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To review radioactive waste disposal and its implications to public health and safety.
The nuclear energy industry is a new and rapidly expanding industry in the United States and throughout the world. In 1968, there were 19 Atomic Energy Commission (AEC) licenses in effect for operating nuclear power reactors, and 45 permits in effect for construction of new nuclear power reactors. In the same year there were 5919 organizations holding AEC reactor byproduct (radioisotope) licenses. These organizations include physicians, medical institutions, industries, universities, laboratories and institutes. In addition, there are others not under direct ACE control. The AEC does not exercise control over natural or accelerator produced radioisotopes, but only over nuclear reactors, reactor produced radioisotopes and special nuclear material ( fissionable material).
The nuclear energy industry is unique in that the potential hazards from its radioactive wastes were quickly recognized during the industry's early stages of development, and a great deal of money and effort has been expended for developing safe and effective methods for handling and disposing of radioactive wastes. Another unique aspect of the nuclear industry is that nuclear reactors must be designed in such a manner that environmental contamination does not occur. Nuclear accident, nevertheless, do occur. Recent examples include the Three-Mile Island and Chernobyl accidents.
In general, radioactive wastes are similar to chemical wastes, except that they contain radioactive materials. Radioactive materials cannot be detected by human or animal senses, but they can cause biological damage by the ionizing radiations they emit. Radioactive materials emit alpha particles beta particles or gamma rays. In general, since alpha and beta particles have a relatively short range, materials emitting these radiations constitute a hazard only when ingested in food or drinking water, or when inhaled. However, gamma emitting radionuclides constitute an external hazard, as their rays may penetrate the body and cause damage at any depth. In general alpha and beta emitters are far more hazardous than are pure gamma emitters when ingested because of their short range and high specific ionization. Another factor contributing to the hazard of ingested radionuclides is that some materials may become incorporated into tissues by normal biological processes. For example, iodine-131 is concentrated in the thyroid, cesium-137 in the muscles, and Br-90, Ca-45, Ra-226 are concentrated in the bones near the highly radiosensitive blood-forming organs. Therefore, even small concentrations of radioactive materials may be dangerous when ingested over long periods of time. Since the hazard presented depends upon the biological fate of the radioactive atom, the half-life and the radiation that are emitted, maximum permissible concentrations (MPC) for drinking water and air have been adopted for each radionuclide which takes these factors into consideration. These MPC’s are based upon recommendations by the International Committee on Radiation Protection (ICRP) and the National Committee on regulations of the U.S. Department of Energy (and the former Atomic Energy Commission), and have the force of law in the United States. Whenever, radioactive wastes are discharged to the environment or sewer, they must not exceed the MPC’s for typical radioactive materials that are given in Table 1. They are given in mg/l so that the maximum permissible concentrations of the radioactive isotopes may be more easily compared with the MPC'S for the corresponding table.
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It can be seen from the above table that the MPC’s for radioactive isotopes are extremely low. The MPC calculations are based on the ICRP recommendations that no member of the public receive a radiation dose exceeding 0.5 rems per year above background and medical exposures. This value is very conservative. However, it conforms to the philosophy that there is no lower limit or threshold below which no radiation damage occurs. In large populations groups, the primary concern is genetic rather than somatic changes.
The single greatest source of radioactive wastes results from the re-processing of spent nuclear fuel elements. This process is economically necessary because valuable fissioned uranium-235 can be recovered for reuse. Plutonium-239 is produced during reactor operation by burying activation of uranium-238.
The reprocessing of nuclear fuel elements greatly compounds the problem of waste disposal. If the nuclear industry had no further need for the spent fuel elements that could be easily disposed of at relatively low cost by burying or storing the intact elements at a carefully controlled disposal site. However, the reprocessing of fuel elements gives rise to enormous quantities of contamination solvents, aqueous solutions, washings, sludges and vapors. Confining, concentration and eventually disposing of these bulky and highly radioactive materials presents a complex and expensive problem for the nuclear industry.
The large quantities of radioactive wastes from fuel reprocessing result from the fission products, since the PU-235 and U-235 are usually recovered with nearly 100 percent efficiency. Table 2 lists some of the predominant fission products and their half lives. From this table it can be seen that two radioactive products, Sr-90 and Cs-137, comprise the principal long term waste disposal problem because of their half lives of 29 and 30 years, respectively. The primary sources for solid radioactive wastes in a nuclear plant are: evaporated bottoms, spent bead resins (from mixed bed demineralizers), spent powered resins, filler sludges, spent filter cartridges, and miscellaneous paper, rags, tools, clothing, etc.
