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.

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...

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.

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.

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: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?

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.