Nuclear Energy - I need to vent/rant

[Schneibster]

A third is that for nations who do not already have an established nuclear industry, creating one from scratch requires an enormous investment. For nations with an established nuclear industry it's "just" those two.

If we build nuclear power plants, we will learn to build better and cheaper (and safer) nuclear power plants. Nations that do not already have the industry will be able to purchase them. Nations that make them will make that easier if they can to help with the global warming problem; it's in their own interest to do so.

And another point: China is not going to stop building coal-fired power plants any time soon unless presented with an alternative, and there are well over a billion Chinese.

Finally, if it's "just" those two, you've had answers to them both. And you have not responded to those answers.

Possibly that is because it's too silly a claim to address seriously, no matter how often it is repeated?

And possibly because it's not refutable and you can't think of anything to do but try to ridicule it and see if anyone believes you. Which is basically what you've done. If it's so silly, surely you can easily refute it. That you haven't speaks volumes.

We could feed them Chicken Little. I think that would solve two problems at once.

Again, ridicule- but no response. See, it's easy to tell when someone's arguments are weak- they pull rhetorical crap like this. I say again, if it's so easily refutable, why don't you do so?

You are welcome to your own opinion. I see no basis in reality for it, but I can't stop you holding it.

And a third time.

You have no evidence to present, so you try rhetoric to draw attention away from that obvious fact. If you have it, bring it. Otherwise, my conclusion is, you don't have a clue what you're talking about.

I guess I'm not clear on who said that it was. Perchance it is a straw man?

And perhaps it's not. It appears that it was you who said:

I've given you the physics already - nuclear fission waste contains radioactive contents with half-lives in the hundreds of years and hundreds of thousands of years. If it's not safe now it's not going to be safe in a thousand years.

So now it becomes eminently clear; you've denied your own words, and called someone's response to them a "straw man." Which means all you have is rhetoric.

Schneibster, exactly what do you think is dangerous about a stockpile of used fuel rods containing U-234?

They're radioactive. Did you have some point or other?

I'm getting the impression that you and Lonewulf don't actually have any clear idea about the matter beyond the vague and cosy impression that the danger is mostly imaginary.

I'm getting the impression that you easily acquire impressions, and that they have little to do with reality.

What we have here is rhetoric, zero information content. You don't appear to know any economics, you don't appear to know any physics, and you don't appear to have a clue how much energy a solar panel or a windmill can make, and how much they cost to build, much less a nuclear power plant. You have an opinion based on hysterical fear of that which you do not understand, and cannot be bothered to actually find out the facts. When presented with them, you engage in an emotional attack based on rhetoric and misunderstanding. As a result of your unreasoning fear, you advocate a course of action that is not only unconscionable, but far more dangerous, both to people and to the environment, than building nuclear power plants.

Now, if you'd like to actually have a rational discussion, then my suggestion is that you begin by reading the points that have been made in this thread and responding to them, rather than engaging in emotional rhetoric. Try doing a little research on half-life, specific activity and its relation to half-life, and then find out where the rice grown in California goes, and where the water that grows that rice comes from and how it gets to where the rice grows. Then have a look at wheat. Then take a look at projected population increases. Then consider the likely climatic effects of global warming on various agricultural regions. You might want to consider geology as well. It would probably also be wisest to find out where fertilizer comes from and how it's made. That would be a good start.

 

[Schneibster]

Okay, I think it's pretty clear where you're coming from. I doubt Schneibster will do any better so I'll stop that line of questioning and just explain what's going on.

Well, before you begin, perhaps we should explore precisely what we're talking about when we talk about nuclear fuel, when it's put in the reactor, when the reactor is running, and when it's taken out.

Nuclear fuel rods for a civilian reactor are enriched to about 5% U-235; the remainder of 95% is U-238, not U-234 (except in vanishingly small amounts). Natural uranium is about 99.3% U-238, 0.7% U-235, and an even smaller amount of U-234 (the enrichment processes, both gaseous diffusion and centrifugal enrichment, favor light isotopes, so both U-234 and U-235 are enriched).

The U-235 (5%) does all the fissioning (except for the very occasional U-234 or U-238 fission; this has a very much longer half-life than radioactive decay of either, however, so only a truly astronomically small fraction of the U-234 and U-238 undergoes fission). As the fuel is used, the U-235 undergoes fission; this results in likely radioactive fission products. The U-238 can absorb neutrons and the newly-formed U-239 will quickly decay to neptunium-239, and then to plutonium-239.

Since plutonium-239 is fissile, like U-235, some of it will undergo fission in the storm of neutrons inside the reactor, forming more fission products; however, this is not a majority process. The majority process is the fission of U-235. That's because the neutrons that are best at being absorbed by U-238 to form Pu-239 aren't the same energy as the neutrons that are best at being absorbed by U-235 and causing it to fission. The structure of the reactor is designed to maximize power, and therefore to moderate the neutrons so that they are the kind that cause the most U-235 fissioning, not the kind that convert U-238 to Pu-239; it's possible to make a reactor that makes neutrons that are the right energy to make Pu-239, of course, and this is called a "breeder" because it makes Pu-239. And, of course, the reason you do is because those neutrons are also not the right energy to cause Pu-239 fission; sure, it happens, but again, that's not the majority process, and that's controlled by the moderation.

