Patent Publication Number: US-2018030576-A1

Title: Uranium hexafluoride off-gas treatment system and method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of U.S. Provisional Patent Application No. 62/368,089, titled “URANIUM HEXAFLUORIDE OFF-GAS TREATMENT SYSTEM AND METHOD”, filed Jul. 28, 2016, which application is hereby incorporated by reference. 
    
    
     INTRODUCTION 
     Hydrogen fluoride gas containing trace amounts of uranium hexafluoride is a byproduct of some methods of making uranium fuel for nuclear reactors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawing figures, which form a part of this application, are illustrative of described technology and are not meant to limit the scope of the invention as claimed in any manner, which scope shall be based on the claims appended hereto. 
         FIG. 1  illustrates an embodiment of a system for implementing the “cold-wall” process for the reduction of UF 6  to UF 4 . 
         FIG. 2  illustrates an embodiment of a method for converting UF 4  to uranium metal. 
         FIG. 3  illustrates, at a high level, an embodiment of a simpler method for treating HF gas containing trace amounts of UF x  such as, for example, UF 6 . 
         FIG. 4  illustrates a general block diagram of a system for treating HF gas containing trace amounts of UF x  such as, for example, UF 6 , based on the method of  FIG. 3 . 
         FIG. 5  illustrates a more detailed system for treating HF gas containing trace amounts of UF x  that utilizes an embodiment of the method of  FIG. 3 . 
         FIG. 6  illustrates an alternative vessel configuration for the reduction reactor, filter vessel, and backup filter vessel that may be used in the system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure describes systems and methods for removing uranium hexafluoride (UF 6 ) and/or other uranium fluoride (uranium fluorides identified herein generally as UF x ) gases from a hydrogen fluoride (HF) gas stream. 
       FIG. 1  illustrates an embodiment of a system for implementing the “cold-wall” process for the reduction of UF 6  to UF 4 . In the system  100  as shown, UF 6 , which may or may not be U-235 enriched or depleted, is provided by an autoclave  102  for blending with fluorine gas. The UF 6  is blended with fluorine gas at a high temperature, for example in a hot box  104  maintained from 95 to 110° C. and fed to a nozzle  106  at the top of a reaction vessel  108  where the gases are mixed with excess hydrogen. The hydrogen and fluorine burn to form HF which generates heat that is used to drive the UF 6  reduction to UF 4 . Thus, the reaction vessel  108  may also be referred to as the reduction reactor  108 . The excess hydrogen will flow through the rest of the system  100  and, ultimately, exit via the off-gas treatment system  118  without further chemical interactions. 
     In an embodiment, the reaction vessel  108  may be a simple tube or column of the appropriate size and volume to allow the reduction reaction to go to completion under the desired throughput rates. In an alternative embodiment, any vessel configuration that allows for gas-solid separation may be used such as cyclone separators. 
     In the simple gravity settling chamber column as shown, the resulting solid UF 4 , in the form of a powder precipitate, drops by gravity through the reaction vessel  108  and is collected in a hopper  110  at the bottom of the vessel  108 , HF off-gas exits the reaction vessel  108  via a primary off-gas filter vessel  112 . In the embodiment shown, the primary off-gas filter vessel  112  may be connected to the reaction vessel using an angled pipe  111  to assist in the collection of any solid UF 4  that may be created or carried into the primary off-gas filter vessel  112 . 
     To enhance the flow of the UF 4  solid down through the reaction vessel  108  and out of the primary off-gas filter vessel  112 , various vibrating elements (not shown) may be used such as vibrating plates. The vibrating element or elements may be spaced along or at various points in the reaction vessel  108 , the primary off-gas filter vessel  112 , or both. For example, vibrating elements may be located at the top and bottom of the reaction vessel  108 . In an alternative embodiment, the entire vessel  108  and/or the primary off-gas filter vessel  112  may be vibrated as a unit to prevent buildup on surfaces of the system  100 . To account for the vibration, flexible connections (not shown) between some or all of the different components, such as the reaction vessel  108 , the primary off-gas filter vessel  112 , the hopper  110 , etc. may be used. 
     In an embodiment, the HF off-gas is filtered in the primary off-gas filter vessel  112  by filters  114  at the top of the primary off-gas filter vessel  112  as shown. Any entrained solid UF 4  is thus prevented from exiting the reaction vessel/filter vessel assembly with the HF off-gas. Any UF 4  that reaches the primary filter vessel  112  is captured and returned to the reaction vessel  108  and, subsequently, to the hopper  110 . 
     The hopper  110  directs the UF 4  powder into a UF 4  storage container  120 . The hopper  110  may be of any convention design and may, or may not, include an active component (such as the screw conveyor  122  as illustrated or any other active element such as a vibrating screen) as shown to assist in the handling and transport of the UF 4  powder to the storage container  120 . 
     In the embodiment shown, the filters  114  in the primary off-gas filter vessel  112  may be occasionally cleaned of any UF 4  particulate on the filter media by backflushing, sometimes also referred to as reverse pulse cleaning. In an embodiment, each filter  114  is independently cleaned with nitrogen on a periodic schedule, e.g., backflush for 0.5 sec after every two minutes of operation, so that at any given time only one of the filters  114  is being backflushed while the remain filters remain in normal operation. Although this embodiment uses nitrogen, any suitable gas may be used including any other inert gas. 
