Patent Publication Number: US-2018050911-A1

Title: System and method for breeding tritium from lithium using a neutron generator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Appl. No. 62/378,078, filed on Aug. 22, 2016 and incorporated in its entirety by reference herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED R&amp;D 
     Some of the work described in this disclosure was made with United States Government support under National Securities Technologies LLC Task Order No. 186303, Subcontract No. 291886-DL-17, awarded under the authority of the U.S. Department of Energy. The United States Government may have certain rights in inventions disclosed herein. 
    
    
     BACKGROUND 
     Field 
     This application relates generally to the production (e.g., generation and collection) of radioactive, nuclear isotopes (often referred to as radioisotopes), and more particularly to systems and methods for producing (e.g., generating and collecting) tritium. 
     Description of the Related Art 
     Historic (e.g., conventional; traditional) methods for special isotope production typically require the total dissolution of the target after an extended irradiation period, and very elaborated separation processes are required to recover the desired isotopes. 
     Historic (e.g., conventional; traditional) tritium production typically requires the dissolution of specially prepared lithium targets. Such methods require the manufacturing of specially designed lithium compounds as target materials and then irradiating the targets in a nuclear reactor. After a predetermined irradiation period, the targets are removed from the reactor and then tritium is recovered by dissolving the entire lithium target. Such historic (e.g., conventional; traditional) methods requires special manufacturing, handling, and material disposal throughout the entire production cycle. 
     SUMMARY 
     Certain embodiments described herein provide a system for producing tritium. The system comprises at least one neutron generator configured to generate neutrons. The system further comprises at least one target comprising a lithium-containing material. The at least one target is configured to be irradiated by at least some of the neutrons and to produce tritium. The system further comprises at least one collection structure configured to receive at least some of the tritium from the at least one target. The at least one collection structure comprises at least one gas conduit having an input configured to receive a carrier gas and an output configured to allow the carrier gas and the received tritium to flow out of the at least one gas conduit after the carrier gas has flowed along the at least one target. 
     Certain embodiments described herein provide a method for producing tritium. The method comprises irradiating at least one target with neutrons. The at least one target comprises a lithium-containing material, and the at least one target is configured to produce tritium in response to neutron irradiation. The method further comprises flowing a carrier gas along the at least one target. The carrier gas is configured to receive at least some of the tritium. The method further comprises collecting the carrier gas and the received tritium after the carrier gas has flowed along the at least one target. 
     Certain embodiments described herein provide a system for producing tritium. The system comprises means for irradiating at least one target with neutrons. The at least one target comprises a lithium-containing material, and the at least one target is configured to produce tritium in response to neutron irradiation. The system further comprises means for flowing a carrier gas along the at least one target. The carrier gas is configured to receive at least some of the tritium. The system further comprises means for collecting the carrier gas and the received tritium after the carrier gas has flowed along the at least one target. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  schematically illustrates an example system for producing tritium in accordance with certain embodiments described herein. 
         FIG. 1B  schematically illustrates another example system for producing tritium in accordance with certain embodiments described herein. 
         FIG. 2A  is a plot of the cross-sections for various nuclear reactions utilizing a deuterium beam as a function of the kinetic energy of the deuterium ions, some of the nuclear reactions resulting in neutron generation in accordance with certain embodiments described herein. 
         FIG. 2B  is a plot of the relative intensities of neutron generation, as functions of the kinetic energy from the D+D, D+T, and T+T nuclear reactions of  FIG. 2A , in accordance with certain embodiments described herein. 
         FIG. 2C  is a plot of neutron spectrum from the combined nuclear reactions of  FIG. 2B  (denoted by a dashed line) in accordance with certain embodiments described herein. 
         FIG. 3A  is a plot of the cross section (in barns) for tritium production by irradiating natural lithium (e.g., having about 7.5%  6 Li and 92.5%  7 Li) with neutrons as a function of neutron incident energy (in MeV) in accordance with certain embodiments described herein. 
