One of the very important aspects of nuclear energy production is the safe treat­ment and storage of nuclear waste. The sources of nuclear waste are as follows:

• The fission products of the (n, f) nuclear reactions,

• Transuranium elements produced in the (n, Y) reactions of uranium, and

• Radioactive nuclides produced in the (n, Y) reactions of the structural material and the environment.

As seen in Section 6.2.1, the fission reaction of 235U (Eq. (6.21)) produces about 300 fission products, many of which are radioactive because the ratio of neutrons to protons is too high for stability (Figure 6.5). The fission products emit negative beta radiation, which are frequently accompanied by gamma radiation. As seen in the last two rows of Table 7.2, the energy of the beta and gamma radiation of the fission products is about 14 MeV, which is about 7% of the total energy released in the fission reaction. The radioactivity of the fission products as a function of time is shown in Figure 7.6.

As seen in Figure 7.6, the two most important fission products are 137Cs and its daughter nuclide, 137mBa, as well as 90Sr and its daughter nuclide, 90Y. Their fis­sion yield is relatively high, and they have relatively long half-lives. Twenty years after the irradiation, the radioactivity of the fission products is almost exclusively due to the presence of these isotopes. About 60% originates from the Cs— Ba

pairing, and about 40% originates from 90Sr—90Y pairing. It should be noted, that cesium and strontium can substitute potassium and calcium in the living organism. Thus, two isotopes are considered to be the most dangerous fission products.

The transuranium elements are formed in the (n, Y) reaction of 238U (Figure 6.22), which composes the main part (>95%) of the fuel elements. Similar reactions produce additional isotopes of the transuranium elements up to 246Pu, 244Am, and some curium isotopes, respectively.

(n, Y) nuclear reactions take place with the structural material and the elements in the environment; for example, with the coolant, the air, and so on. Besides (n, Y),

other nuclear reactions can also produce radioactive isotopes. For example, C-14 isotope can be formed by the (n, p) reaction of the nitrogen in the air: 14N(n, p)14C. Tritium is also formed from the nitrogen by 14N(n,3 4He)T and 14N(n, T)12C reac­tions. The most important radioactive isotopes produced in these reactions are T, C-14, N-15, N-16, O-19, F-18, Ar-41, Cr-51, Mn-54, Fe-55, Fe-59, Co-58, Co-60, Ni-63, Zn-65, and Ag-110.

Nuclear wastes are formed during the mining and refining of uranium ores, the production and reprocessing (see Section 7.3.2) of the fuel element, or in the

industrial, medical, or research isotope laboratories and any applications of sealed and unsealed radioactive sources.

The radioactive wastes are classified based on their activities. The classification is different in different countries; the IAEA also has radioactive waste safety stan­dards. The radioactive wastes can be classified as follows:

• Low-level wastes; for example, the wastes of radioactive workplaces, such as contami­nated tools, clothes, and laboratory vessels.

• Intermediate-level wastes have higher activity and often require shielding. The ion exchange resins, filters, chemical sludge, and other technological wastes of nuclear power plants belong to this group. Under normal operating conditions, these wastes contain fis­sion products, and the radioactive isotopes produced by the nuclear reactions of the struc­tural material and the nuclides of the environment. The quantity of the transuranium elements is very low. The radioactivity of the isotopes in a container filled with typical intermediate-level waste is shown in Figure 7.7. Low — and intermediate-level wastes are frequently handled together.

• High-level waste, such as the wastes formed in the core of the nuclear reactors; namely, the spent fuel elements. In addition, the reprocessing of the spent fuel elements (see Section 7.3.2) produces high-level radioactive waste. Their radioactivity and heat emis­sion is high; thus, they require shielding and cooling by air or in basins filled with water. High-level nuclear waste is stored under these conditions up to approximately 50 years.

In the United States, the transuranium nuclear wastes are also differentiated and treated separately, independent of their alpha activity.