Disposing Of Nuclear Waste As Nuclear Fuel

Saige writes "Nuclear waste has been a contentious issue, recently culminating with fights in the government over Yucca Mountain in Nevada as a proposed storage site. Well, perhaps there's a better way to deal with nuclear waste -by using it in nuclear reactors. A nuclear scientist at the University of Maryland, has come up with CAESAR, a reactor that runs not on the standard U-235, but on U-238. U-238 makes up most of the fuel rods in current reactors, but doesn't contribute to the reaction, and ends up currently as waste." The Yahoo! story linked from this article doesn't seem to open, but here's a story at The Economist.

Comments

Darn links...

[Saige]

Damnit! I accidentally left the wrong link in the original paragraph. I meant to include the Economist link, but I was submitting this to Plastic.com and Slashdot both, and managed to cut and paste the wrong part from Plastic here... *sigh*

If anyone cares, the link to Plastic wasn't what was intended and can be safely ignored.

Cool, but isn't the real problem...

[bscott]

(caveat - I haven't had time to read the article yet, so I'm spouting off without much backing to my opinions, except that I live near Rocky Flats...)

Isn't the real "nuclear waste problem" not just the fuel rods, but the kilotons of contaminated building materials, protective clothing, screwdrivers, air ducts, semi-trailer trucks, topsoil, reactor coolant, baseball caps, human remains...

I'm sure this is a great advance for many reasons, but it's barely gonna scratch the surface of how to deal with contaminated material - or am I wrong?

Re:Cool, but isn't the real problem...

[MacAndrew]

IIRC, that stuff called LLRW (low-level radioactive waste) is not that big a deal, in either strength or half-life radioactivity. It is a lot of volume, and can be toxic, but the really nasty long-lived stuff that must be sequestrered carefully is mostly products of the pile itself, like radioactive plutonium and strontium and so on.

It's fairly hard to make something radioative by exposure. The LLWR is largely stuff that has come into contact with radioactive material, as in processing, hence comtamination.

The biggest problem with LLRW is political -- people don't want it in their back yard. And I don't blame them -- given a choice of your yard or mine, I'd pick yours. :) But the health hazard is exaggerated.

Re:Cool, but isn't the real problem...

[liquidice5]

True

Half Lives:
U-235 = 7.037 x 10^8 years
U-238 = 4.468 x 10^9 years
U-239 = 23.47 minutes
Th-90 (from U-234) = 7.5 x 10^4 years
Ra-88 (from above) = 1.6 x 10^3 years

Strontium:
90 - 28.5 years
94 - 1.235 seconds

Here is a picture similar to to the one in my Physics book on the decay of Uranium(its life forever until its stable)http://www.physics.umd.edu/deptinfo/facilit ies/lecdem/honr228q/notes/U238scheme.gif

From this Page
http://www.physics.umd.edu/deptinfo/facilities/lec dem/honr228q/notes/notesl.htm

Re:So what does the waste turn into?

[TamMan2000]

No, it will be converted to much smaller atoms, so it stores the waste, extracts energy from it, and changes it's form. When CEASER is done with the rods, they will still need to be disposed of carefully, but the energy derived per unit of waste goes up greatly.

So... there is less waste for the same amount of electricity, this is the advantage...

Re:So what does the waste turn into?

[elmegil]

that doesn't make sense. The uranium has to split to generate energy, how can it be more uranium?

Weapons

[MacAndrew]

I think the really interesting thing here is that the reactor cannot be used as a breeder. That would make it an excellent candidate to require nations like North Korea to switch to. (If you think that's the way to go. NK is VERY energy poor now that the Russians no longer send oil. 60 Minutes did an excellent piece on them last night.)