         FIG. 3B  is a plot of the cross section (in barns) for tritium production for various reactions by irradiating natural lithium (e.g., having about 7.5%  6 Li and 92.5%  7 Li) with neutrons as a function of neutron incident energy (in MeV) in accordance with certain embodiments described herein. 
         FIG. 3C  is a plot of the cross section (in barns) for  7 Li nuclear reactions for neutron irradiation of natural lithium (e.g., having about 7.5%  6 Li and 92.5%  7 Li) as a function of neutron incident energy (in MeV) in accordance with certain embodiments described herein. 
         FIG. 3D  is a plot of the cross section (in barns) for D+ 6 Li and D+ 7 Li nuclear reactions for neutron irradiation of natural lithium (e.g., having about 7.5%  6 Li and 92.5%  7 Li) as a function of neutron incident energy (in MeV) in accordance with certain embodiments described herein. 
         FIG. 4  which shows a natural lithium metal sample encapsulated in a small container and submerged in mineral oil in accordance with certain embodiments described herein. 
         FIG. 5  shows an example lithium foil for an example lithium foil target in accordance with certain embodiments described herein. 
         FIG. 6A  schematically illustrates an example apparatus for forming a target comprising lithium foil in accordance with certain embodiments described herein. 
         FIG. 6B  schematically illustrates an example spiral target in accordance with certain embodiments described herein. 
         FIG. 7A-7D  schematically illustrate example collection structures configured to receive at least some of the tritium from the at least one target in accordance with certain embodiments described herein. 
         FIG. 7E  schematically illustrates an example target compatible to be used with the collection structures of  FIGS. 7A-7D . 
         FIG. 8  schematically illustrates an example separation structure in accordance with certain embodiments described herein. 
         FIG. 9A  is a schematic side view of an example system in accordance with certain embodiments described herein. 
         FIG. 9B  is a schematic top view of the example system of  FIG. 9A  in accordance with certain embodiments described herein. 
         FIG. 10  is a schematic view of a plurality of lithium-containing elongate structures to be used as targets in accordance with certain embodiments described herein. 
         FIG. 11  is a schematic top view of another example system in accordance with certain embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Certain embodiments described herein include a system and method for producing (e.g., generating and collecting) tritium from lithium (e.g., natural lithium metal; lithium oxide). In certain embodiments, the system comprises at least one neutron generator (e.g., a “limitless-life” neutron generator) and at least one target comprising lithium and configured to be irradiated (e.g., bombarded) with neutrons from the at least one neutron generator. Certain embodiments include a neutron multiplier (e.g., beryllium; depleted uranium) which increases the number of neutrons irradiating the at least one target, thereby enhancing the tritium production from the lithium of the at least one target. Certain embodiments include at least one neutron reflector (e.g., graphite) which reflects at least a portion of the neutrons from the at least one neutron generator towards the at least one target, thereby enhancing the tritium production from the lithium of the at least one target. 
     In certain embodiments, the system and method for tritium production leverages on the success of a previously patented Mo-99 production methodology which utilizes a neutron generator (see, e.g., U.S. Pat. No. 9,047,997, incorporated in its entirety herein). One feature of this previously patented Mo-99 production methodology is its use of target materials with total surface areas that are at least 4 to 5 orders of magnitude greater than that of a single historical (e.g., conventional; traditional) target. Certain embodiments described herein utilize certain prescribed design features of thin lithium foils coupled to a neutron generator (e.g., a neutron generator as described in U.S. Pat. No. 9,047,997), such that the produced tritium nuclei (e.g., tritons) easily diffuse, migrate, and escape from the surfaces of the thin lithium foils. Certain such embodiments advantageously allow in-situ continual production and collection of tritium gas. 
     In previous tritium generation systems, deuterated titanium targets had short bombardment times and frequent change outs (e.g., after a few hours). Certain embodiments described herein advantageously provide low contamination and continuous irradiation operations and enhanced potential for continual extraction of tritium. 
     The following terms as used in the description herein have their broadest reasonable interpretations and are to be interpreted broadly:
         Non-enriched uranium (“NEU”) or depleted uranium;   Neutron-multiplying material or neutron multiplier   Neutron-reflecting material or neutron reflector;   Fast neutron fission or fast fission; and   Neutron generator.       

