The role of the moderator in a nuclear reactor is a little subtle. When fission occurs both the neutrons and the fission products are ejected at a very high rate of speed. Since the neutrons aren't charged they're apt to exit the reactor post haste without interacting with much of anything, which isn't good if you want to sustain a chain reaction.
What the moderator does is slow neutrons down so that they spend more time in the reactor and have more of an opportunity to meet a uranium nucleus before packing their bags and catching a Greyhound for the big wide world. This slowing down also makes the reaction more controllable, so you can keep it just north of criticality without it getting exponentially more powerful as time passes.
What the TEPCO rep was saying a week or so ago when he said "re-criticality is possible" is that, in certain configurations you can get the conditions for positive fission even without the moderator, and it couldn't be ruled out that the core could reach such configurations. (In practice, it seems that this is happening; just an hour ago I read of blue glows being observed which might be the cerenkov radiation from such events.) What generally happens in such an uncontrolled fission event is that thermal expansion stops the reaction. This is one of the key design problems of making an atomic bomb; if you don't assemble it very quickly it blows itself apart before the reaction can complete. So what happens in the melted reactor core is you get just the right arrangement and a bit of the core starts to fission, but it reaches a very high temperature which pushes the atoms far enough away from one another that they're no longer critical in a few moments. Generally this means the fissioning mass is melted or even vaporized, and it's released a lot of energy, so the mass is free to gurgle on and possibly do it again. And again. And again...
These fission events are very bad for a lot of reasons. They not only keep the mass hot directly, but they produce new fission products that aren't old and decaying into senility. They also produce flashes of extremely strong radiation which are unpredictable and can kill workers in a few minutes if they're too close. They also pretty much prove that a good bit of core is in the form of a puddle. We'd like to hope it's at the bottom of the stainless steel reactor vessel, but it seems a lot more likely now that it's melted through that (or just dripped through bad pipes) and is sitting on concrete in the drywell.
... They also produce flashes of extremely strong radiation which are unpredictable and can kill workers... They also pretty much prove that a good bit of core is in the form of a puddle.....
local,
thanks for taking the time to explain this localized re-critical condition. Your explanation helped me understand something I had heard earlier about short-lived high intensity radiation "beams" being detected near at least one of the reactors.
LocalRoger.
Thank you for the indepth information. It's amazing how much I thought I knew, and just much depth and understanding I didn't have. Again, ^5's for the theory and information; well said and well placed.
My question is: The moving "China Syndrome" is when the core and rods were going to melt through the gound, substained fissuon, and continue melting throgh rock and whatever until it hit a large enuff water supply to cause it to vaporize in a huge steam explosion, or go through the mantle until it hit the hot outter core and then exploded (for some reason I can't seem to recall). For this instance in Japan, is this something that could happen?
KaosKidd, the core is unlikely to get much further than the ground water if it melts through the reactor floor. At that point it will be an extremely hot mass of liquid, and I think the consensus is that the steam explosion would violently disrupt the blob. The steam would probably carry much of the core into the atmosphere. I would expect an ongoing series of releases as the blob steams out until it crusts over, builds up heat until the crust is disrupted, steams out until more crust forms, lather rinse repeat. The key difference between groundwater and the cooling water in the containment is that the amount of water in the containment is finite and it can completely boil off. I'd expect that that would have to happen before the core could heat the floor up enough to melt it. In the ground, no matter how much water boils off there's still more to come. But nobody's ever tried this before so it's hard to be sure.
TEPCO have backed off on some of the measurements that seemed to prove re-criticality was happening. This could mean either (a) they screwed up several unrelated measurements in just the right way or (b) they've decided to lie about it. Either seems plausible given their performance so far.
Even though the core was damped when cooling was lost it is still generating enough energy to melt itself and rock, and will for at least several more months even if it doesn't achieve re-criticality. It's also unknown just how much of the damping rod material is in the blob, how well distributed it is, or how much effect the boron in the water is having.
