Waste disposal, fission reactor

One of the more difficult problems relating to fission-reactor waste disposal is the evaluation of the suitability of a particular site for the long-term storage of actinide by-products from fuel reprocessing. Current U. S. plans call for underground storage in containers placed in a stable geologic environment. 

Even a smoothly operating nuclear power plant has certain inherent problems. Thermal pollution, resulting from the use of nearby natural waters to cool reactor parts, is a problem common to all power plants (Section 13.3). More serious is the problem of nuclear waste disposal. Many of the fission products formed in nuclear reactors have long half-lives, and proposals to bury containers of this waste in deep bedrock cannot be field-tested for the thousands of years that the material will remain harmful. It remains to be seen whether we can operate fission reactors and dispose of the waste safely and economically. 

Some of the rare earth nuclei have small capture cross-sections for neutrons, and some have extremely large capture cross-sections. From the practical viewpoint, since the rare earths are formed in fission, such information plays an important role in nuclear energy technology. A knowledge of the cross-section, lifetimes and chemical behavior of the isotopes is extremely important in the design of reactors, control rods, nuclear poisons and reactor waste disposal processes. 

The fear of accidents like Chernobyl, and the high cost of nuclear waste disposal, halted nuclear power plant construction in the United States m the 1980s, and in most ol the rest ol the world by the 1990s. Because nuclear fusion does not present the waste disposal problem of fission reactors, there is hope that fusion will be the primary energy source late in the twenty-first centuiy as the supplies of natural gas and petroleum dwindle. 

Gauthier-Lafaye, F., 2002, 2 billion year old natural analogs for nuclear waste disposal the natural nuclear fission reactors in Gabon (Africa). C. R. Physiquee 3 839-849. 

The fissioning of U and Pu in a nuclear reactor produces a large number of radioactive fission products. Most of these decay to stable isotopes within a few minutes to a few years after the fuel has been discharged from the reactor and therefore pose no problem in the management of nuclear fuel wastes. There are, however, a number of longer lived radionuclides that must be considered in assessing the environmental impact of any nuclear fuel waste disposal vault in the geosphere. 

The technetium isotope of interest for nuclear fuel waste disposal is Tc. It is a pure 3-emitter (E = 0.293 MeV) with a half-life of 2.13x10 years. Its high fission yield of 6% accounts for the relatively high concentration 0.02% by weight) (1) in fuel discharged from a CANDU (CANada Deuterium Uranium) reactor (burnup 650 GJ/kg U). 

Bebbington, William P., "The Reprocessing of Nuclear Fuel", Scientific American, Vol. 235, No. 6, December 1976, Page 30 Cohen, Bernard L., "The Disposal of Radioactive Waste from Fission Reactors", Scientific American, Vol. 236, No. 6, June 1977, Page 21... 

The valuable fertile elements are recovered from the acid solution by extraction with an organic solvent. The acid residue, containing the extremely radioaetive fission products, is processed to convert the waste into a stable solid form. The fission product waste, in a very concentrated form, is stored for ultimate disposal. This waste represents a different problem than the waste from current burner reactors. Because of the chemical concentration step there is less total mass of material. The same concentration process that reduced the mass of the waste concentrates the radiation produced into a smaller more intense package. This waste is so radioactive that it gets hot and must be actively cooled or diluted to prevent meltdown. Safe storage and disposal methods are very difficult to design. 

A fusion reactor s impact on the environment will be limited to the site it occupies, and the waste heat left over when the reactor heat is used to generate electricity. The only significant radioactive waste disposal problems occur when the reactor has worn out and must be dismantled. In the decommissioning of fusion reactors, the internal parts will be radioactive from years of exposure to the neutrons released by the fusion reaction. The total amount of waste remaining will depend critically on the materials used in the fabrication of the reactor. With selection of the proper elements the radioactive waste disposal problem will be in the range of 10,000 to 1,000,000 times smaller than that involved in the dismantling of a fission reactor and its waste. 

