J. Environ. Sci. Tech: Plutonium risks at Yucca probably overstated.

[NNadir]

First off, let me start this post by stating once again, that I oppose the plans to bury plutonium at Yucca Mountain. As presently envisioned, I can think of few things as dumb as Yucca Mountain. This is because the scheme includes treating plutonium as a waste worthy of dumping. I despise the waste mentality, not only for plutonium but for all of the valuable materials found in spent nuclear fuel.

In my tenure at Democratic Underground, now having generated more than 8,000 posts, there are few in the series on which I worked so hard as those in the long ago "External Energy Thread." In that thread, in some detail, I reviewed some of the chemistry and physics associated with various actinides and fission products, showing how over the long run, many of these materials may be of considerable use for future generations.

(That someone savaged archived thread can be found by accessing my journal or by clicking here: http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=115x5609)

Like Pavlov's unfortunate canines people often, on hearing that I am pro-nuclear energy, immediately come at me, as if they were somehow especially knowledgeable and say: "Yeah, but what do you do with nuclear waste!?!" Usually I respond by asking such people if they know what to do with the wastes of fossil fuels, in particular carbon dioxide, and zero people have demonstrated that they do in fact know what to do with the wastes of fossil fuels. That ought to end the issue, but it doesn't.

Let me take a different tack here and say what I think should be done with spent nuclear fuel, so called "nuclear waste." 1) Hold it where it is for the time being in fuel rods. 2) Build an infrastructure for the chemical separation of the elements therein. 3) Keep the actinides and use them for fuel. 4) Remove the elements that have no long lived isotopes and use them. 5) Use the technetium for technical operations or transmute it into ruthenium and rhodium. 6) Separate the Strontium, recover as much of its heat (energy) as possible, and place it where it can be watched for a few hundred years while it decays to background. 7) Separate the iodine, make it a stable salt and stop worrying about it because the risk is so ridiculously low. 8) Isolate the cesium and see if a space industry ever evolves wherein it can be used. If no such industry evolves, wait a few hundred years until all of the Cs-137 has decayed to background, make synthetic pollucite and stop worrying about it, because Cs-135 is of extraordinarily low risk on the grand scale of things. That is my answer to the ridiculous comment, "Nobody knows what to do with nuclear waste."

I really can't see where such a program worldwide could possibly involve more loss of life than occurs because of air pollution in New York City alone in a single year hear in the dawn of this millennium.

This idea is not unique to me: I recently in another thread referenced a similar idea from the National Academy of Engineering: http://www.democraticunderground.com/discuss/duboard.php?az=show_topic&forum=115&topic_id=53871

So much for the current scheme for using Yucca Mountain.

Still not one of my stated objections references whether or not Yucca Mountain is unacceptably dangerous even as currently conceived.

Generally the subject of "what do we do with the waste," has been ratcheted to more and more extreme insistence on zero risk, which of course is nonsensical in light of the fact that no other continuous scalable form of energy has a risk so low as nuclear energy. The question has become so absurd that the conversation has seriously been held about whether or not some putative rancher in the 26th century in Nevada might get a case of cancer as the result of Yucca mountain, this while millions die now from air pollution and global climate change.

Actually we do know what the fate of nuclear materials in geological systems will be over billions of years, as I often point out, this from the naturally occurring nuclear reactors at Oklo that operated for several hundred thousand years almost 2 billion years ago.

http://www.ocrwm.doe.gov/factsheets/doeymp0010.shtml

The worry about plutonium is usually stated as a series of mutually contingent conditional statements: If the metal canisters in Yucca mountain corrode, and if the borate glasses in which the spent fuel is stored becomes permeable, and if cracks develop in the mountain and if water somehow intrudes into the area, and if the water flows out and if someone drills a well into the groundwater, and if they drink x number of glasses of water they may face an increased risk of cancer. Somehow - mostly through appeals to stupidity - this series of events is unacceptable even if the odds of it happening are less than the odds that you will win the lottery tonight. People insist that proponents of nuclear power prove for all time that this unlikely series of events be proved impossible rather than extremely, incredibly, astronomically unlikely. Further, as part and parcel of these wild suppositions is the implicit unjustified assumption that if something, anything, is unknown, then the worst case is the only case that will prevail. This, too, is pure nonsense.

