Uranium purification

The purification step enables the ‘cleaning’ of uranium and plutonium from the fission products and short-term noxious actinides (Fig. 16.9).

The purification of uranium is performed using liquid-liquid separation in sets of extractors-settlers. Zirconium, thorium, ruthenium (fission products, /emitters) and plutonium and neptunium traces still there are then removed and transferred to the vitrification workshop. Uranium is concentrated (using evaporation) and recovered as uranyl nitrate. Some the uranyl nitrate is used to make the uranous nitrate during the extraction step previously described. The remaining uranyl nitrate can be re-used in a new cycle.

Stripping (de-extraction) returns uranium to the aqueous phase. This is completed by washing the solvent rich with uranium with a high flow rate of slightly acidic water at around 50 °C. These conditions are unfavourable for the stability of the uranium nitrate complexes.

After concentration, uranium is submitted to an additional purification cycle to remove neptunium (Np), the main pollutant of uranium.

Conversion to Pu IV

The processing of the plutonium solutions from the extraction step follows two routes.

The first route is a wet process, which is a chemical treatment. This treatment targets the final production of plutonium oxide.

The second route is a dry process and is a mechanical treatment. This enables the conditioning of the plutonium oxide produced and shipping to MOX production.

Plutonium from the partition in solution it is at valence III. It is oxidized to level IV by spraying the liquid (‘sparging’) into a nitrous vapour.

Hydrazine nitrate is reduced while plutonium is oxidized:

2 Pu3+ + NO3- + 3H+ ^ 2Pu4+ + HNO2 + H2O

Reloaded with acid, the solution can be retreated with additional extraction/ scrubbing cycles.