Patent Grant
8097142
This invention relates to the codeposition of U and Pu in an eletrorefiner from metallic nuclear fuel to prevent segregation of weapons-grade Pu.
Since the early 1980's in the Integral Fast Reactor (IFR) program the Chemical Engineering Division (Chemical Technology Division at that time) at Argonne National Laboratory (ANL) has been developing molten salt electrorefining as a compact process for recovery of uranium from spent metallic reactor fuel, as well as from Light Water Reactor (LWR) oxide fuel that had been reduced to a metal. In the IFR program uranium and transuranic metals (transuranics, or TRUs) were codeposited in a liquid cadmium cathode. The codeposition or co-recovery of uranium and transuranics was desirable for non-proliferation reasons.
The electrorefiner technology has gone through several developmental iterations. The most recent electrorefiner design is called the Planar Electrode ElectroRefiner (PEER). A prototype module has been tested successfully in the Chemical Sciences and Engineering Division at ANL and has met every expectation with respect to the high efficiency cathode scraping and scalability of the design.
In addition to the planar anode baskets and intermittently scraped cathode rods, the PEER design also allows for simple removal of the electrorefined uranium product by collection of the scraped cathode deposit in a basket that is periodically removed from the unit, emptied, and then replaced. Direct removal of the electrorefined product allows the PEER to be passing current, and thus refining the material nearly 100% of the time.
This is a distinct advantage over the Mk-V high throughput electrorefiner that was developed by ANL, and is presently being used to treat EBR-II blanket fuel at Idaho National Laboratory (INL). In the Mk-V electrorefiner, the cathode deposit is not efficiently scraped off and the product falls to a product collector that is attached to the mk-V anode-cathode-module (ACM). To recover the electrorefined uranium product, the entire unit must be removed and the product collector replaced. In a hot cell using remote handling, this process takes up to 12 hours to complete.
Codeposition of U and TRU's at a Solid Cathode in the PEER Design.
In the electrorefining of spent reactor fuel, the actinides, TRU's and active metal fission products are oxidized at the anode and dissolve in the molten salt electrolyte as metal cations. Under normal operating conditions, only uranium is deposited on the cathode. However, if the cathode potential can be made sufficiently negative, uranium and TRUs will codeposit on the cathode. The key to obtaining a sufficiently negative cathode potential is a high cathode current density that depletes uranium from the molten salt electrolyte near the cathode surface. In a practical sense, for this to occur in a uranium electrorefiner in which the electrolyte is molten LiCl-KCl, the anode area must greatly exceed the cathode area. The necessary difference in anode vs. cathode areas is needed because the limiting current density for uranium oxidation at the anode is surprisingly low at potentials that will not oxidize the structural steel in the anode basket. Achieving a sufficiently high anode-to-cathode surface area makes it possible to codeposit U and TRUs on a solid cathode. The PEER design uses multiple planar anode baskets with interleaved linear cathode arrays between the baskets. By disconnecting the linear cathode arrays and adding an additional cathode rod to the side of the anode baskets necessary anode-to-cathode area ratio can be achieved. The U/TRU deposit can be intermittently scraped off the cathode and removed in the same manner as the uranium deposit is scraped off and removed as described previously.
The methods of the present invention have several advantages over existing processes that employ a liquid cadmium or bismuth cathode. First, this invention eliminates the use of cadmium and the engineering challenges of operating a liquid metal cathode. Second, this invention achieves a greater degree of separation between TRUs and rare earths than can ever be achieved with a liquid cadmium cathode. This invention also has a strong nuclear proliferation-resistance aspect, in that as long as the material used in the anode basket is spent fuel that contains a significant fraction of uranium, it is impossible to obtain a pure plutonium (or uranium-free TRU) product at the cathode, because uranium will always be present in the system and codeposit with the TRUs.
