BEFORE THE QUAKE: The Fukushima Daiichi plant as it looked before the 2011 earthquake and tsunami.
Image: WIKIMEDIA COMMONS
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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.
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19 Comments
Add CommentThis is an excellent summary of a topic that is often difficult to understand. Great job, John!
Reply | Report Abuse | Link to thisIt will be interesting to see this accident's effect on the already sputtering nuclear energy development in the U.S. I've just heard on NPR that Congress lined a proposal to halt reactor applications...
Luca Semprini
www.poweringanation.org
Two things come to mind in figuring out a long-term solution to nuclear power generation safety. First, the back up diesel generators should not only be tested, but nuclear plants using this back-up method should be required to run the emergency cooling system for a significant percentage of time, on perhaps a monthly or bi monthly basis where they would be the primarily cooling system for say, 3% to 5% of the time. This way any bugs in the back-up systems (not only diesel) could be ferreted out and corrected. How many times have we went to a backup, only to find out the backup that we depended had failed. That's whether it was a back up floppy (remember those), or a back up generator of a storm. Regardless, as in Murphy's Law, these things tend to fail when you need them most. This may be able to be prevented by a regularly scheduled robust activation and usage of these systems.
Reply | Report Abuse | Link to thisThen in the event of catastrophic failure, we must plan for the unthinkable and the unknown. Now we know that dangerous levels of hydrogen are a factor in a reactor malfunction, so engineer adequate venting into the structure that can relieve the hydrogen build up and possibly avoid the explosions that are occurring at the Fukushima Daiichi power plant.
My two cents; Thanks!
Sierra makes a very good point. Back-ups must be able to back up. Always.
Reply | Report Abuse | Link to thisI'm just a layperson but it seems like there should be more redundancy to keep the water-pumps running in order to keep everything cool. Keeping the core cool is right up there in the list of very important mandatory priorities.
Relying just on diesel generators and the local power grid seems very insufficient. What happens if they fail? Unfortunately we are seeing what happens when they do.
There should be a way to keep the water-pumps going for the time equal to the moderated core temperatures decreasing to a safe level after the fission process has been deactivated.
It seems terribly inefficient to not use the energy already being produced by the moderated fuel rods to power a back-up cooling system.
If that much heat is still being generated when the fission process has been "turned off" use it to power the cooling pumps! Otherwise all that heat energy is completely wasted and potentially dangerous when it leads to a hydrogen explosion - and worse.
It just seems like a huge opportunity is not being used to power those pumps when there is already so much energy already in there. Use that wasted power to control the enemy within: heat.
Using nuclear energy is like being an athlete on steroids. It helps to run very fast in the short term but it destroys us in the long term.
Reply | Report Abuse | Link to thisThanks for another helpful objective account of the technical issues involved in the nuclear mishaps in Japan. Yet what the public needs now is fewer physics and engineering tutorials, and more explanation of how we arrived at this point. The operative question is not whether we should trust nuclear power; it’s whether we should trust the nuclear industry.
Reply | Report Abuse | Link to thisThe industry insists that it is trustworthy and can manage all conceivable risks. So why did they adjudge Japanese fault lines to be good sites for nuclear plants? And why was the Fukushima facility designed to withstand 8.2 magnitude earthquakes when Japan has seen three such events in the past century?
Following what was an entirely foreseeable event (earthquake plus tsunami) these particular plants are clearly not failing safe. There has been significant release of radiation, and great uncertainty as to the survival of the containment vessels. The incident has today been rated as 6 out of 7 - the second gravest nuclear mishap in history. Decision makers on the ground have calculated that the probability of a really serious contamination is sufficient to warrant evacuating over 200,000 people (which is itself a risky thing to do).
In examining the suitability of nuclear power to other lands, we should take no comfort from given places being less earthquake prone than Japan. The lesson is that the nuclear industry puts profits before safety, barely specifying power plants to suit local conditions, whatever they may be.
It sounds like the Diesel back-up generators probably failed due to Tsunami damage following the earthquake.
Reply | Report Abuse | Link to thisThis posability should have been forseen and the generators mounted higher on tsunami and earthquake resistant structures.
BUT, hindsight is always 20/20 and we all need to learn from this and make already safe nuclear plants even more safe.
Possibly a 3rd gravity fed cooling water system with large earthquate resistant reservoir mounted above the structure.
The experts will figgure this out (probably already have)
This is no reason to stop nuclear energy development.
Nuclear is still MUCH more enviornmentally friendly than Coal Mining and Greenhouse gas.
It's the same old story: The Industry concerned becomes essentially profit driven & is subject to poor regulatory supervision, which leads to complacency.
Reply | Report Abuse | Link to thisIn this case the whole decision tree: where it's built, what "events" are allowed for, & what levels of modernisation based on post design experience are effected?
All accidents (as in the airline & oil drilling industry) teach us that complex systems under poor scrutiny will always lead to failure. Have we forgotten already BP's recent demonstration of incompetence in the Gulf?