One of the technical ifs not mentioned here concerns what will happen if the plutonium becomes soluble, because generally plutonium is not very soluble. It does happen though that people have speculated endlessly that plutonium could become soluble.

A diversion:

Were it not so controversial, plutonium would be the subject of considerable scientific excitement and interest. This is not only because of plutoniums remarkable physical properties - it has the most metallic phases of any known element, some brittle, some malleable, and, in fact, to the chagrin of weapons (and some commercial) engineers, the metal machines like a madman's nightmare. Plutonium's history is intimately involved in what we know of stars, it is a place marker for the discovery of why at the end of the day, is tied up in the existence of plutonium, plutonium marking along with its sisters, uranium and thorium, the rhythm of creation. we exist at all, marking along with its sisters, uranium and thorium, the rhythm of creation. Not so long ago on this site, I was explaining that plutonium is an actinide, and I implied in a monumentally simplistic way, that the plutonium was called an "actinide" because its chemistry is "like the element actinium." Actually among elements, plutonium is in many ways unique. It is a damn cool element, with complex redox chemistry, fascinating chemical properties. There's a nice picture over at Wikipedia of some of the solutions of various oxidation states of plutonium

Just click on the test tubes to observe this chemistry in all of its magnificence:

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

Now, in the ASAP section of the scientific journal Environmental Science and Technology, a publication of the American Chemical Society comes this report abstracted here: http://pubs.acs.org/cgi-bin/abstract.cgi/esthag/asap/abs/es052353+.html (The full article requires a subscription.)

Some relevant quotes from the full article follow. Mostly, regrettably, this is technical stuff, but I have put in bold the important point that most people can understand:

Knowledge of radionuclide geochemistry and the surrounding environment is required for predicting subsurface fate and transport. This task grows increasingly complicated for constituents such as Pu, which exhibit complex environmental chemistries. The environmental behavior of Pu can be influenced by complexation, precipitation, adsorption, colloid formation, microbial activity (e.g., direct interaction of biofilms or complexation with exerdates, such as siderophores), and oxidation/reduction (redox) reactions (1-5). The most important of these factors controlling Pu mobility is redox, more specifically, the oxidation state of Pu. This is because Pu(IV) is generally 2-3 orders of magnitude less mobile than Pu(V/VI) in most environments (6). Kaplan et al. (7) showed that Pu(IV) moved 10cm through a vadose zone sediment, with >95% of the Pu remaining within 1.25 cm of the source, after 11 years of exposure to natural rainfall conditions at the Department of Energy (DOE) Savannah River Site (SRS). Pu oxidation and reduction reactions over the 11 year period were found to play an important role in Pu transport through the pH 6.1 system dominated with kaolinite, goethite, and hematite in the clay fraction. Oxidation was attributed to wet-dry cycling and its effects on Fe- and Mn-oxides and bacteria (8). Pu commonly exists simultaneously in several oxidation states (9-10). Choppin (11) reported Pu may exist as Pu(IV), Pu(V), and/or Pu(VI) in oxic natural groundwaters. The pentavalentandhexavalent oxidation states of Pu are typically stabilized in aerated solutions (high EH) and high pH (12). The most common assumed form of precipitated Pu in the environment is PuO2(s) (11). The low solubility of PuO2(s) generally limits subsurface mobility...

...Using micro-XANES, Duff et al. (22) observed oxidation of Pu(V) to Pu(VI) following sorption to a natural zeolitic tuff from Yucca Mt. The tuff contained trace quantities of manganese oxides and more abundant Fe-oxide phases. Elemental maps generated with micro-XRF imaging demonstrated that Pu was preferentially associated with Mn oxides (specifically, ranciete, (Ca,Mn)OMnIVO2â3H2O, an iso-structural form of birnessite, with Ca as the dominant cation in the interlayer) and co-associated smectites rather than with iron oxides or zeolites. These measurements were performed 2 and 6 months after adding aqueous Pu(V) to the tuff...

...Additionally, the Pu is still found to be associated with the mineral phases containing Mn, having an estimated loading of 2000 mg Pu kg-1 at the Mn-rich regions on the tuff. Therefore, the Pu(V) initially equilibrated with Yucca Mountain tuff was oxidized to Pu(VI) or remained unchanged and then reduced to Pu(IV) over time. These data from aged samples appear to conflict with observations that Mn(IV)- containing minerals are capable of oxidizing Pu (15, 23)...

