SEAWATER: Debris float in the Pacific Ocean off the coast of Japan after a magnitude 9.0 earthquake and subsequent tsunami struck the nation on March 11.
Image: COURTESY OF U.S. NAVY, MASS COMMUNICATION SPECIALIST SEAMAN STEVE WHITE
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As the situation at Japan's 40-year-old Fukushima Daiichi nuclear plant goes from bad to worse—four of the plant's six boiling-water reactors have been damaged by explosions or fire, and radiation has begun leaking into the atmosphere—officials there continue to pump seawater into the reactors in a desperate attempt to cool down fuel rods and avoid a complete meltdown that could release radioactive fallout across much of the country and beyond. The move by Tokyo Electric Power Co. (TEPCO), which operates Daiichi, to use seawater doped with neutron-absorbing boron in the reactors' pressure vessels all but ensures that they will never function properly again, permanently damaging one of the world's 25 largest nuclear power stations.
Such extreme measures were necessary because the normal and auxiliary cooling systems, which circulate purified water to keep the fuel rods from melting down, failed. Last week's tsunami not only cut normal electrical power to Daiichi, it also flooded and disabled the backup diesel generators. Three of the plant's six reactors were already shut down for maintenance when the magnitude 9.0 earthquake-induced tsunami struck, but TEPCO has struggled to cool the fuel rods used in the operational reactors. The lack of sufficient water to cover these rods has led to explosions at all three active reactors, likely due to a build-up of hydrogen gas.
A fourth reactor, one of those that had been shut down prior to March 11, experienced a fire Tuesday that threatened to evaporate water in a storage pool for spent nuclear fuel. That fire may have been caused by hydrogen seeping from the pool, Nikkei.com reported.
The prospect of ruining a half dozen nuclear reactors pales in comparison with the alternative—a complete meltdown that would contaminate the ground below the complex with radioactive material that could be spread by wind, rain and groundwater, potentially causing radiation sickness in thousands of people.
Scientific American spoke with Pavel Tsvetkov, an assistant nuclear engineering professor at Texas A&M University in College Station, about why seawater is a last resort for cooling compromised nuclear reactors and TEPCO's options moving forward.
[An edited transcript of the interview follows.]
Why does a nuclear facility normally rely on purified water to cool its reactors? How is it purified?
I'll give you an example: If you have a boiling pot and your water has too many minerals, then condensation will collect inside your boiling pot. When this happens in a reactor, it interrupts the properties of the fuel elements. Energy companies don't want to jeopardize the performance of their reactor materials, so they use purified water, usually from a special water purification plant on site. Purifying the water removes most of the salts and anything that could accumulate on the fuel elements.
Under what circumstances would a nuclear power plant use seawater to cool its reactors?
Using unpurified water is not a normal practice—it's never done. Plants don't take water from the river or the sea to supplement their own internal water, which is in completely closed-loop systems. Of course, they take in some amount of new water periodically to make up for evaporation and other losses like that, but this water is purified before it is used.
TEPCO's reactors lost the water below the normal operating condition, so they had to provide additional water for that. Salt-water obviously has a lot of minerals in it, and if it's taken directly from the sea, it has all sorts of other materials floating in it as well. Even if these things were filtered out, the chemistry of salt-water is not really compatible with what normally goes through the reactor. It's too corrosive for fuel elements. I would guess that after this water was introduced into the reactor cores, those cores would become completely unusable. This is because any materials in the water will attach to the surface of the fuel rods and make heat transfer unpredictable.
What role does the boron in the seawater play?
Boron can be injected into water-coolant systems to control the activity of a reactor core because it is a strong neutron absorber, especially for thermal neutrons. But boron is not usually used in boiling-water reactors such as those at Fukushima Daiichi because boron is also corrosive on fuel elements. In cases of emergency, however, boron and seawater can be used to suppress fission chain reactions in the fuel elements.
So the use of seawater and boron is a last-resort effort to cool a reactor?
Probably if they had more time they would have tried to restore the diesel generators that ran the backup cooling system and circulate the water they already had. But with the water in the core evaporating due to the high temperatures, they needed to add more and more water so they could quickly suppress boiling conditions.
Given that it can cost several billion dollars to build a new reactor, isn't there any way they could clean the fuel elements?
They may try to explore whether the reactor can be restored because it's a significant investment. But I'm not aware of anyone ever using salt-water for any prolonged period of time in their reactors like they are doing now.
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29 Comments
Add CommentAside from the damage to the reactors, can Scientific American provide insight into whether the accumulation of salt inside the reactor chamber under the conditions of a meltdown could create a greater risk of explosions or of the salt boiling off and carrying with it dangerous radioisotopes? Or the possibility that use of sea water now will cause accumulation of salt which then acts as an insulator and limits the effectiveness of later efforts to cool the remaining fuel with water? Or, on the other hand, will the salt effectively encase the fuel and limit the potential for it to escape into the environment?
