Patent Number: 053226445
Section: summary

BACKGROUND OF THE INVENTION Environmental contamination with radioactive materials is a common problem. The problem may occur as a result of mining operations, such as for uranium, or contamination due to operation of nuclear facilities with inadequate environmental controls, or from the disposal of radioactive wastes. Alternatively, contamination may occur as a result of dispersion of uranium billets which have been used as a high density material in military or civil applications as a result of warfare or civil accident. Mining operations have established practical and economic methods for the economic recovery of some radioactive elements from contaminated materials. The objective of mining, however, is usually the economic recovery of materials and secondary waste is rarely the major issue. In environmental clean-up, the economic objective is to complete effective clean-up with minimum secondary waste at minimum cost, and the value of recovered radioactive substances is of secondary importance. Techniques and chemicals which would not be economical or appropriate for mining applications may become practical for environmental clean-up. It is well established that radioactive elements can be recovered from environmental materials by mechanically washing with water with or without surface active additives. However, such procedures are generally limited to the mechanical separation of solids, and will not remove contaminants that are chemically bound to the solid phase. There are established chemical methods for dissolving insoluble radioactive contaminants in concentrated solvents, such as strong acids, in a process known as acid leaching. Such procedures are effective, but are disadvantageous if the spent concentrated solution ultimately becomes waste. In many cases, the concentrated solvents themselves are hazardous in addition to containing the radioactive contaminant that the process is designed to concentrate. The acid leaching and other processes using concentrated solvents to dissolve the radioactive contaminant have the further disadvantage of also dissolving other contaminants that the process was not designed to remove, such as nonradioactive metals. In the decontamination of internal surfaces of nuclear reactor circuits, early processes involved washing with concentrated chemical solutions to dissolve contaminants to yield a concentrated solution containing the contamination. The processing of these waste solutions was found to be difficult and inconvenient and resulted in them becoming waste and requiring disposal. The technology has now progressed to allow the recovery of radioactivity, typically by ion exchange, in a dilute acidic recirculating system. These solutions, being dilute and acidic, do not contain carbonate and are not particularly useful or appropriate for dissolving actinide elements because they do not form soluble complexes with the actinide elements. In reactor decontamination processes, it has been established that certain organic reagents can be used to dissolve contamination and yield it to an ion exchange resin in a recirculating process in such a way that the organic reagent is continuously re-used. Examples of solutions used in acidic reactor decontamination processes are vanadous formate, picolinic acid and sodium hydroxide. Other processes typically use mixtures of citric acid and oxalic acid. These reactor decontaminating solutions have the disadvantage of not being capable of being used in a single one time application to dissolve actinides, radium, and certain fission products, such as technetium. Previous reactor decontaminating solutions do not contain carbonate and are acidic, dissolving the iron oxides of the radioactive elements commonly found in contaminated reactor circuits. This nonselective metal dissolving capacity is a disadvantage of the acidic solutions and makes them unsuitable for decontamination of material such as soil that contains iron and other metals that are not intended to be recovered. Another disadvantage of acidic solutions is that materials such as concrete or limestone are subject to damage or dissolution in an acidic medium. Also, in dealing with previously known washing solutions for treating soil, these solutions contain too many nonselectively dissolved contaminants preventing subjection of the solution to recovery of contaminants and recirculation of the solution to accomplish further decontamination. It has been established that uranium and transuranic radioactive elements can be dissolved in concentrated acidic (pH&lt;1) chemical systems. The acidity poses difficulties as discussed above. Uranium and sometimes thorium are recovered in mining operations in a concentrated basic medium containing carbonate. The use of concentrated solutions is motivated by the need to dissolve materials at a rate economic for mining operations, and such solutions are not particularly suitable where avoidance of secondary waste is of primary concern. There are also references that suggest that uranium and plutonium can be dissolved in a dilute basic solution containing carbonate, citrate (as a chelating agent) and an oxidizing or reducing agent. Such solutions are not, however, suitable for the recovery of radium/barium sulfate because they do not form soluble complexes from barium sulfate. SUMMARY OF THE INVENTION This invention relates to the recovery of radioactive elements, especially technetium, radium, and actinides such as thorium, uranium and transuranic elements, from certain types of contaminated materials. These materials could be natural, such as soil, or man-made materials, such as concrete or steel, which have become subject on a large scale to contamination. The process of the present invention provides that contaminated material is contacted with a dilute, basic, carbonate recirculating dissolving composition that dissolves radioactive contaminants. Contaminated material can be fed in to the process and cleaned material removed continuously therefrom. The contaminants are recovered from the solution by ion exchange, selective adsorption, reagent destruction, filtration or a combination of these techniques. The recovery steps concentrate the contaminants for recovery in such a way that non-residual reagent constituents do not build up in the system. The recirculating dissolving composition can be applied to small particulate materials such as soil in a contained vessel, or to large standing objects such as concrete walls, or steel structures. It is an object of the invention to provide a method to dissolve and concentrate radioactive contaminants from materials. Another feature of the invention is that the concentrated contamination can be further processed for recovery or disposal. It is a further object of the invention to provide a method for the decontamination of soil and the recovery of radioactive contaminants, which uses a dilute basic carbonate solution to achieve dissolution, thereby minimizing risks of environmental or safety hazards, or structural damage. It is an object of the invention to use chemical systems that dissolve the contaminants in a material as selectively as possible and avoid the dissolution of metals, such as iron and lead. It is another object of the present invention to use a recirculating dissolving system wherein secondary chemical waste is avoided, and reagents do not build up in concentration during the application of the process.