered yet another setback today, with water levels in Unit 2 registering 10 million times normal levels. The radiation was so high that workers fled the reactor rather than take a second reading. Radiation levels were an astonishing 1,000 msv/hour (which will cause radiation sickness within an hour and even deaths starting at 6 hours). Given this near-lethal radiation field, workers evacuated Unit 2. 

One question is: where did this radiation come from? Most of it was in the form of iodine-134 (with a half-life of 53 minutes) and iodine-131 (with a half life of 8 days). This indicates that the radiation came directly from the core at Unit 2, rather than the spent fuel pond (where most of the iodine has already decayed). So there seems to be a direct path way from the core to the outside, meaning a breach of containment, similar to the situation in Unit 3. In other words, there could be a crack in the pressure vessel surrounding the super hot uranium core, as well as a crack in the outer primary containment vessel surrounding the pressure vessel. These cracks may allow radiation to escape from the core directly into the environment.

At the very least, this means continued leaks of deadly radiation from the core to the outside world.

But in a worst case scenario, it could be a preview of the day when radiation levels are so high that a complete evacuation is necessary. This means that the cores, without cooling water, will heat up and eventually cause an explosion, (via a hydrogen gas or steam explosion). Remember the only thing preventing this worst case scenario are brave firemen shooting hose water into the cores and spent fuel ponds. Once they are evacuated, then simultaneous meltdowns at 3 nuclear sites is inevitable. Then a steam or hydrogen gas explosion may crack open the pressure vessel, leading to a catastrophic release of radiation, perhaps worse than Chernobyl.

So it is a race against time. On one hand, we have the brave workers trying desperately to keep thecore and spent fuel pond covered with water. On the other hand, the reactors are deteriorating every day, with the possibility of cracks, pipe breaks, secondary earthquakes, which could easily tip the accident into the worst case scenario. At that point, when all the workers have to evacuate, let's hope that the utility took my advice to prepare the Chernobyl option (i.e use the air force for bury the reactors in concrete and sand). However, I doubt that the utility has thought that far ahead.

Hanging by your Fingernails - The Fukushima Meltdown

Michio Kaku on April 4, 2011, 8:30 PM



The situation at Fukushima is relatively stable now, in the same way that you are stable if you hang by your fingernails off a cliff, and your fingernails begin to break one by one....

As we mentioned, the accident so far in Fukushima is progressing in three acts. The first act was the earthquake and tsunami, which immediately wiped out all emergency cooling systems simultaneously at all three reactors and all hell has broken loose.

Act II was the enormous damage done to the cores of these three reactors. With the loss of cooling water, temperatures began to rise rapidly, causing the hydrogen gas explosions and fuel melting. We know that about 70% of Unit 1's core was damaged, and that 33% of Unit 2's core was also damaged. All computer simulations done by various laboratories all show the same thing, that we came perilously close to a full scale metldown at all three reactors, including a spent fuel pond accident in Unit 4.

Against the wishes the utility, the Japanese government ordered flushing the entire reactors with seawater, which temporarily halted the accident from progressing to a full blown tragedy. This stabilized the accident, at present, from going into free fall (but reduced the three reactors to pieces of junk.) Meanwhile, radiation keeps flowing out of the reactors and into food, agricultural products, the oceans, the soil, etc. Now we are entering Act III. With the cores covered with seawater and fresh water, the workers are desperately trying to hit rock bottom, so they can begin recovery operations.

Unfortunately, they have not yet bottom. Its like death from a thousand cuts. Leaks of radiation are being found everywhere. The crucial thing is that the workers do not know precisely where this radiation is coming from. The primary suspect is that there is a direct contact between melted uranium (called "corium") and the cooling water, probably caused by a pipe break or, more ominously, a pressure vessel that has completely melted through.

So the utility is like the little Dutch boy, trying desperately to plug up one leak, only to find another. But until they find the primary source of this leak, there will be damaging reports of radiation being found in more and more places. Time is not on their side. The longer they take to hit rock bottom, the more the danger of evacuations of workers and damage to the economy of the area. Also, secondary earthquakes and pipe breaks can cause the accident to start up all over again with the loss of precious cooling water. It's also a Catch 22 situation. They need to flood the cores with water, but this water becomes contaminated and flows out to the environment. So they are damned if they do, and damned if they don't.