All reactors IIRC produce some plutonium, from bombarded U-238 (virtually all Pu is manmade). Breeders produce a lot. The "waste" which is U-235 depleted but plutonium enriched must be further processed to produce weapons-grade material. For 25 years we have banned reprocessing even to the level needed for use as fuel because of the concern is could be stolen and further enriched. Some countries like France and Japan disagree and do reprocess. The scare in Japan last week illustrates the risk. Most people here would agree there' no such thing as perfect security, esp. with the universal hazards of corruption, accident, and incompetence.

Even if the thieves were unable to purify the material, it would make excellent "dirty bomb" material. Pu is not especially radioactive, absent heavy chain reaction, but it is very toxic and dangerous to ingest or inhale where it might lodge and expose sensitive tissue to prolonged damage.

It's a shame nuclear policy is so constrained by weapons issues.

Re:Weapons

[DuckDuckBOOM!]

I think the really interesting thing here is that the reactor cannot be used as a breeder.

...huh? Quote from The Economist article:

By adjusting the configuration of the core in the right way--by judicious positioning of graphite, for example--almost any civilian reactor can be made to produce plutonium, and thus to make weapons. Access to the core is not necessary with CAESAR, as it could run for decades without any need for refuelling. Thus it could be sealed. Countries could then adopt the design to show that their nuclear intentions were entirely peaceful.

The way I read this, the difference with CAESAR is that the fuel doesn't require periodic replacement, so the reactor could be sealed to prevent, e.g., terrorists from making off with plutonium-laden spent fuel. The key word here is "could". Nothing appears to prevent a gov't from designing a CAESAR to allow harvest of plutonium, or from covertly retrofitting a sealed reactor. (Granted, it almost certainly wouldn't produce as much Pu as quickly as would a fast breeder, but those are purpose-built devices that the nuclear powers would discover & stop in a heartbeat. And you don't need all that much Pu to build a bomb.) The sealed-fuel concept would work well if the government controlling it were trustworthy, or if verification methods were foolproof. Unfortunately, in the case of an Iraq or a N. Korea, those are mighty big IFs.

Re:Weapons

[DuckDuckBOOM!]

I'm picturing a great big seal....

A REALLY big seal, with sharp teeth and attitude...kinda like an elephant seal after exposure to some of those spent fuel rods... :)

I agree that it would probably be easier to detect tampering with a sealed reactor, if only because they'd need a fair amount of time & equipment to crack it, and you could build some of the detectors inside the sealed part where they'd be hard to defeat. The solution N Korea used - throw out the inspectors, shut off the cameras, ignore the griping until you've accomplished what you want to do - does, though, make me a tad nervous re the viability of any preventive measures that have to be carried out on foreign soil. Personally I'd have preferred we built the nukes in S. Korea, and ran a big power line across the DMZ.

Re:Weapons

[MacAndrew]

I don'y usually respond to a response to a response, but...

I like your idea of delivering power from SK. It's probably practical -- power in the U.S. is sold over very long distances. The problem is that NK would not accept it, not least because it would give us a big on/off switch to push them around with.

Re:So what does the waste turn into?

[TamMan2000]

The steam would be used to extract the energy as well. You would control the whole thing, by how you let water in and steam out, then you can control the density of the moderator, and through that the rate of the reaction, and the energy release.

The energy release would modify the steam, but only it's temperature or, if it is at the right temp... it's water fraction, but this would probably not effect the moderation greatly since the mass of the steam is unaffected by temperature...

Sorry for the pun

[moc.tfosorcimgllib]

If they are going to use Radioactive waste as an energy source, more power to them.

Great idea!

[Tuxinatorium]

This is a great idea, but thanks to the morons in washington, reprocessing spent nuclear fuel to recover useful fuel like plutonium has been illegal in the U.S. since the 1970's because they're paranoid that reprocesses plutonium could fall into the wrong hands and result in nuclear proliferation. So instead, instead of reprocessing the waste and thereby getting many times more energy out of the same amount of mined uranium, they store all that stuff underground. Personally, I think if we recycled the damn stuff it would be less likely to fall into the hands of terrorists because there wouldn't be so much nuclear waste crap everywhere. Which do you think is going to be more heavily guarded: a buried nuclear waste dump or a plutonium reprocessing facility. The U.S. law against reprocessing is idiotic and terribly wasteful.

