Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Water – sewage – or other waste water
Reexamination Certificate
2002-03-25
2004-01-27
Phasge, Arun S. (Department: 1753)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Water, sewage, or other waste water
Reexamination Certificate
active
06682646
ABSTRACT:
FIELD OF INVENTION
The present invention is related to the removal of surface radioactive contamination from metallic materials and the collection of the removed contamination in a form suitable for convenient radioactive waste treatment and disposal. More specifically the invention relates to the dissolution of surface contamination with an acid solution, the subsequent conversion of the dissolved ions into metal particles by means of an electrochemical cell and the removal of the metal particles from the decontamination system.
BACKGROUND
Chemical decontamination has become an established method for reducing the radiation exposure of workers at nuclear plants. In this method chemicals are added to a part of the cooling water circuit of the plant and these chemicals dissolve radioactive deposits from the circuit surfaces. The chemicals and the radioactive components are removed from the circulating cooling solution by ion exchange. An example of such a processes is disclosed in U.S. Pat. No. 4,705,573 titled “Descaling Process.”
More recently, the same general principle has been applied to the treatment of components which are no longer required for service in nuclear plants. The decontamination of these components not only reduces the worker radiation exposure, but also reduces the radioactive level so that the decontaminated components may be treated as a lower category of radioactive waste. If the decontamination process reduces the radioactivity below a certain level, the components can be regarded as non-radioactive. These processed components may then be reused as components in other nuclear or non-nuclear applications or recycled. This decontamination processing is sometimes referred to as Decontamination for Decommissioning. An example of this kind of process is disclosed in U.S. Pat. No. 6,147,274 titled “Method For Decontamination Of Nuclear Plant Components.”
The existing Decontamination for Decommissioning processes expose contaminated metal components to a decontamination solution which removes a layer of material. The radioactive solids and cations are then removed from the decontamination solution. The decontamination for decommissioning processes do not generate liquid radioactive waste because the water used to make up the decontamination solutions is returned to a pure deionized form at the end of the process and can be recycled. The decontamination processes also use dilute solutions and avoid the hazards associated with using concentrated chemicals. The decontamination processes are particularly useful for cleaning components of complex shape (such as tube-in-shell heat exchangers) where mechanical decontamination methods are difficult to apply.
A significant problem with existing Decontamination for Decommissioning processes has been the difficulty of handling the secondary radioactive waste. In some countries radioactive waste burial facilities do not exist and it is necessary to store any secondary waste generated indefinitely on-site. For secondary radioactive waste removal the waste must be transferred to a storage facility where storage container integrity and radiation shielding can be guaranteed. The secondary radioactive waste must also be in a form suitable for transportation and burial. In one such method, the secondary radioactive waste is conditioned as a solid monolith and contained in easily handled drums. In all countries it is imperative to maximize the stability and minimize the volume of secondary radioactive waste produced.
Decontamination for decommissioning processes usually generate ion exchange resins as the final secondary radioactive waste form. All the radioactivity from the decontaminated components and any residual chemicals are collected in this ion exchange resin waste. In the United States radioactive ion exchange resin waste is routinely de-watered and sent for burial in high integrity containers. In other countries regulations prohibit handing radioactive ion exchange resin waste in this manner. The resin waste is also not a convenient waste form because it only holds a fraction of its own weight in the form of radioactive or metallic contamination. Thus, the final radioactive resin waste does not consist of just the contamination removed from the component surfaces, but also the organic polymeric materials which make up the ion exchange resin itself. This inefficiency in the waste processing of resin based decontamination is a significant drawback when compared to mechanical decontamination methods where a thin layer of contaminated material is removed from the component surfaces. The only waste produced by mechanical decontamination methods is the material removed from the component.
U.S. Pat. No. 5,078,842A titled “Process For Removing Radioactive Burden From Spent Nuclear Reactor Decontamination Solutions Using Electrochemical Ion Exchange” discloses a process in which ion exchange resin can be used as an intermediate waste form and is hereby incorporated by reference. The '842 patent discloses a three compartment electrochemical ion exchange cell used to remove radioactive cations from a decontamination solution. A decontamination solution passes through a central compartment of the ion exchange cell containing cation exchange resin. The cation exchange resin removes dissolved contaminants and metal ions from the decontamination solution. The ions held on the resin then migrate under the influence of an electric current into a cathode compartment and reduced to a metallic deposit on a cathode. An anionic equivalent of this process is disclosed in U.S. Pat. No. 5,306,399 titled “Electrochemical Exchange Anions In Decontamination Solutions” and is also hereby incorporated by reference.
Although the method described in the '842 patent was extensively tested, the process has not been applied on a full commercial scale for nuclear reactors. A reason for the lack of use is that the process was designed to be part of operational reactor decontamination, which was the subject of commercial interest in the past. Operational decontamination takes place during the nuclear plant maintenance shut down and must be performed in a very short time. Indeed, maintenance shut-downs have become so short at nuclear plants that there is often insufficient time for the decontamination process using a ion exchange cell to be accommodated, let alone an ancillary decontamination solution treatment system. These time constraints require the electrical migration process to happen very quickly, which in turn required a large input of electric power to the ion exchange cell together with heavy, expensive equipment which was commercially unattractive.
The decontamination process disclosed in the '842 patent is much more suited to decontamination for decommissioning applications, where waste volume reduction is imperative and the time constraints are less onerous. In this case the electrical process can take place at low current with modest equipment over a longer period of time. Of crucial importance, the '842 patent does not address the manner in which the radioactive deposit formed on the cathode should be handled. On a laboratory scale the cell described could then be dismantled and the deposit removed mechanically from the electrode surface. Such a method is not feasible at full scale within proper radiation protection constraints. For the method to be fully commercialized it would be necessary to devise a new method for removing the radioactive deposit on the cathode from the ion exchange cell for disposal.
U.S. Pat. No. 4,828,759 titled “Process For Decontaminating Radioactivity Contaminated Metallic Materials” discloses yet another decontamination process in which an acidic decontamination solution is used to remove radio active materials from components. The contaminants and solid impurities are subsequently removed from the decontamination solution by processing with an ion exchange cell through electrochemical means and deposited on a cathode. Like the '842 patent, the '759 patent does not address the manner
Bradbury David
Elder George Richard
Wood Christopher John
Dergosits & Noah LLP
Electric Power Research Institute
Phasge Arun S,.
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