If anyone is interested in detailed reporting on the Fukushima Daiichi disaster, bravenewclimate.com/ has gobs. As a bonus bravenewclimate.com/ lacks FUD, has actual background information, source links, context, with informed (if occasionally wrong) opinion, and well moderated and interesting comments. Btw, this web-site was linked to me by a friend of mine living in japan ~30km north of the Fukushima Daiichi disaster. Finding credible information on the disaster was, understandably, a priority for him.
Among many other problems, it looks like having tons of water sitting in the reactor vessel makes the reactor containment vessel very vulnerable to aftershocks. But because having tons of water inside the reactor vessel is the only way the core can be cooled right now (since the inner cooling tubes have been destroyed) there's no other choice. I wonder how long can this go on?
KaosKidd, the core is unlikely to get much further than the ground water if it melts through the reactor floor. At that point it will be an extremely hot mass of liquid, and I think the consensus is that the steam explosion would violently disrupt the blob. The steam would probably carry much of the core into the atmosphere. I would expect an ongoing series of releases as the blob steams out until it crusts over, builds up heat until the crust is disrupted, steams out until more crust forms, lather rinse repeat. The key difference between groundwater and the cooling water in the containment is that the amount of water in the containment is finite and it can completely boil off. I'd expect that that would have to happen before the core could heat the floor up enough to melt it. In the ground, no matter how much water boils off there's still more to come. But nobody's ever tried this before so it's hard to be sure.
TEPCO have backed off on some of the measurements that seemed to prove re-criticality was happening. This could mean either (a) they screwed up several unrelated measurements in just the right way or (b) they've decided to lie about it. Either seems plausible given their performance so far.
Even though the core was damped when cooling was lost it is still generating enough energy to melt itself and rock, and will for at least several more months even if it doesn't achieve re-criticality. It's also unknown just how much of the damping rod material is in the blob, how well distributed it is, or how much effect the boron in the water is having.
WOW.
Thanks...
I think I can make sence of what's being reported. Again, thanks!
1) Do you think nuclear power makes sense?
2) How would you change reactor design?
1) This question cannot be answered without a whole bunch of qualifiers:
a. On what scale
b. Run by whom
c. Funded how
a: On a small scale (much smaller than we are doing it now) I'd say nuclear energy is an absolute necessity because there are certain things that can't be done without it, including radiomedicine, some space applications, and things like RTG's. Submarines might also qualify, but if you didn't have the nuclear weapons part of the infrastructure you probably wouldn't care about that kind of submarine.
On a much larger scale than we are doing it now, I'd say it doesn't make sense for reason c below.
b: Something as intrinsically dangerous as a nuclear reactor should not be trusted to a private entity which is motivated solely by profit. The temptation to cut corners is too great.
c. Nuclear power should, like any other technology, be able to pay for all of the costs of its deployment, and these should include (but don't, because the government covers them) insurance, disaster cleanups, and the completely kicked to the next generation problem of waste disposal. (I'll note that we are letting other energy technologies slide in a lot of ways, and we shouldn't; we should pay an honest price of our energy usage and perhaps change our lifestyle if we realize the true cost is too high.)
2) Another poster on one of the other boards I frequent made the very astute observation that the worst enemies of today's nuclear industry are the idiots who built half-thought-through rigs like the Fukushima BWR's throughout the 60's and 70's, when we knew better. Assuming we have solved the waste problem (which we haven't) and made the industry pay for its mess (which we don't), THEN what we need to do is tear down all these reactors which require active safety systems to keep them safe and build passively safe designs like pebble bed reactors in their place.
I grew up in a nuclear physics lab and used radiation sources in the science project that took me to the 1981 ISEF. I am not afraid of radiation. I am, however, completely cynical about my fellow human beings and their willingness to treat dangerous things with the respect they deserve when it would be cheaper to cross their fingers and hope for good weather.