Wow.

[DuckDuckBOOM!]

If there's anything to this, it's simultaneously VERY big and VERY scary. Enriching uranium, even to power-plant specs (only 5% or so U235) is extremely difficult, expensive, and both physically & environmentally hazardous, not to mention that the end product must be replaced every few years (resulting in tons of high-level waste) as that small percentage of U235 is "burned up". A workable CAESAR would eliminate nearly all of this, drastically lowering the cost of building & fueling reactors while increasing their fuel supply by a factor of 98 or so. (This of course excludes the artifically inflated costs of the nightmarish regulatory/legal labyrinth builders/operators must run in many countries.)

The dark side of all this is, of course, that a lowered cost of entry makes it just that much easier for "nuclear club" wannabe countries to produce plutonium for less benign applications. The author of the Economist article notes that countries could seal their CAESAR reactors (thus, I assume, burning the created plutonium for power alongside the U238) "to show that their nuclear intentions were entirely peaceful." Yeah, right. I'm sure Saddam Hussein and Kim Jong would be perfectly content to have their CAESARs crank out power, with nary a thought to the goodies sealed therein.

Yet another two-edged sword, but a damned intriguing one.

Well....

[MacAndrew]

Our policy doesn't look so idiotic after what happened in Japan last week, with North Korea right in their back yard and well-known for its infiltration missions. In the U.S. we may feel more secure, but we've had plenty of nasty surprises in the last few years.

Stolen reprocessed plutonium would be very useful for a dirty bomb. It would have to be enriched before making a bomb, but it would be a headstart and obviate a nucleare reactor to make the Pu.

I don't think the problem is that we're overflowing in nuclear waste. The problem is we've had so much trouble confronting where exactly to put the stuff because of political opposition. Countries like Japan and France have far less oil, and this desperation makes the nuclear power sacrifices less daunting.

Old nuclear fuel is sequestered in ceramics and buried. There is not that much need to guard it. Nuclear fuel grade uranium is only 2-3% enriched U-235 to begin with, so its a -long- way from being particularly useful. Old weapons-grade Pu is mixed with material like U-238 to make it useless. With Iraq or N. Korea, the big challenge they face is not getting material, but purifying it.

So ... I think our policy is debatable but not idiotic.

Building a Reactor Using U-238 Is No Big Deal

[kmellis]

...but building a reasonably efficient version for power generation probably is.

At any rate, Fermi's reactor in Chicago (the first) used natural uranium (almost all U-238) as fuel. There wasn't any other choice. Enrichening U-238 to higher quantities of U-235 is a really big deal. Natural uranium contains only about one half of one percent U-235. Fermi's design used highly purified graphite in a honeycomb pattern as the moderator. The Russians, before they got the plans for our reactor, looked at a U-238 design that used heavy water as the moderator (the Germans were going that way, too). Anyway, as this guy has shown, it's all about finding the correct moderator in the right configuration.

That was three questions

[Spamalamadingdong]

So what does the waste turn into?

Unfortunately, the article is so deficient in technical details that it's impossible to answer that question without quite a bit more information. As just one example of how ill-written the article is, there is no explanation of how the reactor is supposed to accomodate the accumulation of neutron-absorbing fission products over its multiple-decade period of operation.

What exactly does the U-238 become after all this?

It becomes fission products. Some of the nucleons (protons and neutrons) become free neutrons which are not absorbed before they beta-decay to protons (hydrogen nuclei).

Wouldn't the steam be affected by the extreme heat?

That depends how extreme the heat is. You will also have some radiolytic decomposition of the steam, and everything else in the reactor. The displacement of atoms within metallic crystals causes "radiation embrittlement", which will put a limit on the run-time of such a reactor even if the fuel is effectively infinite.

