Saturday, December 20, 2008

Cons to nuclear power

Okay. I'm a month or so late... So shoot me.

Why should we NOT increase our use of nuclear power?

There are a number of arguments against nuclear power. The list below is not in any particular order.

1) cost
2) waste
3) proliferation
4) accidents or sabatoge

1) Cost: This is---at best---an inane argument, IMO. We can spend the money now on new technologies or we can spend the money later on health care, global warming mitigation, etc. Saying "it's too expensive to build or develop new nuclear power plants," is just whining. Yes, I am perfectly happy to be dismissive of and condescending to anyone arguing against nuclear power on the basis of cost.

2) Waste: This is the best argument (IMO, of course) against nuclear power.

2A) To get the uranium ore, we must mine for it. This creates long-lived contamination around the mining site. The uranium tailings contain over a dozen radioactive species, including thorium-230, radium-226, radon-222, and polonium-210. These are all hazardous materials, of course. If, as in past practices, the radioactive sand is left on the surface, it could be blown about by the wind, washed away by rains, etc. There is not actually a high concentration (compared with other nuclear waste materials) of hazardous materials by mass, but there will always be a large mass of tailings from any given mine. The most serious and probable human health hazard associated with the tailings is lung cancer caused by inhaling these radioactive products.

Anti-arguments: Here's the thing, though... The most dangerous, common rock, in terms of radioactivity, is granite. Your granite tabletop (if you're so lucky) is continuously producing radon gas. It's not much, and it's certainly not a health concern. Your drywall plaster is probably made of calcined gypsum. That's releasing more ionizing radiation than your granite counter top, and it's in every room of your house. It's still not a health concern. All the portland cement you see and use (home foundations, sidewalks, some freeways, buildings, etc., etc., etc.) are emitting as much or more radon as fly ash from coal-fired power plants. The uranium tailings from old mines are certainly dangerous, but new techniques in dealing with the tailings are in place, and even better ones are being developed. That's a pretty hand-wavy argument that the waste from mining is "safe." It's not safe. Neither is burning coal. Here is a paper that argues that the radiation dosage from mine tailings isn't terribly dangerous. That's only one side of the story, of course. This paper estimates the dosage someone living around a coal-fired plant will be exposed to. This paper discusses the comparison between coal and portland cement sources of radon. We see more cancers from pollution and cement-derived radon in our homes than we do from mine tailings, and increased nuclear power generation will only decrease the overall number of cancers, even if we continued to use the worst methods of dealing with uranium tailings.


2B) To get fissionable material (U235) from the uranium ore (uranium oxide, "yellow cake"), it must go through enrichment. (Stolen from here): Natural uranium is composed of 0.72% U-235 (the fissionable isotope), 99.27% U-238, and a trace quantity 0.0055% U-234 . The 0.72% U-235 is not sufficient to produce a self-sustaining critical chain reaction in U.S. style light-water reactors, although it is used in Canadian CANDU reactors. For light-water reactors, the fuel must be enriched to 2.5-3.5% U-235. Uranium is found as uranium oxide which when purified has a rich yellow color and is called "yellowcake". After reduction, the uranium must go through an isotope enrichment process. Even with the necessity of enrichment, it still takes only about 3 kg of natural uranium to supply the energy needs of one American for a year.

The byproducts of enrichment are basically depleted uranium (DU, which is pretty much just U238) and uranium hexafluoride gas (UF6). U238 is relatively safe, with a half-life of about 4.5 billion years, via the path of alpha decay. Alpha particles are just helium nuclei. They can be dangerous when ingested or inhaled, but are pretty much harmless externally (alpha particles are stopped by a single layer of dead skin cells). That's not to say that alpha decay is harmless. Radon gas, polonium 210, etc. are implicated in cancers and other nasty deaths if inhaled (Pu210 is thought to have been the material used to poison Alexander Litvinenko). U238 is also chemically toxic, and can have seriously bad effects on the liver, once again if ingested. U238 is now being used in a process called "downblending." I'll discuss downblending in the proliferation discussion. UF6 is a different beast altogether. UF6 is an extremely unstable waste compound that's not easy to store or handle. Here you can find a 118 page PDF on safe handling procedures for UF6. I haven't read it. Let it suffice to say that UF6 is mostly bad because of its chemical (not nuclear) toxicity. The uranium part of UF6 is obviously radioactive. The depleted UF6 (with mostly U238, not U235) is stored as waste. Any amount of water mixed with UF6 will turn it into a rather nasty acid, so the storage of UF6 is a difficult task.

I cannot argue against the dangers of depleted UF6... It is as dangerous as any other toxic chemical used in other manufacturing processes. Since it also contains U238, it's scary to a lot of people. I'm not any more concerned about UF6 than any other toxic chemical (melamine?) that is moved around this country and others.