It will be wise to draw detailed plans for decommissioning fusion reactors at the beginning of their development. This will avoid the wretched waste disposal problems that plague the fission reactor industry. The fusion reactors will contain far less radioactive material than a fission reactor. Whatever the amount, one hundredth to one millionth, proper disposal will be required. 

Unlike the waste from fission reactors, none of the radioactive materials present in the decommissioned fusion reactor can be used to make bombs. The expended fusion reactor parts will hold no interest for terrorist groups. The materials are only radioactive isotopes formed in the structural elements of the reactor caused by the neutrons from the fusion reaction. There are no fertile elements or isotopes. No dangerous fission products are produced by the fusion reaction. Disposal of worn out fusion reactors will be safe and simple as compared to the disposal of waste and structures of decommissioned fission reactors. 

Most of the present nuclear reactors have been burning solid fuel elements of either normal or enriched uranium. Thus far, it has been necessary to reprocess fuel in order to recover valuable fissionable or fissile material. It is possible that fuel elements will be developed for future reactors which can be burned to the point where it is not economically justifiable to recover fissionable materials. Obviously, this depends upon the value of these materials. Such a procedure would provide an optimum solution to the major part of the waste disposal problem. The fission products would still be locked in the fuel element, simple disposal techniques could be employed, and in fact, spent fuel elements would probably have secondary uses as radiation sources. 

The structural material will be exposed to high radiation fields, causing radiation damage and induced radioactivity. The preferred material at present seems to be reinforced carbon, special steel and vanadium. Thus, considerable amounts of (ti, 330 d), and possibly also some very long-lived Mn, are formed. This induced activity will be a maintenance hazard, requiring remote control systems. However, compared to a fission reactor of similar size, the fusion reactor will contain less total radioactivity, and (of special importance in waste disposal) be free of long-lived a-activities. 

One namral analogue for geological disposal of nuclear waste is at a site near Oklo, Gabon, West Africa. About 2 billion yr ago, a uranium ore body sustained fission for a period of about 100,000 yr. Most of the fission and activation products from this namral reactor migrated only a short distance from the fission sites, which gives natural supporting evidence for the potential of radionuclide transport from underground waste disposal facilities. 

The major sources contributing to Tc in the environment are fallout from atmospheric nuclear weapons tests and releases from the nuclear fuel cycle, i.e., authorized or accidental releases from nuclear installations (e.g., reprocessing or enrichment plants, nuclear reactors), releases from waste disposal sites, and from dumping of nuclear materials. Contributions from natural processes, i.e., spontaneous fission of in mineral ores such as pitchblende or nuclear reactions in molybdenum ores irradiated with cosmic-ray neutrons are negligible. 

Transmutation of long-lived nuclides (actinides and fission products) in the fuel cycle of fast reactors should proceed, at least, until the biologically equivalent activity of the waste to be buried declines to that of the natural uranium consumed (this condition is referred to as equivalence in terms of radiation and biological hazards). Notably, such equivalence may be attained both by the time of burial and within a historically short and reliably predicted period of, e.g., 200-1,000 years. This approach allows reasonably minimizing the mass and hazard of long-lived waste, while the specific conditions of waste disposal should meet the national regulatory requirements. 

Extensive research work has been going on for the safe disposal of radioactive wastes in different parts of the world, especially in the North American and West European countries where fission reactors are aplenty. Different types of clays and cl -derivatives are found to be more effective in safe disposal of different types of radioactive wastes. The clays are easily available than any other buffer materials, and the procedures of waste disposal with clay buffer are less expensive. This encoimages the researchers to explore further possibilities of application of cl s in this field. 

Write an essay on the pros and cons of nuclear power (based on nuclear fission), paying particular attention to its effect on global warming, nuclear reactor safety and weapon risks, and nuclear waste disposal. 

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