Fission products are lighter nuclei which result from the fission of heavier ones. Some fission products are themselves radioactive, some are not. Pretty much all of them are useless as nuclear fuel.

Radiolysis is the radiation-induced breakdown of chemical compounds. A gamma ray or a fast neutron has more than enough energy to smash a water molecule apart, and this process will produce free radicals such as hydrogen and hydroxyl ions. If those radicals get together, you can get products such as hydrogen gas and hydrogen peroxide, and hydrogen peroxide decomposes pretty quickly to oxygen and water again.

You'd be better off reading an intro on the web, but I hope this whets your appetite for more learning.

Re:So what does the waste turn into?

[Gruturo]

that doesn't make sense. The uranium has to split to generate energy, how can it be more uranium?

The point is not "more uranium". It is about putting U-238 (which is normally considered useless) to a productive use, instead of its more appreciated U-235 brother, which is much less common (0.7%). This way, instead of having to be disposed of as waste, it is used (and at the end of the process you have less waste than you started with).

In nuclear processes, Uranium does not produce more uranium :-), it either gets split into krypton and barium, or absorbs a neutron and through some intermediate decays becomes plutonium.

(apart from the other, naturally-occurring radioactive behaviour of uranium: it emits an alpha particle, becoming Thorium, which then becomes Radium, which becomes Polonium, which finally becomes Lead (not radioactive - it doesn't become anything else)).

Enough with the misconceptions already!

[Spamalamadingdong]

Breeders produce a lot.

Well... no, not really. I'm told that near the end of a fuel cycle, a conventional pressurized water reactor (light water, not a CANDU) is producing the majority of its power output from plutonium fission. The breeder's claim to fame is that it can breed more fissionable fuel than it burns.

The "waste" which is U-235 depleted but plutonium enriched must be further processed to produce weapons-grade material.

Well... no, not a bit. Spent PWR fuel contains quite a bit of plutonium, but it is essentially useless for making bombs. A PWR cycle lasts a couple of years, more or less, and bombards the fuel like mad. U-238 absorbs neutrons and becomes U-239, which beta-decays to Np-239, which beta-decays to Pu-239. While some of the Pu-239 gets fissioned further down the line, some more of it captures a passing neutron and doesn't fission. It becomes Pu-240, or even Pu-241. These are isotopes with very different half-lives (much shorter) and much higher spontaneous fission rates.

This is all-important for making a bomb. U-235 has a half-life of around 700 million years, and making a bomb with it is easy: squeeze together a prompt-supercritical mass, and wait a few milliseconds. Pu-239 is tricky, because its half-life is only about 25000 years and you have very little time to get it into a prompt-supercritical configuration before a spontaneous fission starts the reaction going. If the reaction starts too soon, the bomb blows itself apart into a sub-critical configuration before releasing much energy and all you have is a fizzle. Now imagine dealing with a substantial fraction of Pu-240 (half-life 6564 years or Pu-241 (half-life 14 years).

Bomb-grade material is made in special reactors which allow the fuel to be irradiated relatively briefly at a low level, and then removed and processed to remove the plutonium. This is specifically to avoid the production of enough higher isotopes of plutonium to be a problem. The stuff coming out of a power reactor after a full fuel cycle is dirty as hell, but amateur proliferators are not going to be able to make a serious bomb (as opposed to dirty weapon) out of it. This is why we had few objections to building pressurized-water reactors for North Korea; they are essentially proliferation-proof.

For 25 years we have banned reprocessing even to the level needed for use as fuel because of the concern is could be stolen and further enriched.

I doubt that it's quite that simple. The real problem is that the plant required to refine fuel-grade Pu from spent power reactor fuel uses the exact same chemical processes as the plant which refines bomb-grade Pu from depleted uranium rods held briefly in a neutron flux for transmutation purposes. If you have a world full of people reprocessing it would be very hard to put a finger on the ones who are making weapons, so the US decided we had enough uranium to put the kibosh on all reprocessing just to set a good example.