     The terms “non-enriched uranium” (“NEU”) and “depleted uranium” (“DU”) have their broadest reasonable interpretation and are intended to cover naturally occurring uranium, in addition to any uranium that contains at least as much U-238 as naturally occurring uranium (99.27%) and no more U-235 than naturally occurring uranium (0.72%). Depleted uranium is normally understood to mean uranium that has less than the naturally occurring amount of U-235 (0.72%), but depleted uranium that is used for commercial and military purposes more commonly has less than 0.3% U-235. The terms of NEU and DU are not limited to any form of the uranium, so long as the isotope content meets the above criteria. Such materials can be in the form of bulk solid material, crushed solid material, metallic shavings, metallic filings, sintered pellets, liquid solutions, molten salts, molten alloys, or slurries, and, whatever its form, can also be mixed with other materials that are compatible with the intended use. 
     The terms “neutron-multiplying material” and “neutron multiplier” have their broadest reasonable interpretation and are intended to cover materials that generate more neutrons in response to irradiation by neutrons. Further, while some of the embodiments use neutron-multiplying materials formed into solid structural shapes such as plates, spherical shells, cylindrical shells, tubes, and the like, the term is intended to cover materials that includes small particles such as powders, pellets, shavings, filings, and the like. 
     The terms “neutron-reflecting material” and “neutron reflector” have their broadest reasonable interpretation and are intended to cover materials that reflects or scatters neutrons. While it is preferred in certain embodiments that the scattering be elastic, or largely so, this is not necessary for the definition. Further, while some of the embodiments use neutron-reflecting materials formed into solid structural shapes such as plates, spherical shells, cylindrical shells, tubes, and the like, the term is intended to cover materials that includes small particles such as powders, pellets, shavings, filings, and the like. 
     The terms “fast neutron fission” and “fast fission” have their broadest reasonable interpretation and are intended to cover fission reactions that are caused by neutrons with energies that are above the threshold of 800 keV. 
     The term “neutron generator” has its broadest reasonable interpretation and is intended to cover a wide range of devices and processes for generating neutrons of the desired energies, including but not limited to: neutron source devices which contain compact linear accelerators and that produce neutrons by fusing isotopes of hydrogen together. The fusion reactions taking place in such devices can be initiated by accelerating either deuterium, tritium, or a mixture of these two isotopes into a metal hydride target which also contains either deuterium, tritium or a mixture. As used herein, the term “neutron generator” is defined broadly to include any device that would provide a sufficient number of neutrons of the desired energies. 
       FIG. 1A  schematically illustrates an example system  100  for producing tritium in accordance with certain embodiments described herein. The system  100  comprises at least one neutron generator  110  configured to generate neutrons  112 . The system  100  further comprises at least one target  120  comprising a lithium-containing material (e.g., lithium metal or lithium oxide). The at least one target  120  is configured to be irradiated by at least some of the neutrons  112  and to produce tritium. The system  100  further comprises at least one collection structure  130  configured to receive at least some of the tritium from the at least one target  120 . The at least one collection structure  130  comprises at least one gas conduit  132  having an input  134  configured to receive a carrier gas  136  and an output  138  configured to allow the carrier gas  136  and the received tritium to flow out of the at least one gas conduit  132  after the carrier gas  136  has flowed along the at least one target  120  (e.g., along the lithium-containing material; along a surface of lithium foil; along a surface of the lithium metal or lithium oxide). 
       FIG. 1B  schematically illustrates another example system  100  for producing tritium in accordance with certain embodiments described herein. The system  100  comprises at least one neutron generator  110 , at least one target  120 , and at least one collection structure  130 , and further comprises at least one neutron multiplier  140  and at least one neutron reflector  150 . The at least one neutron multiplier  140  is configured to generate neutrons in response to being irradiated by neutrons, and the at least one neutron reflector is configured to redirect at least some neutrons impinging the at least one neutron reflector. The at least one target  120  is configured to be irradiated by at least some of the neutrons from the at least one neutron multiplier  140  and at least some of the neutrons redirected by the at least one neutron reflector  150 . 
     Example Neutron Generators 
     In certain embodiments, the at least one neutron generator  110  is configured to generate neutrons  112  for irradiating the at least one target  120 . Examples of the at least one neutron generator  110  compatible with certain embodiments described herein include, but are not limited to, one or more of the following:
         DD-109 neutron generator (sometimes referred to herein as a “limitless-life” neutron generator) marketed by Adelphi Technology Inc., 2003 East Bayshore Rd., Redwood City, Calif. 94063. Certain such neutron generators can emit about 3×10 9  neutrons/second. Certain such “limitless-life” neutron generators use a continuous gas stream to produce a plasma and deuterium beam, capable of providing over one thousand hours of non-stop irradiations.   Neutron generator as described in U.S. Pat. No. 9,047,997, which is incorporated in its entirety by reference herein.   Fixed target neutron generator, e.g., as described in G. Voronin, et al., “Development of the Intense Neutron Generator SNEG-13,” Proceedings of the EPAC94, Jun. 27-Jul. 1, 1994, London, V. 3, pp. 2678-2680.   Neutron generator which produce neutrons as a result of a beam of deuterium ions irradiating a target comprising deuterons and/or tritium (e.g., metallic tritide; titanium tritide).       

