PARTIAL MELTDOWN?: Explosions continue to rock the stricken Fukushima Daiichi nuclear power plant in Japan, pictured here, in the wake of the devastating earthquake and tsunami.
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Just after 6 AM local time on Tuesday in Japan, a sound like an explosion was heard near the suppression pool of reactor No. 2 at the stricken Fukushima Daiichi nuclear power plant. This followed an explosion March 11 that ripped the roof off reactor No. 1 and another at reactor No. 3 on March 14 that injured 11 workers. The culprit in all three cases is likely a build-up of explosive hydrogen gas—as occurred at Three Mile Island in the U.S. in 1979 as a result of the meltdown there—caused by nuclear fuel rods experiencing extremely high temperatures stripping the hydrogen out of the plant's steam.
"The hydrogen accumulates outside of containment but inside the reactor building. You get enough and some spark source and you get an explosion," explains nuclear engineer Michael Golay of the Massachusetts Institute of Technology. "The [radioactive] cesium and iodine showing up in releases shows the fuel has been damaged."
The 3.7-meter-long nuclear fuel used at Fukushima is composed of uranium oxide pellets encased in a zirconium cladding. Though control rods have stopped the uranium fission process that drives normal operation of a nuclear reactor, the byproducts of that continue to split and generate heat. If the fuel rods are no longer being cooled—as has happened at all three reactors at the Fukushima Daiichi power plant operating at the time of the earthquake—then the zirconium cladding will swell and crack, releasing the uranium fuel pellets and fission byproducts, such as radioactive cesium and iodine, among others.
The high temperatures that the fuel rods create boil water and continually turn it into steam. If no fresh water is introduced to cool the rods then they continue to heat up. Once the rods reach more than 1200 degrees Celsius, the zirconium will interact with the steam and split the hydrogen from the water. That hydrogen can then be released from the reactor core and containment vessel and, if it accumulates in sufficient quantities—concentrations of 4 percent or more in the air—it can explode, as has apparently occurred at reactors No. 1 and 3, and possibly No. 2 as well. The explosions at reactors No. 1 and 3 destroyed the surrounding buildings but have apparently not damaged the massive steel containment vessel—as much as 20 centimeters thick—that surrounds each reactor's nuclear core.
But the explosion at reactor No. 2 is potentially more serious as it seems to have affected pressure in the suppression pool—a massive pool of water stored in a torus-shaped chamber beneath the reactor itself that both cools and captures any escaping radioactive materials. If the suppression pool is damaged or cracked and can no longer help cool the reactor—and 2.7 meters of the fuel rods in that reactor remain exposed, according to Tokyo Electric Power Company—radioactive material might escape.
And if the temperature continues to rise—and the reaction of zirconium and oxygen produces yet more heat—the cladding itself can spontaneously combust. "If the temperature gets high enough then you get a self-propagating reaction and a fire that would burn like a sparkler," says physicist Arjun Makhijani, president of the Institute for Energy and Environmental Research. "If it catches fire, all bets are off."
The TEPCO operators of the stricken nuclear power plant—a minimal staff of 50 as all non-essential personnel have been evacuated—now face a balance between venting the steam building up in the reactor (since the main danger for a widespread release of radioactive material is steam bursting the thick steel container holding the reactor) and keeping any radioactive materials inside the power plant itself. "There is no way to carry the heat away to cool down the core unless the steam that is generated is vented," Makhijani notes. "That's going to be radioactive steam."
Filters should capture some of the radioactive materials, according to Richard Meserve, president of the Carnegie Institution for Science and former chairman of the U.S. Nuclear Regulatory Commission. "The release of that steam is the way they're getting rid of energy" in order to keep cooling the damaged nuclear fuel rods, he explains.
TEPCO is currently injecting seawater and boric acid—boron absorbs the neutrons that radioactive materials give off to prevent any self-sustaining fission—into two of the three reactors, according to the company's statements. If such cooling is not continued, a full meltdown could occur in which the uranium fuel pellets escape the cladding and form a molten pool in the reactor core—potentially melting through the thick steel containment vessel. "I don't see that they have much choice but to do what they're doing," Makhijani says. If the molten pool melts through the steel containment vessel, large amounts of radiation would be released into the environment.
The Japan Atomic Industrial Forum, an industry body, estimates that core cooling systems are not functioning at all three Fukushima Daiichi operating reactors and two of the four reactors at the nearby Fukushima Daini nuclear power plant are relying on backup cooling systems.
What remains unclear is how much of the nuclear fuel at any of the three Fukushima Daiichi reactors has melted down, though TEPCO has announced that the fuel is likely damaged in all three reactors that were operating there at the time of the earthquake. A TEPCO spokesman said in a press conference on March 15 that the company "cannot deny the possibility that fuel rods may be melting." The truth of the matter may not become clear for years. After all, it took years for the U.S. Nuclear Regulatory Commission to determine that a meltdown had occurred in Three Mile Island—an effort that required sending in robots with cameras.