...That Mn(IV) minerals can oxidize Pu(IV) to the more mobile Pu(V/VI) forms has required regulators and risk assessors to assume that Pu emplaced at some geological waste repositories will be more mobile than may, in fact, be the case. This study lends itself to a new conceptual model describing Pu geochemistry that can be used to more accurately predict the risk associated with the disposal of Pu-containing waste.

...

So there you have again. Let me repeat: "...required regulators and risk assessors to assume...more...than may in fact be the case."

Of course, some people will not be satisfied unless the regulators assume that future generations will - for reasons that must escape us - be spreading plutonium on their breakfast cereal because every atom of plutonium, being invested with supernatural powers, is just struggling to getatcha'.

Actually it happens though that even with the worst case, a hundred Yucca Mountains leaking like sieves would not even remotely approach the consequences not of what could happen, but what is happening from the worst waste dumped (and indiscriminately dumped at that - with no where near as much attention as plutonium gets) by humanity: That would be carbon dioxide.

[Igel]

1. Thanks for the Oklo links; I was

looking for references to them a few weeks ago for some reason that now escapes me.

And I like the Wikipedia quote "Interestingly, in fission weapons, the explosive shock waves used to compress a plutonium core will also cause a transition from the usual delta phase plutonium to the denser alpha phase, significantly helping to achieve supercriticality." How helpful of it. As for the allotrope density ... too weird. (I was an undergrad chem major before I decided I liked languages better.)

As for the zero risk business ... you're right. People do very badly at risk assessment (we really tend to lack an intuitive sense of stats/probabilities, so that's not a surprise). Many of the most dangerous really think we must do everything possible--not just everything feasible--to minimize risks in our environment.

I'll try to wade through the whole article; I assume my wife's VPN connection with the local university will get me to the thing. In any event, I learned a new expression: Vadose zone. (Useful to know, if one needs to talk about the Ural radiologic event in the 1950s.)

[Buzz Clik]

3. If you have trouble getting the pdf...

give a holler.

[Buzz Clik]

2. Excellent post, as always.

I always have been one of those neanderthals hesitant to support nuclear energy production strictly because of the waste issue. Part of it was a technical concern (you've done much to cure that), part was public acceptance of plans to take care of the waste.

Unfortunately, despite your elegant discussion, I see no level of acceptance where it really matters -- on the regulatory end. If the government does not open the door for discussion, it's dead on arrival. How can this change?

Regardless, thanks for the post and all your previous efforts to educate us.

[jpak]

4. First of all, intact spent fuel assemblies - not vitrified waste -

will be entombed at Yucca Mountain. These will be encased in steel casks.

Secondly, rapid corrosion of disposal casks is anticipated (thermo-chemical reactions within the repository is expected to produce highly corrosive aqua regia vapor).

To forestall release of radionuclides from corroding casks, the DOE is spending billions of addition dollars to deploy exotic alloy "drip shields" over them.

Geological characterization of Yucca Mountain is on-going. Preliminary analyses, however, indicate that groundwater within the structure is mobile and, in the event of release from corroding casks, radionuclides would escape the repository rather quickly.

Furthermore, fission rhodium, palladium and other so-called "precious metals" present in spent fuel are radioactive and have little - if any - commercial value.

Radio-iodine is highly volatile, geochemically mobile and readily absorbed by humans - these radioisotopes are extremely dangerous (e.g., Chernobyl). Claims to the contrary are false and laughable nonsense.

Finally, the National Academy of Sciences estimated that actinide burning schemes (if indeed they could be developed at all) would cost taxpayers in excess of $100 billion to develop and deploy.

The rest of the OP is gooble-dee-gook that can be ignored.

The best way to deal with nuclear waste is not to produce it in the first place.

Oh yeah - the cost of Yucca Mountain is currently ~$65 billion and climbing . Taxpayers - not the nuclear plant operators that produced the spent fuel - will pick up the tab for most of this.

Such a fucking deal...