       FIG. 2A  is a plot of the cross-sections for various nuclear reactions utilizing a deuterium beam as a function of the kinetic energy of the deuterium ions, some of the nuclear reactions resulting in neutron generation in accordance with certain embodiments described herein. The nuclear reactions plotted in  FIG. 2A  include the following:
         D+D (neutron branch)→ 3 He+n+2.45 MeV (representing about 50% of the total D+D reactions up to about 4 MeV; denoted in  FIG. 2A  by “D-Dn”)   D+D (proton branch)→T+p+3.02 MeV (representing about 50% of the total D+D reactions up to about 4 MeV; denoted in  FIG. 2A  by “D-Dp”)   D+T→ 4 He+n+14.1 MeV   D+ 3 He→ 4 He+p+14.6 MeV
 
In addition, some neutrons may be generated by the nuclear reaction of T+T→ 4 He+2n+11.3 MeV. The dashed line of  FIG. 2A  denotes a kinetic energy of between 120 keV and 125 keV, which is compatible with operation of the “limitless-life” DD-109 neutron generator marketed by Adelphi Technology Inc.
       

       FIG. 2B  is a plot of the relative intensities of neutron generation, as functions of the kinetic energy from the D+D, D+T, and T+T nuclear reactions of FIG.  2 A, in accordance with certain embodiments described herein.  FIG. 2C  is a plot of neutron spectrum from the combined nuclear reactions of  FIG. 2B  (denoted by a dashed line) in accordance with certain embodiments described herein. While absolute measurements have not been performed, the dashed line represents an approximation of the anticipated combined neutron intensity spectra from the three fusion reactions. Table 1 lists the nuclear reactions and their energy releases. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Energy 
               
               
                 Nuclear 
                 Branching 
                   
                 Yield 
               
               
                 reaction 
                 Ratio (%) 
                 Products 
                 (MeV) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 D + Dp 
                 50 
                 T (1.01 MeV) + p (3.02 MeV) 
                 4.03 
               
               
                 (proton 
               
               
                 branch) 
               
               
                 D + Dn 
                 50 
                   3 He (0.82 MeV) + n (2.54 MeV) 
                 3.27 
               
               
                 (neutron 
               
               
                 branch) 
               
