As mentioned previously in this chapter, high-level nuclear waste (namely, the spent fuel elements) are stored under shielding and cooling in transitional disposals for about 50 years, and then they are deposited in geological repositories for final storage. The spent fuel elements contain the fission products and the transuranium elements. Before final storage, the spent fuel elements have to be treated in differ­ent ways. The aims of these treatments are as follows:

• To utilize the energy of beta and gamma decays,

• To produce additional fuel material (e. g., plutonium),

• To decrease the risk and cost associated with the storage of high-level nuclear waste,

• To decrease the cost of the fuel cycle of nuclear energy production, and

• To gain valuable by-products, e. g., fission products that can be used in other areas.

One possibility of treating the high-level nuclear waste is reprocessing. This is a chemical procedure in which the spent fuel elements are dissolved, and then the fis­sion products, uranium, and transuranium elements are separated. In this way, about 97% of the high-level nuclear waste can be recycled. The steps of reproces­sing are as follows:

• The spent fuel elements are cut into pieces and dissolved in 6—11 mol/dm3 HNO3 solu­tion. If the cladding is zircon or zircalloy, fluoride is also added to the solution. To avoid the chain reaction, neutron absorber (Cd, Gd) is also added.

• The gases released during the dissolution (Kr, Xe, I, T compounds, CO2, etc.) are treated as they would in the normal operation of the nuclear power plant (see Section 7.1.1.1).

• By flowing oxygen gas; if there is any uranium in an oxidation state lower than 6, it is oxidized to uranyl cation (UO2+). As a result of the nitric acidic dissolution, all cations present in the solution are nitrates. The oxidation state of uranium and plutonium is 16 and 14, respectively.

• The uranium and plutonium is extracted by tri-butyl-phosphate dissolved in kerosene. This procedure is called the “PUREX procedure.” The fission products remain dissolved in the aqueous phase.

• The uranium and plutonium are separated by using the reduction of plutonium. For this reason, ferrous(II) sulfamate or U(IV) is added to the kerosene solution. Plutonium is reduced to Pu(III), then extracted by water. The uranium remains in the organic phase (kerosene). If required, this process can be repeated for additional purification.

• The fission products are separated from the aqueous phase using different techniques (precipitation, extraction, ion exchange, etc.). At first, the chemically similar fission pro­ducts are separated, and then the individual isotopes are separated from the groups of the chemically similar elements. An example will be shown in Sections 8.5.2 (Eq. (8.17)) and 8.7.1.4 (Eq. (8.24)).

• The liquid residue of the procedure is solidified in the form of ceramics by the addition of Al(NO3)3 and SiO2, or vitrificated by Al(NO3)3, SiO2, borax, or phosphates.

Besides recycling, isotopes with shorter and longer half-lives may also be sepa­rated during the reprocessing. In this way, both the quantity and the radioactivity of the high-level waste can be significantly reduced, and less-disposal capacity is required.

Another possibility for the treatment of high-level nuclear waste may be the transmutation of the fission products of the spent fuel elements to isotopes with shorter half-lives. During this treatment, the fission products are dissolved in melted salts and bombarded with neutrons with high flux. The neutrons are pro­duced by the spallation reaction of an element with a high atomic number (such as Pb, Bi, or Hg) induced by the bombardment of protons with very high energy (>800 MeV). High-energy protons are generated in linear accelerators. The neu­trons react with the nuclei of the fission products: fission, neutron capture, and then beta decay take place. Finally, radioactive isotopes with shorter half-lives, or even stable isotopes, can be produced. This process is exoergic; about 20% of the released energy is used for the operation of the linear accelerator, and the rest can be utilized for other purposes. Thus, the nuclear energy production becomes more economical. The development of transmutation of spent fuel elements is in the experimental phase at the moment; we may have to wait a long time for the imple­mentation of this process.

Independent of the treatment of spent fuel elements, some amount of high-level nuclear waste is always formed; so final disposal of this waste is always required. Today, the only real option for final disposal is storage in geological repositories; however, presently there is no operating geological repository for high-level radio­active waste. Some countries are researching the construction of such waste reposi­tories, and they are expected to be operational by about 2040. In these repositories, high-level wastes are placed in stainless steel containers surrounded by a bentonite layer and natural geological formations.