     The HF off-gas exiting the primary filter vessel  112  may further be passed through a backup filter vessel  116  as a safety measure as shown. The flow through the backup filter vessel  116  or the pressure drop across the backup filter vessel  116  may be monitored during operation for abnormal conditions (e.g., an unexpected drop in flow or increase in pressure drop across the filter  116 ) indicating that the filters  114  in the filter vessel  112  may have failed and particulate UF 4  is being collected in the backup filter vessel  116 . 
     The HF off-gas that exits the primary off-gas filter vessel  112  and the backup vessel  116  is a stream of gaseous HF, excess hydrogen and nitrogen (from the filter cleaning and system purges). In addition, the HF off-gas will contain some trace amount of gaseous UF 6  and/or other gaseous uranium fluoride specie. The gaseous uranium fluorides as a group will be referred to as UF x  to illustrate that UF 6 , while the likely predominant the predominant species, is not the only uranium fluoride in the HF off-gas stream. In the embodiment shown, the gaseous HF/UF x  off gas stream is cleaned using an off-gas treatment system  118 , which is described in greater detail below with reference to  FIG. 3 . 
     Major inputs to the process (not including the off-gas treatment system  118 ) include UF 6  gas, fluorine, hydrogen, nitrogen purge and blow-back gases. Outputs include UF 4  powder and an HF off-gas stream containing a small amount of uranium as UF 6  and/or UF x . 
       FIG. 2  illustrates an embodiment of a method for converting UF 4  to uranium metal. In the method  200  as shown, uranium metal is made by reducing the UF 4  with calcium metal. The UF 4  is first preconditioned in a preparation operation  202  that may involve a grinder or other system to generate a uniform particulate size of the UF 4 . Preconditioning may also include heating or other physical manipulation of the UF 4 . 
     Properly conditioned UF 4  is then mixed with calcium metal in a mixing operation  204 . This generates a UF 4 /Ca powder blend. After mixing, the UF 4 /Ca powder blend is transferred to a high-temperature reactor having an inert atmosphere. Various additives may be provided also, to assist some aspect of the reaction. For example, metal Iodate (I 2 ) may also be mixed in to lower the CaF melt temperature to assist in separation. Alkali metal peroxide may be added as an ignition agent. 
     In the reactor, the mixture of UF 4  and Ca is heated to ignition (approximately 500-700° C. at 1 atm) in a reaction operation  206 . Upon ignition, the exothermic reduction reaction is initiated and the additional heat generated by the reaction drives up the temperature to above the melting point of uranium (approximately 1132° C.) and, in some embodiments, even above the melting point of calcium fluoride (approximately 1418° C.). For example, reactor temperatures above 1500° C. are possible. The temperature of the reactor may be controlled by the application of active cooling such as by using a cooling jacket or other cooling elements around or in the reactor. As part of the reaction operation  206 , the reactor is operated at a temperature so that molten uranium metal collects at the bottom of the reactor, upon which the much lighter (liquid or solid) CaF 2  will float as a separate phase. 
     The reactor may be lined with a corrosion-resistant liner such as magnesium fluoride or made from a corrosion-resistant material. Any suitable vessel design may be used for the reactor, including that of a simple hollow pressure vessel having suitable inlets and outlets for receiving the powder blend and discharging the liquid product. 
     In batch operation embodiments, after UF 4  and Ca have been reacted by the reaction operation  206 , the collected liquid U and CaF 2  may be cooled to the solid phase and physically separated in a separation operation  208 . The solid formed by such cooling is in two discrete layers, one of substantially pure U metal and the other of substantially pure CaF 2 . Physical separation of the layers may be easily achieved. In an alternative embodiment of the separation operation  208 , the liquid U may be removed from the reactor and collected in a storage vessel in which it is allowed to cool. The liquid CaF 2  may be separately drained from the reactor to a different storage vessel. 
     In an alternative embodiment, magnesium metal may be substituted for calcium metal in the UF 4  reduction process. While less expensive, the magnesium embodiment has higher uranium loss in the magnesium fluoride slag than in the calcium embodiment. 
     The reduction of UF 6  to UF 4  as shown in  FIG. 1  and subsequent reduction of UF 4  to U metal as shown in  FIG. 2 , together, may be referred to as the uranium metallization process. In an embodiment, the U metallization process generates no liquid waste. For example, in an embodiment, the U metallization process waste streams are limited to HF off-gas and solid CaF 2  with trace amounts of uranium, which may be easily disposed of. 
     A historically difficult and important aspect of the uranium metallization process is the treatment of the gaseous HF/UF x  off-gas stream and the design of the off-gas treatment system  118 . Various methods have been used that include dry chemical traps and cold traps to recover small quantities of UF 6  and other UF x  intermediates. For example, some systems have used sodium fluoride or anhydrous calcium sulfate traps to absorb UF 6 . The traps are controlled at elevated temperatures and thermally cycled to remove the UF 6 . A cold trap collection system is required to collect the UF 6 . Then the off-gas is either processed through caustic scrubbers or the HF is recovered in a cold trap recovery system then vented to a caustic scrubber system. At that point, the scrubber effluent is processed in the effluent treatment system. The disadvantages of these approaches include the complexity of the process steps and the associated chemical handling/storage and disposal. 
       FIG. 3  illustrates, at a high level, an embodiment of a simpler method for treating HF gas containing trace amounts of UF x  such as, for example, UF 6 . The method  300  uses a caustic scrubber in which the caustic scrubber solution is provided with trace amounts of particulate CaF 2 . Particulate CaF 2 , for the purposes of this disclosure, refers to solid particles of CaF 2  having an identifiable particle size distribution, e.g., particles of CaF 2  that can pass through a 70 micron filter or through a No. 400 US mesh. For example, as discussed with reference to the embodiment shown in below in  FIG. 5 , the particulate CaF 2  has a particle size sufficiently small to pass through a 5-micron filter press. 