Reply | Report Abuse | Link to thisIf the reactor gets hot enough to melt, or corrode, the cladding (covering) of the fuel pellets, then the uranium will begin to chemically react with the sea water pumped in. If that happens, the fuel will have to be completely removed and reprocessed before it can be used again. Completely refueling 6 reactors is not going to be cheap. However, the station in Japan was scheduled to be decommissioned soon anyway, and the fuel would be removed and reprocessed. So total financial losses due to contamination by sea water are likely to low to negligible. It's dependent on how much physical damage is caused by the explosions and overheating.
Reply | Report Abuse | Link to thisSeriously, who cares about the financial losses to the plant owners? It is quite obvious by now that the facility is beyond all repair and that even if it could be fixed the crisis has made continued operation impossible politically. The real question right now is what reactions could occur if melting fuel contacts the chemicals in sea water. Could this result in an explosion that could compromise containment?
Reply | Report Abuse | Link to this@Seth,
Reply | Report Abuse | Link to thisSo assuming everything you claim is correct. Why would anyone want to build a nuclear reactor? If corruption is happening in Japan what are to make of proposals to build reactors in in underdeveloped nations? Who thinks that the corruption and incompetence you claim we are seeing in Japan is not going to occur in say Nigeria, Albania, etc, etc.
Yet these are the places that will also require nuclear power, if as according to you, nuclear power is the only option. What international mechanism do you propose to prevent corruption and incompetence from occurring again and again. It is patently obvious that no such regime exists now.
I don’t understand why the effect of salt water on the condition of the fuel cladding, which could hardly be any worse already, is relevant to the reactor’s ability to be renovated and used again in future. I suppose the fuel would be removed, hoping that other vessel internals were not too badly damaged, and the reactor refueled. But there’s a reason, obvious to a corrosion scientist, why the reactor could never be rehabilitated – the hot steamy chloride-rich conditions will have already initiated stress corrosion cracking of important vessel internals that are made of austenitic stainless steel.
Reply | Report Abuse | Link to thisAnd what, pray tell, are they going to do with the contaminated sea water?
Reply | Report Abuse | Link to thisI am told that injection of seawater is actually a part of Fukushima Daiichi's formal emergency plan, to the point of having concrete storage tanks from which the seawater can be pumped. Yes, it is certainly a last resort perhaps, but a well thought out and planned for last resort that has done what it was supposed to do (at great cost to the reactor core obviously). It makes sense to have a "beyond design basis" plan in place in a country noted for seismci extremes. As noted in a March 11 SA article, the design basis earthquake for Fukushima is 7.9M, and this one at 9.0 is an order of magnitude larger with 32+ times the energy of the DBE.
Reply | Report Abuse | Link to thisDesperate times require the necessary use of seawater as a last-resort. Nuclear power plants and associated equipment can be replaced regardless of cost. The loss of human lives cannot. I am sure the new designs for nuclear power plants near saltwater sources will include the use of seawater as an alternate cooling water selection. This is not unique for chemical plants and refineries that already have this option already in use for emergency reasons. It is only a matter of cost effectiveness and to save the potentional loss of life in case of a catastrophy.
Reply | Report Abuse | Link to thisI doubt we can do anything about international sales but it is possible a standard requiring regular IAEA inspector on site could be required as a condition of sale.
Reply | Report Abuse | Link to thisModern nukes have passive cooling systems which prevent this sort of accident.
@Sethdayal,
Reply | Report Abuse | Link to this"I doubt we can do anything about international sales but it is possible a standard requiring regular IAEA inspector on site could be required as a condition of sale."
Sale is only one part of construction and you know it. Further once they got the parts what how exactly do you prevent corruption and incompetence?
"Modern nukes have passive cooling systems which prevent this sort of accident."
Have you given thought to what kind of damage corruption and incompetence can do even the best of designs? Any at all?
Once again we have a scare campaign based on ignorance. I fervently hope there are no fatalities but if there are they will be insignificant compared to deaths from coal mining & from particulates including radioactivity from coal burning power houses. In the mean time we have an already traumatised population being scared out of their wits by ignorant scaremongering. Expatriates in particular are fleeing overseas, terrified by alarmist at home begging them to leave. As if the Japanese did not have enough to deal with already, they are being besieged from abroad by people demanding to know the latest & in my view many who are actually hoping for a disaster to prove their anti nuclear mania. It is not going to happen but they will still claim that it could have. There is an absolutely zero chance of a nuclear explosion & even a meltdown ranges from simply a nuisance level to a more serious release of radioactivity but still not catastrophic. The cleanest & safest electricity generation per gigawatt ever devised. Even wind & solar have greater fatalities. There are about fifty technicians actually in the control facility. They are the ones at greatest risk but none of them to date have even received a radiation dosage that would make them sick. No member of the public has been exposed to as much radiation as they would incur forom a single chest ex-ray. I am sure that even if this is resolved without a single fatality after this most extreme event where the Japanese have not only had to deal with one reactor but with a string of them at one time, the alarmists will still try to claim that the technology is inherently unsafe. Garbage.