I think we should have gone with the Integral Fast Reactor, but it seems to have succumbed to the fundamentalist anti-nukes (who probably couldn't figure out that there are medical and explosive grades of nitroglycerine either...).

Actually, it is a very big deal

[Spamalamadingdong]

Fermi's reactor in Chicago (the first) used natural uranium (almost all U-238) as fuel.

But the only part that was actually producing energy (fissions) was the 0.7% which was U-235; the 99.3% which was U-238 was just along for the ride (and eating up the occasional neutron).

There are ways to get energy directly from fission of U-238, but they require very fast neutrons such as are created in a deuterium-tritium fusion reaction.

The Russians, before they got the plans for our reactor, looked at a U-238 design that used heavy water as the moderator...

Then the Canadians must be smarter than the Russians, because the Canadians actually did it.

Re:So what does the waste turn into?

[liquidice5]

We just went over this in Physics, so I will give it a shot..

you dont start out with Pure U-235, only about 4%,
(they get the percentage this high be making a Uranium gas, puting it in a large column, and skimming the lighter U-235 off the top)

So, they still have lots of other stuff, including other types of Uranium.
(only 9.7% of natural Uranium is U-235)

only the U-235 is broken down by the neutrons that the current moderator, water, creating other radioactive elements such as Ba-141 and Kr-92 plus 3 more neutrons, or Xe-140 and Sr-94 plus 2 neutrons.

There is still alot of Uranium in the fission fuel, just not the right kind

(the "235" in U-235 represents its atomic weight, all Uranium has 92 protons in its nucleus)

(you subtract 92 from 235 to find the number of Neutrons in a U-235 atom)

(in order to get Plutonium, you start with U-238, add a neutron going the right speed, then you get U-239*[*=excited], which then beta decays, making a beta particle, a Np-239* and a neutrino. The Np-239* then beta decays and you get Pu-239, a beta particle and a neutrino. This is called "breeding" and is how nuclear bomb fuel is created)

Safer too

[HalfFlat]

IANANP but ...

It seems that this design not only allows the complete removal of the whole enriching process, and the elimination of a good percentage of nuclear waste, but it seems safer too.

The fission is only sustainable when the neutrons are just the right speed, which in turn can only happen when the steam is at just the right density. If anything goes wrong (e.g. steam escapes, etc.) the density changes and the whole chain reaction fissles pretty much to a halt.

However when a good percentage of the U-238 has formed Pu-239, would this level of control still hold? (Thinking that Pu-239 undergoes fission with a wider range of neutron energies.)

Re:But misconceptions fuel great arguments!

[MacAndrew]

Thanks, that's interesting. Of course, it conflicts with a lot of what I've read, but a lot of what I've read conflicts, too. :)

My understanding from many sources is that a breeder can produce material of 20-30% Pu. Yes, it "can breed more fissionable fuel than it burns" but that new fuel is (as I understand it) exactly the Pu-239 we fear. All Pu comes from reactors, anyway, it's just a question of technique, esp. removing the material after a brief bombardment by appropriate-speed neutrons.

Bombarded Pu-rich reactor fuel is not the only problem, there's also the fuel-grade Pu after reprocessing. I've seen a couple of accounts of fuel-grade Pu bombs detonated, and I assume if one had the facilities to purify the fuel it would be even easier.

There are serious technical hurdles to engineering the actual bomb, but here we want to deny them even the fuel. Plenty of countries have surmounted the techincal end, anyway, such as Pakistan. Even a sloppy detonation would be bad enough. BTW, I'm not thinking about terrorists, unless they somehow stole a complete weapon. They'd take the surer low-tech path of a dirty bomb or flying a plane into a building, etc. Terrorists with nukes are a Hollywood thing for now.

