Hazardous or toxic waste destruction or containment – Destruction or containment of radioactive waste
Patent
1997-07-07
1998-12-22
Mai, Ngoclan
Hazardous or toxic waste destruction or containment
Destruction or containment of radioactive waste
588 13, 423 6, 134 2, 134 7, 210682, 376309, 376310, 976DIG376, G21F 900
Patent
active
058527869
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a process for decontaminating radioactive materials.
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 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 the 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 requiring disposal. The technology has 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 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 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 which contain the radioactive elements commonly found in contaminated reactor circuits. This non-selective metal dissolving capacity is a disadvantage of the acidic solutions and makes them unsuitable for the decontamination of material such as soil that contains iron and other metals that are not intended to b
REFERENCES:
patent: 5322644 (1994-06-01), Dunn et al.
Database INIS, International Atomic Energy Agency, AN 26 (22):70940 entitled, "Chemical Method to Recover Uranium and Plutonium from Debris in Spent Fuel Storage Tank" to B. Venkataramani et al. (1995).
Database INIS, International Atomic Energy Agency, AN 27 (+): 2014 entitled, "Plutonium Mobility Studies in Soil Sediment Decontaminated by Means of a Soil-Washing Technology" to M.C. Negri et al. (1995).
Bradbury David
Elder George Richard
Bradtec Limited
Mai Ngoclan
LandOfFree
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