Nuclide waste

Nuclear-physical methods ai e the basic ones in controlling environmental pollution which results from nucleai -power complexes and power plants work. Oil and gas production leads to the extraction of radio nuclides of natural origin in considerable amounts, which later spread from oil-slimes and water wastes in the neighborhoods of oil and gas producing entei prises. Similaidy, toxic and radioactive elements can pollute environment in case of mineral deposits extraction. 

Half-lives span a very wide range (Table 17.5). Consider strontium-90, for which the half-life is 28 a. This nuclide is present in nuclear fallout, the fine dust that settles from clouds of airborne particles after the explosion of a nuclear bomb, and may also be present in the accidental release of radioactive materials into the air. Because it is chemically very similar to calcium, strontium may accompany that element through the environment and become incorporated into bones once there, it continues to emit radiation for many years. About 10 half-lives (for strontium-90, 280 a) must pass before the activity of a sample has fallen to 1/1000 of its initial value. Iodine-131, which was released in the accidental fire at the Chernobyl nuclear power plant, has a half-life of only 8.05 d, but it accumulates in the thyroid gland. Several cases of thyroid cancer have been linked to iodine-131 exposure from the accident. Plutonium-239 has a half-life of 24 ka (24000 years). Consequently, very long term storage facilities are required for plutonium waste, and land contaminated with plutonium cannot be inhabited again for thousands of years without expensive remediation efforts. 

Deep geological disposal is the most favored solution for the permanent disposal of nuclear wastes with long half-lives. Although the locations of the burial places are selected with outmost care to avoid migration of the wastes in nature over a very long period of time, no barrier can be safe forever, so, numerous studies are in progress to determine the main factors that could cause leaks of radioactive nuclides. Soluble compounds in ground water are likely to play a major role in the release of actinides. 

The underground disposal system consists in essence of a series of barriers which prevent disturbance of the wastes and inhibit escape of radioactive nuclides. In general, each barrier alone and independently of the rest is capable, under appropriate circumstances, of retaining the wastes for the requisite time period. Several such barriers together can provide a high degree of assurance that adequate isolation is maintained. The basic components of the system are ... 

This will influence the flow of groundwater to and from the wastes and the chemical composition of. the groundwater. Also It will adsorb water-borne radioactive nuclides and retard their movement. 

The stability of iron oxide suspensions is relevant to fields as varied as the paint industry, extraction of iron from its ores, the structure of soils, hydrometallurgy and waste water treatment. The ease of homogensisation of paint, for example, is controlled by proper adjustment of the stability of the pigment suspensions. In ground waters, the settling behaviour of small iron oxide particles influences transportation of trace elements and radio-nuclides. The stability of a dispersion of magnetic particles can determine the quality of ferrofluids and magnetic tapes. 

Two nuclide concentrations (<10 M and 10"S M) were used, representing slow and fast dissolution of the waste. 

Allard, B., Kipatsi, H., Torstenfelt, B. and Rydberg, J., "Nuclide Transport by Groundwater in Swedish Bedrock", Symp. on Radioactive Waste Management, Boston, Nov. 28 - Dec. 1, 1978, in press... 

Any solid waste form should serve to reduce waste volume and provide short term stabilization under conditions such as fracture, fire, or water immersion which could result from a transportation mishap. In the geologic storage scenarios currently receiving the most scrutiny, the most likely path to the biosphere has been identified as aqueous transport of nuclides via groundwater. Thus an acceptable waste form would also resist dissolution under ambient repository conditions, with the obvious benefit of assuring a sufficiently low nuclide release rate into an aquifer to preclude a significant threat to health and safety. 

Figure 1. Elemental distribution of fission waste nuclides on a sodium titanate column. The distribution shown on the left was determined from qualitative analyses of the numbered column segments. The Cs and Na distributions on the right were obtained by quantitative analyses of each numbered segment. (O), Cs ( ), Na.

The waste nuclides were fractionated on the column into several hands as shown in Fig. 1 for a sodium titanate column. 

The use of ion exchange resins and natural or synthetic inorganic exchange materials in the nuclear industry is well documented ( ). In the waste solidification application, the titanates or niobates offer no unique sorption properties. They do, however, provide a relatively high overall sorption capacity for a variety of nuclides in materials which can be converted into a stable ceramic host for the sorbed ions. After the sorption process, the column bed must be consolidated to reduce surface area. The project emphasis was directed toward a stable waste form and a considerable effort was devoted to producing and characterizing a highly dense form with favorable physical, chemical and thermal properties (l ). 

The consolidated titanate waste pellets are similar in appearance to their glass counterparts, i.e., both are dense, black and apparently homogeneous. Microscopic analyses, however, reveal important differences between these two waste forms. While little definitive work has been done with glassy waste forms, it is apparent that several readily soluble oxide particulates of various nuclides are simply encapsulated in the glass matrix. The titanate waste form has undergone extensive analyses which includes optical microscopy, x-ray, scanning electron microscopy, microprobe, and transmission electron microscopy (l ) The samples of titanate examined were prepared by pressure sintering and consisted of material from a fully loaded titanate column. Zeolite and silicon additions were also present in the samples. 

Figure 4. Comparative accumulation of actinides by small mammals from contaminated soil or sediment relative to the accumulation of plutonium-239. Accumulation factor (AF) = concentration of nuclide in the internal small mammal body -- concentration of nuclide in dry soil. Twelve shrews and seven rats and mice from a floodplain forest were composited to yield four and three separate analyses, respectively. Twelve cotton rats inhabiting the banks of a liquid waste pond (3513) also were analyzed.

In the sediments being studied, it is believed that nuclide migration away from the waste form will primarily be due to molecular diffusion. The simpler mathematical analyses of diffusion which yield anal3rbical solutions show that, to a first approximation, the migration rate of species i should generally be inversely proportional to the magnitude of Kp (l). Hence, the... 

For the nuclides studied (rubidium, cesium, strontium, bariun silver, cadmium, cerium, promethium, europium, and gadolinium) the distribution coefficients generally vary from about 10 ml/gm at solution-phase concentrations on the order of 10 mg-atom/ml to 10 and greater at concentrations on the order of 10 and less. These results are encouraging with regard to the sediment being able to provide a barrier to migration of nuclides away from a waste form and also appear to be reasonably consistent with related data for similar oceanic sediments and related clay minerals found within the continental United States. 

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