Already, radiation at undisclosed levels and in undisclosed forms has been detected on the U.S.S. Ronald Reagan, according to the U.S. Navy, an aircraft carrier that had been 100 miles northeast of the stricken nuclear power plant off the coast of Japan on March 14. Levels detected in a short duration pulse at the plant itself have reached as high as 8,217 microSieverts per hour, or eight times the dose endured in a typical CT scan and four times the normal dose of background radiation in a year. "They're getting out of the way of the plume," Golay says. "There is no reason to stay in the way if they do not have to."
And cooling will need to continue at the Fukushima nuclear power plants for a long time to come. After all, the spent fuel pools that may have been exposed by the power plant explosions contain more than 200 metric tons of used uranium fuel rods that have been cooling for weeks, months or even years—and smoke or steam continues to billow from the exposed spent fuel pool of reactor No. 3. "Their goal is to keep everything stable and keep the radiation bottled up in the reactor vessel," Golay says.
TEPCO continues to pump seawater and boric acid into all three reactors, particularly aiming to cover the partially exposed fuel rods in reactor No. 2, but it is proving difficult to maintain. "They will have to keep on retaining cooling for months," Meserve adds. But "if they can get through the week with this system without a major environmental release, the danger will be less."
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23 Comments
Add CommentCan someone who knows this stuff please give us some idea of what the chances are for the melted core to reach criticality where fission would begin again and what the consequences of that would be. ( I am aware that a nuclear fission explosion is not possible. I am talking about the core sustaining a fission reaction as is done in normal plant operation.) Obviously it is of some concern hence the boric acid injections. I understand that the fuel arrangement and geometry and temperature and density would all have to be favorable and that there would have to be a moderator present to slow the neutrons down. The seawater would be the moderator. If the boron is not present for some unforeseen reason then how likely would the other factors come into line to allow criticality?
Reply | Report Abuse | Link to thisIf criticality was reached would the materials simply heat up and expand beyond critical mass or would loss of the water moderator due to steam formation self-regulate the process? How much energy would be released? Could it cause pressures to rise inside the reactor vessel that would exceed the relief valve capacity?
Vendicar Decarian,
Reply | Report Abuse | Link to thisThanks for your response. I thought I was clear that I do not consider a fission explosion to be even in the realm of possibility. I am trying to use the word "critical" in the same way as I have seen used in various science publications, wikipedia and from what little education I have had. Critical means that a sustained fission chain reaction is taking place. Sub critical means that more neutrons are being absorbed or are escaping the system then are creating new fissions. A working reactor is above critical a scrammed one is sub critical. If the core cannot become critical again under any possible circumstances I must ask why they are injecting boric acid to absorb neutrons? It would not have any effect on the decay of the fuel. I don't think reaching criticality is likely. I don't even know that it would be that consequential. I just want to have some idea of the odds. And what would happen if it became critical and how long that would last, etc.? Maybe the word critical is associated in the public mind with nuclear explosions, I am attempting to use it in a manner I thought was proper.
We do not, today, have the technological capability of safely dealing with nuclear energy.
Reply | Report Abuse | Link to thisWe are running out of the petroleum part of 'our' (i.e., the planet's) fossil fuel assets.
Coal is impossibly dirty.
Solar energy, wind energy, tidal energy - and all other renewable sources of energy - can supply us with only a fraction of what we need to operate the artefacts of our 'civilization'.
Fusion energy - if turns finally out to be clean and safe - is way too far off in the future to be thought of as a realistic alternative.
To me, the answer is clear (and we all should have been wise enough to realise this at least half-a-century ago):
We absolutely have to scale back our 'energy-wasteful' technology very significantly (and also modify our food habits hugely). Probably we shall also have to reduce our population equally significantly - how do we go about doing this? We need to find out just how much time we may have. Best/worst case scenarios need to be worked out most urgently.
Do we have the wisdom to make the right choices?
GSC
Tom - you're correct. Although I can't answer your explosion question, your terminology isn't off. A reactor needs to maintain a criticality of 1. If criticality increases above 1, then the chain reaction will increase. If it drops below 1 then it will die out.
Reply | Report Abuse | Link to thisI'm certain there will be a lot of "lessons learned" from this disaster. I am equally convinced that we must press forward with building a lot more nuclear plants. Shame fusion is still so far off, but I'd still take a lot more nuclear power in exchange for coal. Especially in China and other countries that don't put scrubbers on their stacks.
Compare "estimated" deaths from this disaster (and Chernobyl) side-by-side with the same statistics for coal power. I think nuclear energy compares quite favorably, although I don't have the numbers in front of me at this time.