I agree with LocalRoger's item b, but have a corollary to it:
Something as intrinsically dangerous as a nuclear reactor should not be trusted to a government entity which tends to be motivated by political expediency. The temptation to bury problems under the guise of national security and provide patronage jobs to the unqualified is just too great.
Martin, you definitely have a point; after all, Chernobyl wasn't being run by a private for-profit energy company. They were doing a corner-cutting experiment of trying to mooch more energy out of a reactor that was in the process of being shut down when they blew the joint. All I'd add is that while a government might or might not be trustworthy in such things (and you could sensibly argue that not trustworthy is much more likely), a private for-profit enterprise can't be trustworthy. The cost-benefit analysis between doing things right and just faking the data and hoping the regulators don't slap you too hard is undeniable and unacceptable.
I agree with LocalRoger's item b, but have a corollary to it:
Something as intrinsically dangerous as a nuclear reactor should not be trusted to a government entity which tends to be motivated by political expediency. The temptation to bury problems under the guise of national security and provide patronage jobs to the unqualified is just too great.
'
Who would you push this responsibility onto?
'
I would not recommend B.P. or Exxon for this task.
'
In fact, I couldn't think of any Private company I would trust.
'
I'll stick with the NRC.
I hear Transocean Ltd., the owners of the oil rig that BP helped blow out in the Gulf last year, has an excellent safety record. In fact, their record is so stellar, they recently handed their executives some big bonuses for safety. http://www.cnn.com/2011/US/04/04/gulf.spill.bonuses/index.html?iref=allsearch
Either that, or we can make each and every reactor around the world a pay-to-play on-line video game where contestants vie to push rods in and out of the core. That might help raise funds for research into alternative energy sources. Talk about a killer app.
As pro-private enterprise as I am, it is very difficult to disagree with the views expressed regarding reactor management by the private sector presented here, however, with PROPER government oversight, I think it is entirely feasible. (It sure couldn't be a regulatory agency like the former MMS, though.) Martin mentioned something I have been thinking about for some time . . .
Reactors with active safety systems are just too dangerous for fallible human institutions.
I wonder if it is possible to construct a reactor that has no active safety mechanisms? Such a reactor would not be capable of being refueled. It would be permanently sealed. The primary loop would be lead. The secondary (or even tertiary?) loop would be a traditional water/steam mechanism.
The advantages to such a system--as I think Martin implied--would be that the reactor has no moving parts. Since it would never be refueled, construction could be very heavy duty. When the nuclear fuel "expired", the entire reactor could be simply stacked somewhere, perhaps onsite.
I don't know enough to understand if such a thing is feasible.
Bill, pebble bed reactors are close to that, but they can be opened and re-fueled. They use Helium gas as the coolant and are considered passively stable.
The United States Nuclear Regulatory Commission said Wednesday that some of the core of a stricken Japanese reactor had probably leaked from its steel pressure vessel into the bottom of the containment structure...
That question is important not only for this catastrophe, but in general. It is my observation that true investigative reporting no longer exists due to budget cuts at "news organizations." Such organizations no longer employ people with the education necessary to perform research. This is not a great problem for a society that finds its intellectual stimulation in reality TV and "idol" shows and the like.
Neither television nor radio is in my list of news sources. In fact, I completely avoid both except for the occasional old movie.
Online news organizations suffer from the same problem as "traditional" news entities. If news is not presented to them by a government agency or a company statement or politician's viewpoint or some such, then they are incapable of extended fact gathering and research.
But, if the goal is to read about the latest change in any given celebrity's lifestyle or what happened or may happen in a reality show, then online news sources are certainly the place to look . . . unless it happens to be late at night, on a week end or a holiday, in which case news does not occur.
I didn't mention news neutrality. That hasn't existed in decades.
At least multiple online sources can be quickly accessed and assimilated and the lies, inaccuracies, prejudices, and ignorance cross-checked, thereby allowing one to derive a kernel of fact here and there. (I read a piece about NASA's "Curiosity" rover with eagerness. The entire article described the reporter's novel experience of having to enter a clean room. The only description of Curiosity was a photograph which cut off its wheels.)
