Integral U/TRU recovery cathode system for electrorefining used nuclear fuel, method for electrorefining and harvesting metal from used nuclear fuel

The invention provides a system for collecting metal in an electrorefining process, the system having a hollow cathode; and a container defining an upwardly extending surface adapted to be received by the hollow cathode. An embodiment of the invention provides for metal reduction to occur on laterally facing and medially facing surfaces of the cathode such that electrolyte resides between surfaces of the cathode. Also provided is a metal electrorefining process having the steps of subjecting molten salt containing metal moieties to electrolysis wherein reduced metal accumulates in a cathode-cup construct in a first position; raising the construct to a second position above the molten salt while subjecting the construct to heat from the molten salt; withdrawing the cathode from the construct into a vestibule to the electrorefiner to a third position; and removing the cathode and cup from the electrorefiner to a fourth position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a system and method for harvesting metal from used nuclear fuel and more specifically this invention relates to a system and method for collecting and removing reduced metal from cathodes in electrorefining processes.

2. Background of the Invention

Electrorefining operations require scrupulous control of reaction environment conditions. For example, inert atmospheres are usually required. Also, operating temperatures of between 500 and 650° C. are employed, inasmuch as molten salt electrolyte is required to recover targeted metal product at desired purities. Lastly, harvesting and removal of any metal reduced during the process must be extremely efficient such that the vast majority of the metal is deposited in ingots or other storage containers while the molten salt draining therefrom is still flowing.

Electrorefining is also associated with highly radioactive fission products such as noble metals, active metals and gases, which must be controlled. As such, these processes are typically performed in inert atmosphere hot cells, and manipulated remotely using robots.

A need exists in the art for a method and system for harvesting transuranic metals during electrorefining operations. The method and system should minimize loss of metal during harvesting. The method and system should also maximize collection of reduced metal without exposure of personnel or the environment to the radiation and toxicity normally associated with the metals.

SUMMARY OF INVENTION

An object of the invention is to provide a method and system for harvesting metal (i.e., a co-deposited uranium-transuranic element product) from used nuclear fuel that overcomes many of the drawbacks of the prior art.

Another object of the invention is to provide a system for harvesting reduced metal from an electrorefiner. A feature of the invention is a removable collection cup that reversibly receives a cathode so as to encircle the sides and bottom of the cathode. An advantage of the invention is that the cup collects reduced metal as it is formed at the cathode, and maintains electrical contact between the collected metal and the cathode, therefore minimizing metal loss to electrolyte due to undesired parasitic reactions.

Yet another object of the invention is to provide a system and method for harvesting metals from used nuclear fuel. A feature of the invention is a metal-collection vessel that is sized to limit the amount of metal collected at any one harvesting event and configured to minimize neutron levels within the collected metal and vessel. An advantage of the invention is that the collection vessel prevents the metal collected there from reaching a critical reaction state, thereby maximizing criticality safety of the system.

Briefly, the invention provides a system for collecting metal in an electrorefining process, the system comprising: a hollow cathode; and a container defining an upwardly extending surface adapted to be received by the hollow cathode.

Also provided is method for harvesting metal, the method comprising supplying a molten electrolyte liquor containing salts of the target metal; contacting the liquor with a cathode defining medially directed and laterally directed surfaces; and surrounding the medially directed surfaces and laterally directed surfaces with a container for collecting the metal forming on the cathode.

The invention further provides a metal electrorefining process comprising: subjecting molten salt containing metal moieties to electrolysis wherein reduced metal accumulates in a cathode-cup construct in a first step or position; raising the construct in a second step to a second position above the molten salt while subjecting the construct to heat from the molten salt; withdrawing the cathode from the construct in a third step maintaining the cathode and the cup in a third position within a vestibule to the electrorefiner; and in a fourth step removing the cathode from the vestibule while maintaining the cup within the vestibule in a fourth position.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing summary, as well as the following detailed description of certain embodiments of the present invention, will be better understood when read in conjunction with the appended drawings.

As used herein, an element or step recited in the singular and preceded with the word “a” or “an” should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly stated. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.

Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property.

The invented system and method facilitates collection of co-deposited metallic uranium and transuranic element (U/TRU) product in electrorefining processes. The system includes a uniquely configured cup to collect reduced metal as the latter is plated onto the cathode. The cup is adapted to slidably communicate with interior aspects of the cathode such that the cup encircles depending surfaces and longitudinally extending surfaces of the cathode. Other aspects of the system include a plurality of heat shields to minimize heat loss from the system during replacement of the cathode and during withdrawal of the cathode from the collection cup.

FIG. 1Adepicts a metal harvesting collection system designated as numeral10. The system10is designed to interact with a cabinet assembly11(depicted inFIGS. 5A-C) which serves as the semi-permanently mounted roof section of the electrorefiner. The cabinet11is a vestibule to the entry ports into the electrorefiner. The vestibule keeps heat from escaping to the ambient environment, and conversely, maintains components withdrawn from the refiner at above salt solidification temperatures. Thus, the vestibule keeps the components from cooling off too quickly before their further processing. Throughout this specification, “cabinet” and “vestibule” may be used interchangeably.

Generally elongate in configuration, the system10comprises a first or depending end12and a second end20positioned superior to the first end12. As depicted inFIGS. 1B-1E, a cathode14is collinearly arranged with the elongated configuration so as to be slidably received by the second end20. The cathode may be one continuous tube or a plurality of tubes, collinearly, coaxially joined. A myriad of means (element43inFIG. 1D) for coaxially joining the tubes are commercially available, including snap fit, spring clip, male-female thread configurations, and combinations thereof. This joining means43is positioned from the distal or depending end of the cathode tube at a distance greater than the length of the cup center post38.

The first end12is adapted to removably receive a metal recovery cup18. As such, the first end12terminates in a metal recovery cup support plate16that is generally horizontally disposed. The cup support plate16defines a notch15extending from the periphery of the plate to the center of the plate, and is adapted to receive vertically disposed aspects of a cup pedestal17, described infra.

The second end20of the collection system is superior from the first end12and connected thereto via a plurality of longitudinally extending struts22, such that the struts are vertically disposed. The struts22are positioned along the periphery of the system such that the struts are parallel both with each other and with the longitudinal axis of the system10. The struts22are radially spaced, relative to each other and to allow access to interior aspects of the system, such as the recovery cup18. The spacing of the struts from each other provide a means for guiding the cup in and out of the interior aspects of the system.

The upwardly extending ends of the struts terminate in a horizontally disposed plate23such that the upwardly extending ends of the struts22are attached thereto. Depending ends of the struts are mounted to the similarly disposed cup support plate16. Therefore, the attachment of the ends of the struts to the plate23and the cup support plate16maintain the positioning of the struts in relationship to each other.

Both the first end12and second end20of the system terminate in a superior heat shield24and an inferior heat shield25, respectively. The heat shields minimize heat flow out of the electrorefiner when the cathode is replaced or withdrawn from the electrolyte bath.FIG. 1Adepicts the shield24positioned beneath the strut positioning plate23.

The second (i.e., superior) end20of the assembly10is adapted to removably receive an insulator plug26, such that the assembly10is mounted to a downwardly facing surface of the plug26. The plug26is annular in shape so as to define a central shaft or longitudinally extending aperture32to slidably receive the cathode14. Inferior aspects (e.g., depending regions) and circumferential regions of the plug26are configured to reversibly latch, couple with, frictionally engage or otherwise mate with the cup holder assembly10.FIG. 1Bshows the plug26positioned above the first superior heat shield24, with the strut plate29positioned between the plug26and the shield24.

As depicted inFIGS. 1B and 1C, an upwardly ending end of the plug26supports a lift block62for moving the entire module by an overhead handling system (not shown). Specifically, circumferentially extending regions of a superior aspect of the plug26define a radially extending lip28such that the lip28cantilevers over longitudinally extending walls of the plug. Diametrically opposed regions of the lip define lateral projections40, each projection40adapted to support two hinged levers68.