Reply | Report Abuse | Link to thisThis disaster happened off the scale!
Reply | Report Abuse | Link to thisI'm 62, this quake was a 9.0. I think only one of TWO during my life.
While this is a horrible accident, I don't think you can say every plant should be built to withstand a category 10 event.
But, since Three Mile Island put a 30 year hold on nuclear plans here, this six reactor accident will probably doom nuclear power for the rest of my life.
Too bad, that. We need nuclear. There is no such thing as Clean Coal. There is horrible pollution with Frakking. There are unacceptable costs to oil.
But these problems will be overcome with time.
My heart goes to the 50 who volunteer to stay the bastions and man the gear. May God keep them in palm of his hand!
These are *very* special people. Japan is cursed with this accident. Japan is blessed by the presence of these people!
There a was a news item a day or so ago that said the Plant was due to closed in February (It is 40 years old) but Government decided to keep it going .How long for?
Reply | Report Abuse | Link to thisDon't know, but it won't be repaired as more modern designs will be more efficient and this old one is rather battle damaged.
What we are looking at happening is this:
Reply | Report Abuse | Link to thisThe Fukushima facility consists of six boiling water reactors. One reactor's core has most likely breached it's containment vessel, and it is in uncontrolled meltdown state and is emitting large amounts of radiation. The Japanese have abandoned the entire facility. THE OTHER FIVE REACTORS HAVE BEEN ABANDONED Also. Now are looking at six reactors in the next few days melting down. We will unfortunately witness the worst nuclear disaster in human history. God be with the Japanese....Dr Craven
What is your optimism based on. Are you a nuclear engineer like myself? Please write me why you feel secure about this situation. Thanks...Dr Drle Craven IV
Reply | Report Abuse | Link to thisIf you cool a reactor with seawater, the salinity of the water in the reactor increases as it emits steam. The boiling point also rises one half degree Celsius for every 58 additional grams of salt dissolved each kilogram of water. To your question...the corrosive effects on the core itself will be negligible. Some reactors are actually cooled with liquid salt. As far as the reaction is concerned the salt is really moot. The volume of the saltwater that is delivered to the reactor cores is the most important aspect in this current dilemma...Drle
Reply | Report Abuse | Link to thisCarlyle,
Reply | Report Abuse | Link to thisWhat you said is..."Once again we have a scare campaign based on ignorance."
Please pardon my ignorance...Dr D
I did not claim that I liked the situation. I claim it must be kept in perspective. By the way, the Japanese have not abandoned it. The temperature is declining but steam is carrying enough radiation out for it to have reached a level where it is becoming dangerous for those right at the site. They expect the temperatures to continue declining so why put the men at further risk. You say you are a nuclear scientist. That covers everything from nuclear medicine to physicist. I could claim something like that. I mined & processed uranium back in the early '60s. & have maintained an interest ever since. Just which branch do you claim expertise in? I have a few questions in mind. Perhaps you could help me. I can not find your name listed as a nuclear physicist anywhere. Fourth generation no less?
Reply | Report Abuse | Link to thisI beg your pardon. Nuclear engineer. The English call a mechanic an engineer.
Reply | Report Abuse | Link to thisSlowhand. Your post No. 18. The Japanese have abandoned the entire facility. THE OTHER FIVE REACTORS HAVE BEEN ABANDONED Also. Now are looking at six reactors in the next few days melting down. We will unfortunately witness the worst nuclear disaster in human history. God be with the Japanese....Dr Craven
Reply | Report Abuse | Link to thisMore allarmism. They were only breifly withdrawn according to the BBC.
I rest my case. If you are a nuclear engineer, you should be ashamed of yourself.
I agree with your assessment. My optimism is based on the following: first, it appears that most of the fuel in the affected cores is still in the rods and at least half is underwater; second, none of the containment vessels seem to have been seriously compromised; and third, the radiation released in steam seems to be well within safe limits everywhere except possibly on the actual plant premises.
Reply | Report Abuse | Link to thisReports say the containment vessel at reactor 2 (and now 3) may be damaged, but this does not appear to be serious damage of the sort that would cause radioactive material to escape even in the case of extensive fuel melting. Moreover, the building at reactor 2 seems to be relatively intact. JAIF says "slightly damaged."