     This method  300  takes advantage of the following chemistry: a) the trace amount of UF x , upon contact with the scrubber solution, oxidizes to form oxidized uranium fluoride compounds (UO x F y ), such as UO 2 F 2 ; b) the oxidized uranium fluoride compounds will sorb to particulate CaF 2  in the caustic solution; and c) the HF, upon contact with the caustic scrubber solution, reacts to form soluble KF that remains in the caustic solution. Without being bound to any particular theory, it is believed the uranium compounds resulting from reaction with the caustic solution are one or more species of UO x F y  that form some kind of a complex or agglomerate with individual particles of CaF 2 , resulting in a larger, stable particle or agglomerate of uranium compounds and CaF 2 . Furthermore, it appears that the oxidized uranium fluoride compounds interact with the surface of particles of CaF 2 . Thus, as long as sufficient surface area of particulate CaF 2  and sufficient mixing are provided, the actual particle size of the particulate CaF 2  will not be important to the UO x F y  removal. This can be easily achieved by providing relatively more particulate CaF 2  based on the surface area of the particles used. However, because there is only trace amounts of uranium in the caustic liquid to begin with, it is anticipated that an excess of particulate CaF 2  is very easy to achieve regardless of the particle size chosen. The term “sorb” is used herein to refer to this interaction resulting in the stable uranium compound/CaF 2  particle. Thus, the resulting uranium-bearing particles will be referred to as particulate CaF 2  with sorbed U compounds. 
     At a high level, the method  300  can be described as contacting operation  302  followed by a separation operation  304 . In the contacting operation  302 , the uranium-bearing HF off-gas stream is brought into contact with a caustic solution, such as potassium hydroxide (KOH) solution, containing particulate CaF 2 . As described above, the HF off-gas stream includes HF, excess hydrogen and nitrogen, and some trace amount of gaseous UF x . Because of the reactions described above, this contacting consumes the HF and results in a liquid-gas mixture that includes a uranium-bearing caustic solution and non-condensable components including H 2  and N 2 . Depending on how the efficient the contacting is between the two reactants, the reaction may be almost instantaneous. 
     The particulate CaF 2  may be provided in any manner and may have any particle size. For example, in an embodiment an amount of particulate CaF 2  may be mixed into a batch of caustic solution to create a caustic/CaF 2  mixture for use in the method  300 . In a particularly elegant embodiment, discussed in greater detail below, the particulate CaF 2  may be a byproduct of a caustic recycling operation, such as recycling operation  306 . 
     The resulting liquid-gas mixture will readily separate into a uranium-bearing liquid stream and a hydrogen/nitrogen gas stream. Thus, in one embodiment, the separating operation  304  involves simply collecting the liquid-gas mixture from the contacting operation  302  and holding it for a time sufficient to allow the reaction to occur and the phases to separate. 
     The efficiency of the contacting operation  302  may be improved by actively mixing the off-gas stream and the caustic solution. In an embodiment, discussed in greater detail below, this may be achieved by using a venturi to ensure adequate mixing of the off-gas stream with the caustic solution. In this embodiment, the off-gas stream is delivered to the suction inlet of the venturi and is drawn into the venturi through the suction created by the flow of the caustic through the motive inlet of the venturi. The two streams become well mixed as they pass through the throat of the venturi and exit as a liquid-gas mixture at the venturi&#39;s discharge. The flows can be controlled so that the reactions are driven substantially to completion within the venturi such that the discharge from the venturi consists of a liquid-gas mixture of a uranium-bearing liquid stream and non-condensable hydrogen and nitrogen gas. 
     Other methods and systems of mixing or combining a gas stream with a liquid stream to encourage a chemical reaction are also possible. Many such systems are known and any suitable technology may be used. For example, in an alternative embodiment, a packed column, a bubble column, a spray tower, a plate column, a falling-film column, a diffusion tank, or a rotating disc diffuser, to name but a few, may be contemplated. 
     The separating operation  304  may also include separating the uranium-bearing CaF 2  particulate from the caustic solution. In practice, it has been determined that the uranium-bearing CaF 2  particulates are easily filtered from the caustic solution when passed through a 50- to 70-micron filter when the original particulate CaF 2  in the solution have a particle size of less than 10 microns. It is believed a CaF 2  particle size greater than 10 microns could also be effective. 
     The method  300  may also include an optional caustic recycle  306 . This operation recycles the caustic solution so that it may be reused in the contacting operation  302 . In a KOH caustic embodiment, for example, the KF created by the reaction with the HF in the off-gas may be recycled back into KOH by using Ca(OH) 2 . 
     In an embodiment, particulate CaF 2  may be added as byproduct of the optional caustic recycle operation  306 . In an embodiment of a recycle operation, the filtered caustic solution will have some amount of the cation of the caustic in the form of a fluoride, e.g., KF if the caustic solution is a KOH solution. The fluoride may be removed by adding Ca(OH) 2  to the filtered caustic solution, so that the fluoride forms a particulate CaF 2  precipitate and new KOH. Again, using the example of KOH, the reaction is as follows: 
       Ca(OH) 2 +2KF→2KOH+CaF 2 (solid particulate)  
 
     So much CaF 2  may be created in the recycling operation  306  that most CaF 2  may need to be removed before the recycled caustic solution can be reused. However, the recycling operation  306  may include allowing some of the particulate CaF 2  to remain in the recycled caustic solution. Thus, the recycled caustic solution can be reused in the contacting operation  302  (as illustrated) as is without the need to introduce additional fresh particulate CaF 2 . 