So what we've got when we start is mostly U-238, with one atom in 20 being U-235, and one in a few thousand being U-234. As it's bombarded with neutrons, the U-235 fissions a lot, the U-238 is occasionally converted to Pu-239, the Pu-239 fissions sometimes, the U-234 is very occasionally converted to U-235, and occasionally a fission product (which was likely already radioactive) is converted to another isotope by the neutrons. As time goes on, the amount of U-235 decreases. As it does so, the amount of fission going on drops; this means that the moderation must be adjusted to keep the reaction going on. There's another problem, too; these products are also very often neutron absorbers and as their concentration increases, they damp the reaction by eating neutrons. Eventually, no amount of moderation decrease can keep the reaction going; there are too few neutrons being made, and too many things absorbing them. By this time, the amount of U-235 has decreased to perhaps 1-2% (Hindmost could probably come up with that figure; I'm too lazy to hunt further for a reference, but it's not all that important). A few percent of the U-238 has converted to Pu-239, and some of the Pu-239 has fissioned. This has taken a decade or more.

What you've got is still well over 90% U-238. Of the remainder, a couple percent is U-235, and a couple percent is Pu-239. The remaining six or eight percent is fission products, of varying levels of activity; a couple percent of this is high activity nuclear waste. But the higher level activity it has, the faster it decays. The bulk of the material is U-238, which has a half-life of 4.5 billion years; in terms of radiation hazard, not much because it has such a long half-life. There are high-activity isotopes in it, but storage for a few decades reduces this to the point where it is very little more active than the U-238 already is anyway. That's because the concentration of highly active isotopes was already in the single digits, and they decay very quickly; that's what "high activity" means.

Now, I won't misrepresent this; it's still radioactive after it's done its decades cooling off. But handling it for a limited period would be unlikely to harm the average person. You wouldn't want to handle it with your bare hands for a period of weeks on end, but more because you'd be likely to ingest some than because of anything it might do to your hands. You wouldn't want to eat it much of it; uranium is a low energy alpha emitter, so your skin (as has already been pointed out) is sufficient to stop the radiation, but if you were to ingest it, it could be a problem later on; however, it's important to point out here that the hazard would be far more a chemical poisoning risk than anything to do with radiation. In amounts small enough not to cause heavy metal poisoning and kidney failure, it would not pose much of a radiation hazard. No increase in human cancer has ever been reported in the scientific and medical literature as a result of exposure to natural uranium, and after the high-level isotopes have decayed, the spent fuel rods are not a great deal different.

Now, those are the facts. Let's see how many of them you already knew.

First things first. Atom for atom long-lived isotopes like U-234 are not as dangerous as short-lived ones in and of themselves, you and Schneibster were right that far.

Well, gee, I'm sure glad you validated that from your infinite wisdom. I have a question: where is all this U-234 coming from? It's only five thousandths of a percent in the first place, even after enrichment, and U-234 doesn't get made by likely interactions inside a reactor.

The fault in your analyis is that you conclude from this that since atom-for-atom U-234 is less dangerous than something else, then it follows that the amount of U-234 in spent fuel rods is safe. You are missing the other side of the coin, which is that if you have enough U-234 atoms in one place to be a danger it will continue to be a danger for an extraordinarily long time.

So? It still only decays at a rate so low that only one half of it has done so in the lifetime of the Earth. And I repeat, why do you keep talking about U-234?

The other assumption you seem to be making is that once an atom of U-234 or another dangerous isotope decays, poof, it's gone. That is not how it works. If the result of radioactive decay is another radioactive element then you can get a second decay event, and a third, and so on down the line until the decay chain reaches a stable isotope. U-234 becomes thorium 230 becomes radium 226 becomes radon 222 by a chain of alpha emissions, each product having a shorter half-life than its parent. So it's not just a matter of keeping U-234 in a box until it goes poof, because its decay chain leads to radioactive radon gas. Plus the other unstable isotopes in the fuel rods are also doing their own individual slide down the periodic table.

But it STILL only happens to half the atoms in a sample in four and a half billion years. I repeat, so? The incidence of it is still low. Furthermore, you're still talking about U-234. Which you have not demonstrated is relevant to the conversation since it only represents five one-thousandths of a percent of enriched uranium, and five ten-thousandths of a percent of natural uranium. Where is all this U-234 coming from?

Lastly, you seem to be assuming that we can just stick something in a vault and it will be held harmless for hundreds of thousands of years without ever getting in the atmosphere or the water supply, and that strikes me as a failry crazy assumption.

Why does it have to be held in a vault for hundreds of thousands of years? In a hundred years, it's no more radioactive than natural uranium. And in thirty, it's only barely so.

We've never developed the technology to build something to last like that, because we have never needed it. Handwaving this issue away is in my mind an immoral way of foisting the bill for today's air conditioning onto our descendants in the form of radioactive waste.

And ignoring the facts is hysteria.

