Byproducts of uranium enrichment

March 16, 2006 10:44 AM

I'm being told that cesium and strontium are by-products of the enrichment of plutonium, which is a step beyond what takes place in a uranium-enrichment plant. True or false?
posted by tizzie to science & nature (8 comments total)

No, strontium and cesium are byproducts from the fission that occures inside a reactor or released from a nuclear explosion. (lots of google results)
posted by furtive at 11:46 AM on March 16, 2006

Plutonium is not naturally occurring, so you would have to isolate it from the fission products of a nuclear reactor. The fission products would also include cesium and strontium. I don't think 'enriching' is quite the right term for isolating plutonium from the fission products though..

Here is a pretty interesting Scientific American article related to your question - Smarter Use of Nuclear Waste.
posted by Chuckles at 11:53 AM on March 16, 2006

IANA(nuclear physicist), but plutonium, unlike uranium, is produced only in reactors of some kind; it's a synthetic element. The spent fuel rods from a power reactor contain a mixture of all sorts of elements and isotopes, because when a uranium nucleus fissions it leaves behind fragments which are lighter elements, and the neutrons (also produced in fission) can further transmute these. Anyway, when you recycle a spent fuel rod, basically you sort out its constituent elements. Many of them are inert, and can be safely discarded; many of them are useful, as radiation sources (eg in medicine) or as more reactor fuel. And any reactor is going to produce some amount of plutonium, which is quite useful as reactor fuel, and also, unfortunately, as an explosive.

This is, as I understand it, the reason that the US doesn't recycle its spent fuel and instead leaves it in holding tanks where it can leach into the groundwater: the recycling operation would produce some plutonium, and there's the fear that some terrorist would steal some and make a bomb.

Cesium and strontium can be extracted from things other than reactor waste (both elements were known before anyone built a reactor), but maybe some specific, useful isotopes of them mostly come from reactor waste.
posted by hattifattener at 12:03 PM on March 16, 2006

The word "enrichment" doesn't apply to plutonium production.

Naturally occurring Uranium consists of 0.6% U-235 and the rest is U-238. U-238 isn't fissionable and in fact is almost not radioactive; it's U-235 which does the work in a power plant or a bomb. But chemically they're both uranium, and for most purposes 0.6% U-235 isn't a high enough proportion to be used. (There's a Canadian reactor design which works with natural uranium but most plants won't.)

So they have to increase the percentage of U-235, and that's what "enrichment" refers to. There are several ways this can be done but they all involve creating uranium hexaflouride gas and taking advantage of the slight difference in weight between U-235 and U-238.

Production of plutonium is entirely different. What you do is to create a reactor which produces a lot of neutrons, and then in most designs you use cooling water rich in deuterium to slow the neutrons down. U-238 is placed around the core and hopefull individual atoms will absorb a neutron and convert to U-239.

U-239 then undergoes a pretty rapid two-step beta decay and gets converted into Pu-239, which is fissionable. The problem is that it also is much better at absorbing neutrons than U-238. So if you let this process run for too long, what you get is an increasing amount of Pu-239, but also Pu-240 and Pu-241, which are not fissionable.

What you have to do is to pull it out after not too long, and then dissolve it with a powerful acid (like sulfuric). Because plutonium is chemically different than uranium, it's possible (although not easy) to separate it from the uranium without messing around with mass spectroscopy approaches. If you are careful about how long you leave the U-238 exposed to the neutron flux, what you get is a small amount of Pu-239 but almost no Pu-240 or Pu-241.

But that process of separating plutonium from uranium isn't referred to as "enrichment". And in any case it doesn't produce cesium or strontium.

Cesium and strontium are produced by fission, which takes place in the core which is producing the neutrons. But they'd be produced even if no plutonium was being deliberately manufactured.
posted by Steven C. Den Beste at 12:09 PM on March 16, 2006

hattifattener, the reason we don't reprocess spent fuel rods is that the plutonium in them is a mix of Pu-239, Pu-240, and Pu-241, and the latter are generally present in sufficient quantities to "poison" the plutonium and make it so that it won't fission. Separating the Pu-239 from the others requires an approach similar to enriching uranium, but it's even more difficult and less efficient because the mass difference is so much smaller.

You can't produce weapons grade plutonium by reprocessing spent fuel rods from a normal reactor.
posted by Steven C. Den Beste at 12:12 PM on March 16, 2006

Thank you for all of that. I probably need to ask a more direct question: Would either cesium or strontium have been produced during the operations that went on here? We are buying land very close to the remediated site, and want to be certain we're having adequate tests done.
posted by tizzie at 1:52 PM on March 16, 2006

I would suspect not. They don't seem to have been working on or with anything that had spent any significant time inside an operating reactor, and thus there shouldn't be any significant cesium or strontium in what they did work with.
posted by Steven C. Den Beste at 3:42 PM on March 16, 2006

Thank you. Despite the testing we've had done, I was still getting a lot of conflicting advice. I appreciate your input.
posted by tizzie at 5:15 PM on March 16, 2006