Reprocessing processes

Introduction
In the “closed” fuel cycle, spent nuclear fuel is reprocessed so that uranium and transuranium elements can be further used in nuclear industry. However, only uranium and plutonium are recycled by the PUREX process. Currently, several advanced processes (DIAMEX, SANEX, GANEX) are under development to achieve the challenging goal of complete partitioning of spent nuclear fuel. Treatment of the spent nuclear fuel may be performed by hydrometallurgical and pyrometallurgical processes.

Hydrometallurgical processes
Hydrometallurgical processes use various chemicals (usually extractants and diluents) and methods for the selective separation of a specific element or group of elements from their mixture in a solution.
The extraction methods can be divided into liquid-liquid extraction, gas-liquid extraction, and solid-liquid extraction. For instance, the scheme of liquid-liquid extraction is shown in Figure 1. In the beginning, the element of interest is present in one phase (liquid A). By adding another proper phase (liquid B), immiscible with liquid A, the element of interest can be selectively transferred from the original one to the new phase.

Figure 1 - Scheme of liquid-liquid extraction.

As mentioned earlier, there exist several hydrochemical partitioning processes. On an industrial scale, plutonium and uranium are usually separated using the PUREX process. The PUREX process was developed within the Manhattan Project in the USA in the late 1940’s. Its first purpose was to isolate plutonium as fissile element for nuclear weapons.
During this process, the spent nuclear fuel is dissolved into a highly concentrated nitric acid (aqueous phase), and then both elements (U and

Pu) are extracted with TBP molecules (TriButyl Phosphate), which is shown in Figure 2, into the organic phase. The organic phase consists of 30 % tributyl phosphate in kerosene.

Figure 2 - Chemical structure of Tributyl Phosphate (TBP).

All metals ions must be extracted as neutral molecules to go through the interface from the aqueous phase into the organic phase. One neutral molecule consists of one uranyl cation (UO₂ ²⁺), two nitrate anions (NO₃ ^-) and two molecules of tributyl phosphate (TBP) according to the following equation:

In the next step, a reducing agent is used to reduce plutonium(IV) to plutonium(III). The reduced plutonium goes from organic solvent to aqueous phase. Finally, uranium(VI) complexed in the organic phase is stripped (moved) to aqueous phase by back-extraction into low-concentration nitric acid solutions.
The obtained plutonium and uranium nitrate solutions are then converted to plutonium and uranium dioxide. This process consists of two steps – oxalic precipitation and air calcination. Afterwards, plutonium and uranium can be converted into a mixed oxide to obtain the MOX fuel (Mixed OXide) to be used in a nuclear reactor.
The treated aqueous phase (the waste from the TBP extraction) is called PUREX raffinate and contains all of the fission products and minor actinoids. It may be further concentrated and denitrified yielding the so-called PUREX concentrate.
A modified PUREX process has been developed to separate neptunium, iodine, and technetium. However, this process has not yet been broadly used on an industrial scale.
Chromatographic processes are based mainly on

extraction chromatography or ion exchange. These methods are slower than the liquid-liquid extraction methods but allow more efficient partitioning of elements with similar properties, such as transplutonium elements.

Pyrometallurgical processes
Similarly to hydrometallurgy, the pyrometallurgical processes aim at separating uranium, plutonium, and minor actinoids from the other fission products. In these processes, spent nuclear fuel is dissolved in e.g. molten fluoride or chloride salt systems or in molten metals, such as cadmium, bismuth or aluminum. Once the fuel is melted, the melt is processed e.g. electrochemically or by liquid-liquid extraction into another immiscible melted phase.
Pyrometallurgical technologies could play an essential role in the fuel cycle of the Generation IV reactors. Two promising processes were designed: Electrolytic refining (electrorefining) on solid aluminum cathodes in molten chloride and liquid-liquid reductive extraction in a liquid aluminum-molten fluoride system. The setup proposed for electrorefining is shown in Figure 3.

Figure 3 - The scheme of electrorefining.

Comparison of hydrochemical and pyrometallurgical processes
Both processes have their advantages and disadvantages. Pyrometallurgical methods may process spent nuclear fuel after shorter cooling time than hydrochemical processes. On the other hand, molten salts are highly corrosive and special safety and resistant materials must be used for the construction of the equipment. Another advantage of pyrometallurgical processes is higher radiation stability, and as far as molten salts are not neutron moderators, larger amounts of fissile materials can be handled.