Wikipedia:Reference desk/Archives/Science/2021 June 27

Science desk
< June 26	<< May | June | Jul >>	June 28 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is a transcluded archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

June 27

Reprocessing nuclear fuel vs breeder reactor

Dear Wikipedians:

As per this article and this video, I still don't quite get what the difference is between reprocessing/recycling of nuclear fuel and breeder reactors.

The way I understand it: in the standard reactors that are now used everywhere in the world, a proportion of the U-238 always gets transmuted to Pu-239. In once-through model (the US), once a fuel rod is spent, it is buried as nuclear waste. In recycling/reprocessing model (France), once a fuel rod is spent, it is shipped to a reprocessing plant, which takes the said fuel rod apart, separates the U-238 and Pu-239 from the fission products, repackages the U-238/Pu-239 combo into a "new" fuel rod ("recycling"), and sends the new fuel rod back to the power plant to be re-burned in the same way as the original fuel rod, as Pu-239 is equivalent to U-235. This recycling can be repeated many times (since each time a proportion of the U-238 remaining in the fuel rod will always be transmuted to Pu-239), until all the U-238 in the original fuel rod is eventually turned into Pu-239 and burnt.

But isn't this how a breeder reactor works? By turning U-238 into Pu-239? So why bother with complicated breeder reactors when you can achieve the same effect of "breeding" U-238 into Pu-239 through conventional regular nuclear reactors plus reprocessing/recycling plants to close the nuclear fuel cycle by reprocessing/recycling a fuel rod repeatedly until all its U-238 is converted into Pu-239?

Also I don't get why in the US recycling/reprocessing of nuclear fuel is illegal. Fuel rod recycling/reprocessing sounds like such a beautiful solution that solves two problems with one stone (making use of the U-238, and solving the nuclear waste problem), that it would be crazy to deem it illegal, no?

172.97.226.26 (talk) 21:06, 27 June 2021 (UTC)

The 'nuclear waste problem' is not 'solved', because Pu-239 is itself very radioactive (particularly hazardous if the dust is inhaled) as well as highly toxic (as are all isotopes of plutonium). The illegality is in part motivated by the fact that Pu-239 is also a prime ingredient of nuclear weapons. If there were a thriving industry moving and storing large quantities of Pu-239, it would present opportunities for terrorists to obtain some and create a bomb, which even if it didn't work flawlessly as a fission bomb, would double as a dirty bomb. {The poster formerly known as 87.81.230.195} 2.125.73.1 (talk) 07:23, 28 June 2021 (UTC)

Nah, Pu-239 isn't very radioactive, but radioactive enough to be a concern. But it is indeed quite long-lived. Reprocessing and breeder reactors reduce the amount of Pu-239, so reprocessing reduces the nuclear waste problem. Breeder reactors can almost eliminate the long-lived waste. A terrorist wanting a dirty bomb may prefer hospital waste. PiusImpavidus (talk) 10:53, 28 June 2021 (UTC)

When using nuclear reprocessing, you start with slightly enriched uranium fuel (U-238 and a few percent U-235, which is the actual fuel). U-235 is burned while a fraction of the U-238 is turned into Pu-239. Part of the Pu-239 is burned, part of it is turned into Pu-240. Pu-240 is something you don't want to make, as it has no use. That puts a limit on the process. You can't burn all U-238, as part of it will end up as Pu-240. Run on a low ratio of Pu-239 to U-235, and you use up your U-235 without consuming your Pu-239 and U-238, run on a high ratio of Pu-239 to U-235, and fuel gets wasted as it's turned into Pu-240.

During reprocessing, the Pu is extracted and mixed with fresh enriched U and returned to the reactor. Compared to a single pass, this can more or less double the amount of energy extracted from a given amount of natural U. Instead of burning 1% of the natural U, you can burn 2%. At the same time, the process lowers the amount of long lived radioactive waste, which contains a high fraction Pu.

A breeder reactor goes a step further. It doesn't mind converting Pu-239 into Pu-240, because it can turn that into Pu-241 and use that as fuel. So, it can burn 100% of the natural U. This leads to a large increase in energy extracted from a given amount of natural U (or Th) and greatly reduces the amount of long-lived, medium-radioactive waste.

The main problem of nuclear reprocessing and why it was banned in the USA is that it's the easy way to make nuclear weapons. To make a nuclear weapon based on U, you need highly enriched U, which is very hard to make. Making slightly enriched U for a nuclear power station is much easier. Use that to produce some Pu-239. If you run the fuel rod for a short period, the Pu will be almost pure Pu-239, which can be chemically separated and used in a nuclear weapon. No need for complex ultra-centrifuges, as you don't have to separate the isotopes. If you keep the fuel in the reactor longer or start with fuel which already has Pu-239 mixed in, the fraction of Pu-240 increases, making it less fit for nuclear weapons. That's why the Soviets designed the RBMK (a.k.a. Chernobyl-type reactor), a type of reactor to allow quick cycling of fuel to produce a lot of weapons-grade Pu.

Back in the 1970s, it was still thought that the nuclear industry would be a normal kind of industry and it would be safer to keep weapons-grade fissile material out of civilian hands. It appears now that controls on the civilian nuclear industry are pretty strict and there are reprocessing facilities in France, Russia and other countries anyway, so the USA lifted the ban later. As U and Pu (from dismantled nuclear weapons) is quite cheap nowadays, reprocessing is hardly economically viable and mostly done to reduce the long-lived nuclear waste. PiusImpavidus (talk) 10:53, 28 June 2021 (UTC)

Thanks everyone for your detailed inputs! I now have a more authentic grasp on the realities of reprocessing/recycling. I also understood why the ban on reprocessing/recycling was in place in the US for a period of time. 172.97.226.26 (talk) 12:25, 28 June 2021 (UTC)

Resolved