I understand that they are using sea water to take the heat away. However sea water is extremely corrosive and in the coming days it will be more difficult to remove the heat due to slack formation. Full meltdown is probably inevitable.
Reply | Report Abuse | Link to thisI was surprised to learn that the water circulation system depended on diesel generators and batteries for back-up in the event of power failure. In my 20 years in petroleum refinery operation and design, STEAM turbine driven pumps were the standbys for certain critical systems. Steam is always available in abundance where nuclear reactors are concerned. Why are steam turbines not employed?
Reply | Report Abuse | Link to this01:11 PM 3/15/11
Reply | Report Abuse | Link to this"Why are steam turbines not employed?" they are employed how do you think they convert the nuclear energy to electricity they use the steam to turn turbines to generate electricity so why wouldn't they run that to the pumps as well or at the least a backup to the diesel
@drafter: While that might seem like a useful design, the mechanical controls that needs to be exerted over a boiler and steam turbine make that an unpredictable option to rely on for accident planning. Having said that, most modern Gen. Electric BWR's (boiling water reactors) do have the ability to inject condensate via a HPCI turbine powered by steam. See http://abbreviations.yourdictionary.com/hpci-turbine
Reply | Report Abuse | Link to thisI've already debunked this nitwit's claptrap. He refuses to show us any calculation or links for his nonsense.
Reply | Report Abuse | Link to thisActually it would take much less than 50K nuke to supply the current world population with the same energy consumption as the US today - Much less with a conversion to nuclear and electric transportation fuels, and effective mass transit. Annual annual energy use much less than US average in warm states like CA. Most of the world population is in warm places.
World population projections are hard to make as birth rate drops quickly to less than replacement levels when women achieve education and job access.
In 1943 with 1% of today GDP Americans production of liberty ships was the equivalent of a nuke a day. The world could easily mass produce 10 times that today - so 10 years job done.
Core melt rates on old nukes are not relevant to new ones which are thousands of times less likely. In any case it appears the reactor just doesn't get anywhere hot enough to melt steel much less 6 inches of it once the fuel rods melt even without coolant. TMI proved that.
Veindicar: You have many compelling points however I strongly contest your initial posting. Fifteen Billion people???? Where on earth did you come up with that? I do not know your age so you may not have been around for the initial ZPG movement of the late 60's. IN any case even a cursory review of population predictions since that time can only lead to one conclusion: they are incorrect! Please if your are going to posit hypotheticals please use data that is a bit closer to reality.
Reply | Report Abuse | Link to thisSecond, it may be a point of semantics however referring to what is happening in these facilities as a "Failure" is misleading. While I grant you that placing nuclear plant in areas known for their seismic activity is far from prudent; this was not a failure of human operation, which you infer. This is an extremely rare event that is unlikely to hit other Nuk plants due to their distances from major geologically active areas. Consequently your predictions of doom are to say the least over rated.
The issue is risk and how we manage it. You awoke this morning therefore there is risk in your life. I can guarantee that today you took voluntary risks far more hazardous to your health and well being than the possibility of ever being affected by this or any other Nuk power plant problem and yet you thought nothing of it. Our illusions regarding control cause us to dismiss the dangers of proximal risk and amplify those of distant and unlikely risks. In a nutshell, as we are enjoying this exchange of ideas, there is a guy somewhere who just finished his third beer while driving down a highway and he is worrying about dying in an asteroid hit. This is irrational but is is us.
Our future with power generation will be challenging and barring miraculous breakthroughs will utilize more nuclear. However, the current problems were brought about by monoculture. We will need to exploit a combination of generation sources if we are to succeed. This means we will have to use photovoltaic, wind, tidal,coal and other fossil fuels in those areas close to population centers or where the risks of geologic activity is high. In others where population and other risks are low, Nuclear power makes sense. While this does not fit with those powers looking for monopolistic control of energy production it is the only real solution we have.
earthquake of this magnitude
I noticed that no one has suggested getting a surplus of iodine for their families while it is still available,a fall back is of course salt but even that may become scarce if the radiation gets much worse.The time to act is now.
Reply | Report Abuse | Link to thisVendicar is a typical zero knowledge antinuclear zealot spewing drivel with each comment
Reply | Report Abuse | Link to thisThe weight of steel in 3 liberty ships is the equivalent weight of the steel in a nuke plant almost all it rebar.
A nuke plant is really not much different than a coal plant with nuclear heating elements inserted in the heat transfer water instead of an external heat source. Much less complex than a Liberty ship.
Note that vendicar is refusing to backup his absurd claim of 200k nukes required to carry the world's energy load.
Reply | Report Abuse | Link to thisHe also refuses to justify his claim that modern nukes have the same chance of core damage as the ancient 50's design's that have caused problem.
He just makes up nonsense in an effort to impress himself.
So what would the last step be if there is a total failure and nuclear fuel leaks and/or burns out into the environment?
