Thorium has a distribution coefficient in TBP smaller by an order of magni­tude than uranium. This makes thorium extraction by TBP more difficult than that of uranium. In particular, much larger fluxes of TBP are required for the same extracted quantity. Such an extraction, called the Thorex process, has been achieved at a pre-industrial stage in two instances [138]:

• At Hanford more than 200 metric tons of low-burn-up aluminium-clad thorium fuel were processed [142].

• In the frame of the THTR (thorium high-temperature reactor), laboratory extraction of thorium and uranium was carried out at Jiilich on highly irradiated fuels [143].

Aside from a low distribution coefficient, thorium extraction is made difficult by the existence of an additional organic phase which appears for a rather modest thorium nitrate concentration in the aqueous phase. This concentration decreases as a function of the concentration of nitric acid. Furthermore it was found that at moderate nitric acid concentrations, needed for good separation of thorium from fission products (around 1.5mol/l), fission products, if present in large quantities, might precipitate. This has led [143, 144] to a rather complicated extraction process where an initial pre-decontamination of thorium and uranium with low thorium (1.15mol/l) and medium nitric acid (1mol/l) concentrations, was followed by an extraction stage with lower nitric acid concentration (0.15mol/l) and, finally, separation and purification of thorium and uranium. The Purex process is unable to separate thorium from protactinium with good efficiency. An improved separation can be obtained by adding phosphoric acid H3PO4 which complexes readily with protactinium.

An interesting aspect of the Jiilich experiment is that it involved the treatment of fuel particles embedded in graphite and silicon carbide spheres. The graphite was burned in oxygen to CO2. Because of the presence of 14C this has to be sequestered in the form of CaCO3, for example.

Thus the treatment of thorium fuels by the Purex process has been demonstrated but leads to much larger radioactive effluents than does the Purex process for uranium and plutonium extraction.