Yes you are correct lufry, nuclear energy releases zero greenhouse gases. However, I disagree with you. First let me ask you where is the best spot in the world to store radioactive materials (low-level and high-level)? the answer is no where. the spot where the world could store these needs to be completely secure from geological disasters. There is no way scientists can determine this. the half life of uranium 253 is 700 million years, so wherever we store it, it must have to stay away from the environment. this is virtually impossible and even if it was, how much money would it cost to store it, who will pay for it, what country would take the responsibility to safeguard these? while greenhouse contamination does affect our environment a lot, radiation from nuclear waste damages the environment FOR EVER. once something has been exposed, it is virtually toxic for ever due to the half life of radioactive elements. Look at what happened with the Chernobyl accident and what problems they still face today. http://www.youtube.com/watch?v=bSRC1_OZPIg
Reply | Report Abuse | Link to thisNuclear energy is an alternative to meet our energy demands, it is not the correct alternative though.
Thanks John. A great summary.
Reply | Report Abuse | Link to thisAbout radioactive waste:
Reply | Report Abuse | Link to thisWhy do we have to look for a safe storage for millions of years? We could find a safe storage for 500 years and leave it to better ideas / technology to find a better way. We just cannot dump it any more, we have to store it properly.
Even if you are against nuclear energy (as I am) you still have to deal with the waste!
Very good article, thanks.
Reply | Report Abuse | Link to thisBut I have one more question: How long does it take until the core is cooled down if properly cooled, or the other way around: how many tons of water can you boil with the remaining heat in a reactor like that?
I have seen this reported in two places now, that the situation in Fukushima "has been rated a 6 on the INER scale." Who is it who has rated it a 6? Can you post a link supporting this assertion?
Reply | Report Abuse | Link to thisThe article mentions coal and gas plants as larger power sources. So my question, if answerable, is "What would be the situation if Fukushima Daiichi were coal- or gas-, rather than nuclear-, fired?
Reply | Report Abuse | Link to thisSo it appears the key problem is cooling on hand is cooling the damn reactor. The Core is still highly radioactive and hot, so I presume the Lead would melt, I'm not sure what transformations will take place to the molten lead. But clearly the core needs to be cooled. If that's the case can we inject Liquid Nitrogen in large enough quantities to bring this sucker down? The question is how does one make large quantities of liquid nitrogen. Fractional distillation of air will produce Liquid Nitrogen, but can we set up a plant quickly enough to get sufficient quantities into the core?
Reply | Report Abuse | Link to thisHell, Nitrogen is only 70% of the atomosphere.. any engineers out there willing to take a crack at this?
What happens if we dump a several tons of Lead (Pb) on the reactor core. I was asked this question last weekend and unfortunately my Chemistry knowledge is no longer what it used to be.
Will Lead kill the reacto,r or at least slow it down, at the very least it should form a radioactivity shield...In addition if we throw in sufficient Lead, it could help arrest the radiation...
thoughts?
I see that physicists usually are very confident that a meltdown is just about raising temperatures and melting which leads to leaking radioctive material in the environment. What about the risk that the fuel core material seperates from the neutron absorbing material because of the melting (do all of the materials have the same melting point??). Could the core or parts of the core than get reactivated and what will happen than?? What explains the rising temperatures in units 4 to 6 in the Fukushima plant. Is this caused by heat from unit 3 or because of the fact that the resting the fuel rods in unit 4 to 6 get reactivated by an external neutron bombardment and if so where is this coming from??? It seems that radiation levels in the vicinity of the plant are very high. The point is we never dealt with a real meltdown except that of Tjernobyl and we assume they all will follow the same paths independent from their exact constitution. Now we can only see how this one evolves or revolves.
Reply | Report Abuse | Link to thisThank you John Matson for explaining what happens during a nuclear meltdown. I was not exactly sure until I read your article. It can be really difficult material to understand but you made it clear to me.
Reply | Report Abuse | Link to thisI agree with you 100% Sierra....maybe YOU should be the person advising these crazy people. I know that accidents happen and the unknown is just that...the UN-known. However, with nuclear power and the terrible, potentially LONG lasting consequences of melt down etc, it seems that the folks involved here would be required to take EVERY imaginable precaution to predict and plan for the unknown. And with all the brilliant physicists/chemists in the world....how did they not foresee that hydrogen could pose a problem?
Reply | Report Abuse | Link to thisAn excellent summary. But it is very important to assess the vulnerability of nuclear plant to hostile action (attacks by bombs/ missiles) and sabotage (terrorist attacks). For the latter, it would be a good idea to assign the equivalent of graded 'seismic' zones (terrorist activity,or TA, zones)to various territories. For countries in high risk TA zones, the vulnerability of nuclear plants to hostile action or sabotage assumes critical importance and needs to be subjected to a rigorous scientific analysis. Governments that favour nuclear power tend to sweep this under the carpet.
Reply | Report Abuse | Link to thisI'm writing an apocalyptic type book and could really use some insight from you guys if you have time.
Reply | Report Abuse | Link to thisIf all of the fuel rods overheated, what would be the short and long term affects?
How long would it be before crops could be planted that are edible and won't make anyone sick?
Would the water ever be potable and if so, how long before it is?
Would all animals and plants be destroyed throughout the world?
How long after power outages REALISTICALLY speaking, would it be before ALL of the nuclear plants would overheat?
I'd appreciate you help in this. It's a sci fi book and I'm trying to make it as realistic as possible.