Radioactivity fission products and

One of the key processes here is the dissolution of the spent nuclear fuel matrix in groundwater liberating radioactive fission products and actinides. Without this process no radioactivity will be released to the biosphere. 

The use of a nuclear reactor, the generation of a range of highly radioactive fission products and the use of a number of organic chemicals is likely to make the detection of a Pu-based weapons programme simpler than detection of a U-based programme. 

The first step in the production of nuclear data for applied purposes is measurement. Nuclear theory cannot provide accurate nuclear data. Nevertheless theory plays an important part in the interpretation of measurements, and is used for interpolation and extrapolation of measured data. Theory also provides much of the data for reactions of lesser importance, such as secondary energy and angular distributions of inelastically scattered neutrons, and also capture cross-sections for materials which are difficult to measure, such as radioactive fission products and minor actinide isotopes. If the resonance structure of a cross-section needs to be known, when... 

The fate of actinide elements introduced into the environment is of course not merely a scientific issue. The disposal of the by-products of the nuclear power industry has become a matter of public concern. For each 1000 kg of uranium fuel irradiated in a typical nuclear reactor for a three-year period, about 50 kg of uranium are consumed. In addition to a large amount of energy evolved as heat, 35 kg of radioactive fission products and 15 kg of plutonium and transplutonium elements are produced. Many of the fission-product nuclides are stable, but others are highly radioactive. All of the fission products are isotopes of elements whose chemical properties are well-understood. The transuranium elements produced in the reactor by neutron capture, however, have unique chemical properties, which are reasonably well-understood but are not always easily inferred by extrapolation from the chemistry of the classical elements. Plutonium is fissile and can be recycled as a nuclear fuel in conventional or breeder reactors, but the transplutonium elements are not fissile to the extent of supporting a nuclear chain reaction, and in any event they are produced in amounts too small to be of interest for large-scale uses. The transplutonium elements must therefore be secured and stored. 

Safety. A large inventory of radioactive fission products is present in any reactor fuel where the reactor has been operated for times on the order of months. In steady state, radioactive decay heat amounts to about 5% of fission heat, and continues after a reactor is shut down. If cooling is not provided, decay heat can melt fuel rods, causing release of the contents. Protection against a loss-of-coolant accident (LOCA), eg, a primary coolant pipe break, is required. Power reactors have an emergency core cooling system (ECCS) that comes into play upon initiation of a LOCA. 

The Natural Reactor. Some two biUion years ago, uranium had a much higher (ca 3%) fraction of U than that of modem times (0.7%). There is a difference in half-hves of the two principal uranium isotopes, U having a half-life of 7.08 x 10 yr and U 4.43 x 10 yr. A natural reactor existed, long before the dinosaurs were extinct and before humans appeared on the earth, in the African state of Gabon, near Oklo. Conditions were favorable for a neutron chain reaction involving only uranium and water. Evidence that this process continued intermittently over thousands of years is provided by concentration measurements of fission products and plutonium isotopes. Usehil information about retention or migration of radioactive wastes can be gleaned from studies of this natural reactor and its products (12). 

Sepa.ra.tion of Plutonium. The principal problem in the purification of metallic plutonium is the separation of a small amount of plutonium (ca 200—900 ppm) from large amounts of uranium, which contain intensely radioactive fission products. The plutonium yield or recovery must be high and the plutonium relatively pure with respect to fission products and light elements, such as lithium, beryUium, or boron. The purity required depends on the intended use for the plutonium. The high yield requirement is imposed by the price or value of the metal and by industrial health considerations, which require extremely low effluent concentrations. 

Stanley G. Thompson joined my group on October 1, 1942 and it fell to his lot to discover the process that was chosen for use at Clinton Laboratories (in Tennessee) and the Hanford Engineer Works (in the state of Washington) for the separation of plutonium from uranium and the immense intensity of radioactive fission products with which it was produced in the nuclear chain reactors. Again I turn to my journal to tell the story ... 

The fact that spent fuel reprocessing and recycle are essential components ofgood nuclear non-proliferation and radioactive waste management practices. These actions are needed so that more efficient use can be made offissionablc materials, and unwanted radioactive fission products can be disposed of without need for permanent safeguards. In addition, potential weapons usable materials are destroyed through beneficial use. 

Synthesis of plutonium in significant quantities requires a sufficiently long reactor fuel irradiation period. Uranium, plutonium, and the fission products obtained after neutron irradiation are removed from the reactor and stored under water for several weeks. During such cooling periods most neptunium-239 initially formed from uranium and present in the mixture transforms to plutonium-239. Also, the highly radioactive fission products, such as xenon-133 and iodine-131 continue to decay during this period. 

Xenon occurs in the atmosphere at trace concentrations. It also occurs in gases from certain mineral springs. Xenon also is a fission product of uranium, plutonium, and thorium isotopes induced by neutron bombardment. The radioactive fission product, xenon-135, has a very high thermal neutron cross-section. The element has been detected in Mars atmosphere. 

Touring the formation of radioactive fallout particles, one of the most important processes is the uptake, in the cooling nuclear fireball, of the vaporized radioactive fission products by particles of molten soil or other environmental materials. Owing to the differences in the chemical nature of the various radioactive elements, their rates of uptake vary, depending upon temperature, pressure, and substrate and vapor-phase composition. These varying rates of uptake, combined with different residence times of the substrate particles in the fireball, result in radiochemical fractionation of the fallout. This fractionation has a considerable effect on the final partition of radioactivity, exposure rate, and radionuclides between the ground surface and the atmosphere. 

Radioactive Debris in Cows. Nuclear detonations produce a complex mixture of radionuclides that includes both the fission products and those induced by neutron activation. Experiments relating to such events... 

For a nuclear weapon hurst in air. all materials in the fireball are vaporized. Condensation of fission products and other bomb materials is then governed by the saturation vapor pressures of the most abundant constituents. Primary debris can combine w ilh naturally-occurring aerosols, and almost all of (he fallout becomes tropospheric or stratospheric. If the weapon detonation takes place within a few hundred Icet of (either above or below) a land or water surface, large quaniilies of surface materials are drawn up or thrown into the air above Ihe place ol detonation. Condensation of radioactive nuclides in this material then leads in considerable quantities of local fallout, but some of the radioactivity still goes into tropospheric and stratospheric fallout. If the hurst occurs sufficiently fur underground, the surface is not bruken and no fallout results. 

The radioactive wastes associated with nuclear reactors fall into two categories (1) commercial wastes — the result of operating nuclear-powered electric generating facilities and (2) military wastes—the result of reactor operations associated with weapons manufacture, Because the fuel in plutonium production reactors, as required by weapons, is irradiated less than the fuel in commercial power reactors, the military wastes contain fewer fission products and thus are not as active radiologically or thermally. They are nevertheless hazardous and require careful disposal. 

The third principal component of environmental radioactivity is that due to the activities of humans, the anthropogenic radionuclides. This group of nuclides includes the previously discussed cases of 3H and 14C along with the fission products and the transuranium elements. The primary sources of these nuclides are nuclear weapons tests and nuclear power plant accidents. These events and the gross nuclide releases associated with them are shown in Table 3.1. Except for 14C and... 

In recent years, acute air pollution problems have been associated with large power plants. Stack discharges depend on the type of power plant. In oil-fired power plants, the emissions are mainly S02 and NOx. In coal-fired operations, emissions include S02, NOx and a variety of radioactive nuclides derived from coal. In nuclear power plants, emissions are limited to small amounts of radioactive fission products. 

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