     In a batch embodiment of the method  300  using KOH solution as the caustic solution, an amount of initial KOH solution with a pH 14 is provided. The contacting operation  302  and separation operation  304  are performed with the KOH solution until the pH drops below a threshold, for example 12, or for some predetermined period of time, after which some or all of the KOH solution is replaced with fresh or recycled KOH solution. Continuous embodiments are also possible in which a portion of the KOH solution is being continuously recycled by the recycle operation  306  so that the pH and KOH concentration of the caustic solution are maintained at some steady state value. 
       FIG. 4  illustrates a general block diagram of a system for treating HF gas containing trace amounts of UF x  such as, for example, UF 6 , based on an embodiment of the method of  FIG. 3 . The system  400 , as illustrated, continuously receives the incoming HF off-gas stream  402  and treats it in a gas-liquid contactor  404 . As discussed above, the gas-liquid contactor  404  may be of any suitable type and may include one or more contacting stages, vessels, venturis, valves, nozzles, flowmeters, temperature sensors, pressure sensors, and other individual components. For example, in an embodiment the contactor  404  includes a gas inlet which may include a nozzle that directs the received gas into a reaction vessel containing a caustic solution where the mixing occurs. In an alternative embodiment discussed in greater detail below, the gas-liquid contactor  404  may include a venturi that receives the inlet gas and caustic solution and discharges the combined product into a holding vessel. In yet another embodiment, the contactor may consist of a venturi followed by a liquid-gas separator. 
     In the gas-liquid contactor  404 , the received HF/UF x  gas  402  is mixed with a fresh caustic treatment solution  406 , a fresh KOH treatment solution is illustrated in  FIG. 4 , that contains the particulate CaF 2 . In an embodiment, the fresh KOH treatment solution  406  may be obtained from an optional reservoir  430 , such as a holding tank, or directly from the uranium separator  410  and/or the spent KOH treatment solution recycler  420  as discussed below. The received gas  402  may be mixed with the fresh KOH treatment solution  406  so that some, substantially all, or all incoming HF is consumed and converted to KF and H 2 O. In addition, the UF x  gas is partially or completely contacted with the fresh KOH treatment solution  406  as a result of the mixing, thereby causing the UF x  to react with the KOH treatment solution to form the oxidized uranium fluoride compounds. 
     The non-condensable H 2  gas from the HF off-gas is allowed to separate from the contacted solution and is vented from the contactor  404  for subsequent treatment and eventual release to the atmosphere by an H 2  gas treatment system  412 . Alternatively, the H 2  gas may be flared or otherwise collected and burned instead of being released to the atmosphere. Note that the received HF/UF x  off-gas  402  may include N 2  gas. Any such N 2  gas will be chemically unaffected by the system  400  and will be discharged along with the H 2  gas  414  through gas treatment system  412 . 
     After separation of the H 2  (and any N 2 ) gas, the liquid effluent of the contactor  404  is a uranium-bearing KOH solution  408 , which is then transferred to a uranium separator  410 . In this embodiment, the uranium-bearing KOH solution  408  will be a solution of KF, KOH and particulate CaF 2 , in which at least some of the CaF 2  will have combined with U compounds generated by the reaction of UF x  with the water in the solution. 
     In an alternative embodiment, the gas-liquid contactor  404  may be operated so that only partial conversion of HF is achieved. In this embodiment, the contactor  404  may be operated so that all or substantially all of the UF x  gas is contacted by or solubilized into the fresh KOH treatment solution  406  but that excess HF gas remains. This may be suitable when the goal is to remove all uranium but retain some HF for sale or later use. However, for the purposes of the remaining discussion, it will be assumed that the contactor  404  is operated to achieve complete, or near complete, conversion of both the HF and UF x  received by the contactor  404 . 
     The contactor  404  may be operated in a batch, semi-batch, or continuous fashion. When in continuous operation, the HF/UF x  gas  402  and the fresh KOH treatment solution  406  are received as separate streams that are combined by the contactor  404 . The contactor  404  further operates to continuously separate the effluent streams, i.e., the non-condensable H 2  gas stream  414  and uranium-bearing KOH solution  408  stream. The non-condensable gas stream  414  is discharged to the gas treatment system  412  to be vented to atmosphere. In an embodiment, the gas stream  414  is diluted by the gas treatment system  412  to reduce the concentration of H 2  to acceptable levels. The uranium-bearing KOH solution  408  stream is passed to the uranium separator  410  for uranium removal. 
     The uranium separator  410  removes the particles of CaF 2  along with any sorbed uranium compounds from the uranium-bearing KOH solution  408 . This may be done by filtration or by any other suitable liquid-solid separation technique. When done by filtration, the uranium compounds will collect on the filter media until the media is replaced. The uranium-bearing filter media may then be disposed as a solid waste product  418  of the separator  410 . When the separation is achieved using other techniques, the uranium-bearing solid waste  418  may be in some other form, such as a precipitate, agglomerate, or complex. Depending on the embodiment, particles of CaF 2  that have not combined with U, referred to as “free CaF 2 ” to distinguish it from particles of CaF 2  with sorbed U, may or may not be removed by the uranium separator  410 . In the embodiment shown, the uranium separator captures particles of CaF 2  with sorbed U but passes free CaF 2 . This may be achieved, for example, by using a filter sized to remove particles of CaF 2  with sorbed U but to pass the free CaF 2  particles. In this embodiment, the effluent of the uranium separator  410  is a filtered KOH treatment solution  416  that contains free CaF 2  suitable for sorbing with uranium compounds in the next pass through the contactor. 