Let's talk about reality. The problem here isn't with civilian waste. The problem is with military waste. Military reactors use highly enriched uranium, and can "burn" a great deal more U-235 because they have a great deal more in the first place. This results in a very high concentration of fission products, and those products are very "hot;" furthermore, they remain so for a much longer time. Thousands or tens of thousands of years is not an unreasonable time to discuss spent military fuel remaining very dangerous in radiological terms. But if civilian use becomes widespread, the waste that it makes remains much lower level, and is therefore much less of a disposal problem than military waste. So basically, what opponents of civilian nuclear power are doing is equating military waste with civilian waste and claiming they're the same thing; add a dose of hysteria over teh invisuble nucular cancer rays, and there you have it.

And why do military reactors use HEU? That would be because most military reactors are on ships and submarines and they have to carry the fuel; that means that the more enriched the fuel, the less weight they have to carry per unit power, and the faster they can go. Simple, easy, obvious.

All of this is information you could easily have found out for yourself; much of it is already available in Wikipedia. Try "nuclear fuel cycle" and "uranium" for highly relevant articles.

Yes, yes, all caution regarding radiation is ill-informed gibbering. We get it.

Speaking of straw men...

So again, what we have here is hysteria driven emotional rhetoric. You don't know the difference between U-234, U-235, and U-238; you don't know that military and civilian fuel are different; you don't know the relation between specific activity and half-life; you don't know the difference between a breeder and a power reactor; you don't know the civilian "use once" cycle; you don't know what an integral fast reactor is; you don't know that there is thousands of times more uranium in seawater than in all the deposits of natural uranium we have ever discovered; you don't know that natural uranium is insufficiently radioactive to cause a single known case of cancer documented in the scientific or medical literature; and you don't know enough math to understand why if one in four and a half billion uranium atoms decays in a year, it means that one atom in 142,009,200,000,000,000 decays every second.

Overall, I'd have to say you know little of physics, and less of nuclear engineering. Which means that your opinions are based, not on facts, but on emotion. Which is basically what I said before. I'm pretty certain that there's no point in moving on to economics until you demonstrate a much firmer grasp of physics. So I think I'll stop here and wait to see if you have learned anything before I waste any more time.

 

[luddite]

Schneibster, I know you're not talking to me, but I actually want to thank you for that description.

I will interject that I don't think you're being fair to Kevin, though. You're addressing issues he never stated and misunderstanding what he said. The U234 for example, was brought up by Lonewulf, and Kevin only continued with that example.

I'm going to try to rephrase Kevin's concern. What Kevin said was that if you have a given amount of material, a long half life will be safer to a short half life. However, if you have enough material to be dangerous, the long half-life means it will continue to be dangerous for a very long time (10 half-lives, I believe is the usual measure). You ridiculed his understanding of half-life. I probably understand less than Kevin, but it makes sense to me.

I hope you'll correct any inaccuracies with what I'm about to say and address any concerns. You've said that natural uranium is not dangerous. First of all, I'm not even sure that's correct, but I have to ask what you mean. It was my understanding that even people who live near natural uranium deposits have a statistically higher incidence of certain cancers and other disorders. But it is a minor problem.

Spent fuel rods have a lot more uranium than natural uranium deposits. So even before fissioning in a reactor, they would be more dangerous than living near naturally occurring uranium deposits. In other words, even just the U238 would be more concentrated and more dangerous in spent fuel than in a similar amount of naturally occurring uranium ore. Correct? I assume this is what you meant by natural uranium?

Next, because fuel rods are enriched (not in Canada, which contributes to my lack of understanding), there is a much higher proportion of the U235 and U234 in them, both of which are more dangerous because they have a faster decay rate. U235, for example, has a very long half-life at 70,400,000,000, but that's still 63 times more active than U238. And U234 is almost 20 million times more active than U238. Concentrating U235 from 0.7% to 5% (and in the process increasing the U234 as well) again elevates the danger above both naturally occurring uranium, and even above concentrated uranium in naturally-occurring proportions. Even after half or more of the U235 fissions, there is still more than in naturally occurring concentrations.

More importantly, after undergoing fission in a reactor, there are fission products that are much more active. Just looking at the decay chain (which I certainly don't know off the top of my head), I see U235 decaying to Th231, with a half-life just over a day, and then to Pa231 with a half-life of 32760 years. So I assume that after some weeks or months of "cooling", the majority of activated decay pauses for a while at this stage. Yes, 32760 years is long, but it is a lot shorter than U238, with a half-life of 4,468,000,000,000 or its grandparent U235, with a half-life of 70,400,000,000. So fissioning makes that 3% of the fuel rod 2 million times more active than enriched uranium, which is already more dangerous than concentrated uranium, which is already more dangerous than naturally occurring uranium, which you'd rather not live around if you can avoid it.

So having tons of this stuff can't be good in your backyard. And at this point the long half-life doesn't really help. It just means that you have to take care of it for a very long time. You have to keep track of it and you have to make sure it isn't getting in the water and food.

Then you get the daughters of this decay. You ridiculed Kevin for talking about decay products, but I'm not sure why. In a National Geographic article, there was this interesting tidbit:

The Environmental Protection Agency has ruled that DOE must demonstrate that Yucca Mountain can meet EPA standards for public and environmental health for 10,000 years. Does that mean radioactivity won't be a threat after 10,000 years? Nope. The peak radiation dose to the environment will occur after 400,000 years, according to DOE.

