Individual operations making up the nuclear fuel cycle for light-water power reactors of the type developed in the United States are shown in the pictorial flow sheet, Fig. 1.14. This follows case II of Fig. 1.11.

The first step is mining of uranium ore, which typically contains only a few pounds of uranium per ton. Uranium values in the ore are concentrated in a uranium mill, which is located near the mine, in order to reduce subsequent shipping charges. Concentration processes frequently used include leaching, precipitation, solvent extraction, and ion exchange. The principles of solvent extraction are described in Chap. 4; applications of solvent extraction and ion exchange to uranium ore processing are taken up in Chap. 5. Uranium concentrates are

(b) MOLTEN-SALT BREEDER REACTOR

Figure 1.13 Fuel processing flow sheets for reactors using thorium as fertile material. Basis: 1 year, 80 percent capacity factor.

Figure 1.14 Fuel-cycle operations for light-water reactor.

known commercially as “yellow cake,” because the sodium diuranate or ammonium diuranate commonly produced by uranium mills is a bright yellow solid. Figure 1.15 is a photograph of the uranium mill of Union Carbide Corporation.

Concentrates are shipped from the uranium mill to a uranium refinery or conversion plant. Here chemical impurities are removed and the purified uranium is converted into the chemical form needed for the next step in the fuel cycle. Figure 1.14 shows concentrates being converted into uranium hexafluoride (UF6), the form used as process gas in the gaseous diffusion process for enriching 235 U. Other possible products of a uranium refinery used in other fuel cycles are uranium metal, uranium dioxide, or uranium carbide. Uranium purification and conversion processes are also described in Chap. 5.

Light-water reactors must be supplied with uranium having a higher content of fissile material than the 0.711 w/o 235U present in natural uranium. This can be done by enriching 235 U in an isotope separation plant as depicted in Fig. 1.14, by adding plutonium to natural uranium, or by some combination. The gaseous diffusion process is the principal process that has been used thus far for enrichment of uranium on a commercial scale. As working fluid it uses UF6, the only stable compound of uranium that is volatile at room temperature. UFe melts at 64° C, at which its vapor pressure is 1.5 atm. Natural UF6 is shipped in large steel cylinders. As UF6 reacts readily with water and organic materials, it must be handled in clean equipment, out of contact with moist air.

A gaseous diffusion plant consists of many gaseous diffusion stages connected in series. Each stage contains many porous tubes made of membranes with very fine holes, termed diffusion barriers. UF6 gas at a relatively high pressure flows along the inner wall of these tubes, whose outer wall is maintained at a relatively low pressure. The UF6 gas flowing through the tube wall is slightly enriched in 233 U relative to the gas remaining on the high-pressure side. Since one gaseous diffusion stage can increase the ratio of 235 U to 238 U by no more than a factor of 1.0043, it is necessary to repeat the process in hundreds of stages to obtain a useful

degree of separation, recompressing the UF6 between stages. Large quantities of UF6 must be recycled, and the power consumption is enormous. To produce 1 kg of uranium enriched to 3 percent 23SU while stripping natural uranium to 0.2 percent requires about 13,000 kWh of electric energy. The U. S. Atomic Energy Commission built three large gaseous diffusion plants at a cost of $2.3 billion. When operated at capacity they consume 6000 MW of electric power. Figure 1.16 is a photograph of the plant at Oak Ridge, Tennessee. The large number of stages is suggested by the repetition of the basic building structure. These plants and the gaseous diffusion process are described in more detail in Chap. 14.

Enriched UF6 is shipped to the plant for fabricating reactor fuel elements in monel cylinders whose size is determined from the 235 U content, so as to prevent accumulation of a critical mass. At the fuel fabrication plant UF6 is converted to U02 or other chemical form used in reactor fuel. For light-water reactors the U02 is pressed into pellets, which are sintered, ground to size, and loaded into zircaloy tubing, which is Filled with helium and closed with welded zircaloy end plugs. These individual fuel rods are assembled into bundles, constituting the fuel elements shipped to the reactor. Conversion of UF6 to U02 is described in Chap. 5. Extraction of zirconium from its ores and separation of zirconium from its companion element hafnium is described in Chap. 7.

The length of time that fuel can be used in a reactor before it must be discharged depends on the characteristics of the reactor, the initial composition of the fuel, the neutron flux to which it is exposed, and the way in which fuel is managed in the reactor, as described in more detail in Chap. 3. Factors that eventually require fuel to be discharged include deterioration of cladding as a result of fuel swelling, thermal stresses or corrosion, and loss of nuclear reactivity

as a result of depletion of fissile material and buildup of neutron-absorbing fission products. A typical fuel lifetime is 3 years.

When spent fuel is discharged from the reactor, it contains substantial amounts of fissile and fertile material, which, in the case of light-water reactors, are valuable enough to offset part or all of the cost of reclamation. Because of the fission products, spent fuel is intensely radioactive, with activities of 10 Ci/g+ being common. Spent fuel is usually held in cooled storage basins at the reactor site for 150 days or more to allow some of the radioactivity to decay. If to be reprocessed, spent fuel would be shipped in cooled, heavily shielded casks, strong enough to remain intact in a shipping accident.

In the fuel reprocessing plant, fuel cladding is removed chemically or mechanically, the fuel material is dissolved in acid, and fissile and fertile materials are separated from fission products and from each other. The Purex process, commonly used in reprocessing plants, is described at somewhat greater length in Sec. 7, below, and in more detail in Chap. 10. Figure 1.17 is a photograph of the Purex plant of the U. S. Department of Energy at Hanford, Washington. The massive, windowless, concrete building is characteristic of these radiochemical fuel reprocessing plants. In the case of light-water reactor fuel, the most valuable products of the fuel reprocessing plant are plutonium, usually in the form of a concentrated aqueous solution of plutonium nitrate, and uranium, most conveniently in the form of UF6. Some individual fission products such as 137 Cs, a valuable gamma-emitting radioisotope, may be separated for industrial or medical use. The remaining radioactive fission products are held at the reprocessing site for additional decay, then converted to solid form, packaged, and shipped to storage vaults where they

t Curies per gram.

must be kept out of human contact for thousands of years. Procedures for handling radioactive wastes are described in Chap. 11.

Plutonium nitrate from the reprocessing plant is converted to metal, oxide, or carbide and used in fuel for fast reactors or recycled to thermal reactors. UF6 from the reprocessing plant is recycled to the gaseous diffusion plant to be reenriched in 235 U.