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

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.

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.