Nevertheless, and despite objections from many scientists, EPA decided on 10,000 years because of "tremendous uncertainties" beyond that period.

http://magma.nationalgeographic.com/ngm/0207/feature1/fulltext.html

So you've got stuff that will peak in its radiation after 400,000 years and you need to babysit it until the radiation subsides. 400,000 years ago was before the Neanderthals, and that's when it will peak in terms of harm. You need to babysit it a lot longer than that.

Why does it have to be held in a vault for hundreds of thousands of years? In a hundred years, it's no more radioactive than natural uranium. And in thirty, it's only barely so.

I may be missing something, but in my cursory understanding, I think you're wrong. It's a lot more radioactive than natural uranium, and for a lot longer than thirty years.

 

[Lonewulf]

The U234 for example, was brought up by Lonewulf...

It was?

I don't know why people are ignoring this bit, though.

U-234 can just be left in. You don't really need to seperate it. It has a very large thermal neutron cross section. That means that in reactor fuel for just a standard normal light water reactor it will quickly convert into U-235. Reprocessed uranium from a normal uranium reactor actually has LESS than is found in natural uranium because it converts into U-235 that easily.

No, use it as fuel. I just pointed out that this stuff can bred in to usable fuel. And it doesn't even require what you currently might think of as a breeder reactor to do it. U-234 can be bred in to U-235 in a thermal reactor.

 

[luddite]

Sorry, I just looked back, and I was mistaken. The first mention of U-234 was, in fact, Kevin's.

The question of breeder reactors is a whole nuther question, which is not what I was talking about. I could talk about it. But I think there are more knowledgeable people than me making any of my points better than I could.

The question I was addressing was just the one about waste from currently running reactors.

Dr. Buzzo's comments I believe. But they are similarly not related to what we do with spent fuel. I have no idea what the final proportions are of the elements in spent fuel, which is why I didn't focus on U-234.

 

[luddite]

Question out of curiosity.

What exactly is in spent nuclear fuel? I suppose it depends somewhat on the reactor. Is there a list or a series of lists somewhere?

Thanks.

 

[Ziggurat]

I hope you'll correct any inaccuracies with what I'm about to say and address any concerns. You've said that natural uranium is not dangerous. First of all, I'm not even sure that's correct, but I have to ask what you mean. It was my understanding that even people who live near natural uranium deposits have a statistically higher incidence of certain cancers and other disorders. But it is a minor problem.

Uranium is chemically toxic, just like lead is. But its chemical toxicity is really the only significant health risk for naturally occuring uranium.

Spent fuel rods have a lot more uranium than natural uranium deposits.

No, they have a higher concentration of uranium than uranium ore, but concentration and amount aren't the same thing.

Next, because fuel rods are enriched (not in Canada, which contributes to my lack of understanding), there is a much higher proportion of the U235 and U234 in them, both of which are more dangerous because they have a faster decay rate. U235, for example, has a very long half-life at 70,400,000,000,

704,000,000 years. You're off by two orders of magnitude. Half-lives can be found here:
http://www.webelements.com/webelements/elements/text/U/radio.html

And U234 is almost 20 million times more active than U238.

U234 isn't a problem. It's the product of U238 decay, not fission, and as such it gets generated very slowly even if you've got a chain reaction going. Furthermore, inside a nuclear reactor, it gets bumped up to U235 VERY quickly, so the abundance of U234 in spent fuel is not a concern.

More importantly, after undergoing fission in a reactor, there are fission products that are much more active.

Indeed some of the products are much more active.

Just looking at the decay chain (which I certainly don't know off the top of my head), I see U235 decaying to Th231, with a half-life just over a day, and then to Pa231 with a half-life of 32760 years.

If you're looking at a decay chain, you're looking at what happens with spontaneous decay. A fission chain reaction, however, is driven by processes which are not spontaneous - that's why it's a chain reaction. The uranium breaks apart into much lighter atoms, releasing multiple neutrons in the process. The decay chain you refer to is not really the relevant one for the fission byproducts of a nuclear chain reaction.

So you've got stuff that will peak in its radiation after 400,000 years and you need to babysit it until the radiation subsides. 400,000 years ago was before the Neanderthals, and that's when it will peak in terms of harm. You need to babysit it a lot longer than that.

This doesn't make any sense. I'm not sure what exactly this journalist is talking about, but the activity level of the waste is not going to peak in the future, it's going to decline monotonically over time. Either he misunderstood what the DOE said, or he's leaving out some critical fact needed to make sense of that claim.

 

[Schneibster]

Schneibster, I know you're not talking to me, but I actually want to thank you for that description.

I will interject that I don't think you're being fair to Kevin, though. You're addressing issues he never stated and misunderstanding what he said. The U234 for example, was brought up by Lonewulf, and Kevin only continued with that example.

Not as far as I could tell, either. Kevin seems to have thought it up all by himself. Furthermore, U-234 is extremely rare, and its concentration is so small even in enriched uranium that unless its activity were so high that its half-life were weeks, it would make no difference to the radioactivity of enriched uranium. And we know its half-life cannot be that short, because it appears in measurable (though extremely small) quantities in natural uranium.