[Dead_Parrot]

6. Non-vitrified waste is indeed a turkey...

I have to agree (oow, the pain!) with some of your points. Yucca is turning into a clusterfuck. :(

Edit: although "the National Academy of Sciences estimated that actinide burning schemes (if indeed they could be developed at all) would cost taxpayers in excess of $100 billion to develop and deploy." would have to be compared to $10 billion/month for a pointless war-for-oil in Iraq. And given Europe is well on the way to such schemes, that would save a lot of the R&D cost...

...But that would depend on reading NNadir's posts - and understanding them - which the fossil fuel lobby refuse to do.

Weather that includes you is up for debate.

[Dead_Parrot]

5. I remain unconvinced...

...That the US Govt is going about Yucca in the right way: there seems to be too many question marks over the methods of storage, the geology of the area and the safety procedures. This does not mean that geologic disposal per se is a bad thing - the Finns and Swedes have no problem with it - but I have to ask if the US government isn't taking a leaf from the "Soviet Book of Nukes" and just making the rules fit the game.

That they seem committed to this path without looking at transmutation doesn't help, to be honest...

Edit: BTW, thanks for journaling the "pay with your flesh" thread - it's a lot easier to find, now!

[NNadir]

7. I very much doubt that Yucca Mountain under any circumstances

will lead to a tremendous loss of life under any circumstances.

Most of the objections to it involve, from what I've seen, some very tortured arguments.

I will find them more credible when the same tortured arguments, particularly with respect to risk are applied to all fuels.

The Oklo reactors existed. They would not have existed were they not in highly permaeable sandstone, in an extremely wet area, an area that at least until recently was rain forest.

I don't think that their is anything similar to the Soviet practices at Yucca. I think the concerns are largely manufactured through misinterpretation and misrepresentation, and the setting of deliberately unachievable standards by the (foolish) opposition. I do note that the even the Soviet practices, as bad as they were, have resulted in tremendous loss of life. I am sure that the loss of life connected with coal burning in Nevada has already exceeded any loss of life that may someday be connected with Yucca Mountain.

People here love to carry on about our weapons plant at Hanford, where some pretty terrible practices were used. The decision to build that plant there had no concern whatsoever with the geology.

I don't approve of nuclear weapons - I want them banned - and I don't approve of nuclear weapons plants, but the fact remains that even Hanford has not lead to a wholesale destruction of life along the Columbia river. In fact the worst effect on the river, so far as its damage to ecosystems from human interference are concerned, so far has involved the presence of dams. The tanks are leaking, and radioisotopes are measurable in the groundwater and the groundwater, but they are moving very, very, very slowly are are extremely dilute.

To get an idea of exactly how absurdly tortured these kinds of analyses can get, let's look at the definition of the "MEI" which is government talk for "Maximally Exposed Individual" at Hanford:

The MEI is a hypothetical person who lives at a location and has a postulated lifestyle such that it is unlikely that other members of the public would receive higher radiation doses. This individual's characteristics were chosen to maximize the combined doses from all realistic environmental pathways of exposure to radionuclides in Hanford effluents. In reality, such a combination of maximized parameters is unlikely to apply to any single individual.
The location selected for the MEI can vary from year to year depending on the relative importance of the several sources of radioactive effluents released to the air and to the Columbia River from Hanford facilities. Historically, two separate locations in the Hanford environs have been identified as potential sites for the MEI: the Ringold area, 26 km (16 mi) east of the 200 Areas separation facilities, and the Riverview irrigation district across the river from Richland (Figure 6.0.1). The principal differences between the two MEI locations are that Ringold is closer than Riverview to the Hanford facilities, which had been the major contributors of airborne effluents in the past, but the MEI at Ringold does not drink water derived from the Columbia River. The MEI at Riverview, although farther from the Hanford sources of airborne radionuclides, can be exposed to the one additional pathway of irrigation water derived from the Columbia River.

During 1994, the hypothetical MEI (assumed to be located 1.5 km <1 mi> directly across the Columbia River from the 300 Area) was calculated to have received a slightly higher dose in 1994 than an MEI located at either Ringold or Riverview. The farms located across from the 300 Area use water obtained from the Columbia Irrigation System far upstream of the Hanford Site for irrigation and well water for sanitary purposes. Foods grown there would only contain radionuclides released with airborne effluents of Hanford origin. Therefore, the conservative assumption was made that the diet of the MEI residing across from the 300 Area consisted totally of foods purchased from the Riverview area, which could contain radionuclides present in both liquid and gaseous effluents from Hanford. The added contribution of the radionuclides in the Riverview irrigation water maximizes the calculated dose from all air and water pathways combined.

