Pursuant to 35 U.S.C. §119(a), this application claims the benefit of earlier filing date and right of priority to Korean Application No. 10-2012-0043417, filed on Apr. 25, 2012, the contents of which is incorporated by reference herein in its entirety.
1. Field of the Invention
This specification relates to a decontamination method and apparatus for cladding hull wastes generated during pyroprocessing of spent nuclear fuel.
2. Background of the Invention
In general, after disassembling and shearing of nuclear fuel assembly in a pretreatment stage of a nuclear non-proliferation reprocessing technology called pyroprocessing, which is developed for efficient treatment and recycle of spent nuclear fuel, cladding hull wastes, structural component wastes and the like are generated as metallic wastes, which are left after unloading the spent nuclear fuel. Such wastes are generated as much as more than about 3.5 tons per 10-ton spent nuclear fuel. Especially, cladding hull wastes occupy about 2.5 tons of the total quantity of wastes, and the cladding hulls are all sorted as high level wastes because the spent nuclear fuel remains still within the cladding hulls or several μm of fission products are penetrated into the cladding hulls. However, if only spent nuclear fuel stuck on the cladding hull wastes and high irradiative nuclides are removed or main elements constructing the cladding hull wastes are merely extracted, disposal of the cladding hull wastes into intermediate/low level wastes or low level wastes are feasible.
For example, if cladding hulls which are occupied by zirconium (Zr) by more than 98% are treated through electrolytic refining or chlorination process, zirconium which is more than about 99% pure could be recovered. However, those processes require the zirconium (Zr) recovery, and thereby show shortcomings in the aspects of a large volume of a treatment apparatus and a long reaction time.
On the contrary, if only an extremely small quantity of spent nuclear fuel remaining still in the cladding hull wastes and the fission products penetrated into the cladding hulls are decontaminated, such shortcomings may be overcome. To this end, a chemical etching method may be used. Here, an exemplarily used chemical is an aqueous solution in which nitric acid (HNO3) and hydrofluoric acid (HF) are mixed. However, the use of the aqueous solution of the nitric acid and the hydrofluoric acid may enable separation/extraction of sensitive materials such as uranium (U) or plutonium (Pu). Accordingly, the use of such aqueous solution is inhibited in the aspect of nuclear proliferation or should be subject to strict management.
To achieve these and other advantages and in accordance with the purpose of this specification, as embodied and broadly described herein, there is provided a decontamination method for cladding hull wastes in a method of decontaminating cladding hull wastes generated from spent nuclear fuels, the method including inserting the cladding hull waste into an anodic basket, immersing a reference electrode and an cathodic electrode as well as the anodic basket in a molten salt, dissolving a surface of the cladding hull waste by applying a voltage or current to the anodic basket with respect to cathodic electrode or the reference electrode, removing the anodic basket, and removing a salt remaining on the surface of the cladding hull waste.
In one aspect of the present disclosure, a temperature of the molten salt may be in the range of 400 to 900° C.
In one aspect of the present disclosure, the anodic basket may be made of a mesh, a porous metal layer or a ceramic.
In one aspect of the present disclosure, a material of the anodic basket may exhibit a reduction potential higher than a material of the cladding hull waste.
In one aspect of the present disclosure, the molten salt may be one of LiCl, LiCl—KCl, NaCl, NaCl—KCl, LiF—NaF and LiF—KF—NaF.
In one aspect of the present disclosure, the molten salt may further contain an initiator.
In one aspect of the present disclosure, the initiator may be one of ZrCl4, ZrF4, K2ZrF6 and LiI.
In one aspect of the present disclosure, the molten salt may further contain an additive.
In one aspect of the present disclosure, the additive may comprise fluoride and iodide.
In one aspect of the present disclosure, a voltage in the rage of −1.5 V ˜+1.0 V or a current in the range of 0.1 A/cm2˜2A/cm2, with respect to an Ag/AgCl reference electrode, may be applied depending on a material of the cladding hull waste, to dissolve the surface of the cladding hull waste.