So it sounds to me like there is no significant danger to any human or to the environment. I will refrain from speculating on the causes of the sensational and hysterical tone that characterizes much of the mainstream reporting on this incident.
Thank you pollack.
Reply | Report Abuse | Link to thisEverything I've been able to find on this situation completely contradicts the mindless panic you seem to be suffering. None of the reactors have been abandoned. They have all been shut down for safety reasons but crew are still there working on insuring no catastrophic melt down occurs. Non-essential personnel have left to minimize danger to them.
Reply | Report Abuse | Link to thisDoes anyone know a good URL for updates on the plants? My morning paper sys they pulled all workers out because of radiation levels. You guys are saying that's not so. Google news says nothing. What is a good source of info?
Reply | Report Abuse | Link to thisAlso, if the worst were to happen and there was a spent fuel fire or reactor containment failure, would the resulting radiation levels preclude any further human intervention in the remaining 5 reactors? In other words, if one goes do they all eventually go?
If that were the case, just as a thought experiment, would it be desirable to wait until the winds are definitely and persistently blowing out to see and blow the whole mess to hell with a large thermonuclear device , tossing most of the stuff downwind into the sea rather than have 6 plants spewing things out for a protracted time with heavy fallout into Tokyo when the winds blow that way? I know it's crazy but just as a mental exercise.
Who really knows what is going on over there? The company or government haven't been honest or forthcoming during this who incident.
Reply | Report Abuse | Link to thisThe reporting has been disgusting in the MSM. For example it is still being widely reported that the workers have been withdrawn from the nuclear plants when in fact they were only withdrawn for an hour. I have not found a genuinely reliable single source. They all seem to favour sensationalising the situation over factual reporting.
Reply | Report Abuse | Link to thisThis site gives some factual information. You can also find tome facts on the BBC if you are patient & weed out the BS.
The evacuation order was issued at 10.40am local time and about an hour later workers were allowed to return when the radiation spike subsided, Kyodo News reported.
The nuclear safety agency said the level around the plant peaked at 6.4 millisieverts at 10.45am local time. Within 10 minutes it had fallen to 2.9 millisieverts, AFP reported.
(A chest ex-ray exposes you to 20 millisieverts & early ex ray machines exposed people to as much as fifteen hundred times as much as modern machines)
The reporting has been disgusting in the MSM. For example it is still being widely reported that the workers have been withdrawn from the nuclear plants when in fact they were only withdrawn for an hour. I have not found a genuinely reliable single source. They all seem to favour sensationalising the situation over factual reporting.
This site gives some factual information. You can also find tome facts on the BBC if you are patient & weed out the BS.
The evacuation order was issued at 10.40am local time and about an hour later workers were allowed to return when the radiation spike subsided, Kyodo News reported.
The nuclear safety agency said the level around the plant peaked at 6.4 millisieverts at 10.45am local time. Within 10 minutes it had fallen to 2.9 millisieverts, AFP reported.
(A chest ex-ray exposes you to 20 millisieverts & early ex ray machines exposed people to as much as fifteen hundred times as much as modern machines)
http://www.heraldsun.com.au/news/special-reports/japan-earthquake-new-explosion-rocks-fukushima-nuclear-power-plant/story-fn7zkbgs-1226021415043
China, for their ultra-high altitude train to Lhasa, had to keep the permafrost beneath the tracks frozen and for this purpose they used large, hollow metal cylinders (about 10" in diameter) with radiative fins at the top. Inside they put liquid ammonia which evaporates as it collects heat from the earth and is then cooled in the freezing air above with the help of the radiators and recondenses to start the cooling cycle all over again. It's a totally passive system. Would such heat siphons be of use in Japan's situation?
Reply | Report Abuse | Link to thisIn the event of a total melt down isn't likely that if it makes it out of all containment's. It will melt until it hits the water table or they put it out.................................................... Then its a hole different story,that I have yet to hear.
Reply | Report Abuse | Link to thishttp://www.iaea.org/newscenter/news/tsunamiupdate01.html
Reply | Report Abuse | Link to thishttp://www.world-nuclear.org/
Both of these links contain current, technically reliable information. The IAEA posts regular updates throughout the day and as new developments occur. Everything that I have seen in the mainstream media, the one possible exception being Reuters, has been comprised primarily of the author's misunderstanding or personal interpretation of the events as they are reported by the IAEA and the WNA.
The IAEA provides detailed information with regard to actual radiometric measurements, injuries, fatalities in addition to technical updates on the reactors themselves... Hope this helps. I will also note that although these are both industry sites, they have been forthcoming about the risks involved. For example, when the damage below the reactor core was first "discovered" at the Diachi(sp) 2 the headline at WNN read something along the lines of "Dramatic Escalation...." The WNA/WNN reports include more context and interpretation than the IAEA reports which are more of a fact reporting service.