First hand accounts, such as we have read concerning disasters right on this forum, are reliable news sources. Of course, they are naturally limited in scope.
I beg forgiveness for "going off" on this subject. As a news junkie, it is something that tortures me.
Online news organizations suffer from the same problem as "traditional" news entities. If news is not presented to them by a government agency or a company statement or politician's viewpoint or some such, then they are incapable of extended fact gathering and research. ...
This seems especially true when it comes to anything technical or scientific. I've been impressed by the false impression many news channels have given about the condition of the Fukushima reactors. News outlets have been reporting that TEPCO has managed to get cooling systems back up and running to the reactor cores, but in reality two of the reactors are being cooled only by water being sprayed onto them by fire sprinkler systems, which is not exactly the same thing as a reactor cooling system. And yet to get the detailed information, you really don't have to be a super sleuth news reporter: all you have to do is read what's on the IAEA's website, which is updated at least once a day:
In Unit 1 fresh water is being continuously injected into the reactor pressure vessel through the feed-water line at an indicated flow rate of 6 m3/h using a temporary electric pump with off-site power. Fresh water is being injected continuously into the RPVs through the fire extinguisher lines in Units 2 and 3 at indicated rates of 8 m3/h and 7 m3/h respectively using a temporary electric pump with off-site power.
The SUN fuses two hydrogen atoms into one Helium atom.
'
Our outer--space is full of helium
That generates a couple of severely off-topic questions; How much gamma radiation are astronauts exposed to? How are engines cooled? (I know these should be in a new thread.)
Comments
http://www.iaea.org/newscenter/news/tsunamiupdate01.html
http://www.slideshare.net/iaea/summary-of-reactor-unit-status-3-april-2011-0600-utc
I only wish somebody were creating a real-time map of radioactivity in that area.
What the moderator does is slow neutrons down so that they spend more time in the reactor and have more of an opportunity to meet a uranium nucleus before packing their bags and catching a Greyhound for the big wide world. This slowing down also makes the reaction more controllable, so you can keep it just north of criticality without it getting exponentially more powerful as time passes.
What the TEPCO rep was saying a week or so ago when he said "re-criticality is possible" is that, in certain configurations you can get the conditions for positive fission even without the moderator, and it couldn't be ruled out that the core could reach such configurations. (In practice, it seems that this is happening; just an hour ago I read of blue glows being observed which might be the cerenkov radiation from such events.) What generally happens in such an uncontrolled fission event is that thermal expansion stops the reaction. This is one of the key design problems of making an atomic bomb; if you don't assemble it very quickly it blows itself apart before the reaction can complete. So what happens in the melted reactor core is you get just the right arrangement and a bit of the core starts to fission, but it reaches a very high temperature which pushes the atoms far enough away from one another that they're no longer critical in a few moments. Generally this means the fissioning mass is melted or even vaporized, and it's released a lot of energy, so the mass is free to gurgle on and possibly do it again. And again. And again...
These fission events are very bad for a lot of reasons. They not only keep the mass hot directly, but they produce new fission products that aren't old and decaying into senility. They also produce flashes of extremely strong radiation which are unpredictable and can kill workers in a few minutes if they're too close. They also pretty much prove that a good bit of core is in the form of a puddle. We'd like to hope it's at the bottom of the stainless steel reactor vessel, but it seems a lot more likely now that it's melted through that (or just dripped through bad pipes) and is sitting on concrete in the drywell.
local,
thanks for taking the time to explain this localized re-critical condition. Your explanation helped me understand something I had heard earlier about short-lived high intensity radiation "beams" being detected near at least one of the reactors.
Thank you for the great explanations. I seem to remember a tiny bit about the role of the moderator, which is counter-intuitive.
--Bill
Thank you for the indepth information. It's amazing how much I thought I knew, and just much depth and understanding I didn't have. Again, ^5's for the theory and information; well said and well placed.
