Radioactivity from solid waste form

One of the more important factors affecting the isolation of radioactive waste is the rate of release of the radioactivity from the solid waste form to the environment. The most probable mechanism for release and transport of radioactivity from a solid waste form is by leaching of radioactive isotopes with groundwater. The objective of leach-testing various waste forms is to evaluate the rate at which specific hazardous radionuclides migrate from waste if and when the waste form comes in contact with groundwater. In this paper, measurement of leach rates of radioactive waste by a method which incorporates neutron activation is described. 

The chapters of this volume are organized into sections that cover the chemical aspects that are important to understanding the behavior of disposed radioactive wastes. These aspects include radionuclide sorption and desorption, solubility of radionuclide compounds, chemical species of radionuclides in natural waters, hydrothermal geochemical reactions, measurements of radionuclide migration, solid state chemistry of wastes, and waste-form leaching behavior. The information in each of these sections is necessary to predict the transport of radionuclides from wastes via natural waters and thus to predict the safety of the disposed waste. 

All of these complexes, including those with the lanthanide cations, are prepared directly from W04 and the appropriate An" (Ln ), and have only a narrow pH-range of stability (ca5.5-6.5) in aqueous solution. However in the solid state the ammonium salt of [Th(W50is)2] yields the cubic tungsten bronze ThxWOs (x 0.1) when heated in an inert atmosphere to 725°C. It has been proposed that bronzes prepared in such fashion from appropriate POM salts could be used as inert waste forms for actinides and radioactive lanthanides [40]. 

As for the material, the primary distinction is between solid, liquid, and gaseous waste. Solid waste includes any kind of contaminated or activated plant components, tools, filters, and protective clothing. Most of the liquid wastes are aqueous solutions or sludges. The term gaseous waste will be used for radioactive gases recovered from off-gas streams and contained in an appropriate form. 

The waste resulting from the above activities comes in various forms (i.e., gaseous, liquid, or solid). These wastes have different characteristics. For safety and technical reasons, the various forms of wastes are usually categorized by their levels of radioactivity, heat content, and potential hazard. 

Zirconia is an important refractory ceramic. It also forms the basis of solid electrolytes in systems such as Zr02-Y203. Both zirconia and hafnia figure prominently in refractory ceramics proposed for the containment of uranium and plutonium radioactive waste. The monoclinic form of Zr02, baddeleyite, is a widespread accessory mineral. Zirconia undergoes transitions from monoclinic to tetragonal to cubic, with thermochemical parameters listed in Table 5. 

The second step is to dissolve the metal oxide fuel using strong nitric acid. The object is to bring all the fission products, uranium, and transuranics, into solution to feed the extraction process. Some of the fission products exceed solubility limits and the fine solids formed must be removed before extraction. Provisions to recover nitrogen oxides and collect gaseous fission products released during this step must be in place. The stainless steel and zircaloy fuel jackets from the fuel assemblies do not dissolve and are separated from the solution, washed, checked for radioactivity, and packaged for disposal as low-level radioactive waste. 

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. 

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