The following exposure pathways were included in the calculation of doses potentially received by the hypothetical MEI for 1994: inhalation of and submersion in air downwind of the Site, consumption of foods contaminated by radionuclides deposited from the air and by irrigation with water from the Columbia River, direct exposure to radionuclides deposited on the ground, consumption of fish taken from the Columbia River, and external radiation during recreation activities on the Columbia River and its shoreline. The MEI for 1994 was postulated to be an individual who:


was a resident of the closest farm 1.5 km (1 mi) across the Columbia River from the 300 Area
consumed foodstuffs irrigated with Columbia River water grown in the Riverview Irrigation District
used the Columbia River extensively for boating, swimming, and fishing and consumed the fish caught
drank well water that did not contain radionuclides of Hanford origin
was exposed via inhalation and external exposure to the airborne radionuclides released from Hanford facilities.

There is no evidence that the MEI would drop dead immediately under any circumstances. Driving the tractor on his or her farm is probably the most dangerous thing he or she does, especially if he or she breathes the diesel fumes, unless of course, he or she is using biodiesel, with slightly lower particulates.

I'm sure that Yucca opposition is all about similar MEI's. I'm sure that the conversation is not about real people, but hypothetical people representing an improbable worst case. I don't think that the Yucca facility should be built as imagined, and I'm happy on some level about the delays, since it gives more time for people to become realistic, but I find that since there are no MEI's for coal ash - the discussion of an MEI in the 30th century who might live in a city that might exist near Yucca Mountain if the area ever has sufficient water just a little bit nuts. There's nothing Soviet about it.

[Dead_Parrot]

8. I agree, but...

There do seem to be some shenanigans over the way Yucca is being developed. It's not enough for nuclear power to be safe, but it has to be seen to be safe, beyond doubt.

Let's face it, you could use a once-through fuel cycle and dump the used rods in Central Park encased in Emmental, and it would still be safer than coal in the long run, but in a country where 30% still is Bush is the right man for the job, you can't rely on people being smart enough to see past the tabloid headlines.

[NNadir]

9. The time to stop catering to the dumb 20% or 30% is now.

I think that time in which reality trumps fantasy is upon us.

I don't think that there is one so called shenanigan at Yucca Mountain that is reflective of risk assessed reality. The perception that such shenanigans exist, that they are serious, and that they somehow are a threat to someone somewhere at some time is manufactured news.

Yucca mountain could open next week with the latest scenario for operation and probably not result in a single loss of life for centuries, if ever. If you look at the objections, they usually are about unfilled paperwork, incomplete data and blah, blah, blah. Essentially they are nonsense.

The unfilled paperwork and especially the incomplete data on every other form of energy is vast, enormous and unaddressable. The only case where energy waste disposal issues can be addressed without enormous loss of life is at Yucca Mountain.

That said, Yucca should not open. I like the fact that we are being forced to hold the fuel above ground for another 10 or 20 years. That gives time to come to our collective senses and stop indulging stupidity.

I have been very happy to learn in recent weeks, having looked into it, more details about the latest fuel cycles being developed in Europe and other places, things with intersting names like CORAIL, MIX-UE, APA etc. They seem to address almost everything I've been saying for years, and I'm very excited to see new approaches to light water transmutation of minor actinides, plutonium inventory management with changes in isotopic composition, and isolation of intermediate life fission products like Sr-90 and Cs-137. There are some very exciting approaches to fuel that do not require vast infrastructure changes. They are available to us now because of our ability to bring vast computational power to bear on the problem of fuel burn-up, something that was not with us until recently. The technical work is certainly under way in France, and France has developed several scenarios that seem to fit the bill for the next century. I will write more about these approaches later, probably in a new thread.

The French seem to be thinking of avoiding long term geological disposal for many decades. This is a good thing, because for the first time in many years, people are looking at the problem with a long term nuclear energy future in mind. The French experience is cutting edge. They are leading the world, and the sense I get is that for the first time, many people are impressed, even jealous, of their success. Let's be clear. La Hague is not Sellafield. Every drop of plutonium recovered at La Hague is now considered fuel. It is being burned right now, even as we speak.