2A and 2B are the wastes from the processing and creation of nuclear fuel. There are wastes that come out the back end of the nuclear power generation process.

2C) Spent fuel rods contain fisson products that emit beta and gamma particles. They also contain actinides that emit alpha particles. These products include uranium-234, neptunium-237, plutonium-238 and americium-241, and even sometimes some neutron emitters such as californium (Cf). None of these are pleasant products. Some are so radioactively hot that they're thermally hot.

Many countries, the US excluded, reprocess these products to extract any left-over U235 for re-use as a nuclear fuel. This increases the concentration of the radioactive products in the waste. It also increases the concentration of the toxic chemicals used to process the materials. The US just stores the waste as-is, without reprocessing. One can argue either way about what to do with these products. Since we're in the US, I'll briefly mention Yucca Mountain.

Some geologists have certified that Yucca Mountain would be a safe place to store these waste products. Others have argued that it is not safe. I'm not sure who is correct. I'm not sure how dangerous these products are in the long term. I am sure that if we were to spend some money paying scientists and engineers, they could figure out a workable solution. It would not be ideal, it would not please everyone.

So, waste is the biggie. It's not solved, but it's not nearly as bad as many people think.

I think this post is long enough for now. I'll post arguments 3 and 4 later...

5 comments:

Red Craig said...

What an excellent article! It's hard to put all this in perspective but you've done it and in a small space as well.

One point I didn't get was about UF6. Couldn't that be reacted into U308 and some innocuous fluoride compound?

I am Moses. said...

Thanks! I just stole information from others. :)

U3O8 (O as in oxygen) is yellowcake (uranium oxide), which is the original source of the uranium used in the enrichment process.

See
http://www.chemistrydaily.com/chemistry/Uranium_hexafluoride

For the below paragraph.

Milled uranium ore -- U3O8, or "yellowcake" -- is dissolved in nitric acid, yielding a solution of uranyl nitrate UO2(NO3)2. Pure uranyl nitrate is obtained by solvent extraction , then treated with ammonia to produce ammonium diuranate (ADU ). Reduction with hydrogen gives UO2, which is converted with hydrofluoric acid (HF) to UF4. Oxidation with fluorine finally yields UF6.


The UF6 is then used in the gas diffusion process to separate the U238 from the U235. What's left is nearly pure U(238)F6. I'm not aware of a safe way to dispose of (chemically or otherwise) UF6.

When UF6 reacts with water, it creates hydrofluoric acid (HF) and UO2F2, Uranyl fluoride. HF is a weak acid. UO2F2 is a pretty toxic material and inhalation or ingestion is likely to cause death long before the radioactivity can harm you.

Red Craig said...

Oh.

Well, the uranium that goes into fuel rods is some kind of oxide. I'm thinking that if UF6 is highly corrosive then it ought to be easy to treat so it is less chemically reactive. Making stuff less chemically reactive is probably the easiest thing chemists have to do.

I am Moses. said...

Right. UO2, uranium dioxide, is the oxide in the pellets.

After some more digging and more careful reading, this is what I understand is happening, and I see that there is a way to deal with the UF6 without having to store it. Of course, there are still problems, but...

First, UF6 is produced as discussed earlier. During the gas diffusion process, UF6 is separated into two forms: U(238)F6 and U(235)F6. The U(238)F6 is depleted UF6 (let's call it dUF6). dUF6 is no longer useful in the nuclear fuel cycle, but is still toxic.

U(235)F6 needs to be chemically decomposed into the final form of UO2 for use in the nuclear reactor. I haven't found the full chemical path for this yet...

However, UF6 (in either form) can be decomposed into U3O8 via the reaction:

UF6 + 3 H2O --> 1/3 U3O8 + 6HF + 1/6 O2.

This is done in a water vapor plasma, so at very high temperatures (1050--1400 K; 1430--2060 F).

It's not clear to me why this is not used to decompose the dUF6 into a more stable compound (U3O8). Perhaps this is the method of dealing with new dUF6 as it is produced; we still have many containers of dUF6 in storage around the country.

Again, one of the chemicals produced from decomposition of UF6 is HF. While HF is extremely corrosive (it dissolves glass), it has many uses (pharmaceuticals, teflon, etc.). Unfortunately, it is also considered a contact poison (it'll dissolve your flesh on contact, and your body will absorb it); contact can lead to amputation if not medically treated immediately [wikipedia].

Here's an interesting article on the question of dUF6 treatment:
http://www.earthisland.org/yggdrasil/duf6.html

The biggest problem with any scenario for the disposal/storage of dUF6 seems to be the likelyhood of HF release into the environment.

Grumpator said...

Thanks for pulling this together - I know how much time a well-researched blog post can take! I don't blame you for the lateness of it. Can't wait for part 2!