One solution is to put a special TV camera into the contaminated water to actually photograph the bottom of the reactor vessel, which is underwater, to see precislely the nature of this leak, whether its just a simple crack or a full blown melt-through of the reactor pressure vessel. If its just a crack, then it might be possible to drain the water and then weld the crack shut. But if it is a melt through, then it is much harder to seal up the hole. Either way, robots or suicide squads of workers will be necessary to seal up the leak. In  best case scenario, the situation continues on for months and years. At TMI, it took years before a camera was finally able go underwater to photograph the state of the core. There, the reactor vessel was not damaged. But at Fukushima, it is likely that the pressure vessel is cracked or partially melted through, which makes clean up much more difficult. Workers have a long, long way to go. For example, although electricity was been brought onto the site, a great disappointment is that the pumps cannot be turned on, because they are broken, or there is too much hydrogen gas, or it is too radioactive to make repairs. Until the pumps are turned on, workers have to use the Stone Age method of using fire men to shoot hose water into the reactor. (This is a problem if radiation levels continue to rise due to the failue to find the leak in the reactor.)

I get a headache just thinking of all the measures that have to be taken just to reach rock bottom and then begin clean up operators. Meanwhile, the clock is ticking. Let's hope there are no more secondary earthquakes, or pipe breaks, or full scale evacuations of workers, which could start up the nightmare all over again.

Lets hope no more fingernails crack.

Machines clear rubble as Japan ranks crisis with Chernobyl

by Tim Hornyak   April 11, 2011 8:20 PM PDT


TEPCO workers remotely operate heavy machinery at the Fukushima Daiichi nuclear plant.
(Credit: TEPCO)

TOKYO--Robots and remote-controlled heavy machinery finally got to work at the crisis-hit Fukushima nuclear plant in an effort to minimize human exposure to radiation as Japan raised the severity of the disaster from 5 to 7, putting it on par with the 1986 Chernobyl catastrophe.

A month after the 9.0-magnitude March 11 earthquake, operator Tokyo Electric Power Company (TEPCO) deployed three remote-controlled excavators equipped with cameras to clear radioactive debris around the unit 3 reactor, according to a TEPCO spokeswoman.


The unmanned machinery was donated by Shimizu and Kajima corporations.
(Credit: TEPCO)

The excavators were donated by two Japanese construction companies. Remote-operated power loaders sent to Japan by Qinetiq North America are still being evaluated before deployment to the plant.

Meanwhile, TEPCO launched a Honeywell T-Hawk micro air vehicle to survey the plant from above. As seen in the video below, the MAV recorded footage of the reactors and turbine structures from about 500 feet up, showing extensive damage to the buildings from the tsunami and hydrogen blasts.

TEPCO has also used two of Qinetiq's portable Talon robots--often used for ordnance disposal and reconnaissance--to take additional footage of the plant and the obstacles preventing the deployment of more machines. The bots have audio and video feeds as well as chemical, biological, radiological, nuclear, and explosive detection kits.

Powerful aftershocks shook Tokyo and northeast Japan again on Monday and Tuesday, temporarily cutting power to the plant, but cooling operations are continuing.

Meanwhile, Japan's Nuclear and Industry Safety Agency raised the severity of the crisis to 7, the highest on the International Nuclear Event Scale (INES).

The Fukushima crisis had been ranked at 5 on the INES, the same level as the Three Mile Island accident. A rank of 7, or "major accident," involves a "major release of radioactive material with widespread health and environmental effects," according to an INES pamphlet.

TEPCO says there are some 900 workers at the Fukushima Daiichi and Daini plants tackling the crisis, but none has exceeded the annual radiation dosage limit of 250 microsieverts. About 20 workers have reached the 100-microsievert level.

Japan also expanded the 12-mile evacuation zone on Monday to include the communities of Katsurao, Namie, and Iitate, as well as parts of Kawamata and Minamisoma. The government has estimated that the radiation dose in part of Namie could exceed 300 millisieverts over a year.

It's another sign that parts of Japan may be permanently abandoned as nuclear wastelands.

http://www.youtube.com/watch?feature=player_embedded&v=rW_CaBaK0bo