Accordingly, the present invention provides a method of simultaneously removing uranium and transuranics from metallic nuclear fuel containing both uranium and transuranics in an electrorefiner having a solid cathode and an anode basket containing the metallic nuclear fuel and a molten halide electrolyte. The process comprises (a) establishing a potential difference between the anode basket and solid cathode, thereby creating a diffusion layer of uranium and transuranic ions at the solid cathode, a first current density at the anode basket, and a second current density at the solid cathode; (b) establishing a ratio of anode basket area to solid cathode area dependent on the total concentration of uranium and transuranics in the molten halide electrolyte and the effective thickness of the diffusion layer at the solid cathode, such that the established first and second current densities result in both codeposition of uranium and transuranics on the solid cathode and oxidation of the metallic nuclear fuel in the anode basket; (c) maintaining the first and second current densities at levels sufficient to codeposit uranium and transuranics on the solid cathode; (d) removing deposited material from the solid cathode; and (e) controlling the first current density at the anode basket to prevent substantial oxidation of the anode basket during operation of the electrorefiner.
The present invention also provides a method of simultaneously removing uranium and transuranics from metallic nuclear fuel containing both uranium and transuranics in an electrorefiner having a solid cathode and an anode basket containing the metallic nuclear fuel and a molten halide electrolyte. In this aspect, the process comprises (a) establishing a potential difference between the anode basket and solid cathode, thereby creating a diffusion layer of uranium and transuranic ions at the solid cathode, a first current density at the anode basket, and a second current density at the solid cathode; (b) establishing a ratio of anode basket area to solid cathode area dependent on the total concentration of uranium and transuranics in the molten halide electrolyte and the effective thickness of the diffusion layer at the solid cathode, such that the first current density is maintained in the range of about 70 mA/cm2 to about 100 mA/cm2, and the second current density at the solid cathode is maintained in the range of greater than about 200 to about 1400 mA/cm2, resulting in both codeposition of uranium and transuranics on the solid cathode and oxidation of the metallic nuclear fuel in the anode basket; (c) maintaining the first and second current densities at levels to codeposit uranium and transuranics on the solid cathode; (d) removing deposited material from the solid cathode; and (e) controlling the first current density at the anode basket to prevent substantial oxidation of the anode basket during operation of the electrorefiner.
In addition, the present invention provides a method of simultaneously removing uranium and transuranics from metallic nuclear fuel containing both uranium and transuranics in an electrorefiner having a solid cathode and a plurality of electrically connected anode baskets containing the metallic nuclear fuel and a molten halide electrolyte, the anode baskets including opposed planar meshes establishing contact between the metallic nuclear fuel and the molten electrolyte. In this aspect, the process comprises (a) establishing a potential difference between the anode baskets and solid cathode, thereby creating a diffusion layer of uranium and transuranic ions at the solid cathode, a first current density at the anode baskets, and a second current density at the solid cathode; (b) establishing a ratio of total anode basket area to solid cathode area dependent on the total concentration of uranium and transuranics in the molten halide electrolyte and the effective thickness of the diffusion layer at the solid cathode such that the established first and second current densities result in both codeposition of uranium and transuranics on the solid cathode and oxidation of the metallic nuclear fuel in the anode baskets; (c) maintaining the first and second current densities at levels to codeposit uranium and transuranics on the solid cathode; (d) removing deposited material from the solid cathode; and (e) controlling the first current density at the anode baskets to prevent substantial oxidation of the anode baskets during operation of the electrorefiner.
The invention includes of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.
For the purpose of facilitating an understanding of the invention, there is illustrated in the accompanying drawings a preferred embodiment thereof, from an inspection of which, when considered in connection with the following description, the invention, its construction and operation, and many of its advantages should be readily understood and appreciated.
A basic electrolytic cell consists of an anode and a cathode immersed in an electrolyte. The cell is typically operated under controlled current or controlled voltage conditions. When a voltage is applied between the two electrodes (controlled potential mode) or an electric current is passed between the electrodes (controlled current mode) two electrochemical reactions occur simultaneously, namely an oxidation reaction at the anode and a reduction reaction at the cathode. Electrorefining is one type of electrolytic process. In an electrorefining cell an impure metal is electrochemically oxidized at the anode and high-purity metal is electrochemically reduced and deposited at a cathode. The anode and cathode reactions are shown below in Equations 1 and 2 respectively. For the specific case of electrorefining a metal R, which represents the reduced species, is in the metallic state and O, which represents the oxidized species, is a metal cation.