               
                 D + T 
                   
                   4 He (3.54 MeV) + n (14.06 MeV) 
                 17.6 
               
               
                 D +  3 He 
                   
                   4 He (3.66 MeV) + p (14.6 MeV) 
                 18.3 
               
               
                 T + T 
                   
                   4 He (2.1 MeV) + 2n (9.2 MeV) 
                 11.3 
               
               
                   3 He + T 
                 51 
                   4 He + p + n + 12.1 MeV 
                 12.1 
               
               
                   
                 43 
                   4 He (4.8 MeV) + D (9.5 MeV) 
                 14.3 
               
               
                   
                 6 
                   5 He (2.4 MeV) + p (11.9 MeV) 
                 11.4 
               
               
                   3 He +  3 He 
                   
                 He + p + p 
                 12.9 
               
               
                   
                   
                   
                 ΔE = 
               
               
                   
                   
                   
                 105.2 MeV 
               
               
                   
               
            
           
         
       
     
     Reactions with kinetic energies greater than 50 keV can be referred to as DD catalyzed reactions. For reactions with kinetic energies less than 50 keV, the reaction D+ 3 He is not significant. Across these nuclear reactions, 6D are fused, generating 2 p, 2  4 He, and 2 n, and releasing energy of 43.2 MeV or about 43/6=7.2 MeV per D. T and  3 He can act as catalysts in the overall reactions. The two neutrons have energies at: 2.54 MeV and 14.1 MeV. The T+T reaction can also be important in terms of total neutron production, producing a white neutron spectrum with the 9.2 MeV distributed between the two neutrons. As a result, each of the two neutrons can have energy ranges from 0 to 9.2 MeV (e.g., one neutron has an energy of E 1 , with E 1  in a range from 0 to 9.2 MeV and the other neutron has an energy of E 2 =9.2 MeV−E 1 , with E 2  in a range from 0 to 9.2 MeV). 
     Example Targets 
     Certain embodiments described herein utilize at least one target  120  configured to be irradiated by at least some of neutrons  112  generated and emitted by the at least one neutron generator  110  and to produce tritium. Examples of the at least one target  120  compatible with certain embodiments described herein include, but are not limited to, one or more of the following:
         Natural lithium materials (e.g., 7.5%  6 Li);
           Lithium can be in solid or liquid form, e.g., metal, molten, or compounds (e.g., sintered);   
           Enriched lithium materials (e.g., having a higher percentage of  6 Li than occurs in natural lithium; e.g., any combinations of percentages of  6 Li and  7 Li in which the  6 Li percentage is greater than 7.5%; can have percentage of  6 Li as high as 100%);
           Enriched lithium can be in solid or liquid form, e.g., metal, molten, or compounds (e.g., sintered);   
           Lithium oxide (e.g., lithium oxide blanket configurations);
           Lithium oxide (e.g., LiO 2 ) can be in the form of pellets or powders;   
           Other forms or compounds comprising lithium
           e.g., liquid eutectic (e.g., Pb—Li); molten salts (e.g., F—Li—Be; F—Li—NaBe); solid Li-ceramics (e.g., Li 4 SiO 4 ; Li 2 TiO 3 ; Li 2 O; LiHCO 3 ; Li 2 Cr 2 O 7 ; Li 2 CrO 4 ; Li 3 P; Li 2 HPO 4 ; LiNO 2 ; Li 2 CO 3 ; Li 2 SO 4 ; LiHSO 3 ; Li 2 C 2 O 4 ; Li 2 SO 3 ; LiCl; LiI; LiOH; Li 3 N; Li 2 S; LiBr; LiNO 3 ; LiF; LiHSO 4 ; Li 2 S 2 O 3 ; LiNO 3 ; LiH 2 PO 4 ; LiCH 3 COO; LiClO 2 ; LiSCN; LiClO 3 ).   
               