The hinged lever68(a detailed depiction of which is inFIG. 4) engages longitudinally extending regions of the cathode via horizontally disposed notches70along those regions. As shown in greater detail inFIG. 4, the lever68comprises a tongue72with a proximal end74in rotatable communication with an axle-hub construct76. A distal end78of the tongue72terminates in an edge adapted to be received in one of the aforementioned notches70. As such, the cross sectional topography of the distal end78is dimensioned slightly smaller than the cross section topography of the notch70into which the distal end nests. The axle-hub construct76is mounted such that the tongue72extends medially at approximately a right angle to the longitudinal axis a of the system. (Several pairs of tongue/axle-hub constructs are positioned along the longitudinal lifting path of the system10. Each of these construct pairs may engage pairs of notches70spaced along longitudinally extending surfaces of the cathode to provide different vertical positioning points for the assembly during withdrawal or insertion into the electro refiner. For example, each pair of notches can define an engagement point with two tongues coplanarly arranged to each other. Given two notches for two tongues, three pairs of notches facilitate serial engagement with three pairs of tongues.)

The axle-hub construct is not involved in supporting the assembly10on the bottom of the cabinet. Rather, when the assembly10is fully engaged within the salt bath (seeFIG. 5A), it rests on the bottom of the cabinet11via downwardly facing surfaces of the radially extending regions40. The weight of the system allows it to remain in position within the salt bath.

The lift block62is configured to reversibly receive mechanical jaws, pinchers, forks etc. As such, circumferential aspects of the block62may define grooves63extending through the block as a plurality of chords located at diametrically opposed regions of the block.FIG. 1CandFIG. 4show the lift block62positioned at the end of the cathode tube14. Positioned between the block and tube is an electrical conductor plate60, discussed supra.

FIGS. 1D and 1Edepict an alternative means for manipulating the assembly10. Diametrically opposed regions of the lip are in pivotal communication with a loop, latch, or similar handle configuration30. This handle30is provided to facilitate manipulation of the plug by an overhead handling system such as a crane, pick, remote arm, etc. The handle configuration depicted inFIGS. 1D and 1Eis an upside down “U” such that the ends of the “U” are each pivotally mounted to the lip regions. The “U” shape facilitates a straight vertical lift, thereby assuring that all devices are centered during insertion and removal.

Detail

The cathode14comprises a first upwardly extending end34and a depending or downwardly extending end36. The first end34mates with a bus bar adapter or a electrical conductor plate60so as to cause the cathode to become electrically charged. The second end36is configured as a tube to slidably receive regions of the metal collection cup18.

The bottom or floor of the metal collection cup defines an annular space having a predetermined volume. The volume is calculated to prevent an over abundance of metal from collecting, which would otherwise lead to critical reactions occurring. An upwardly extending region38of the floor, reminiscent of a post, is coaxial with the longitudinal axis of the cup. This upward extension conforms the interior space of the cup to an annular shaped void with the post at its center. This upwardly extending region38also siphons, collects or otherwise diverts neutrons from the metal forming on the cathode and accumulating in the cup. This neutron diversion feature further enhances criticality safety.

FIGS. 2A, 2B and 3depict two different configurations for manipulating the recovery cup18. One embodiment depicted inFIG. 2Adepicts a first configuration17afor a cup pedestal. This configuration comprises a depending construct56defining a channel58. The construct is configured as a “U” with its two ends attached to the downward facing surface of the cup18. The construct56is dimensioned so as to be slidably received by the notch15of the cup support plate16so as to reside below the plane formed by the plate16. The channel58may be closed at its bottom portion (as shown) or open. The channel is adapted to receive a mechanical grabbing means such as a boss, pinchers, or expanders. The channel may be configured as a threaded female aperture to receive a similarly dimensioned threaded rod.

FIG. 2Bdepicts an alternative configuration17bfor the cup pedestal. In this embodiment, the depending or downwardly facing end of the cup terminates in pedestal structure resembling a spool. The spool17defines a first horizontally disposed substrate50, a second horizontally disposed substrate52and a center axis54coaxial with the longitudinal axis of the assembly10, the center axis54separating the first and second substrates, such that the first substrate50is positioned above the second substrate52. The open sides defined by the spool configuration allows the longitudinal axis to be accessible from lateral aspects of the pedestal. As such, the open sides allow for initial engagement with a pincher or fork having a breadth or spread wider than the diameter of the substrates50or52.