R→ne−+On+ anode reaction (1)
On++ne−→R cathode reaction (2)
Most industrial electrolytic processes are operated under controlled current conditions. In this mode of operation, the electrode potentials are controlled by mass transport. At steady state conditions a concentration gradient forms near the anode and cathode surfaces. The region where this concentration gradient occurs is called the diffusion layer. In the limiting case the concentration of the oxidized species at the anode surface is the concentration of the pure oxidized species in units of mol/cm3. The concentration of the oxidized species in the bulk electrolyte ranges from zero to the saturation concentration of the oxidized species in the electrolyte. This concentration gradient typically occurs over a short distance (about 10−2 cm). Based on this simple concentration gradient model, the mass-transport limited current ilim can be calculated using Equation 3.
ilim=nFAΔCD/δ (3)
In Equation 3, n is the number of electrons transferred and has the same value as in Equations 1 and 2, F is Faraday's constant (96485 coulombs/equivalent), A is the electrode area in cm2, ΔC is the change (mol/cm3) in concentration of the oxidized species (O) in the diffusion layer, D is the diffusion coefficient (cm2/sec), and δ is the thickness of the diffusion layer in cm. The term D/δ can be combined to give a “velocity” term, v which has units of cm/sec as shown in Equation 4.
ilim=nFAΔCv (4)
This invention specifically addresses electrorefining in a molten chloride electrolyte, a metal alloy containing uranium, transuranics, less “active” metals (e.g. iron, zirconium, molybdenum, rhodium), and active metal fission products (e.g. cesium, strontium, barium, and rare earth metals). Such an alloy arises from spent metallic nuclear reactor fuel or spent oxide nuclear reactor fuel that has been reduced to a metallic state. Numerous experiments at small and large scales have demonstrated that the limiting current per unit area for oxidation of uranium metal in a molten chloride electrolyte is about 0.1 A/cm2. The maximum value for ΔC at a uranium anode undergoing oxidation can be approximated by the concentration of pure UCl3 (about 1.5×10−2 mol/cm3). Inserting these values along with a n value of 3 into Equation 4 affords a vox value of 2.3×10−5 cm/sec for electrochemical oxidation of uranium in a molten chloride salt. The electrochemical reduction of UCl3 to U metal at a cathode has also been well-studied. Experimental measurements of D and δ for electrodeposition of uranium at a cathode in a molten chloride salt have established a vred value of 2×10−3 cm/sec for reduction of UCl3 to U-metal at a cathode in a molten chloride salt. It is reasonable to assume the vred values for transuranic chlorides such as PuCl3, AmCl3, and NpCl3 would be comparable to that for UCl3.
Equations 5 and 6 describe the condition when both electrodes are operating at their maximum, mass-transport-limited current. When both electrodes are at the mass-transport limited current, the anode and cathode are polarized to the same degree.
ilim, cathode=ilim, anode (5)
n·F·Acathode·ΔCcathode·vred=n·F·Aanode·ΔCanode·vox (6)
If the concentration of the oxidized species in the molten salt electrolyte is small (i.e. <10 wt %) the limiting value of ΔCanode is about 1.5×10−2 mol/cm2 as stated above. Similarly, in the limiting case, when the concentration of the oxidized species at the cathode/electrolyte interface is zero, ΔCcathode is simply the concentration of the oxidized species in the bulk electrolyte Cox. Making these substitutions into Equation 6 as well, along with an algebraic manipulation provides Equation 7, which shows the relationship between the ratio of the anode and cathode areas to the concentration and velocity terms.
Aanode/Acathode=(Cox·vred)/(vox·1.5×10−2 mol/cm3) (7)
Using the experimentally determined values for vox and Vred for uranium electrorefining derived above, Equation 7 simplifies to Equation 8 in which the Aanode to Acathode ratio in the case where both electrodes are operating at their respective mass-transport limits where Cox is the concentration of the oxidized species in the bulk electrolyte.
