Because of the importance of reactor fuel reprocessing in nuclear power technology, some further discussion of this topic is warranted in this introductory chapter.

In addition to fissionable isotopes (235U, 233 U, or plutonium) and fertile isotopes (238U or thorium), spent fuel from a reactor contains a large number of fission product isotopes, in which all elements of the periodic table from zinc to gadolinium are represented. Some of these fission product isotopes are short-lived and decay rapidly, but a dozen or more need to be considered when designing processes for separation of reactor products. The most important neutron-absorbing and long-lived fission products in irradiated uranium are listed in Table 1.4.

Processing of spent reactor fuels is made especially difficult by their intense radioactivity. The process equipment must be surrounded by massive shielding, provision must be made to remove the substantial amounts of heat that are associated with this radioactivity, and in some instances damage to solvents and construction materials from the radiations emitted by the materials being processed is a problem. Another difficulty is the critical-mass hazard, which is present whenever fissionable material is handled at substantial concentrations. This often requires a limitation in the size of batches being processed or in the dimensions of individual pieces of equipment. A third difficulty is the high degree of recovery that is usually required because of the great value of the fissionable materials being processed. A fourth is the high degree of separation specified for the removal of radioactive fission products; in present

Table 1.4 Important isotopes in irradiated uranium

Long-lived radioactive fission products

processes it is necessary to reduce the concentration of some of these elements by a factor of 10 million. Another difficulty is the large number of components present, with elements of such diverse properties as the alkali cesium and the manufactured elements technetium (resembling manganese) and promethium (one of the rare earths). A final difficulty, and one that was not originally anticipated, is the chemical similarity between uranium and plutonium.

The principle of the Purex process, now commonly used for processing irradiated uranium by solvent extraction, is illustrated in Fig. 1.18. The solvent used in this process is a solution of tributyl phosphate (TBP) in a high-boiling hydrocarbon, frequently и-dodecane or a mixture of similar hydrocarbons. TBP forms complexes with uranyl nitrate [U02(N03)2] and tetravalent plutonium nitrate [Pu(N03)4] whose concentration in the hydrocarbon phase is higher than in an aqueous solution of nitric acid in equilibrium with the hydrocarbon phase. On the other hand, TBP complexes of most fission products and trivalent plutonium nitrate have lower concentrations in the hydrocarbon phase than in the aqueous phase in equilibrium.

In the Purex process, irradiated U02 is dissolved in nitric acid under such conditions that uranium is oxidized to uranyl nitrate and plutonium to Pu(N03)4. The resulting aqueous solution of uranyl, plutonium, and fission-product nitrates is fed to the center of counter­current solvent extraction contactor I, which may be either a pulse column or a battery of mixer-settlers. This contactor is refluxed at one end by clean solvent and at the other by a dilute nitric acid scrub solution. The solvent extracts all the uranium and plutonium from the aqueous phase and some of the fission products. The fission products are removed from the solvent by the nitric acid scrub solution. Fission products leave contactor I in solution in aqueous nitric acid.

Solvent from contactor I containing uranyl nitrate and Pu(N03)4 is fed to the center of contactor II. This is refluxed at one end by clean solvent and at the other by a dilute nitric acid solution of a reducing agent strong enough to reduce plutonium to the trivalent form, but not so strong as to reduce uranium from the hexavalent form. Ferrous sulfamate is frequently used. In contactor II plutonium is transferred to the aqueous phase, while uranium remains in the solvent. Solvent from contactor II is fed to one end of contactor III, which is stripped at

the other end by water, which transfers the uranium to the aqueous phase leaving the contactor.

After chemical treatment to remove degradation products, the solvent leaving contactor Ш is reused in contactors I and II.

This brief discussion of the Purex process is expanded in Chap. 10, which discusses other processes for treating irradiated fuel and which deals with novel aspects of processing highly radioactive and fissile materials.