The work being pioneered there represents about the only hope we have under the circumstances.

[Dead_Parrot]

10. I hope so.

The dumb have been running the planet for long enough, it seems: It would be Nice if people who can comprehend 50 years into the future could get a go. We'll see.

As for geologic disposal, I'd agree that burying a used fuel rod is analogous to putting your car in a crusher because the gas tank's empty. However, it's also my understanding that the bulk of "nuclear waste" - by volume - is the intermediate waste: everything from processing sludge to used reactor parts. I'm guessing that this has limited options for using as fuel, so most it may need some sort of disposal.

What I don't have any information on is the sorts of half-lives we're looking at for this. Some of it is doubtless safe after a few months or years, some will run to centuries or millenia... Have you stumbled any sort of breakdown of this?

I defiantly think there's a problem with the perception - and the practice - of handling everything that comes out of a reactor as "nuclear waste". To pick a convoluted example, say you get some sulphur, aluminium and uranium out of an old reactor: The uranium you'd want to keep to reprocess; the sulphur you could just stick in a box for a few years until it drops to harmless levels; and the aluminium you might want to bury - If it includes ²⁶Al, it's probably not worth the aggravation.

I believe Europe does handle intermediate waste in a slightly more sane way that the US in this respect: But what I'd like to see is a deep storage facility (rather than "disposal") where everything is labled and catagorised as it goes in: if someone builds a CANDU-like reactor in 20 years time that'll run of ²⁶Al, you'd feel a bit daft if you'd buried a ton of it under 500m of concrete.

[NNadir]

11. The half-lives and fission yields are given in the Table of Nuclides.

http://atom.kaeri.re.kr/

(One needs to click on the various colored nuclei in the chart to get the properties. The blue nuclei are the stable nuclei and the blue staircase is known at the line of nuclear stability.)

Here one can also find various evaluated nuclear data, such as various types of cross sections, and cross section graphs from evaluated nuclear data files.

Knowing that you are a physicist, I will explain in a little more detail, although necessarily, I will only sketch out what can be sketched in words without access to equations.

If one spends enough time with this table, one can get a crude estimate of the constituents of spent fuel. More precise calculations however depend on access to various types of computer programs which "solve," analytically a set of differential equations known collectively as the fuel depletion equations.

The variables in these equation sets are, for a jth nucleus, are the average neutron flux, the fission yield, the macroscopic fission cross section of the fissioning nucleus, the summation of the capture cross sections of the ith nuclei for all i that can yield the jth nucleus, as well as the decay constant (ln(2)/half-life) of any parent nuclei that decay into the jth nucleus, as well as the cross section for neutron capture in the jth nucleus and its own decay constant. One analytically integrates these equation sets to calculate the accumulation of nuclei in a particular fuel scheme.

Of course, the results are then tested experimentally, but as the evaluated data files get better, and computing power becomes greater, the problem is far more trivial than it was when the first generation of nuclear reactors were built.

You also refer to neutron capture by structural materials, which is, in fact, not much of a radiotoxicity issue. In general, one is interested, for these purposes, in neutron rich nuclei. The only source of Al-26 in a nuclear reactor is the improbable (n, 2n) reaction in Al-27 used as a structural material - as it was in Magnox reactors. The probability of this reaction over the fission spectrum average is a rather unimpressive 670 microbarns, as you can see from the use of the table. Is there some Al-26 in such structural materials? Yes. Is it important? No. It is essentially impossible for a person to eat enough aluminum to be affected by it without first being killed by the chemical toxicity of the aluminum itself. With the exception of control rod materials and burnable poisons, all of the structural materials selected for use in nuclear reactors are selected precisely because they don't interact much with neutrons. Structural materials have a radiotoxicity greater than that of natural uranium used to fuel the reactor for only about 10 years after shutdown.

Thus almost all of the putative radiotoxicity from the use of nuclear power comes from the fission products and the actinides. For the first few hundred years, the radiotoxicity of the relatively short-lived fission products dominates. Primarily after 50 years, this toxicity is dominated by just two nuclei, strontium-90, and cesium-137. After about 200 years, the toxicity of the fission products has fallen below the radiotoxicity of the original uranium ore used to fuel the reactor. Thereafter all radiotoxicity is dominated by actinides. The radiotoxicity of the actinides in turn, is dominated by the type of fuel cycle being used.