My question is: The moving "China Syndrome" is when the core and rods were going to melt through the gound, substained fissuon, and continue melting throgh rock and whatever until it hit a large enuff water supply to cause it to vaporize in a huge steam explosion, or go through the mantle until it hit the hot outter core and then exploded (for some reason I can't seem to recall). For this instance in Japan, is this something that could happen?
KK
TEPCO have backed off on some of the measurements that seemed to prove re-criticality was happening. This could mean either (a) they screwed up several unrelated measurements in just the right way or (b) they've decided to lie about it. Either seems plausible given their performance so far.
Even though the core was damped when cooling was lost it is still generating enough energy to melt itself and rock, and will for at least several more months even if it doesn't achieve re-criticality. It's also unknown just how much of the damping rod material is in the blob, how well distributed it is, or how much effect the boron in the water is having.
Lawson
P.S. At least read the latest article on brave new climate Measuring our Monsters in the Midday Sun.
http://www.nytimes.com/2011/04/06/world/asia/06nuclear.html?pagewanted=1&hp
1) Do you think nuclear power makes sense?
2) How would you change reactor design?
WOW.
Thanks...
I think I can make sence of what's being reported. Again, thanks!
KK
1) This question cannot be answered without a whole bunch of qualifiers:
a. On what scale
b. Run by whom
c. Funded how
a: On a small scale (much smaller than we are doing it now) I'd say nuclear energy is an absolute necessity because there are certain things that can't be done without it, including radiomedicine, some space applications, and things like RTG's. Submarines might also qualify, but if you didn't have the nuclear weapons part of the infrastructure you probably wouldn't care about that kind of submarine.
On a much larger scale than we are doing it now, I'd say it doesn't make sense for reason c below.
b: Something as intrinsically dangerous as a nuclear reactor should not be trusted to a private entity which is motivated solely by profit. The temptation to cut corners is too great.
c. Nuclear power should, like any other technology, be able to pay for all of the costs of its deployment, and these should include (but don't, because the government covers them) insurance, disaster cleanups, and the completely kicked to the next generation problem of waste disposal. (I'll note that we are letting other energy technologies slide in a lot of ways, and we shouldn't; we should pay an honest price of our energy usage and perhaps change our lifestyle if we realize the true cost is too high.)
2) Another poster on one of the other boards I frequent made the very astute observation that the worst enemies of today's nuclear industry are the idiots who built half-thought-through rigs like the Fukushima BWR's throughout the 60's and 70's, when we knew better. Assuming we have solved the waste problem (which we haven't) and made the industry pay for its mess (which we don't), THEN what we need to do is tear down all these reactors which require active safety systems to keep them safe and build passively safe designs like pebble bed reactors in their place.
I grew up in a nuclear physics lab and used radiation sources in the science project that took me to the 1981 ISEF. I am not afraid of radiation. I am, however, completely cynical about my fellow human beings and their willingness to treat dangerous things with the respect they deserve when it would be cheaper to cross their fingers and hope for good weather.
Yes, and the very fact insurance companies will not cover such accidents should give us all a hint.
hint... hint....
Something as intrinsically dangerous as a nuclear reactor should not be trusted to a government entity which tends to be motivated by political expediency. The temptation to bury problems under the guise of national security and provide patronage jobs to the unqualified is just too great.
Homer Simpson might take exception to this...
Who would you push this responsibility onto?
'
I would not recommend B.P. or Exxon for this task.
'
In fact, I couldn't think of any Private company I would trust.
'
I'll stick with the NRC.
I hear Transocean Ltd., the owners of the oil rig that BP helped blow out in the Gulf last year, has an excellent safety record. In fact, their record is so stellar, they recently handed their executives some big bonuses for safety. http://www.cnn.com/2011/US/04/04/gulf.spill.bonuses/index.html?iref=allsearch
Either that, or we can make each and every reactor around the world a pay-to-play on-line video game where contestants vie to push rods in and out of the core. That might help raise funds for research into alternative energy sources. Talk about a killer app.