I'm going to try to rephrase Kevin's concern. What Kevin said was that if you have a given amount of material, a long half life will be safer to a short half life. However, if you have enough material to be dangerous, the long half-life means it will continue to be dangerous for a very long time (10 half-lives, I believe is the usual measure). You ridiculed his understanding of half-life. I probably understand less than Kevin, but it makes sense to me.

First, Kevin never said anything about a longer half-life being safer. Second, the nature of the risk when dealing with radioactive materials varies greatly on a number of factors: what type of radiation the material emits, how energetic that radiation is, the specific activity and therefore the half-life, how easily the material is incorporated into biological systems, what it decays into and what the type and energy of it's radiation is, and it's biological activity, and most importantly, the concentration of the material. And it was concentration that I was addressing. Lonewulf has been making the same point with his coffee example, and Dr. Buzzo with his bananas example and others.

I hope you'll correct any inaccuracies with what I'm about to say and address any concerns. You've said that natural uranium is not dangerous. First of all, I'm not even sure that's correct, but I have to ask what you mean. It was my understanding that even people who live near natural uranium deposits have a statistically higher incidence of certain cancers and other disorders. But it is a minor problem.

It's not been shown that cancer is increased by this factor in any peer-reviewed medical or scientific study to the best of my knowledge. Anecdotal evidence is worthless; it might be the uranium, or it might be a chemical associated with it, or used to mine it, and it might be a chemical problem not a radiation problem. As far as "other disorders," I'm not sure what "other disorders" you think you can get from radiation; there's radiation burns, and there's cancer, and as far as I know, that's about it. You'd have to do something incredibly stupid to get radiation burns from natural uranium.

Spent fuel rods have a lot more uranium than natural uranium deposits.

Excuse me? Uranium has more uranium in it than uranium? You perhaps were unfamiliar with the terminology I was using. By "natural uranium," I didn't mean ore; I meant refined uranium, either as the dioxide or the metal, but unenriched from its primordial (natural) isotope proportions. Such uranium is unusable in all but the most primitive reactors, because it is 99.3% U-238, which will not fission (except with an extremely low probability- not within many orders of magnitude of enough to support a nuclear chain reaction). For civilian use, the U-235, which is only 0.7% of natural uranium, must be enriched to about 5%; for military use, it is generally enriched to over 90%. That is what is used in military reactors (which are almost exclusively for ships and submarines) and in nuclear weapons.

When I spoke of the relative inertness of "natural uranium," I meant the refined metal or the dioxide. It isn't even powerful enough for the radiation to penetrate your skin. And even military-grade uranium, enriched to over 90% U-235, isn't. U-235 has a half life of over 700 million years, so even it has a very low specific activity, about six times that of U-238; six times almost nothing is still almost nothing.

And as I showed, spent fuel rods from a civilian reactor contain over 90% U-238 in any case; they have to. They started out containing 95% U-238, and nothing happened in the reactor to change that by more than a few percent. They are, therefore, little different by composition from natural uranium; and natural uranium is fairly safe to handle. On the other hand, they do contain some pretty nasty radioisotopes, so they need to "cool off" for a while until those short-lived isotopes have decayed. As I stated, most of them are gone within a few decades, and once they are, there's little left but U-238, which is what we started with, and which remains relatively safe.

So even before fissioning in a reactor, they would be more dangerous than living near naturally occurring uranium deposits. In other words, even just the U238 would be more concentrated and more dangerous in spent fuel than in a similar amount of naturally occurring uranium ore. Correct? I assume this is what you meant by natural uranium?

Not really; please read the above.

Next, because fuel rods are enriched (not in Canada, which contributes to my lack of understanding), there is a much higher proportion of the U235 and U234 in them, both of which are more dangerous because they have a faster decay rate. U235, for example, has a very long half-life at 70,400,000,000, but that's still 63 times more active than U238.

That would be 704 million years in the real world, which would be six times almost nothing, remaining almost nothing.

And U234 is almost 20 million times more active than U238.

That would be 18,000 times in the real world; the half-life of U-234 is 245,000 years. Furthermore, with a concentration in natural uranium of 0.0054%, there is so little of it, and the specific activity of U-238 is so low that even at 18,000 times the activity, it makes essentially no contribution to the radiation of natural uranium.

Concentrating U235 from 0.7% to 5% (and in the process increasing the U234 as well) again elevates the danger above both naturally occurring uranium, and even above concentrated uranium in naturally-occurring proportions. Even after half or more of the U235 fissions, there is still more than in naturally occurring concentrations.

"More" as in, enough to be dangerously radioactive? No, not really. Not to mention you seem to keep dropping and adding zeros; first it was 70 million instead of 700 million, and then it was 20 million instead of 20 thousand.

This is what I mean about hysteria; there's this urge to inflate all the numbers. Funny how both your errors inflated how dangerous it all is.

More importantly, after undergoing fission in a reactor, there are fission products that are much more active.

And which therefore have much shorter half-lives.

Just looking at the decay chain (which I certainly don't know off the top of my head), I see U235 decaying to Th231, with a half-life just over a day,

In the real world, U-235 decays to Th-231 with a half-life of 704 million years, as we discussed above. Th-231 does indeed have a half-life of just over a day, but it is a beta emitter. Beta particles are electrons. Just electrons. That means that in any sample of uranium, the amount of thorium-231 is the ratio of 704 million years to one day; an amount so small it's not chemically detectable. We only know it's there because of the specific energy of the beta particles it gives off, and because we deduce it from the fact that uranium gives off alpha particles.

and then to Pa231 with a half-life of 32760 years. So I assume that after some weeks or months of "cooling", the majority of activated decay pauses for a while at this stage.