     For example, the at least one target  120  can comprise lithium metal or lithium oxide containing lithium having about 7.5%  6 Li and 92.5%  7 Li, having more than 7.5%  6 Li and less than 92.5%  7 Li, an isotope abundance ratio of  6 Li: 7 Li equal to or greater than a naturally-occurring isotope abundance ratio of  6 Li: 7 Li (e.g., equal to or greater than 7.5:92.5). 
     In certain embodiments, natural lithium metal, containing about 7.5%  6 Li and 92.5%  7 Li, can be used as the target material.  FIG. 3A  is a plot of the cross section (in barns) for tritium production by irradiating natural lithium (e.g., having about 7.5%  6 Li and 92.5%  7 Li) with neutrons as a function of neutron incident energy (in MeV) in accordance with certain embodiments described herein. The plot of  FIG. 3A  includes a first line showing the cross section for tritium production via the  6 Li(n, t) reaction and a second line showing the cross section for tritium production via the  7 Li(n, n′T) reaction. In certain embodiments in which the at least one target  120  comprise natural lithium, the  6 Li can be the primary source material for tritium production for a range of neutron energies below about 5 MeV. Table 2 lists the nuclear reactions for neutrons and natural lithium, along with their energy releases. The first two reactions listed in Table 2 can be considered to happen simultaneously to generate two tritons to form tritium gas and can be expressed as: n+ 6 Li+ 7 Li→T+T+ 4 He+ 4 He. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Energy 
               
               
                   
                   
                 Yield 
               
               
                 Nuclear reaction 
                 Products 
                 (MeV) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   6 Li (n, T)  4 He 
                   4 He (2.05 MeV) + T (2.73 MeV) 
                 4.78 
               
               
                   7 Li (n, n, T)  4 He 
                   5 He (2.1 MeV) + T (2.7 MeV) 
                 −2.47 
               
               
                   
                   5 He → n + α (t 1/2  = 2 × 10 −21  s) 
               
               
                   6 Li (n, D)  5 He 
                   5 He → n + α 
                 −2.21 
               
               
                   6 Li (D, α)  4 He 
                   4 He +  4 He 
                 22.40 
               
               
                   6 Li (n, 2n)  5 Li 
                   5 Li → p + α 
                 −5.66 
               
               
                   7 Li (p, n)  7 Be 
                   7 Be 
                 −1.64 
               
               
                   
                   7 Be +  4 He →  11 C + γ (t 1/2  = 53.3 d) 
                 (7.54) 
               
               
                   7 Li (n, D)  6 He 
                   6 He →  6 Li + β −  (t 1/2  = 0.8 s) 
                 −7.75 
               
               
                   7 Li (D, α)  5 He 
                   5 He +  4 He 
                 14.39 
               
               
                   
                   5 He → n + α (t 1/2  = 2 × 10 −21  s) 
               
               
                   7 Li (n,  5 He) T 
                   5 He (2.1 MeV) + T (2.7 MeV) 
                 −3.20 
               
               
                   
                   5 He → n + α (t 1/2  = 2 × 10 −21  s) 
               
               
                   7 Li (n, 2n)  6 Li 
                   6 Li + n + n 
                 −7.25 
               
               
                   7 Li (n, α)  4 H 
                   4 H 
                 −4.07 
               
               
                   
                   4 H → T + n (t 1/2  = 1.4 × 10 −22  s) 
                   
               
               
                   
                   
                 ΔE = 
               
               
                   
                   
                 7.32 MeV 
               
               
                   
               
            
           
         
       
     
