In these processes the spent fuels are first decladded by shearing and sawing. A dissolution of the oxide fuel in hot nitric acid follows. At present, on an industrial scale, uranium and plutonium are extracted by the Purex process.

9.2.1 The Purex process

Solvent properties

The Purex process uses an organic phase consisting of tributyl phosphate (TBP) soluted in a hydrocarbon diluent as extractant. The formula of TBP is (C4H9)3PO4. A possible structure for TBP is:

H

C

C

C

C

O

H—C—C—C—C—O—P—C—C—C—C—H

O

It is thought that TBP forms bonds via the electron of unsaturated oxygen.

This molecule forms a complex with uranyl nitrate UO2(NO3)2 which is soluble in the hydrocarbon diluent but not in water. Similarly it can form a complex with plutonium nitrate Pu(NO3)4. The main complex forming equilibrium reactions for U and Pu read [138]

UO2+(aq) + 2NOf(aq) + 2TBP(o)^ UO2(NO3)2-2TBP(o) (9.11)

Pu4+(aq)+4NOf(aq)+2TBP(o)^ Pu(NO3)4.2TBP(o) (9.12)

which occur at the interface between the aqueous phase (aq) and the organic one (o).

The equilibrium concentrations are correlated by the law of mass action:

Distribution coefficients measure the ratio between the molar concentra­tions in the organic phase and those in the aqueous phase. For example, for uranium in the TBP-aqueous system:

which, using equation (9.13), reads

Du = Ku[NOf (aq)]2.[TBP(o)]2.

From equation (9.16) it appears that the concentration of uranium in the organic phase increases with the concentration of TBP in the organic phase as well as with the concentration of NOf in the aqueous phase. It is important to note that here we are dealing with free TBP concentration. In particular, a larger uranium concentration in the aqueous phase leads to a smaller free TBP concentration in the organic phase (for fixed total TBP concentration) since more TBP is used in the formation of the complex.

The concentration of NOf in the aqueous phase can be adjusted by adding more or less nitric acid, which reacts with TBP according to

H+(aq)+NOf(aq)+TBP(o)! HNO3TBP(o)

which leads to the equilibrium equation

Finally DU can be expressed as a function of the aqueous concentrations [H+(aq)], [NO^(aq)], UO2+(aq) and the total concentration of TBP in the organic phase. Typical values of KU and KH are 5.5 and 0.145 respectively. Table 9.1 from reference [138] illustrates these considerations. It shows that the concentration of the uranyl ions strongly influences the value of DU. The influence of the nitric acid concentration is less dramatic and becomes small for a concentration larger than 1.5 moles/l.

Figure 9.2 shows that TBP has much larger distribution coefficients for actinides than for most fission products. The separation properties can be controlled by playing on the concentration of nitric acid in the aqueous phase and on the uranium concentration in the organic phase.