# How does nuclear fuel reprocessing work?

As far I know, the depleted fuel cells are crushed, and solved in nitric acid.

What is coming after that? This nitric acid should probably contain a very wide spectrum of different salts (practically, all of the elements between 35-65, and yet a lot of transurans, and a lot of uranium (both of 235 ad 238), and plutonium).

To reach an efficient reprocessing, the uranium (or, at least, the actinid elements) should be somehow separated from this solution. But, AFAIK, they have very different chemical properties. How is it possible to separate only the transuranic materials?

• Should we rephrase the question to be about only the seperation of fission products and uranium (Which seems to me the main question)? Because pre and postprocessing is a pretty big engineering topic, too. – John H. K. Jan 21 '15 at 20:08

Fission Products Extraction

The seperation of plutionium and uranium from the other fission products is done with the organic molecule tributyl phosphate by liquid-liquid-extraction. You have two phases, one organic and one aqueous. The fission products will solve in the aqueous phase and uranium/plutonium will solve in the organic phase with the tributyl phosphate.

Uranium/Plutonium Separation

To seperate uranium from plutonium you have to reduce the plutonium with uranium nitrate. Again, you have two streams: One with the uranium/plutonium from before and one aqueous stream with uranium nitrate (U4+). Plutonium will be reduced and solve in the aqueous solution.

To accomplish the aforementioned chemical processes, the liquid-liquid extraction you can use the following desgins:

Mixer-Settler

(source)

Pulse Column

(source)

The principle is always: Organic phase is lighter than the aqueous phase. Both phases are first separated, than mixed, and then again separated. During the mixing the chemical reactions occur.

You could try Wikipedia, but they note that the article may not meet there quality standards, so I'll stay away from there as a source. The World Nuclear Association seems more credible, anyway. Their page discusses the process. Quoting the section entitled "Reprocessing today – PUREX" (emphasis mine):

All commercial reprocessing plants use the well-proven hydrometallurgical PUREX (plutonium uranium extraction) process. This involves dissolving the fuel elements in concentrated nitric acid. Chemical separation of uranium and plutonium is then undertaken by solvent extraction steps (neptunium – which may be used for producing Pu-238 for thermo-electric generators for spacecraft – can also be recovered if required). The Pu and U can be returned to the input side of the fuel cycle – the uranium to the conversion plant prior to re-enrichment and the plutonium straight to MOX fuel fabrication.

Block of text $\to$ coherent summary: Chemical solvents are used.

Further below, the WNA notes

The used fuel is chopped up and dissolved in hot concentrated nitric acid. The first stage separates the uranium and plutonium in the aqueous nitric acid stream from the fission products and minor actinides by a countercurrent solvent extraction process, using tributyl phosphate dissolved in kerosene or dodecane. In a pulsed column uranium and plutonium enter the organic phase while the fission products and other elements remain in the aqueous raffinate.

In a second pulsed column uranium is separated from plutonium by reduction with excess U4+ added to the aqueous stream. Plutonium is then transferred to the aqueous phase while the mixture of U4+ and U6+ remains in the organic phase. It is then stripped from the organic solvent with dilute nitric acid.

The plutonium nitrate is concentrated by evaporation then subject to an oxalate precipitation process followed by calcination to produce PuO2 in powder form. The uranium nitrate is concentrated by evaporation and calcined to produce UO3 in powder form. It is then converted to UO2 product by reduction in hydrogen.

Compounds of importance: tributyl phosphate, kerosene and dodecane, and aqueous affinate are used as solvents after the nitric acid. Then there is a reaction with $U^{4+}$, and uranium and plutonium are separated.

That's the chemical part. I haven't been able to find much information on the specific mechanical methods (i.e. the pure engineering you were looking for). In fact, the original patent for PUREX does not cover the mechanical processes, leading me to believe that the exact setup can vary.

This pdf is somewhat helpful. It says that the input materials are first chopped up. Perforated baskets are then used to separate this machinery from the rest of the solution; a "wheel dissolver" may be used.

I do have some information, albeit for a non-PUREX method. For aqueous reprocessing (go to page 12), mixer settlers (mixing chambers with settling chambers), columns and centrifuges are the main pieces of equipment.