The world became aware of the Chernobyl accident not by an announcement from the Soviet government but from a Swedish nuclear power plant worker who came to work on April 27 and set off radiation alarms because radioactive material from the accident had blown over Sweden and gotten on his clothes. The Soviet Union did not admit the disaster until April 28, two days after the accident. The plume of radiation resulting from the explosion and fires spread through the troposphere, with the heavier particles of debris falling locally and smaller radioactive particles and gases traveling over the continent of Europe. The deposition pattern from the radiation plume depended on wind direction and rains, so some areas in Belarus, Russia, and Ukraine, far from Chernobyl, received high levels of radiation, while others received very little. Many parts of Europe got some radiation from the acci­dent, but far below the normal background levels, and some radiation was even measured in the United States (15).

As the uranium fuel in a reactor burns, hundreds of fission products are pro­duced, as discussed in Chapter 9. The radioactivity is almost all from p and у radiation, the least damaging type of radiation. The majority of these are very short-lived, so the amount of radiation after a nuclear accident decays away rap­idly over time. Table 9.1 in Chapter 9 listed some of the major isotopes that build up in nuclear fuel as it burns; three of them that are particularly important in a nuclear accident such as Chernobyl—iodine-131 (131I), cesium-137 (137Cs) and strontium-90 (90Sr)—were highlighted. Because they are so important in deter­mining the radiation hazards after an accident, other critical physical, chemical, and biological properties of these isotopes are listed in Table 10.1.

131I is produced in high amounts in a reactor and it has unique properties that make it very hazardous. It has a short half-life of eight days, so it decays away quickly, but that actually means that a given mass of it is more radioactive than isotopes with a longer half-life. The good thing is that within a few months it is no longer a hazard. It boils at a low temperature, so it is readily volatilized in a loss of coolant accident such as at TMI or Chernobyl. It is readily assimilated and concentrates in the thyroid, since iodine is an essential element for the proper functioning of the thyroid. It has a biological half-life (the time it takes for half of it to be excreted from the body) that is longer than its physical half-life, so nearly all of it that is ingested will decay in the thyroid. Finally, there is a clear pathway for human consumption. It falls from the cloud of radioactivity and deposits on grass and other plants, cows eat the grass and rapidly incorporate it in their milk, then people—especially children—drink the milk. This whole cycle can occur within two days, and if that happens, the 131I concentrates in the thyroid, which is a radiosensitive tissue with a tissue weighting factor (WT) of 0.05.

Fortunately, it is relatively easy to avoid the problem by not drinking contami­nated milk and also by taking iodine tablets that prevent uptake of the radioactive iodine. Unfortunately, because the Soviet government was not forthcoming about the accident, they did not warn people or distribute iodine pills to enough people in time to prevent high doses in many children and young adults. The iodine distribution was very uneven, with the citizens of Pripyat—the nearest town to the reactor where many workers lived—getting iodine pills immediately, but peo­ple in other towns not getting them early enough or at all (16). The Chernobyl

accident released about 1760 PBq of 131I (15) and was the main health risk.2 This is about 3 million times as much as was released from TMI.

137Cs is the next most important radioisotope. It has a half-life of 30 years, so it can still be hazardous for about 300 years, depending on the concentration. It p-decays to 1 37Ba (barium), which is unstable and promptly emits a у ray, so both p and у radiation come from 137Cs decay. Its boiling point is much higher than iodine, but low enough that in a core meltdown it is also likely to volatilize. Also, 137Xe, a gaseous fission product, decays to produce 137Cs. Another isotope of cesium, 134Cs, is also produced in large quantities in reactors, but it has a half-life of 2.1 years so it not a long-term problem. Cesium mimics potassium, so it is read­ily taken up in muscle and other tissues throughout the body that have an aver­age WT of 0.10. It can fall on plants and be ingested, or it can get incorporated in plants that grow in soils that have high levels of cesium and then get into animals and humans that eat the plants. However, its biological half-life is only 110 days, so a single dose of 137Cs will be excreted in two or three years. Cesium tends to bind to clay soils, so it often becomes relatively immobilized in the soil, though this is strongly dependent on the soil type. About 85 PBq of 137Cs was released in the Chernobyl accident (15) and is the main source of long-term problems with soil contamination and potential health effects.

90Sr is the other important radioisotope that was released from Chernobyl. It has a similar half-life as that of 137Cs at 29 years, but its boiling point is so high that it is very unlikely to ever volatilize in a reactor. The only reason that it was widespread after Chernobyl was because of the explosion and fire that contin­ued to hurl small fuel particles into the air. Strontium is chemically similar to calcium so it primarily concentrates in the bones, which is a radiation-resistant tissue with a WT of 0.01. Only about 30% of strontium uptake is actually assimi­lated in the body and 85% of that is assimilated in tissues that rapidly excrete it. About 6% goes to hard bone that has a long biological half-life, so it remains in the body much longer than cesium. About two-thirds of it is removed from bone by 1,000 days (17). It can be ingested from eating contaminated plants and soil but it is not highly mobile in most ecosystems. About 10 PBq of 90Sr was released in the Chernobyl accident (15) but it is not a major health concern.

Helen Caldicott said that there was enough plutonium released at Chernobyl to kill every person on earth (18). In reality, only 0.013 PBq of 239Pu was released (15) and it was from the explosion, since Pu has a boiling point of5,842°F so it does not volatilize. Because it is so dense, and because it was mostly associated with large fuel particles, nearly all of the plutonium that was released fell in a very localized region, so it was not a health hazard to anyone outside the exclusion zone.