You assume incorrectly. The ratios of radioisotopes in a sample are, at maximum, the ratios of their half-lives, and that doesn't actually start to be true until the passage of one or more half-lives of the longest-lived isotope in the chain. And we haven't been refining uranium for 700 million years yet. With a ratio on the close order of 150,000 to one, the amount of protactinium in any uranium sample is on the close order of 0.004%, at maximum; in any sample younger than that, it will be smaller, and in a sample only decades old, there will be essentially no protactinium in any uranium sample, from either the standpoint of the radiation it produces, or the standpoint of chemical analysis.

Yes, 32760 years is long, but it is a lot shorter than U238, with a half-life of 4,468,000,000,000 or its grandparent U235, with a half-life of 70,400,000,000. So fissioning makes that 3% of the fuel rod 2 million times more active than enriched uranium, which is already more dangerous than concentrated uranium, which is already more dangerous than naturally occurring uranium, which you'd rather not live around if you can avoid it.

You have so completely misunderstood what's going on that I can only wait until you correct your errors and tell you to try again. The production of daughter isotopes in radioactivity has nothing to do with the production of fission products. The production of daughter isotopes in a uranium sample happens so slowly that even after many human lifetimes, a sample of purified uranium is still purified uranium, only a few hundreds of thousandths of a percent less pure.

The danger in a spent fuel rod is from fission products, and as the name implies, fission products aren't a few atomic numbers from their parent, they are instead halfway down the periodic table, fifty atomic numbers different or thereabouts. Fissioning uranium-235 generally splits about 60/40, so on average, you see two sets of products, one around atomic number 53 and one around atomic number 39. Most radioisotopes of elements in these ranges are extremely active, with half-lives ranging from microseconds to a few years. It is these isotopes that make spent nuclear fuel dangerous; but the fact that the half-lives are so short for these isotopes means that almost all of them is gone in a few decades at most. A very few somewhat longer-lived isotopes (half lives in the decades or centuries) remain problematic, but their concentrations are already so low (at least in civilian grade fuel) that they don't pose much of a threat.

The above paragraph by you is filled with incorrect numbers, exaggerated in your favor by up to three orders of magnitude; complete misunderstanding of the difference between fission and radioactive decay; and finally with a figure that plain ignores concentration as a measure of activity. You are also, however, incorrect about one point that is in your favor; you do at least understand more than Kevin does, and although your calculations are incorrect, they're at least an attempt at the right way to go about this; three orders of magnitude isn't bad for a first attempt, and at least you got the mantissas approximately right.

So having tons of this stuff can't be good in your backyard. And at this point the long half-life doesn't really help. It just means that you have to take care of it for a very long time. You have to keep track of it and you have to make sure it isn't getting in the water and food.

And here, you've drawn an incorrect final conclusion from your incorrect reasoning. Overall, the long half-life helps a great deal, and the fact that the initial concentration of materials with short half-lives (and therefore high activities) is already small helps even more.

Then you get the daughters of this decay. You ridiculed Kevin for talking about decay products, but I'm not sure why.

Because he made the same mistake you're about to.

In a National Geographic article, there was this interesting tidbit:

Which completely ignores the fact that what they're talking about is military nuclear waste, not civilian nuclear waste, with the difference in concentration of several orders of magnitude that implies. Furthermore, it also contains no reference to the DOE calculations so that they can be checked; I have no idea what the author is talking about when he claims that the DOE says that "the peak radiation dose to the environment will occur after 400,000 years." First, it's not clear whether they are talking about waste from plutonium manufacturing or from reactors; second, it's not clear what isotopes are involved; and third, no matter all of the above, it IS clear that they're talking about military waste, not civilian waste.

So you've got stuff that will peak in its radiation after 400,000 years and you need to babysit it until the radiation subsides. 400,000 years ago was before the Neanderthals, and that's when it will peak in terms of harm. You need to babysit it a lot longer than that.

And again, that's military waste, and it's not even clear it's from a reactor and not plutonium production for the manufacture of nuclear weapons.

I may be missing something,

I will restrain myself; I would say you've missed a LOT.

but in my cursory understanding, I think you're wrong. It's a lot more radioactive than natural uranium, and for a lot longer than thirty years.

I think you've got a lot more research to do before you're prepared to say anything of the kind, and if you do that research, I'm pretty certain that what you'll find is what I've already told you.

 

[Lonewulf]

First, Kevin never said anything about a longer half-life being safer.

Well, that might make him look weak in the eyes of the enemy.

 

[Schneibster]

Question out of curiosity.

What exactly is in spent nuclear fuel? I suppose it depends somewhat on the reactor. Is there a list or a series of lists somewhere?

Thanks.