The once-through uranium cycle to which the United States remains (nominally) committed since the days of Jimmy Carter, leaves the most residual radiotoxicity, which remains higher than natural uranium for about 10 million years. After 10,000 years, in the once through cycle almost all of this toxicity is associated with neptunium-237 that arises from the decay of plutonium-241 and its daughter, americium-241. In the once through cycle, the radiotoxicity peaks about 200-300 years after shutdown, where it is about 900 times that of natural uranium used to fuel the reactor.

Plutonium-241 however is an extremely fissionable nucleus. In plutonium recycling schemes, much of it is fissioned, and thus it is less of a problem. The formation of Am-241 and Np-237 is greatly reduced long term. Of course, the plutonium-239 is also fissioned under these circumstances. Since the radiotoxicity of fission products dominates only for a few hundred years, and further since an equilibrium is established because of the physical nature of the problem - the simultaneous presence of a formation reaction (fission) and a decay/transmutation set of reactions - the fissioning of the actinides reduces the radiotoxicity enormously. It can be shown from solving simplified versions of the fuel depletion equations that the accumulation of fission products asymptotically approaches a maximal value, after which no further accumulation is possible at a fixed power level. I will post a link in a subsequent post showing graphically the radiotoxicity lifetimes of various fuel cycles. (I have taken my discussion here from my favorite nuclear reactor physics text, Stacy's Nuclear Reactor Physics, Wiley, 2001, page 223-239 (I often use this text for my discussions here.)

The monorecycle of plutonium (recycling plutonium only one time) results in total reduction of the radiotoxicity of the planet to that below that of natural uranium in about 1000 years. Some cycles with multiple plutonium recycling, and the recycling of minor actinides (for instance including 1% Americium in the fuels) achieve this happy result in a few hundred years. All recycling schemes of course enjoy better resource utilization and much less mining. What they do not necessarily enjoy is better economics, although this consideration will only last for a few more decades. If humanity survives global climate change, of course, the economics of uranium will change in such a way that recycled fuel will become competitive with mined uranium, including that recovered from seawater.

Note that radiotoxicity is determined on the basis of a person deliberately eating radioactive materials. It does not account for the low probability that one will actually be able to eat these materials even if one wanted to do so. For instance, if one wanted to eat spent fuel at Yucca Mountain, one would need to dissolve a lot of borate glass to do it, after first drilling into the repository. One would also need to bring tools to break the drums open. As a practical matter, that ain't gonna happen. As for the matter happening unintentionally, I note that the materials have to move a great distance under essentially chromatographic conditions, and then find their way to the surface and into food or water or air. That also ain't gonna happen realistically.

[Dead_Parrot]

12. well, I said it was convoluted...

:D

I can't think of any real-life circurcumstance where disposal of ²⁶Al (or it's escape into the enviroment) is going to be a burning issue. Except at Greenpeace, of course... :evilgrin:

Thanks for the kaeri link, btw. I could loose a lot of time in there... :)

[Javaman]

13. I wonder what the next evolution of human will think in 40,000 years...

when they unearth that place?

[NNadir]

14. Why are you so sure they will unearth it?

Do modern day Europeans deliberately dig up every lead mining tailing dump from the Roman era?

I suspect that if Yucca Mountian is built, future generations, will, in fact be inspired to dig it up, especially if the current plans go through and they know what is in there. In fact, I'll be that it will take no where near as long as 40,000 years. I predict it will happen within several hundred years, if not sooner.

This is because they will need the resources therein. They are likely to be worth a lot of money in the not too distant future.

I expect they will be highly annoyed that the chemical form of the spent fuel will be so stable and intractable, since this will complicate their recovery and raise their costs.

Unquestionably they will shake their heads at our stupidity in just not using the stuff in the first place. I imagine they will have lots of other reasons to hate us though. Probably their most cogent hatred will focus on the fact that we were so obsessed with so called "nuclear waste" that we completely ignored the problem of the far more dangerous waste carbon dioxide. For that, they will probably never forgive us, nor should they.