The cup18is supported on an upwardly facing surface of the first substrate50. In an embodiment of the invention, the cup18is removably attached to the upwardly facing surface of the first substrate. In another embodiment of the invention, the cup is permanently attached to the upwardly facing surface of the first substrate50of the pedestal17.

The cross section of the center axis54of the pedestal17is dimensioned to be slidably received by the notch15formed in the cup support plate, that notch so depicted inFIG. 3. Once so nested, the support plate resides between the first and second substrates of the pedestal17. In other words, once the pedestal is slid into position, it sandwiches the support plate16between its two horizontally disposed substrates50,52.

The first and second substrates are spaced apart so as to facilitate grabbing of the axis54with a tool (not shown), once the pedestal/cup construct is fully extracted from the confines of the electrorefiner. The pedestal configuration assures that the tool will securely fasten onto the pedestal and allow the pedestal to be upended when cup emptying procedures commence.

As depicted inFIG. 1BandFIG. 2, when the cathode14is in a fully extended configuration, it completely envelopes the aforementioned upwardly extending region38of the cup18. As such, the depending end of the cathode is positioned in close spatial relation to the floor of the cup when the cathode is fully extended downwardly. In this fully nested position, two annular spaces41,42are created in which reprocessed metal may accumulate. The first annular space41(FIG. 2b) is defined by medially facing surfaces of the cathode tube14(i.e., interior surfaces of the cathode tube) and laterally facing surfaces of the upwardly extending region38of the cup. The second annular space42is defined by laterally facing surfaces (i.e., exterior surfaces) of the cathode tube14and medially facing surfaces of the peripheral walls of the cup.

An embodiment of the invention prevents metal from remaining in an annular space42. Otherwise, large amounts of metal accumulation there may stymie easy removal of the cathode from the extending region38of the cup18at the end of a collection cycle. One way for preventing metal accumulation within the first annular space41is to position the depending end44of the cathode to be above the floor of the cup a distance which facilitates liquid drainage from that annular space41.

In another embodiment of the invention, the lip of the depending end44of the fully extended cathode14is sufficiently offset radially from opposing surfaces of the upwardly extending region38or post of the cup so as not to hinder drainage of reduced metal formed on the interior of the cathode from seeking the level of the metal in the cup forming during electrolysis. This configuration increases the efficiency of the cathode inasmuch as it facilitates metal formation and collection on medially facing surfaces and laterally facings surfaces of the cathode viz. the collection cup. Therefore, this configuration provides two annular spaces in which metal formation occurs, both of the annular spaces confined to the same collection cup.

Alternatively, the medially facing surfaces of the cathode tube can be coated with a non-conductive film to prevent deposition of material on that surface. This non-conductive film may provide a means for preventing the tube from freezing or otherwise sticking to the cup.

Operational Detail

In full operational mode, the system and therefore the cathode is fully inserted into the electro-refiner as shown inFIG. 5A, with the cup immersed within molten electrolyte, and the cathode fully extended into the molten electrolyte. The upwardly extending end34of the cathode14is in electrical communication with an electrical supply block82positioned on the floor13of the cabinet11as depicted inFIG. 5A.

FIGS. 4 and 5Adepict the upwardly extending (i.e., superior) end34of the system in electrical communication with a proximal end of the electrical conductor plate60attached to the floor13of the cabinet11via an electrically charged pin or post66. These figures show the system fully inserted into the electrorefiner, which lies below the floor13of the cabinet11.

The plate60is positioned between the end of the rod14and the lift block62, the latter of which provides a means for gripping the cathode rod14to facilitate cathode lowering and raising operations. The lift block62may define a threaded aperture64so as to mate with the upwardly extending end34of the cathode, which terminates in a threaded rod. A transverse aperture may be provided in the plate such that the aperture is collinear with the aforementioned threaded aperture of the lift block and the end34of the cathode.