Aanode/Acathode=(Cox·6×103 cm3/mol) (8)
This derivation illustrates that at Aanode/Acathode ratios larger than the value calculated using Equation 8, the electrorefining cell will operate at the mass transport limit only at the cathode and the cathode will be polarized to a greater degree than the anode. Conversely operation of the cell at Aanode/Acathode ratios smaller than the value calculated using Equation 8 results in mass-transport limited conditions occurring only at the anode and the anode will be polarized to a greater degree than the cathode.
This invention has significant implications for the design of electrorefiners for treating spent nuclear fuel. If the cell is operated at currents that exceed the limiting current for oxidation of uranium, the potential at the anode will shift to increasingly positive values until oxidation of a more noble metal such as iron can occur to supply the necessary current or a potential that oxidizes U+3 to U+4. Because uranium electrorefiners typically use steel anode baskets and the anode feed material can include steel or zircaloy cladding, anode potentials that result in the oxidation of iron or zirconium are usually not desirable. Oxidation of U+3 to U+4 decreases the current efficiency of the process because it creates a parasitic redox reaction as U+4 is reduced back to U+3 at the cathode. Just as multiple anode oxidations can occur at the anode, depending on the cell current and resulting anode potential, multiple reduction reactions can likewise occur at the cathode. If the cathode current exceeds the limiting current for reduction of UCl3 and the electrolyte contains other, more stable metal chloride species such as PuCl3, AmCl3, NpCl3, or rare earth chlorides, the cathode potential will shift to more negative values where reduction of these other species occur. This results in the codeposition of uranium and transuranics at a solid cathode at sufficiently high values of the ratio Aanode/Acathode. For the case where uranium and transuranics codeposit, the effective value for Cox is approximated as the sum of the UCl3 and transuranic chlorides concentrations in the electrolyte. To ensure that the cathode potential does not shift too negative, a reference electrode may be used to measure the cathode potential versus the reference electrode.
By adjusting the relative electrode areas and the composition of the electrolyte and thereby the relative degree of anode and cathode polarization, it is possible to operate an electrorefiner for spent reactor fuel to obtain a high purity uranium product or a mixed uranium-transuranic product. Allowing for some error in the values of vox and vred, and converting the concentrations (Cox) from mol/cm3 to weight percent (Cox, wt %) the following guiding calculations (Equations 9 and 10) can be used to ensure that the electrorefiner is operating in a cathode or anode limited mode. Experiments in which the degree of anode and cathode polarization were measured over a range of Aanode/Acathode demonstrated behavior that is consistent with Equation 8.
Anode Limited Mode Aanode/Acathode≦(Cox, wt %·0.1 wt %−1) (pure uranium cathode deposit) (9)
Cathode Limited Mode Aanode/Acathode≧(Cox, wt %·10 wt %−1) (uranium-transuranic codeposit) (10)
Typical uranium concentrations in this field range from 1 to 7 wt % and combined uranium+transuranic concentration in this work range from 2 to 8 wt %. Inserting these typical concentration ranges in Equations 9 and 10 results in an anode:cathode area ratio ranging from less than 0.1 (1 wt % uranium) to less than 0.7 (7 wt % uranium) to ensure a pure uranium cathode deposit. As electrorefining progresses the anode:cathode ratio will become smaller as the anode area decreases and the cathode area increases. In the case where deposition of high-purity uranium is desirable a low initial anode:cathode ratio will be used and the ratio will further decrease as the anode is consumed. This means that conditions favoring deposition of high-purity uranium will prevail throughout the process operation.
Similarly, an anode-cathode area ratio ranging from greater than 70 (7 wt % uranium in electrolyte) to at least about 10 (1 wt % uranium in the electrolyte) will make it possible to achieve cathode potentials at which uranium and transuranics will codeposit. This analysis suggests that the anode:cathode ratios at lower concentrations (i.e. 1-3 wt % uranium) are more practical.