In one aspect of the present disclosure, the dissolving of the surface of the cladding hull waste may be performed to electrochemically dissolve the surface of the cladding hull waste so as to remove or reduce spent nuclear fuel residues and fission products.
In one aspect of the present disclosure, the method may further include reducing radioactivity of the cladding hull waste by removing or reducing the spent nuclear fuel residues and fission products, and reducing a quantity and volume of high-level wastes by disposal of the cladding hull waste as an intermediate/low level or low level.
In one aspect of the present disclosure, the method may further include repetitively performing the dissolution after treatment of the cladding hull waste, so as to remove radioactive nuclides on the surface of the cladding hull waste.
In one aspect of the present disclosure, the removing of the salt may be performed to evaporate the salt under a vacuum or inactive gaseous atmosphere of 500 to 1200° C.
To achieve these and other advantages and in accordance with the purpose of this specification, as embodied and broadly described herein, there is provided a decontamination apparatus for cladding hull wastes in an apparatus for decontaminating cladding hull wastes generated from spent nuclear fuels, the apparatus including a crucible containing a molten salt, an anodic basket immersed in the molten salt and containing cladding hull wastes, and a reference electrode and a cathodic electrode immersed in the molten salt, wherein a surface of the cladding hull waste may be dissolved by applying a voltage or current to the anodic basket with respect tocathodic electrode or the reference electrode.
Further scope of applicability of the present application will become more apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description.
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments and together with the description serve to explain the principles of the invention.
In the drawings:
Technical terms used in this specification are used to merely illustrate specific embodiments, and should be understood that they are not intended to limit the present disclosure. As far as not being defined differently, all terms used herein including technical or scientific terms may have the same meaning as those generally understood by an ordinary person skilled in the art to which the present disclosure belongs, and should not be construed in an excessively comprehensive meaning or an excessively restricted meaning. In addition, if a technical term used in the description of the present disclosure is an erroneous term that fails to clearly express the idea of the present disclosure, it should be replaced by a technical term that can be properly understood by the skilled person in the art. In addition, general terms used in the description of the present disclosure should be construed according to definitions in dictionaries or according to its front or rear context, and should not be construed to have an excessively restrained meaning.
A singular representation may include a plural representation as far as it represents a definitely different meaning from the context. Terms ‘include’ or ‘has’ used herein should be understood that they are intended to indicate an existence of several components or several steps, disclosed in the specification, and it may also be understood that part of the components or steps may not be included or additional components or steps may further be included.
It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure.
Embodiments of the present invention will be described below in detail with reference to the accompanying drawings, where those components are rendered the same reference number that are the same or are in correspondence, regardless of the figure number, and redundant explanations are omitted.
In describing the present invention, if a detailed explanation for a related known function or construction is considered to unnecessarily divert the gist of the present invention, such explanation has been omitted but would be understood by those skilled in the art. The accompanying drawings are used to help easily understood the technical idea of the present invention and it should be understood that the idea of the present invention is not limited by the accompanying drawings.
Hereinafter, description will be given of a method for treating cladding hull wastes in accordance with an exemplary embodiment with reference to
A decontamination apparatus for cladding hull wastes in accordance with an exemplary embodiment may include a crucible containing molten salt, an anodic basket immersed in the molten salt and containing cladding hull wastes, a reference electrode and a cathodic electrode immersed in the molten salt, and a power supply unit to apply a voltage or current to the electrodes. Surfaces of the cladding hull wastes may be dissolved as the voltage or current is applied to the anodic basket with respect to the cathodic electrode or the reference electrode.
First, to remove spent nuclear fuel residues remaining on a surface of a cladding hull waste and fission products through electrochemical dissolution, the cladding hull (cladding, cladding tube, cladding hull waste) is collected after unloading (extracting) the spent nuclear fuel within the cladding hull (S1).
The cladding hull is inserted into a basket made of a metal (for example, stainless steel) (S2). The cladding hull inserted into the basket may be in plurality.
After inserting the cladding hull into the metallic basket (for example, stainless steel basket), the basket is connected to a anode.