Reply | Report Abuse | Link to thisAll the "experts" have been saying this won't happen and that won't happen and you know what it has all happened.
So, I'm curious what is the worst case scenario for what is going on in Japan, the absolute worst, because it seems like that is where we are headed.
If you are the management of a 40 year old nuclear power plant, that is already giving a lot of trouble, you must consider de-commissioning the plant. Costs of decommissioning demolish the economics of nuclear power.
Reply | Report Abuse | Link to thisThe cheapest way to decommission a nuclear power plant is to have its core meltdown. If done during a natural disaster, we can blame nature and shift costs to the taxpayers. Just turn off all cooling, the core does the rest, in fact it can be part of the control program, so no employee on site is aware of any wrongdoing.
If you are caught: Humbly apologise. Remember Tuna quotas?
What you talking about, it helps us get rid of CO2, which according to all the hypocrites is the cause of global climate change.
Reply | Report Abuse | Link to thisPopulation levels drive CO2, global climate change and the sixth extinction. Nuclear core meltdowns control Homo sapien population levels, since they cluster around power plants. Great for the planet, and in the short term (1-2Kyrs) for the biosphere too.
The answer is Homo sapien population control. No more than 2G of this stupid species on this planet.
Reply | Report Abuse | Link to thisOne day you need to decommission, a meltdown is the cheapest solution.
Reply | Report Abuse | Link to thisWhy not put an enclosure completely around the reactor like an above ground pool and then fill it to the brim with water?
Reply | Report Abuse | Link to thisI think the water lines have to run to the fuel rods to cool them and this way the dammed water would keep the roods cool and keep it from overheating and escaping as steam or worse, blowing up?
Engineers should be able to get a pool like enclosure around the reactor in no time and since most the area is awash and radiated, even if some water leaks; water can be kept pumped into the pool or dam enclosure until the rods finally cool off themselves.
Charles Slakan
And although my suggestion might sound a little simple, in situations that are untried in the real world experience, maybe we all should keep an open mind.
Reply | Report Abuse | Link to thisAnd those of you that can propose my suggestion because of your credentials, please don't feel ashamed to do so.
Everything the Japanese have tried thus far has not been effective.
At this point, any idea should be reviewed and not dismissed.
Thanks for bearing with me,
Charles Slakan
The explosions at the plants are clear indicators that 3 if not 4 of the reactors have suffered at least a partial melting of the core. Nuclear plants do not store large amounts of hydrogen gas. The gas is created when the water surrounding the fuel rods is heated to steam, and then heated to over 900 degrees where the water molecules break into hydrogen and oxygen, when the percentage in the air reaches over 4%, any spark will cause the gas to explode. The hydrogen gas explosions at the plant have been cause by a meltdown of the nuclear material in the reactor. Unless they can get enough cooling water into the reactor area the mass will continue to heat up and will eventually drop to the bottom of the steel containment vessel and burn through, there into another water area surrounding the containment vessel, surrounded by concrete.
Reply | Report Abuse | Link to thishttp://www.eurekalert.org/features/doe/2004-06/dnl-npm061404.php
A vast number of variables at this point, but its entirely possible the concrete structure surrounding the reactor has been cracked by either the quake or the explosions.
As you indicate there is no risk of a NUCLEAR explosion, but its possible to have an non-nuclear explosion of the material as it comes in contact with the water in the outer containment vessel.
The risk is radioactive material being vented into the atmosphere as a result of these events.
What is possibly a bigger risk is the spent fuel rods (spent in a reactor sense, not a nuclear sense) which must be constantly cooled for long periods of time. The cooling system at the plant has totally failed, and in at least one pond there is no water at all. THe risk there is the spent rods heating to the point where they actually burn, releasing cesium 137 into the atmosphere which has a half life of 30 years and is assimilated into plant and biologics the same way potassium is.
There are between 50 and 100 tons of spent rods next to each reactor.
Kent
A couple more comments.
Reply | Report Abuse | Link to thisYou should consider the inherently safer Candu reactor design. With the american design, if all systems fail, core can melt down. With the Candu design, if systems fail, reaction fails.
Nothing is foolproof, but the design is safer.
As for fossil fuels, think you will find that fossil fuels never came from fossils.
Supply in some depleted fields has rebounded.
Think about it, all the worlds oil came from the oils from dead critters that seaped through the earths crust to form in pools hundreds and thousands of feet below the surface. Billions and billions of barrels worth.
I dont think so.
I want to clarify the assertion that a concentration of 4% hydrogen in air is the lowest explosive level (LEL) for hydrogen gas--this is incorrect. The LEL of hydrogen is 18%--much different than the lowest flammability level (LFL) which is at concentration of 4% hydrogen in air. Thus at 4%, it will only ignite. It is also important to note that its stoichiometric mix is at 29%.
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