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

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...

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.

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.

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: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

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