     In an alternative embodiment, the uranium separator  410  may remove all the particulate CaF 2  regardless of whether it is sorbed to uranium or not. In this embodiment, additional CaF 2  particulate may be added to the filtered KOH treatment solution  416  from an optional particulate CaF 2  source  424 . 
     In the embodiment shown, the filtered KOH treatment solution  416  and particulate CaF 2  is transferred back to the gas-liquid contactor  404 , either directly or via the optional reservoir  430 , and reused as fresh KOH treatment solution  406  in contacting additional HF/UF x  gas  402 . Thus, the contactor  404  and separator  410  may be operated as a closed loop system that receives HF/UF x  gas  402  and discharges H 2  gas  414  and a uranium-bearing solid waste product  418  stream. 
     The separator  410  may be operated as a batch, semi-batch, or continuous stage of treatment. For example, in an embodiment the contactor  404  and the separator  410  are operated continuously as a closed loop treatment of the HF/UF x  gas  402  until such time as the KOH treatment solution is considered spent. In an alternative, batch embodiment, the contactor  404  is operated continuously until a sufficient amount of uranium-bearing KOH treatment solution  408  has been generated, at which time the uranium-bearing KOH solution  408  is transferred to the separator  410  for treatment in a batch operation. 
     In a continuous embodiment, reuse of filtered KOH treatment solution  416  as KOH treatment solution  408  may be performed until the filtered KOH treatment solution  416  exiting the separator  410  is spent and no longer has sufficient KOH in solution to convert the incoming HF gas  402 . However, complete consumption of the KOH treatment solution is not necessarily efficient or preferable in some situations and the KOH treatment solution may be recycled or replaced based on a mass balance taking into account the amount of HF removed, based on the volume of HF gas  402  treated, based on a monitored pH of the treatment solution  408 , or based on the time since the last replacement. 
     In the embodiment illustrated in  FIG. 4 , an optional KOH treatment solution recycler  420  is provided to recycle spent KOH treatment solution by converting the KF back into KOH. The recycler  420  may receive spent KOH treatment solution either from the gas-liquid contactor  404  or from the uranium separator  410 . In an embodiment the recycler  420  converts KF back into KOH utilizing the reaction previously cited above by adding Ca(OH) 2    426  to the spent solution. This reaction also generates solid particulate, free CaF 2 . This newly generated particulate CaF 2  may be removed in whole or in part from the recycled KOH treatment solution. In an embodiment, the recycler  420  is operated so that enough free CaF 2  generated by the addition of Ca(OH) 2  is retained in a particulate form to replace the amount of CaF 2  removed by the uranium separator  410  as part of the uranium removal. 
     In this embodiment, the output of the recycler  420  is a recycled KOH treatment solution  422  having some particulate free CaF 2  ready for use as the input of fresh KOH treatment solution  406  for the gas-liquid contactor  404 . In the embodiment shown, the recycled KOH treatment solution  422  is transferred back to the gas-liquid contactor  404 , either directly or via the optional reservoir  430 , and reused as fresh KOH treatment solution  406  in contacting additional HF/UF x  gas  402 . 
     As with the separator  410  and contactor  404 , the recycling may be done as a batch, semi-batch or continuous operation. For example, in the embodiment shown the recycler  420  is operated as a batch operation, illustrated by the inflows and outflows being shown as a dashed line. In this embodiment, upon determination that the KOH treatment solution needs to be recycled, which may be determined by monitoring the pH or the amount of CaF 2  of the treatment solution  406 , some amount of KOH treatment solution from the system may be transferred to the spent KOH recycler  420  for treatment. For example, if the monitored pH of the KOH treatment solution at any spot in the system falls below a selected lower threshold, the batch recycling operation may be performed. Suitable pH thresholds may be 11, 12, 12.5, 13 or 13.5. In an alternative embodiment, the spent KOH recycler  420  may be continuously operating on a side stream of treatment solution to maintain the pH or CaF 2  content of the fresh KOH treatment solution  406  at a selected constant level, such as at a pH level from 11 to 14. In a simpler embodiment, the recycling may be done periodically on a schedule based on the amount of HF gas treated or based on a monitored KOH concentration in the solution  408 . 
     The major inputs to the HF gas treatment system  400  are the HF/UF x  gas  402 , hydrated lime  426  and, possibly, a small amount of makeup potassium hydroxide, particulate CaF 2 , and deionized water. The output streams are a relatively large stream of calcium fluoride solid  428 , a much smaller stream  418  of uranium-bearing calcium fluoride captured on filter media, and scrubbed process off-gases  432 . The system  400  is effective in isolating the uranium into a relatively small, solid waste stream  418  that is easily handled and efficiently disposed of. 
       FIG. 5  illustrates a more detailed system for treating HF gas containing trace amounts of UF x  that utilizes an embodiment of the method of  FIG. 3 . In the system  500  as shown, the mixed HF/UF x  gas is received, such as from the UF 6  to UF 4  reduction process described above with reference to  FIG. 1 , and then mixed with a KOH treatment solution in a venturi to generate a venturi discharge stream. In the embodiment shown, the KOH treatment solution is passed through the motive inlet of the venturi  502  and the inlet gas connected to the suction inlet so that pumping the treatment solution through the venturi draws in the inlet gas at a rate that is a known function of the treatment solution flowrate. 
