First of all a word of hope to the Japanese People: Goiania’s population, together with the Brazilian authorities, got rid from a serious leak of cesium-137 in the year 1987 (grade 5 in the international scale of nuclear accidents) where just those in direct contact with the radioactive material had serious problems. After a few years the land area and neighbouring places started recovering the real value; nowadays -less than 30 years after the accident- it is a place for touristic visitation.
Since the accident at the nuclear plants in Fukushima, Japan, the world news has been warning of the danger of the isotopes 137Cs and 131I, without naming other isotopes, present and certainly more abundant in those plants. The third plant, for example, where radioactive material leaked, 235U and 239Pu are the fissionable elements. The fission process, corresponds to the nuclear-break – in the case of 235U nucleus (92 protons and 143 neutrons), after absorbing an additional neutron that is transformed into 236U; due to its high instability it breaks into two smaller nuclei-usually 142Ba and 92Kr + 2 neutrons which will feed into new fission reactions. This process is accompanied by a reduction in the overall mass of the order of 1 / 1000 of the original mass that is transformed into energy in the proportion E= Δm c2 according to Einstein’s famous equation.
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Why then there is so much talk in the 137Cs and 131I? The explanation lies in the fact that, apart from 142Ba and 92Kr nuclei, the 137Cs and 131I nuclei are produced, even though a minority, in occasional reactions of fission of 236U, for example, like others they are radioactive-emitting gamma radiation, capable to cause genetic mutations and cancer, and for that reason, and I explain below, they are very hazardous to our health and the environment.
I explain then why this risk-somehow disposed to the other cores majority cited: Discard the danger of 142Ba, even radioactive, because it has a high melting point (727C = 1340 F) and have a half-life (time taken for half the existing radioactive nuclei to decay to other states – often stable, ie non-radioactive) very short 10.6 minutes and is not volatile do not worry because it will hardly go into the atmosphere. The 92Kr could raise eyebrows because it is a gas, but its half-life of 1.8 seconds. Since 137Cs is radioactive, volatile (T melting = 28 C or 82 F) and the worst: a half-life of about 30 years. For an initial amount drops to ~ 1% from the beginning, we would have to wait about 200 years (~7 half-lives). The 131-I problem is a little diferent: its half-life is relatively short (8 days) and after about 2 months it has fallen to 1% of the original radioactive material, BUT our body absorbs iodine (accumulates in the thyroid gland) and in 2 months it is capable of causing cancer. Those who have saturated amounts of iodine in their thyroids are at less risk. Children under growth stage do not have this protection, as they continue increasing the amount of iodine, especially the younger ones, and this is a very high risk to be avoided at all costs