The basket connected to the anode (i.e., anodic basket) and a cathodic electrode are immersed into a molten salt (S3). A reference electrode may be added into to the molten salt if necessary. The anodic basket may be made of a mesh, a porous metal film or a ceramic to allow dissolved materials to be discharged therethrough with maintaining conductivity in a contact state with the cladding hull waste. The molten salt may be filled in a crucible (not shown).
The cathodic electrode may be implemented by using molybdenum (Mo), tungsten (W), iron (Fe), nickel (Ni) and the like or alloy thereof, and the reference electrode may be implemented by using Ag/Ag+, Ni/Ni2+, Na/Na+, Al/Al3+, Pt/Pt2+ and the like.
After immersing the anodic basket and the cathodic electrode in the molten salt, a preset voltage or current is applied to the anode, dissolving the surface of the cladding hull waste (S4). That is, to electrochemically dissolve the cladding hull waste, a voltage or current appropriate to oxidize main components of the cladding is applied to the cathodic basket. For example, a voltage in the range of 0.1V to −1.0V with respect to Ag/AgCl reference electrode is applied to zircaloy or zirlo containing zirconium, to allow the zirconium (Zr) to be oxidized and dissolved to Zr2+ or Zr4+. That is, for treatment of the zirconium-based cladding hull such as the zircaloy or zirlo, a positive potential rather than a balancing reduction potential of the zirconium is applied to induce oxidization and dissolution of the zirconium on the surface of the cladding hull waste. The current and voltage may change into a positive direction to increase the oxidation speed.
Therefore, the spent nuclear fuel residues and fission products may be removed or reduced by dissolving the surface of the cladding hull waste through the electrochemical dissolution.
Also, with the removal or reduction of the spent nuclear fuel residues and fission products, radioactivity of the cladding hull waste may be reduced and the cladding hull waste may be disposed as an intermediate/low level or a low level, resulting in decreasing a quantity and volume of high-level wastes. With the reduction of radioactivity of the cladding hull wastes by virtue of the removal or reduction of the spent nuclear fuel residues and fission products, the cladding hull wastes may be recycled as an additive or a nuclear reactor component or container upon disposal of nuclear fuel and high-level radioactive wastes of Sodium-cooled Fast Reactor (SFR).
As the immersing solvent, a molten salt such as LiCl, LiCl—KCl, NaCl, NaCl—KCl, LiF—NaF, LiF—KF—NaF, or the like may be used.
For more effective dissolution of the surface of the cladding hull waste, an initiator such as ZrCl4, ZrF4, K2ZrF6, LiI or the like may further be contained in the molten salt.
Fluoride such as NaF, KF, LiF or the like and iodide such as LiI may further be contained in the molten salt (for example, chloride-based molten salt). That is, the further addition of the initiator and/or the additive into the molten salt may result in an effective dissolution of the surface of the cladding hull waste.
After removing the treated anodic basket (S5), the salt remaining on the surface of the cladding hull waste is removed (S6). For example, after taking the cladding treated by the electrochemical dissolution out of the molten salt, the salt is evaporated in a vacuum or inactive gaseous atmosphere of 500° C. to 1200° C. The salt absorbed onto the surface of the cladding hull waste is thus removed.
A material of the anodic basket may preferably have a reduction potential higher than a main material of the cladding hull such that the anodic basket cannot be affected by the electrochemical dissolution. That is, the basket may be made of a metal whose reduction potential is higher than the main component of the cladding hull. For example, if the cladding hull is made of zirconium-containing zircaloy or zirlo, the basket may be made of molybdenum (Mo), tungsten (W), iron (Fe), nickel (Ni) and the like or alloy thereof.
After disposal of the cladding hull waste, the second or third dissolution may be repeatedly performed, if necessary, to remove radioactive nuclides on the surface of the cladding hull waste. For example, when the spent nuclear fuel residues or fission products are removed through the electrochemical dissolution, the dissolution time may extend or the second or third electrochemical dissolution may be performed in the molten salt, enhancing decontamination effect.
An experiment may be performed within a glove box filled with inactive gas during the disposal of the cladding hull waste, thereby adjusting a concentration of oxygen and moisture to several to several tens of ppm.