     The venturi discharges into a vessel identified as the KOH scrubber  504  in  FIG. 5 . In the embodiment shown, the scrubber  504  is a closed holding tank having an inlet for the venturi discharge, a gas outlet to a packed column  506  and a liquid outlet, or drain, for the uranium-bearing KOH treatment solution. The KOH scrubber  504  provides additional holding time to the discharge stream from the venturi, which allows for additional contact time for the reaction to occur and for the non-condensable H 2  and N 2  gases to separate from the treatment solution. In an embodiment, the holding tank is only partially full of KOH treatment solution, the rest of the tank being a headspace. The venturi discharge may be directed into the scrubber  504  through the top as shown or may be into the side or the bottom of the tank, depending on the amount of secondary mixing and agitation desired. 
     The packed column  506  is provided as a secondary treatment of the H 2  (and any N 2 ) gas to prevent any unreacted HF or UF x  gas from exiting the gas outlet of the scrubber  504 . In the embodiment shown, the packed column  506  includes a flow, under gravity, of KOH treatment solution which discharges into the scrubber  504  through the scrubber&#39;s gas outlet. 
     During normal treatment, uranium-bearing KOH treatment solution is pumped from the scrubber through one or the other of the filters  508 . In the embodiment shown, two filters  508  are provided so that one filter may be easily removed and replaced with a new filter without interrupting the continuous treatment of the KOH treatment solution. Filters may be deemed spent and replaced based on activity, time in service, throughput, differential pressure drop indicative of clogging of the filter, or any other suitable method. For example, a differential pressure drop threshold across the filter at a designated flowrate may be used to determine when to replace a filter. Activity is also easily monitored and a filter  508  may be replaced upon determination that the measured activity is at or above some predetermined threshold. The filtrate effluent of the filters  508  is a filtered KOH treatment solution which is returned to motive inlet of the venturi  502 . 
     A condenser  528  may be provided as shown to maintain the temperature of the caustic solution in the scrubber circuit. Optionally and additionally, another condenser (not shown) may be provided in the gas outlet line after the packed column  506  to capture any volatilized caustic solution from the gas effluent and to return the captured solution to the KOH scrubber  504 . 
       FIG. 5  also shows that the KOH treatment solution discharged from the scrubber  504  may be alternatively pumped to a spent KOH solution holding tank  510 . This holding tank  510  is provided to allow the recycling system of the system  500  to operate in a batch mode. That is, an amount of treatment solution may be transferred to the holding tank  510  at any time (with fresh, makeup treatment solution being supplied from the filtrate tank  518  to keep the amount of treatment solution in the scrubber  504  at a desired level). An additional filter (not shown) may be included before the spent KOH solution holding tank  510  in order to prevent any uranium in the spent KOH solution from entering the recycle loop. Such a filter may be sized much smaller than the scrubber filters  508  to reduce the chance that any uranium-bearing solids may pass into the spent KOH tank  510 . 
     The spent KOH solution holding tank  510  feeds a reaction tank  514  in which hydrated lime is mixed with the spent KOH solution. Hydrated lime may be used or, as discussed above, any form of calcium oxide, hydroxide or equivalent base may be used. In an embodiment, the reaction tank  514  is operated as a batch reactor in which all the removed spent KOH solution is transferred into the reaction tank  514  and treated at one time. As discussed above, the KF in the spent KOH solution is converted into CAF 2  and KOH by the reaction. In an embodiment, the concentration of KF of the contents of the reaction tank  514  is determined, either through direct measurement or estimated based on the volume of gas treated, and then additional hydrated lime is added in an amount sufficient to completely convert the KF to KOH. In an alternative embodiment, other methods may be used to determine when the spent KOH solution has been sufficiently regenerated, for example by calculation, by monitoring other parameters such as KOH concentration, KF concentration, etc. 
     The contents of the reaction tank  514 , after the reaction is deemed sufficiently complete as indicated based on the pH or some other parameter, are transferred through a filtration system, illustrated as filter press  516 . The filter press  516  may be of any suitable type including manual or automatic plate and frame presses and/or recessed plate presses. Alternatively, any other filtration or liquid-solid separation system may be used. 
     One aspect of the filter press  516  is that it can be operated so that some amount of small particulate CaF 2  can be allowed to pass through the press. This provides a ready source of fresh CaF 2 , and the fresh, particulate CaF 2  is already in the recycled KOH treatment solution that exits the filter press  516 . In an embodiment, a filter press  516  nominally sized to remove particulate larger 5 microns has been found to pass sufficient particulate CaF 2  to be used without needing any further addition of particulate CaF 2  to the KOH solution. A precoat may be used to assist consistent filtration in the filter press and compressed air may be used depending on the design of the filter press. The CaF 2  that does not exit with the KOH solution will be collected as a filter cake and disposed of as a solid waste. 
     In the embodiment shown, the effluent of the filter press  516  is passed to a filtrate holding tank  518  where it can be held until the next batch recycle operation. At that time, the recycled KOH solution with particulate CaF 2  in the filtrate holding tank  518  may be pumped to the scrubber  504 , as shown, to make up for the volume of spent KOH removed, as discussed above. Makeup KOH and deionized water may also be added to the filtrate holding tank  518  as necessary to keep the KOH treatment solution at the desired pH and KOH concentration. 
     The system  500  is also illustrated with a backup scrubber  512  for safety. The backup scrubber  512  is optional and may be any type of gas-liquid contactor that contacts KOH treatment solution with the gas discharged from the packed column  506 . 