The list is huge. When U-235 splits, it can do so in many, many ways. Each way creates a different pair of isotopes, far down the periodic table. U-235 contains 92 protons and 235 - 92, or 143, neutrons. Both prompt (that is, ejected during fission) and delayed (that is, ejected by the fission products) neutrons are produced; an average of 3.2 neutrons is produced in each fission, with less than one of them delayed (and this is pretty obvious, because if the multiplication factor were less than one, there could be no chain reaction). Fission mostly if not exclusively involves the nucleus splitting in two; I am not aware of any three-nuclide fissions, but I haven't checked thoroughly. Most fission products have too many neutrons to be stable; the ratio of neutrons to protons needed to keep a nucleus stable increases with the number of protons. The half-lives of isotopes of lighter elements are generally quite short, though there are a few longer-lived light isotopes. This is because the energy "hill" that light isotopes can "roll down" to achieve stability is much "higher" due to their lightness. By the time you get to the actinides, atomic numbers in the 80s, stable isotopes are few and far between; above uranium, stabilities are so low that we don't find any of these elements naturally, they've all decayed away in the lifetime of the Earth. Only one element below the actinides has no stable isotopes; technetium is this element. It was first identified in the fallout from nuclear weapons explosions, and being man-made, was given its name from that fact.

 

[luddite]

Thanks Schneibster, that's helpful.

Obviously, I've got some work to do.

 

[luddite]

And thanks Ziggurat. I missed your answer earlier, but I find it even easier to read.

 

[Schneibster]

Let me emphasize this point once again: the waste from military uses of nuclear power is of a very different character than that from civilian nuclear power, and the waste from a breeder yet another thing, and from making nuclear weapons again yet another thing. Plutonium is a much more dangerously radioactive substance than uranium; the half-life of Pu-239 is 24,000 years, and it is therefore highly active. It is also chemically poisonous in almost exactly the same way as uranium is. However, use of plutonium in the civilian fuel cycle would not concentrate it any more in the fuel than U-235 is concentrated there; at least not if the people doing it are sane.

The use of highly purified Pu-239 or highly enriched U-235 in a reactor is a very tricky and ticklish business. The Soviet Union lost more than one nuclear submarine to nuclear accidents involving reactors using them. The US has never lost a nuclear submarine to a nuclear accident, although they have lost two nuclear submarines to other things.

 

[DRBUZZ0]

Question out of curiosity.

What exactly is in spent nuclear fuel? I suppose it depends somewhat on the reactor. Is there a list or a series of lists somewhere?

Thanks.

Well if it's a standard uranium-based reactor:

Mostly U-238: which is the "Non fissionable" uranium, which is what most of it is when you put it in. (this is generally more than 80-90%)

"Unburned" U-235: This is the fissionable uranium. You start off with about 3% U-235 in most reactors. Depending on the reactor and the enrichment of the fuel anywhere from 25% to 50% or more remains and never actually fissioned.

Heavy Elements: Mostly plutonium but also a small amount of Amercium and neptunium. These can be used as fuel because they are generally fissionable or at least "fertile" meaning they will become fissionable after absorbing a neutron. They may account for up to 10% but usually are only a few percent. Also, they are crappy for weapons. You get "reactor grade" plutonium with is too much Pu-240. Works fine for reactor fuel but not bombs.

U-236: Occasionally a U-235 atom will absorbe a neutron and not fission but enter a ground state and become U-236. Uranium-236 has a low neutron cross section but can be useful in creating specialty isotopes. It's not especially toxic or dangerous, so it's not usually a big deal. At worst it is a nusence.

Fission bi-products: These are the "spint" atoms which are left from the U-235. Of all the material generated these are the only ones that don't generally have any use in making new fuel. Most (95%+) are short lived enough that there is no disposal problem. Just hang onto the fuel for a few years and they're gone.

A few last millions of years, but the yeild of these is low and their long half life leads to them being not all that dangerous.

Finally the problematic ones are the ones that last decades. These last too long to just hang onto. But they're short enough to be dangerous. These are like Sr-90 and Cs-137.


I recall reading that the actual material in a bundle of spent fuel (about 24 rods 16 feet long) that is radioactive and not usable for more fuel is about the size of a tablet of asprin.

Of course, these do have uses. Cs-137 is used for cancer treatment and for sterilizing food and Sr-90 is used for power sources for oceanographic equipment and for keeping the components warm on satellites.

 

[Octavo]

This has been an excellent and very informative thread. It's taken me a few days to trawl through the whole thing, but I'm really glad I did.

To all the regular participants - Schneibster, DRBUZZ0, Lonewulf, luddite and others, thank you for this - I really appreciate all the research and fact-finding and explanation that was done. It has certainly increased my understanding a great deal!

I don't have anything technical to add - just some trivia about the nuclear industry in South Africa

Here in SA we have 1 nuclear plant (Koeberg) that has faithfully and quietly been producing power for the last 20 years and helping to keep Cape Town beautiful. Owing to our lack of energy sources, the only alternative for generating power in the Cape would have been to build a coal-fired plant and then use railways to transport the coal down from the northern reaches of the country. It would have been exceptionally expensive and absolutely filthy. The apartheid government already had enriched uranium and a fair amount of nuclear expertise from our secret nuclear weapons program, so we built Koeberg instead.

On the back of the incredibly cheap power supplied by Koeberg (and our abundance of naturally occurring coal in the north), for a while SA had the cheapest electricity in the world - of course Eskom (the para-statal power utility that owns all these plants) didn't bother investing in new capacity, so about 2 years ago we started feeling the pinch.