The anode:cathode area ratio specifications described above can also be recast in terms of anode and cathode current densities. As stated above, at an anode current density of 70 to 100 mA/cm2, only the spent metallic fuel and not the cladding and steel basket hardware will be oxidized because the anode potential is not sufficiently anodically polarized for oxidation of iron. This limiting current density is essentially independent of the typical range of UCl3 or transuranic chloride concentrations in the molten salt electrolyte. Consequently, a plot of anode current density vs. anode polarization should show no dependence on concentration of UCl3 in the electrolyte. This has been observed experimentally. Also as stated above the cathode current density must be sufficiently large to require the reduction of transuranic chlorides in addition to the reduction of uranium chlorides and thus shift the cathode potential to more negative values. This cathode limiting current density for uranium and transuranic codeposition is dependent on the combined concentration of uranium and transuranic chlorides dissolved in the molten salt electrolyte. Thus, unlike the case for the anode, a plot of cathode current density vs. cathode polarization will have an increasing slope with increasing values for Cox. Therefore Equation 11 can be written to describe the relationship between the combined concentration of uranium and transuranic chlorides (Cox, wt %) in the molten salt electrolyte and the limiting current density at the cathode, jlim,cathode. The 200 mA/(cm2·wt %) coefficient of Equation 11 includes a roughness factor and an allowance for the increase in cathode surface area as the cathode deposit grows. Therefore, the to Acathode term in Equation 11 is the geometric area of the cathode.
jlim,cathode=ilim,cathode/Acathode>[200 mA/(cm2·wt %)]·Coc, wt % (11)
As stated above, typical values for Cox, wt % range from 1 to 7 wt %. Using these values in Equation and allowing for surface roughness and a growing surface area, uranium and transuranics will codeposit at a metal cathode under typical conditions at current densities ranging from greater than 200 and 1400 mA/cm2, respectively.
The ability to achieve codeposition of uranium and transuranics at a solid cathode is both a surprising and significant achievement for the pyrochemical treatment of spent reactor fuel. Prior to this invention, codeposition of uranium and transuranics was thought to require a large (i.e., >30) PuU concentration ratio in the molten salt electrolyte or the use of a liquid metal cathode that shifted the reduction potential of the transuranic chlorides to more positive values. In fact it has been demonstrated many times that the use of liquid cadmium or bismuth cathodes does allow for codeposition of uranium and transuranics. However, these liquid metal cathodes also shift the deposition potential for rare earth chlorides to more positive potentials as well. Unfortunately, at the liquid cadmium cathode, the potential shift to less cathodic potentials for the rare earth chloride deposition is typically larger than the cathodic shift in transuranic deposition potential. This larger shift in rare earth chloride deposition potential decreases the degree of separation that can be achieved between the transuranics and the rare earths. If the uranium and transuranics are to be recycled into fresh fuel, rare earth contamination is not desirable. A much greater degree of separation between the transuranics and the rare earths can be achieved using a solid cathode. In fact comparing transuranic-rare earth separation factors for a solid cathode and a liquid cadmium cathode shows that compared to a liquid cadmium cathode, the transuranic-rare earth separation factors are 100,000 times larger even at low PuU concentration ratios in the molten salt. This huge increase in transuranic-rare earth separation factor means that the uranium-transuranic codeposit can be recycled into fresh fuel, and meet the low-rare-earth content specifications for the new nuclear fuel.
For illustration purposes only, not to be construed as limiting, an electrorefiner 10 such as schematically illustrated in
Periodically a smaller surface area cathode 16 can be inserted into the electrorefiner or some of the cathode rod assemblies 14 can be switched out of the circuit, such that an anode-to-cathode area ratio that satisfies Equation 10 is reached. Passing current between the full set of anode baskets and this smaller-area cathode results in codeposition of uranium and transuranics on cathode 16. Alternatively, a second power supply circuit with the anode baskets 12 connected to the positive terminal of the power supply and the small-surface-area cathode 16 connected to the negative terminal of the power supply and using both power supplies to simultaneously deposit uranium on the multiple cathode arrays 14 and codeposit uranium and transuranics on the small-surface-area cathode 16.
The electrode arrangement shown in
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.
This application claims the benefit of U.S. Provisional Application Ser. No. 61/067,568, filed on Feb. 29, 2008, which is incorporated herein by reference.
The United States Government has rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the United States Government and The University of Chicago and/or pursuant to Contract No. DE-AC02-06CH11357 between the United States Government and UChicago Argonne, LLC representing Argonne National Laboratory.
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