To dissolve the surface of the cladding hull waste, a voltage in the rage of −1.5 V ˜+1.0 V or a current in the range of 0.1 A/cm2˜2A/cm2, with respect to Ag/AgCl reference electrode, may be applied depending on a main material of the cladding hull waste.
As shown in
During decladding for unloading the spent nuclear fuel in the pretreatment stage of the pyroprocessing, the cladding hull waste may be oxidized under air or oxygen atmosphere of about 400˜700° C. For zircaloy-4, a zirconium oxide layer may have a thickness in the range of several to several tens of μm according to temperature and time. This may affect the electrochemical surface dissolution. Hence, the same experiment has been performed using the oxidized cladding hull waste.
The results for the cladding hulls oxidized at 400° C., 500° C. and 600° C. are shown in
On the contrary, for the cladding hulls oxidized at 500° C. and 600° C., the oxide layers of the surfaces of the cladding hulls are about 1.3 μm and 4.5 μm thick, respectively. Referring to
For example, when the zircaloy-4 cladding hulls oxidized at 400° C., 500° C. and 600° C., respectively, for 5 hours, are equally connected to the electrodes and immersed into the same molten salt, and a voltage of −0.78 V is applied to the corresponding cladding hulls, the current-time graphs are represented as shown in
Referring to
In order to check whether or not the zirconium oxide layer on the surface of the cladding within the molten salt of high temperature is removed electrochemically, a piece (fragment) of the cladding hull oxidized at 500° C. for 5 hours was immersed into a molten salt, which contained LiCl—KCl eutectic salt and 4 percent by weight of ZrCl4, at 500° C. for 1 hour. Afterwards, the surface of the cladding was analyzed by means of X-ray Photoelectron Spectroscopy (XPS). The results were shown in
As shown in
As illustrated in the exemplary embodiment, as the oxidation temperature increases during decladding of the spent nuclear fuel, the thickness of the oxide layer of the cladding hull increases. This may extend a time taken to dissolve the surface of the cladding hull. However, it can be observed that the surface of the cladding hull oxidized at about 500° C., which is an optimum decladding condition, is easily dissolved through the electrochemical dissolution.
As described above, in accordance with the decontamination method and apparatus for the cladding hull waste according to the exemplary embodiments, the cladding hull waste may be decontaminated in a molten salt through an electrochemical dissolution. It may make it possible to effectively remove residual spent nuclear fuel (products) remaining still on a surface of the cladding hull waste or fission products contained in an oxide layer in a lift-off manner.
In accordance with the decontamination method and apparatus for the cladding hull waste according to the exemplary embodiments, a deep dissolution of the surface of the cladding hull may be enabled, resulting in decontamination of fission products penetrated into the surface of the metallic cladding hull.
In accordance with the decontamination method and apparatus for the cladding hull waste according to the exemplary embodiments, since the residual spent nuclear fuel or fission products remain in the molten salt together with rare earth elements, jewelries and various nuclides, they may be recollected or treated in a high nuclear proliferation-resistant manner upon following treatment or decontamination.
In accordance with the decontamination method and apparatus for the cladding hull waste according to the exemplary embodiments, a process time may be more reduced than electrolytic refining or chlorination method of extracting and collecting main components of cladding hull wastes, and an additional process of treating recollected components may also be reduced, resulting in reduction of process costs.
In accordance with the decontamination method and apparatus for the cladding hull waste according to the exemplary embodiments, a quantity of high level wastes can be remarkably reduced by treatment of cladding hull wastes and the treated cladding hull wastes can be recycled, which may arise an additional economic gain.
The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present disclosure. The present teachings can be readily applied to other types of apparatuses. This description is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. The features, structures, methods, and other characteristics of the exemplary embodiments described herein may be combined in various ways to obtain additional and/or alternative exemplary embodiments.
As the present features may be embodied in several forms without departing from the characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the appended claims.
Number | Date | Country | Kind |
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10-2012-0043417 | Apr 2012 | KR | national |