Besides natural radioactive isotopes, artificial radioactive isotopes are present in

the environment. They originate from different anthropogenic activities:

1. Radioactive wastes of isotope laboratories, including the research, medical, and industrial laboratories.

2. Radioactive wastes of nuclear energy production and reprocessing technologies (see Section 7.3). From nuclear plants, radioactive isotopes can be introduced into the environment in accidents (Section 7.2) and by regular emissions (Section 7.1.1.1), of which the emission of gaseous radioactive isotopes (T, 14C, 85Kr, 133Xe, 135Xe, and I isotopes) is the most important. A nuclear reactor with 440 MW electric power produces 2.7 X 1019 Bq/year radioactivity. Some important fission products are shown in Figure 13.1. The fission products are present in nuclear waste, as discussed in Section 7.3 and shown in Figures 7.6 and 7.7.

In addition, there are some isotopes which emit beta particles with low energy (79Se 95Zr, 109Pd, and 135Cs). Their radioactivity is not very high; however, they have long half-lives, so they will be present in nuclear waste disposal and may remain in the environment for a long time.

3. The radioactive isotopes of nuclear bombs and the experimental nuclear explosions. As discussed in Section 7.5, the first nuclear explosions that affected the atmosphere were two US explosions in Japan (Hiroshima and Nagasaki in August 1945), a Soviet explo­sion (1949), a British explosion (1952), and a Chinese explosion (1964). When exploding a nuclear bomb equivalent to a 1000 ton traditional trinitro-toluol (TNT) bomb, 48.5 g of fission products is emitted into the atmosphere. This mass seems to be low; however, the radioactivity is extremely high (3.7 X 1021 Bq). A significant portion of the fission products has a short half-life, so the radioactivity decreases rapidly. After 24 h, it is 5.9 X 1016 Bq. This is still a very high level of radioactivity. The radioactive isotopes of the nuclear explosions have the following half-lives (see Figure 7.7): t1/2 < 1 day for 131 isotopes; 1 < t1/2 < 10 days for 117 isotopes; 10 < t1/2 < 30 days for 9 isotopes; 30 days < t1/2 < 1 year for 12 isotopes; 1 year < t1/2 < 10 years for 7 isotopes; 10 years < t1/2 < 100 years for 3 isotopes; t1/2 > 100 years for 10 isotopes.

The longtime pollution obviously originates from the isotopes with long half­lives. The most important polluting radioactive isotopes are 14C, 90Sr, 137Cs, 95Nb, 106Ru, 106Rh, 140Ba, 140La, 144Ce, 144Pr, and Pu. Before 1963, 1.2 X 1016 Bq (about 400 kg) of 239Pu were emitted into the atmosphere.

The nuclear wastes of isotope laboratories and nuclear energy production are treated and stored under very strictly checked conditions (as discussed in Sections

7.3 and 8.9). The radioactive products of the nuclear explosions, however, freely got into the environment. In 1963, the United States, the Soviet Union, and the United Kingdom signed the Limited Test Ban Treaty, pledging to refrain from test­ing nuclear weapons in the atmosphere, underwater, or in outer space. The treaty permitted underground tests. Many other nonnuclear nations have acceded to the Treaty; however, some countries, which possess nuclear weapons, have not. As a result of the Limited Test Ban Treaty, the radioactivity of the atmosphere
originating from the nuclear explosions has decreased, and that from nuclear energy production has increased. The radioactive pollution reached its maximum between 1961 and 1965. Additional significant radioactive pollution entered the environ­ment during the Chernobyl and Fukushima accidents (described in Section 7.2).