       FIG. 3B  is a plot of the cross section (in barns) for tritium production for various reactions listed in Table 2 by irradiating natural lithium (e.g., having about 7.5%  6 Li and 92.5%  7 Li) with neutrons as a function of neutron incident energy (in MeV) in accordance with certain embodiments described herein.  FIG. 3C  is a plot of the cross section (in barns) for  7 Li nuclear reactions for neutron irradiation of natural lithium (e.g., having about 7.5%  6 Li and 92.5%  7 Li) as a function of neutron incident energy (in MeV) in accordance with certain embodiments described herein.  FIG. 3D  is a plot of the cross section (in barns) for D+ 6 Li and D+ 7 Li nuclear reactions for neutron irradiation of natural lithium (e.g., having about 7.5%  6 Li and 92.5%  7 Li) as a function of neutron incident energy (in MeV) in accordance with certain embodiments described herein. 
     In certain embodiments, using at least one target  120  comprising natural lithium metal can provide one or more of the following advantages:
         Capable of being purchased on-line from commercial suppliers, thereby potentially shortening deliverables times (see, e.g., https://unitednuclear.com/index.php?main_page=page&amp;id=25&amp;zenid=584e0975a2475780acac08b602291872);   Shipped in small containers in which the lithium is encapsulated and submerged in mineral oil (see, e.g.,  FIG. 4  which shows a natural lithium metal sample encapsulated in a small container and submerged in mineral oil);   Minimizing contamination;   Ease of sample handling and change-outs;   Tritium counting can be accomplished without taking the lithium metal outside the small containers. For example, since the produced tritium atoms can be released and flow into the air-space of the container and their activities can be easily determined using one or more residual gas analyzer (RGA) spectrometers. For another example, certain amount of the produced tritium atoms will exchange with the hydrogen atoms in the oil, so the activities of the oil can be measured using one or more beta spectrometers. In certain embodiments, the resulting total activities produced from multiple samples can be used to correlate the specific sample locations with different neutron energy regimes.       