Koeberg shut down in mid-winter that year - 1 reactor was shut down for planned maintenance and refuelling, the other tripped for some reason and then off-site power failed or something and basically they had to shut the second reactor down until the nuclear regulator was satisfied that everything had been sorted out and gave the all-clear to bring it back online. With 1600Mw suddenly missing, Cape Town was plunged into darkness and rolling blackouts for about a month.

It's a long story, with government crapping on Eskom and Eskom blaming government etc. but essentially Eskom have now accelerated plans to have the French build us another "Koeberg" along the coast and have also decided to build the first full-scale PBMR demo reactor at Koeberg as well.

Eskom hopes that PBMR's will prove succesful and hopefully catch on in the rest of the world. Some linkage for the interested:

http://www.pbmr.com/

http://en.wikipedia.org/wiki/Pebble_bed_modular_reactor

 

[luddite]

Thank you Buzzo.

 

[Hindmost]

Let me emphasize this point once again: the waste from military uses of nuclear power is of a very different character than that from civilian nuclear power, and the waste from a breeder yet another thing, and from making nuclear weapons again yet another thing. Plutonium is a much more dangerously radioactive substance than uranium; the half-life of Pu-239 is 24,000 years, and it is therefore highly active. It is also chemically poisonous in almost exactly the same way as uranium is. However, use of plutonium in the civilian fuel cycle would not concentrate it any more in the fuel than U-235 is concentrated there; at least not if the people doing it are sane.

The use of highly purified Pu-239 or highly enriched U-235 in a reactor is a very tricky and ticklish business. The Soviet Union lost more than one nuclear submarine to nuclear accidents involving reactors using them. The US has never lost a nuclear submarine to a nuclear accident, although they have lost two nuclear submarines to other things.

I am pressed for time of late, but I wanted to add about plutonium. If swallowed, it will just run through your body and end up in the toilet. Not as much of a problem actually. It has heavy metal toxicity--but no more than other heavy metals and it is not as toxic chemically as uranium, but the biggest problem is if it is inhaled. It stays in the lungs causing problems and is a bone seeker. When it gets into your bones, it will kill the marrow fairly quickly--and you don't have to inhale very much. Sr-90 is also a bone seeker and has the same type of issue.

glenn

 

[Hindmost]

I'm not sure how many times I can rephrase this without just repeating myself.

We have non-proliferation treaties in place to limit the spread of nuclear weapons. The problem with these treaties is that given a civilian nuclear program it is possible to covertly run a weapons programme, as India, Pakistan and Israel demonstrated. Breeder reactors make the process much easier.

If as a planet we end up using breeder reactors to supply our power needs, we are going to inevitably end up with breeeder reactors in countries that we do not want developing nuclear weapons. What else do you expect them to do, put up solar panels?


....snip

I suppose we can disagree on this, however, I don't see any causality with the US building breeder reactors and proliferation in other countries. The US has no plan to sell breeder technology to anyone. Iran has signed the non-proliferation treaty.

Civilian light water technology is essentially useless for building weapons grade materials. India constructed weapons production reactors. Pakistan originally enriched uranium but now has some production reactors under construction...both have not signed the NPT.

glenn

 

[Schneibster]

Dr. Buzzo, nice. Well done.

Hindmost, I didn't know that about plutonium.

Octavo, I'm very pleased to hear that someone is finally doing something with PBR technology. It's the safest way to build a nuclear plant that anyone's come up with. And it has an added bonus I don't think anyone's realized yet. It USES carbon- what I mean is, you can make use of all that carbon that has to be sequestered from cement and steel manufacturing to make pebbles.

 

[Hindmost]

Dr. Buzzo, nice. Well done.

Hindmost, I didn't know that about plutonium.

Octavo, I'm very pleased to hear that someone is finally doing something with PBR technology. It's the safest way to build a nuclear plant that anyone's come up with. And it has an added bonus I don't think anyone's realized yet. It USES carbon- what I mean is, you can make use of all that carbon that has to be sequestered from cement and steel manufacturing to make pebbles.

Of course pu has a reputation as the most toxic substance even though many things are much worse....now, this guy is crazy...and I don't know if I would consider this stuff reasonable.

http://russp.org/BLC-3.html

Adding to what you and DRBUZZO have done so well.

On the miliary reactor thing. Typically, the cores are highly enriched as previously stated. This give two advantages. One--with a very robust core design...reactor power on demand and also length of service. Refueling is truly a pain on subs and carriers and extending core life has been an ongoing goal. The cores are loaded with burnable poisons to keep the reactor's reactivity relatively constant over time--however, neutron absorbing poisons eventually get them too. When the core is removed, there is still a fair amount of usable U235 that was reprocessed in the past. The plant were not built for efficiency since they weren't design to make electricity. The two US subs at that bottom of the oceans have not leached anything thing from the cores...however...I am dated on that note. The wiki article indicates it is still true...but obviously it is wiki.

glenn

by the way...at the end of cycle in a comercial plant, 1-2% u235 remaining is accurate. What got me a long time ago was the fact that 30% of reactor power comes from burning Pu at end of cycle. With increased burnup designs, I am sure this has changed a bit.