On policy, here is a rather different account of why we don't reprocess -- economics. According to this account, Reagan vacated the Carter order in 1981. Truth?

Seems like a lot of trouble

[robert0122]

Couldn't we just compost the nuclear waste? ;-)

Precisely

[Spamalamadingdong]

And the neutrons which caused the U-238 to fission did not come from the U-238, but from the D-T fusion reaction. This is why I'm skeptical about this U-238 reactor; I have no information to indicate that the neutron spectrum emitted by fissioning U-238 is capable of sustaining a chain reaction at all.

Similar to Boiling Water CANDU

[radtea]

IAANP, and from what I can tell from the article, the design looks similar in concept to the boiling water CANDU, which didn't quite live up to its promise.

There are a couple of problems with steam-moderated designs:

• maintaining a given density is not trivial
• steam pipes tend to erode

The first problem is a polite way of saying there may be reactor stability issues. If CAESAR uses super-heated steam this may be less of an issue, but otherwise the ratio of steam to liquid in the cooling circuit is a function of pressure and temperature in ways that can create problems if there is an unexpected excursion.

The second problem is a major issue, especially when coupled with the long-term affects of radiation on materials. Intense neutron bombardment is a good way to introduce defects in metallic lattices. Defects are a good place for corrosion, cracking and other bad things to start. Ergo, the odds of a sealed reactor lasting for more than a decade or so are not good. Retubing of reactors is an ongoing maintenance problem even in conventional designs.

Nuclear power is an option that we may in the end decide we have to go with, and it's good that advanced reactor designs continue to get consideration, but the engineering challenges are still severe and the proliferation potential is large.

--Tom

CAESAR page at U of M

[toddzilla]

Here's the U of Maryland page on the CAESAR project.

http://www.caesar.umd.edu/

Separating plutonium is not remotely comparable

[phr2]

to separating uranium isotopes from each other. Getting the U235 out of natural uranium is so difficult because U235 and U238 are the same chemical element; any separation process has to distinguish the isotopes based on the small difference in atomic weight. That means centrifuges, gaseous diffusion, lasers, and other cumbersome means.

Separating plutonium from U238 in spent reactor fuel is much easier. Plutonium and uranium are different chemical elements and can be separated by chemical processes. It's not something you can do in your kitchen, but atom bomb designer Ted Taylor, in John McPhee's excellent book The Curve of Binding Energy, compares its difficulty to that of building, say, a large scale drug lab.

We know perfectly well that criminal organizations manage enough chemical engineering to produce refined heroin and cocaine by tens of tons even without any governments supporting or protecting them. Separating Pu from U in quantities of only a few kilograms, as an official project of the local rogue government, appears quite achievable in the face of that knowledge.

Re:I don't pretend to understand how...

[Drishmung]

Probably not. It's likely responsible for heat in the mantle though. This Scientific American article gives more detail.

The core is hot because... it hasn't cooled down yet. The earth is a very poor conductor of heat, so it takes a very long time for the heat at the core to radiate out to space. I remember doing the math as an excerise to show that the 4Gy age of the earth was not great enough to account for a significant radiative transfer of heat from the core---which is why volcanos and such have to get their energy from somewhere else. Friction and fission being the most likely candidates.

As to safety, well, let us rather look at cost effectiveness, for total lifetime cost. Against the value of the energy produced, set the cost of:

• Building the reactor
• Fueling the reactor
• Running the reactor
• Maintaining the reactor
• Assuring 'other people' that you are not making bombs
• Disposing of the waste
• Decommisioning the reactor when its done with

The last three items are not usually significant with 'conventional' energy sources. Or rather, it's not nearly as expensive to render acceptable the output of an oil-fired generator as it is radioactive isotopes with ky half-lives.

So, which is cheaper? Note, I don't know. There is so much mis- and dis-information on both sides it's tough to decide.