     An air injection system  520  is further illustrated that supplies air to the gas discharged from the scrubber  504  for the purpose of diluting the H 2  gas sufficiently to make it safe to vent to the atmosphere, such as to less than the Lower Explosive Limit (LEL) for H 2  in air. An H2 monitor  526  may be provided to ensure this treatment is being achieved and to control the amount of air being added. A HEPA (high efficiency particulate air) filter  522  is also provided for cleaning of the gas discharge. 
     The scrubber circuit including the scrubber  504  and filters  514  is a closed, airtight system to prevent exposure of the solution and gas streams to the atmosphere. In the embodiment shown, an oxygen sensor  524  is provided in the gas output to detect the presence of oxygen in the gas stream. The presence of oxygen is indicative of a leak in the process equipment and is potentially a safety concern. 
     The major inputs to the off-gas treatment system  500  are hydrated lime and a small amount of makeup potassium hydroxide and deionized water. Outputs are calcium fluoride filter cake, calcium fluoride with some uranium contamination captured on filter media, and scrubbed process off-gases. 
     A different embodiment of the above systems and methods does not use a KOH solution with particulate CaF 2  but, rather, uses a CaF 2  contacting vessel to remove the uranium. In this embodiment, the KOH solution that does not have any CaF 2  is used to contact the inlet HF/UF x  gas to obtain a uranium-bearing KOH solution. The uranium-bearing KOH solution is then passed through a CaF 2  contactor. The CaF 2  contactor may be a simple vessel containing a packed bed of particles of CaF 2 . In an alternative embodiment, the CaF 2  contactor may be one or more filters in which the filter media contains particulate CaF 2  or some type of media with exposed surface area of CaF 2 . 
     Schematically, such embodiments would appear similar to those systems provided in  FIGS. 4 and 5 . In  FIG. 4 , the uranium separator  410  would include the CaF 2  contactor, the other main difference in systems being that the various KOH solutions no longer require particulate CaF 2 . Likewise, in  FIG. 5 , the filters  508  would be replaced by the CaF 2  contactor (which, as discussed above, may be a filter with CaF 2  on the filter media) and, again, the main difference in systems being that the various KOH solutions no longer require particulate CaF 2 . 
       FIG. 6  illustrates an alternative vessel configuration for the reduction reactor, filter vessel, and backup filter vessel that may be used in the system of  FIG. 1 . In the embodiment shown, the reduction reactor  608  and the filter vessel  612  are parallel columns of the same length and diameter attached at the bottom to a symmetrical Y-shaped connector  611 . These take the place of the reduction reactor  108 , primary off-gas filter vessel  112  and angled pipe  111  in FIG.  1 . The reduction reactor  608 , the filter vessel  612 , and Y connector  611  assembly may be supported by a support structure attached to the Y connector  611  or by the attachment to the hopper (not shown). In the illustration, the high angle of the Y connector improves the collection of the solid UF 4  by eliminating horizontal and near horizontal surfaces where the powder can easily collect. A vibrator may be easily attached to the assembly to further improve the solid collection. One or more filters (not shown) may be included in the filter vessel  612  and, additionally, in the backup filter vessel  616 . In an alternative embodiment, the reduction reactor  608  and the filter vessel  612  may or may not be parallel columns and may or may not be the same length or the same diameter. 
     Notwithstanding the appended claims, the disclosure is also defined by the following clauses: 
     1. A method for treating hydrogen fluoride gas containing trace amounts of uranium fluoride gas comprising: 
     contacting the hydrogen fluoride gas containing trace amounts of uranium fluoride gas with a caustic solution containing particulate CaF 2 ; and 
     separating the particulate CaF 2  from the caustic solution after the contacting operation. 
     2. The method of clause 1 wherein the solution is an aqueous solution having a pH of greater than 11. 
     3. The method of clause 1 or 2 wherein the solution is an aqueous KOH solution having a pH of 12 or more. 
     4. The method of any of clauses 1-3 wherein the separating comprises: 
     passing, after the contacting operation, the solution containing particulate CaF 2  through a filter sized to remove at least some of the particulate CaF 2 . 
     5. The method of any of clauses 1-4 wherein hydrogen fluoride gas further includes hydrogen gas wherein the contacting comprises: 
     combining the hydrogen fluoride gas containing trace amounts of uranium fluoride gas and the solution containing particulate CaF 2  using a venturi to generate a combined gas/solution/particulate stream; 
     collecting the combined gas/solution/particulate stream in a vessel; and 
     allowing the combined gas/solution/particulate stream to separate into a hydrogen gas stream and a uranium-bearing caustic solution containing particulate CaF 2 . 
     6. A method for collecting uranium from a HF gas containing trace amounts of UF 6  comprising: 
     mixing a raw gas stream of the HF gas containing a first concentration of UF 6  with a KOH treatment solution, the KOH treatment solution being an aqueous solution of KOH having a pH of greater than 11 and including an amount of particulate CaF 2 , thereby generating a uranium-bearing combined stream containing KOH and KF in solution and particulate CaF 2  with sorbed U compounds. 
     7. The method of clause 6 further comprising: 
     separating the particulate CaF 2  with sorbed U compounds from the uranium-bearing liquid stream, thereby generating a filtered KOH solution stream; 
     wherein the separating includes passing the uranium-bearing liquid stream through a filter sized to remove at least some of the particulate CaF 2  with sorbed U compounds. 