As pro-private enterprise as I am, it is very difficult to disagree with the views expressed regarding reactor management by the private sector presented here, however, with PROPER government oversight, I think it is entirely feasible. (It sure couldn't be a regulatory agency like the former MMS, though.) Martin mentioned something I have been thinking about for some time . . .
I wonder if it is possible to construct a reactor that has no active safety mechanisms? Such a reactor would not be capable of being refueled. It would be permanently sealed. The primary loop would be lead. The secondary (or even tertiary?) loop would be a traditional water/steam mechanism.
The advantages to such a system--as I think Martin implied--would be that the reactor has no moving parts. Since it would never be refueled, construction could be very heavy duty. When the nuclear fuel "expired", the entire reactor could be simply stacked somewhere, perhaps onsite.
I don't know enough to understand if such a thing is feasible.
--Bill
That's a shame. There will be no helium left very soon. http://www.scientificamerican.com/blog/post.cfm?id=the-coming-shortage-of-helium-2010-06-30
from here:
http://www.nytimes.com/2011/04/07/world/asia/07japan.html?_r=1
Some informative graphics here:
http://www.nytimes.com/interactive/2011/04/05/world/asia/20110405-japan-leak.html?ref=asia
Neither television nor radio is in my list of news sources. In fact, I completely avoid both except for the occasional old movie.
Online news organizations suffer from the same problem as "traditional" news entities. If news is not presented to them by a government agency or a company statement or politician's viewpoint or some such, then they are incapable of extended fact gathering and research.
But, if the goal is to read about the latest change in any given celebrity's lifestyle or what happened or may happen in a reality show, then online news sources are certainly the place to look . . . unless it happens to be late at night, on a week end or a holiday, in which case news does not occur.
I didn't mention news neutrality. That hasn't existed in decades.
At least multiple online sources can be quickly accessed and assimilated and the lies, inaccuracies, prejudices, and ignorance cross-checked, thereby allowing one to derive a kernel of fact here and there. (I read a piece about NASA's "Curiosity" rover with eagerness. The entire article described the reporter's novel experience of having to enter a clean room. The only description of Curiosity was a photograph which cut off its wheels.)
First hand accounts, such as we have read concerning disasters right on this forum, are reliable news sources. Of course, they are naturally limited in scope.
I beg forgiveness for "going off" on this subject. As a news junkie, it is something that tortures me.
--Bill
This seems especially true when it comes to anything technical or scientific. I've been impressed by the false impression many news channels have given about the condition of the Fukushima reactors. News outlets have been reporting that TEPCO has managed to get cooling systems back up and running to the reactor cores, but in reality two of the reactors are being cooled only by water being sprayed onto them by fire sprinkler systems, which is not exactly the same thing as a reactor cooling system. And yet to get the detailed information, you really don't have to be a super sleuth news reporter: all you have to do is read what's on the IAEA's website, which is updated at least once a day:
http://www.iaea.org/newscenter/news/tsunamiupdate01.html
'
Self canceling reactors.
'
'
The SUN fuses two hydrogen atoms into one Helium atom.
'
Our outer--space is full of helium
That generates a couple of severely off-topic questions; How much gamma radiation are astronauts exposed to? How are engines cooled? (I know these should be in a new thread.)
I think the Sun and most moderately-sized stars primarily fuse 4 hydrogens into a single helium. Look up "astrophysical reaction chains. " http://en.wikipedia.org/wiki/Nuclear_fusion#Hot_fusion
You're probably thinking about fusion reactions done on earth, which try to fuse isotopes of hydrogen like deuterium and tritium.
http://nycaviation.com/2011/04/video-antonov-an-124-loads-up-giant-concrete-pump-in-atlanta-for-delivery-to-crippled-japan-nuke-plant/
Will these become too "Hot" to return?
I stand corrected.
'
Thanks