RADIOLOGICAL IMPACT OF FERTILIZERS: PRODUCTION AND USE - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 280

Figure 1. Decay chain of the natural uranium series (4n+2). Graphic from (Tosaka, 2008).
This phenomenon is usually known as global fallout. Most of these radionuclides were
short-lived and have already decayed, as is the case with 131 I (T ½ = 8.02 d). However, 137 Cs
and 90 Sr have longer half-lives (30.07 and 28.79 yr, respectively), and are actually the most
important man-made radionuclides released into the environment. The estimated quantities of
activity released were 948 and 622 PBq for 137 Cs and 90 Sr, respectively. Some radioisotopes
of plutonium were also released but in lower amounts - 10.87 PBq for 239+240 Pu. The annual
deposition of 137 Cs and 90 Sr due to global fallout is shown in Figure 2. There was a maximum
around 1962, in which year the greatest number of tests were carried out. As the majority of
tests took place in the northern hemisphere, the deposition there was greater than in the
southern hemisphere.