Radioactive Fallout, the radioactivity from nuclear explosions. Initially the term applied only to the radioactive products of nuclear explosions deposited on the surface of the earth. Later, in its broadest sense, it came to signify ali manmade radioactivity, airborne as well as deposited, released to the environment.
Sources. Nuclear explosions create a very large number of radioactive products by three processes: (1) the fission of uranium-235 or plutonium-239; (2) the fusion of heavy hydrogen; and (3) the interaction of neutrons, produced by fission and fusion, with surrounding materials. In the fission process, heavy atoms are split and a great amount of energy is released. For each atom fissioned, two large atomic fragments that are called fission products are produced. In the fusion process, two heavy hydrogen isotopes, deuterium and tritium, eombine to form helium and release energetic neutrons plus thermal energy. The principal radioactive by-product of fusion is tritium. Neutrons produced by the fission and fusion processes react with surrounding materials to produce a myriad of radioactive products or escape to produce carbon-14 by reaction with atmospheric nitrogen. Because fusion reactions release about ten times as many neutrons as do fission reactions per unit of energy release, thermonuclear explosions create correspondingly greater amounts of carbon-14 and other neutron interaction products.
Transport. Radioactive fallout can be variously classified as local, tropospheric, and stratospheric. Local fallout is produced only by surface or near surface nuclear explosions. It involves radioactive products attached to large soil particles that settle out rapidly around the explosion site and downwind. Tropospheric fallout consists of debris from surface or lower-atmosphere explosions of about 100 kilotons or less. For such explosions the radioactive debris is confined to the lower atmosphere, where it is redistributed by zonal winds and removed from the atmosphere on a time scale of weeks. Stratospheric fallout results from large nuclear explosions with energy yields ranging from hundreds of kilotons to tens of megatons and whose clouds rise up into the stratosphere. Stratospheric fallout mixes down slowly on a time scale of years and is distributed widely throughout the hemisphere in which the explosion took place. Except for the gaseous radioactivities krypton-85 and carbon-14, only a small fraction of stratospheric fallout mixes into the opposite hemisphere.
The fallout distribution is different for high-altitude explosions whose clouds penetrate up into the mesosphere (the top one thousandth of the atmosphere) and above. In this case the radioactive products spread over the whole globe and mix down very slowly over both hemispheres. Stratospheric fallout, as well as that from high-altitude explosions, undergoes extensive radioactive decay within the atmosphere. The remaining radioactivity is diluted by widespread mixing before it is deposited on the earth’s surface.
In the lower atmosphere, fallout is removed by several processes. Precipitation scavenging is the main mechanism for the removal of fallout particles. Radioactive particles and gases are also removed by eddy mixing down to vegetation surfaces where they become attached or adsorbed. This mode of deposition is especially important for radioactive fallout released near the earth’s surface, from vented underground tests, nuclear accidents, and the like. Except for the local fallout of Iarge particles, gravitational sedimentation plays only a minor role in fallout deposition.
The nature and distribution of radioactive fallout from underground and nuclear excavation tests are less predictable than that from atmospheric tests. For nuclear cratering shots much of the fallout is trapped within pulverized surface material, which falls back into the crater and onto the crater lip. Some of the debris is deposited downwind as local fallout. The upper portion of radioactive clouds from cratering shots is highly enriched with strontium-90, cesium-137, iodine-131, and other radioisotopes that have gaseous or volatile precursors or are themselves gaseous or volatile. Radioactive effluents from vented underground tests are similarly enriched with these same dangerous radioisotopes. Because these radioactive products are released into the lowest layers of the atmosphere, they are rapidly deposited downwind, giving rise to very high concentrations of radioactive fallout in some areas. Radioactive effluents from nuclear reactor operations and accidents would be distributed in a similar fashion.
Radioactivity and Food Chains. Investigations of the biological distribution of radioisotopes have revealed that some trace elements are selectively adsorbed and highly enriched relative to others as they pass from soil and water into plants and living organisms.
Strontium-90, a fallout radioisotope having a 28-year half-life, is chemically similar to calcium and follows calcium from soil into plants and through cows into milk, where, like calcium, it is incorporated into human bone. Strontium-89 and barium-140, bone-seekers of short half-life, follow the path from fallout on vegetation through fresh dairy products into man. The short-lived radioactive iocline-131, which is adsorbed in the thy-roid gland, also follows this short path into the human system.
Another long-lived radioisotope, cesium-137, is adsorbed into muscular tissue by grazing animals and by freshwater fish. Thus radioactive cesium-137 is present in meat and fish as well as on ali plant foods. In subarctic regions, cesium-137 falls out on lichen and other vegetation and is highly concentrated in the meat of caribou, reindeer, and other foraging animals. As a consequence, Lapps, Eskimos, and others whose diets inçlude much meat and fish have cesium-137 body concentrations about 100 times the average for people of the Northern Hemisphere. Radioecological concentration processes also occur in high mountain environments because of the exceptionally high concentration of radioactive fallout in melting snow.
Similar concentration processes are at work in freshwater and marine environments. Radioactive fallout and liquid effluents from nuclear industry accumulate by sedimentation and by adsorption on sediments. Aquatic organisms selectively adsorb cesium-137 and other radioisotopes. Phosphorus-32, zinc-65, manganese-54, cobalt-60, and sodium-24 are among the most prominent radioisotopes found in the tissues of fish, oysters, and other aquatic foods. In the open oceans, zinc-65, manganese-54, and radioisotopes of cobalt and iron are selectively concentrated in plankton and are progressively more concentrated in higher forms of marine life.
Effects on Man. Exposure to nuclear radiation results in a wide range of adverse effects on man, including life shortening, cancer and leukemia deaths, and mutations that result in stillbirths or abnormalities. The radiation contributed by fallout and radioactive pollution from other sources increases the normal occurrence of such effects from the natural background radiation.
Radioisotopes emit ionizing radiation that causes celi damage by dissociation of molecules within the cells. Some fallout constituents emit gamma rays that are more penetrating than X rays and contribute to human radiation exposure from external as well as internal sources. Other radioisotopes emit beta radiation (energetic electrons), which are less penetrating. For internal beta emitters the radiation damage is confined mainly to the organ in which they are incorporated.
In general, the inhalation of radioactive fallout particles is not a significant hazard. However, accidents involving plutonium bombs and fires in plutonium processing plants release submicroscopic particles of plutonium oxide to the atmosphere. These particles are insoluble and intensely radioactive. When inhaled they persist in the lung and may produce lung cancer. This is the principal threat to public health posed by accidents in fast breeder reactors and plutonium processing facilities.
Outlook. During periods of major nuclear testing, the radiation from radioactive fallout approached that from natural sources over most of the Northern Hemisphere and temporarily exceeded it in some areas. Due to radioecological concentration processes the radiation exposure of limited segments of the population has exceeded that from natural sources. The cessation of most atmospheric nuclear tests following the Nuclear Test Ban Treaty of 1963 has reduced radioactive fallout effects to a small fraction of those from natural radioactivity.
At present, the most serious source of manmade radiation exposure of people in advanced countries is that due to medical and dental X rays. In the future such exposures can be greatly reduced without loss of the benefits of diagnostic X rays.
Barring a nuclear war, the most important potential sources of radioactive contamination inçlude that associated with the growing number of nuclear power plants, nuclear fuel reprocessing facilities, radioactive waste handling and storage, and perhaps also that from peaceful uses of nuclear explosives.