A distal end of the plate is in electrical communication with an electrical contact pin66so as to facilitate quick connect-disconnect of the system10to the electrical supply block82. The pin66depends from a bottom facing surface of the plate so that electrical disconnect occurs when the system is pulled upwardly, and out of the electrolytic bath.

During this phase of the electrorefining process (i.e., where the system is fully inserted into the electrorefiner), metal collecting on the cathode clones off of it almost immediately due to gravity, and comes to rest within the confines of the recovery cup18. Metal plating occurs along the entire length of the cathode14and not just on that portion of the cathode directly overlying the recovery cup. As the cup fills with metal, it displaces the electrolyte residing therein.

Extraction Sequence

Detail

The cathode extraction sequence for the system10is multi-fold inasmuch as the cathode rod14is first partially withdrawn, then the entire assembly10is partially withdrawn. A salient feature of this sequence is that it minimizes heat loss from the crucible to the ambient environment, and even to the interior of the semi permanent cabinet11, which serves as a vestibule to the electrorefiner proper.

Cathode movement within the system is facilitated with the aforedescribed tongue and groove interlocking mechanism, such that the tongue, mounted to the system, mates with the grooves formed in longitudinally extending regions of the cathode, which itself is in slidable communication with the system. Similar tongue and groove arrangements allow for sequential, step-wise withdrawal of the entire system10from the confines of the electrorefiner. When the system10is fully engaged (FIG. 5A), it is supported at the floor13of the cabinet11by the radially extending cantilevered regions40of the plug26.

When the system10is partially extracted from the electrorefiner (FIGS. 5B, 5C), downwardly facing surfaces of the radially extending cantilevered regions40again support the system10. In this instance, the cantilevered regions40contact either medially directed shelves87or other similarly situated structures within the cabinet, or with upwardly facing surfaces of the aforementioned tongues72, which project from the distal ends of such shelves or structures. This partial system10extraction position allows any salt or metal to drip off the cup, while still relegating the cup to the heated confines of the electrorefiner.

When the system10is completely withdrawn from the electrorefiner (FIG. 5D), the system is again supported by the radially extending cantilevered regions40of the plug26, but this time those regions are supported by the roof of the cabinet11. This complete withdrawal, such as depicted, allows for the removal of the currently used metal collection cup after the cathode14is extracted upwardly, as described infra.

First Extraction

Step Detail

The system10is shown inFIG. 5Aas fully deployed within the salt bath. The cathode14is shown as locked into that position via the tongue72and groove mechanism described supra, wherein the relatively stationary tongue (anchored to the floor of the cabinet11) is nested within the superior notch70formed within the cathode14.

Upon completion of the electrorefining process, and starting at a position depicted inFIGS. 1B and 5A, an overhead system with an end attachment means such as a hook, clamps or pinchers, engages the cathode lift block62and raises the cathode until a radially extending boss19or other protuberance from the cathode engages or otherwise contacts the downwardly depending surface of a scraper/guide plate29. The scrapper/guide plate29is depicted inFIGS. 1A and 1Bas residing above the surface of the salt bath. The superior heat shield24is also seen residing above the surface of the salt bath and fully nested within the floor13of the cabinet11.

This first extraction step is illustrated inFIG. 5B. Movement occurs up to the point of protuberance19engagement with the scraper/guide plate29, and causes the cathode to slide upwardly and through a central aperture31of the scraper/guide plate29. The protuberance engagement with the downwardly facing surface of the scraper/guide plate29provides a tactile cue to the operator that the first height of the cathode extraction has been reached. This protuberance engagement limits movement of the cathode guide tube a predetermined distance. This predetermined distance is sufficient to disconnect power to the cathode, while still allowing the depending end of the cathode14to be engaged in the cup post38, so as to confer stability to the depending end of the cathode. In an embodiment of the invention, this first lifting distance is approximately 13 inches.

At this engagement point, a second set of cathode notches70, seen inFIG. 1Cas inferior to the most superior notch, engages with the tongue72anchored to the floor13of the cabinet.