     8. A system for removing uranium from a hydrogen fluoride gas containing at least some uranium hexafluoride, the system comprising: 
     a gas-liquid contactor that
         a) receives the hydrogen fluoride gas containing uranium hexafluoride at a gas inlet,   b) receives a KOH treatment solution containing particulate CaF 2  at a solution inlet, and   c) discharges a uranium-bearing KOH solution including KOH, KF, and particulate CaF 2  with sorbed U compounds from a solution outlet;       

     a first separator that receives the uranium-bearing KOH solution from the solution outlet, the first separator adapted to separate particulate CaF 2  from the uranium-bearing KOH solution to obtain a filtered KOH treatment solution containing KF and KOH and a solid residue of particulate CaF 2  and sorbed U compounds; and 
     a KOH treatment solution recycler that receives either uranium-bearing KOH solution or filtered KOH treatment solution and adds Ca(OH) 2  to the received solution to convert the KF into KOH and particulate CaF 2 , thereby generating the KOH treatment solution containing particulate CaF 2 . 
     9. The system of clause 8 wherein the gas-liquid contactor comprises: 
     a venturi that has the KOH treatment solution inlet and the hydrogen fluoride gas inlet and a discharge that discharges a mixed venturi output stream created by the mixing of the hydrogen fluoride gas, uranium hexafluoride, KOH solution and particulate CaF 2  within the venturi; and 
     a KOH holding vessel having a mixture inlet and the uranium-bearing KOH solution outlet, wherein the KOH contacting vessel receives the discharged mixed venturi output stream. 
     10. The system of clauses 8 and 9 wherein the first separator comprises: 
     at least one filter, the filter having a filter media sized to retain the particulate CaF 2  with sorbed U compounds on the filter media. 
     11. The system of clause 10 wherein the at least one filter has a filter media sized to pass particulate CaF 2  but to retain particulate CaF 2  with sorbed U compounds. 
     12. The system of any of clauses 8-11 wherein the KOH treatment solution recycler comprises: 
     a Ca(OH) 2  mixing vessel that mixes Ca(OH) 2  with the received solution containing KF and KOH to convert the KF into KOH and particulate CaF 2 , thereby generating a recycled KOH treatment solution stream containing particulate CaF 2 , 
     a second separator that separates at least some particulate CaF 2  from the recycled KOH treatment solution stream; and 
     a makeup vessel that receives a fresh KOH solution and the recycled KOH treatment solution to generate the KOH treatment solution containing particulate CaF 2 . 
     13. The system of any of clauses 8-12 wherein the hydrogen fluoride gas containing at least some uranium hexafluoride further includes at least some hydrogen gas and the system further comprises: 
     the KOH holding vessel having a treated gas outlet in addition to the mixture inlet and the uranium-bearing KOH solution outlet, wherein the KOH contacting vessel receives the discharged mixed venturi output stream which then separates into a vessel gas and the uranium-bearing KOH solution, the vessel gas being discharged from the KOH contacting vessel via the treated gas outlet and the uranium-bearing KOH solution being discharged from the uranium-bearing KOH solution outlet; and 
     a packed column having a vessel gas inlet, a filtered KOH treatment solution inlet and a treated hydrogen gas outlet, wherein the packed column contacts the vessel gas with filtered KOH treatment solution prior to discharging the treated hydrogen gas. 
     The system of claim  13  wherein the at least one filter has a filter media sized to pass particulate CaF 2  but to retain particulate CaF 2  with sorbed U compounds. 
     15. The system of claim  11  wherein the KOH treatment solution recycler comprises: 
     a Ca(OH) 2  mixing vessel that mixes Ca(OH) 2  with the received solution containing KF and KOH to convert the KF into KOH and particulate CaF 2 , thereby generating a recycled KOH treatment solution stream containing particulate CaF 2 , 
     a second separator that separates at least some particulate CaF 2  from the recycled KOH treatment solution stream; and 
     a makeup vessel that receives a fresh KOH solution and the recycled KOH treatment solution to generate the KOH treatment solution containing particulate CaF 2 . 
     16. The system of claim  12  wherein the hydrogen fluoride gas containing at least some uranium hexafluoride further includes at least some hydrogen gas and the system further comprises: 
     the KOH holding vessel having a treated gas outlet in addition to the mixture inlet and the uranium-bearing KOH solution outlet, wherein the KOH contacting vessel receives the discharged mixed venturi output stream which then separates into a vessel gas and the uranium-bearing KOH solution, the vessel gas being discharged from the KOH contacting vessel via the treated gas outlet and the uranium-bearing KOH solution being discharged from the uranium-bearing KOH solution outlet; and 
     a packed column having a vessel gas inlet, a filtered KOH treatment solution inlet and a treated hydrogen gas outlet, wherein the packed column contacts the vessel gas with filtered KOH treatment solution prior to discharging the treated hydrogen gas. 
     It will be clear that the systems and methods described herein are well adapted to attain the ends and advantages mentioned as well as those inherent therein. Those skilled in the art will recognize that the methods and systems within this specification may be implemented in many manners and as such is not to be limited by the foregoing exemplified embodiments and examples. In this regard, any number of the features of the different embodiments described herein may be combined into one single embodiment and alternate embodiments having fewer than or more than all of the features herein described are possible. 
     While various embodiments have been described for purposes of this disclosure, various changes and modifications may be made which are well within the scope contemplated by the present disclosure. For example, in the embodiment described with reference to  FIG. 4 , free CaF 2  need not be provided by the recycler  420 . Instead, the recycler may remove all CaF 2  generated from the recycling reactor and free CaF 2  may be added as a separate operation so that the exact amount and size of the CaF 2  in the system can be controlled. Numerous other changes may be made which will readily suggest themselves to those skilled in the art and which are encompassed in the spirit of the disclosure.