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In the reprocessing, the fuel elements are stored for 3 or 4 months are to allow the high activity, short-lived fission products to decay. The "cooled" elements, which have reduced in activity by about ten thousand fold, are still a biological hazard and must be handled remotely. The stainless steel jackets of the elements are cut open and the contents are dissolved with the jackets in nitric acid. The plutonium-239 and uranium-235 are then recovered by one of three different extractions. The remaining fission products may be further treated for recovery of Sr-90 or Cs-137 (which have some commercial value). The processing wastewater is then concentrated, neutralized and stored in stainless steel and concrete underground tanks. Table 3 gives some typical characteristics of reactor fuel reprocessing wastes.
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Because of the high cost of storing liquid wastes, the great increases in nuclear power, and the possibility of tank leaks and corrosion, there have been many efforts directed toward solidifying these high level fuel reprocessing wastes for eventual underground disposal in salt domes or other formations. Some of the methods currently being investigated for solidifying high level radioactive wastes include pot calcinating spray solidification, phosphate glass formation and fluidized bed calcination.
In addition to the high level fuel reprocessing wastes in their cooling systems. There are two sources of radioactive contamination in reactor cooling water. Impurities in the water may be activated by neutrons in the reactor core to form radioactive products. Also, if a fuel element should rupture or develop a leak during operation, radioactive fission products may escape into the cooling water. To minimize the possibility of these contaminants from being released to the secondary cooling system, the primary coolant (usually pressurized water) circulates through the reactor core to remove heat. It then transfers heat to a secondary cooling system. The secondary coolant, after its heat has heen removed by power generation, is then discharged with negligible radioactivity. The primary coolant is passed through an ion exchange system to remove contamination before it is re-circulated through the reactor. The ion-exchange resins are removed and properly disposed of as the radioactivity builds up to prescribed levels.
A widespread source of relatively low volume and low level radioactive wastes are the many facilities that use radioisotopes for tracers. These facilities include medical institutions, universities and research laboratories. Many of these institutions are non-AEC users, in that their radioisotopes are not reactor experiments, decontamination of equipment, laboratories and clothing. The most common method of disposal is to utilize the dilution capacity of sewers, streams and the atmosphere. For example, a survey made in 1952 showed that 41 percent of 1027 users of radioisotopes disposed of their waste by dilution and discharge to sewers (4). In addition to diluting wastes, it is a common practice to hold short half-lived materials in order to allow the activity to decay to lower levels. For example, if one were to store wastes for ten half-lives, the activity would be reduced to about 1/1000th of its original activity.
Long lived radioactive wastes and solid wastes, such as laboratory glassware, disposable gloves, filters and experimental animal carcasses, are frequently disposed of by burial at AEC burial sites. For example, it is the policy of Purdue University to dispose of all long-lived radioactive wastes by burial at an AEC burial site in Illinois.
In some instances certain radionuclides may be biologically concentrated by organism in the environment when disposed of in very low concentrations. For this reason it is sometimes desirable to add some stable isotopes of the same element to the radioactive waste. For example if P32 labeled phosphoric acid is to be disposed of by way of the sewer, some stable phosphate may be added to reduce the extent of reconcentration of the P32 microorganisms. This technique is referred to as carrier dilution.
There are some special techniques for renewing or concentration on the low level radioactive wastes that are practiced by some of the larger AEC operated laboratories. The Knolls Atomic Power Laboratory in New York, for example, evaporates its wastewater to about 40% solids, then dries it with vacuum driers. The dried radioactive material is sealed in steel in steel drums and buried.
Chemical precipitation is used at Argonne and Oak Ridge National Laboratories for treating low level wastes. Phosphates are most often used as a precipitating agent because most of the radioactive elements found in wastewater from insoluble phosphate compounds. Several other methods of concentrating low level wastes have been studied, such as solvent extraction, electrolytic separation, biological concentration and ion exchange with clay or resins, however, these method appear not to be widespread.
All treatment methods discussed for radioactive wastes involve one of three principles
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