This initial cathode retraction automatically decouples the electrical connection between the cabinet and the cathode14.

Second Extraction

Step Detail

In a second extraction step, depicted inFIG. 5C, the entire assembly10is lifted upwardly, such that the cathode and the system move in tandem. This tandem lift is facilitated by the protuberances19of the cathode still engaging the underside of the scraper/guide plate29. The entire assembly comes to rest at a second, superior set of tongues72which are anchored within the cabinet and approximately midway along the vertical axis of the cabinet at a mid level support structure86of the cabinet11.

This second extraction configuration allows the exterior surfaces of the cup18to drip dry inasmuch as the cup is no longer in contact with the salt bath. However, the cup18remains within the headspace of the electrorefiner so as to stay relatively warm. This will facilitate drainage of salt from the cup18while minimizing the risk of salt freezing on the cup.

Third Extraction

The system10is further retracted from the refiner as depicted inFIG. 5D. Retraction of the system10is facilitated by aligning the radially extending cantilevered regions40of the plug26with mating apertures formed within the roof of the cabinet11. Then the system is lifted such that the superior end of the plug26is lifted above the cabinet11, rotated along an arc angle sufficient to take the cantilevered regions out of alignment with the mating apertures, and lowered such that the system rests on the roof of the cabinet.FIG. 5Ddepicts the system rotated approximately 45 degrees.

Upon the completion of this third extraction step, the cathode14is still engaged with the superior tip48of the cup post38. Also, the second, inferior heat shield25is nested within the aperture33formed in the floor of the cabinet11. (The aperture33provides a means of ingress and egress of the assembly10in and out of the confines of the electrorefiner.) Such nesting of the inferior heat shield25provides a means for preventing heat loss from the head space above the bath into the cabinet/vestibule interior.

Fourth Extraction

Step Detail

FIG. 5Eshows the system after a fourth extraction step. This fourth step is initiated when the cathode rod14is rotated about 90 degrees until its protuberances19align with diametrically opposing notches21(seeFIG. 1B) positioned along the central aperture31of the scraping/aligning plate29. Then, while the system is kept in contact with the roof of the cabinet11, the cathode14is raised to allow passage of the protuberances through the notches and similarly aligned notches along the central aperture of the first, superior heat shield24. This raising action may result in any residual material being scraped from the cathode14. This raising action also results in disengagement of the cathode14from the center post38of the cup18.

As the cathode tube is raised past the notches in plate29, a third set of notches identical to item70come into view and allow the cathode system to be positioned in stationary position above the collection cup. The third set of notches in item70engage the hinged lever,68and72-78, to position the cathode above the cup. The notches are not visible in the drawings because they are hidden by the heat shield,24, and insulation,26.

Travel of the cathode is approximately 7 inches before the third set of notches engage the hinged lever

FIGS. 5A-Cdepicts other aspects of the interior of the cabinet11, including a plurality of storage areas19for placement of additional metal recovery cups18. Two such storage areas84are depicted as arranged on the same side of the cathode system10, with one storage area arranged beneath the other. However, other configurations may be suitable.

The overhead handling system, utilizing the same grasping means as was used to grasp the lift block62, is used to grasp the pedestal engagement portion54inFIG. 2, or element56inFIG. 3, and place it in a superior positioned cup storage space84in the cabinet. The handling system then harvests an empty cup, on its own pedestal from the lower storage shelf84and installs it in the module10.

Inasmuch as the molten electrolyte and metals will approach 650° C., suitable materials are selected to withstand reaction temperatures. For example, suitable materials for the cup18are any nonelectrically conductive substrate, such as ceramic. Cathode materials are electrically conductive materials selected from the group consisting of tungsten, molybdenum, tantalum, and combinations thereof.

One skilled in the art will also readily recognize that where members are grouped together in a common manner, such as in a Markush group, the present invention encompasses not only the entire group listed as a whole, but each member of the group individually and all possible subgroups of the main group. Accordingly, for all purposes, the present invention encompasses not only the main group, but also the main group absent one or more of the group members. The present invention also envisages the explicit exclusion of one or more of any of the group members in the claimed invention.