     In certain embodiments, lithium metal targets  120  can be formed using lithium carbonate, which is an inorganic compound, the lithium salt of carbonate with the formula Li 2 CO 3 . This white salt is widely used in the processing of metal oxides. Lithium carbonate (Li 2 CO 3 ) exists only in the anhydrous form (see, e.g., Greenwood, N. N.; &amp; Earnshaw, A. (1997), Chemistry of the Elements (2nd Edn.), Oxford: Butterworth-Heinemann. Pages 84-85, ISBN 0-7506-3365-4). In other words, water molecules are not bound or attached to the compound as a hydrate. 
     In certain embodiments, a room-temperature ionic liquid (RTIL) can be used to dissolve the enriched  6 Li 2 CO 3  compound and through electrochemistry to collect the  6 Li metal onto electrodes. For example, the RTIL can be de-hydrated and electrochemical deposition of  6 Li metal onto electrodes (Au or Graphite) can be performed. In this way, in the conversion of the  6 Li 2 CO 3  compound to  6 Li metal, no oxygen atoms are presented or get carried over when formation of the metal occurs according to: 2H + +2RTIL − + 6 Li 2 CO 3 →2 Li-RTIL (complex)+H 2 O+CO 2  (water and carbon dioxide are off-gassed and removed in the process to deposit lithium metal by Ar purging, roto-evaporation, and/or water gettering prior to deposition). Preparation of the Li ionic liquid can be achieved by direct dissolution or cation exchange on a column very easily.  6 Li-RTIL→ 6 Li metal can be yielded on an over-potential in the RTIL matrix selected which can be collected, pressed into pellets, then in the form for use directly to be neutron irradiated and resulting formation of tritium during breeding. The  6 Li 2  metal can be transferred into one or more quartz tubes (e.g., 10 cm in length×1.5 cm in diameter, with wall thicknesses of about 0.2 cm) under an inert gas atmosphere and sealed at both ends. For example, 20 tubes can be made with each tube containing about 2 Moles of  6 Li (˜12 grams). Regarding the conversion, 72 grams of  6 Li 2 CO 3 =&gt;1 Mole, and 1 Mole of  6 Li 2 CO 3  produces 2 Moles of  6 Li metal=&gt;12 grams. 
     In certain embodiments, the at least one target  120  can comprise lithium foil.  FIG. 5  shows an example of Lib-LiF-30m lithium foil from MTI Corp. of Richmond, Calif. for an example lithium foil target  120  in accordance with certain embodiments described herein. This lithium foil is in the form of a roll of foil having a length of 30 meters, a width of 3.5 cm, and a thickness of 0.017 cm, a total mass of 96 grams, a purity of about 99.99%, and a total surface area of about 21,000 cm 2 . Lithium foils with other dimensions are also compatible with certain embodiments described herein. In addition, other forms of lithium are also compatible with certain embodiments described herein. 
       FIG. 6A  schematically illustrates an example apparatus  200  for forming a target  120  comprising lithium foil in accordance with certain embodiments described herein.  FIG. 6B  schematically illustrates an example spiral target  120  in accordance with certain embodiments described herein. While  FIGS. 6A and 6B  schematically illustrate the formation of a target  120  using two lithium foils, in certain other embodiments, other numbers of lithium foils (e.g., one, three, four, or more) can also be used. 
     The apparatus  200  can comprise a rotatable mandrel  210  and one or more sizing rollers  220 . The mandrel  210  can be configured to receive a first portion  232  (e.g., a first end) of a first lithium foil  230  and a second portion  242  (e.g., a second end) of a second lithium foil  240 . In certain embodiments, the first lithium foil  230  and at least one first spacer  234  can be sandwiched together to form a first layer structure  236 , and the second lithium foil  240  and at least one second spacer  244  can be sandwiched together to form a second layer structure  246 . An end portion of the first layer structure  236  can be coupled to (e.g., inserted into) a first portion  212  of the mandrel  210  and an end portion of the second layer structure  246  can be coupled to (e.g., inserted into) a second portion  214  of the mandrel  210 . For example, as schematically illustrated by  FIG. 6A , the mandrel  210  is further configured to receive the first lithium foil  230  with the at least one spacer  234  and to receive the second lithium foil  240  with the at least one second spacer  244 . 
     By rotating the mandrel  210  (e.g., as denoted by arrows in  FIG. 6A ), the first layer structure  236  (comprising the first lithium foil  230  and the at least one first spacer  234 ) and the second layer structure  246  (comprising the second lithium foil  240  and the at least one second spacer  244 ) can be wound together to form a target  120  having a spiral configuration, with the at least one first spacer  234  and the at least one second spacer  244  separating portions of the first lithium foil  230  and the second lithium foil  240  from one another. In certain embodiments, the one or more sizing rollers  220  are configured to control an outer diameter of the spiral target  120  (e.g., by providing compressive forces on the spiral target  120 ). In certain embodiments in which the target  120  comprises one lithium foil or more than two lithium foils (e.g., three, four, or more), the mandrel  210  can comprise a corresponding number of portions configured to receive the lithium foils (e.g., with corresponding spacers) and to wind the lithium foils to forma spiral structure. 
     After the target  120  is removed from the mandrel  210 , the central portion of the target  120  can comprise a gas conduit  126  configured to receive tritons generated by the first lithium foil  230  and the second lithium foil  240  and configured to allow carrier gas to flow therethrough (e.g., across the lithium-containing material; across a surface of the lithium foil). In addition, regions of the target  120  between the first layer structure  236  and the second layer structure  246  can comprise one or more gas conduits  128  configured to receive tritons generated by the first lithium foil  230  and the second lithium foil  240  and configured to allow carrier gas to flow therethrough (e.g., across the lithium-containing material; across a surface of the lithium foil). The size of the one or more gas conduits  128  can be selected to be sufficient for the carrier gas to flow therethrough at a predetermined rate. The shape of the gas conduit  126  can be determined by the shape of the mandrel  210 , the overall shape of the target  120  can be determined by the shape of the mandrel  210 , and the size of the target  120  can be determined by the amount of lithium foil and spacers, as well as by the sizing rollers  220 . 
     In certain embodiments, the at least one first spacer  234  and the at least one second spacer  244  comprise gas conduits  128  (e.g., pores) positioned between adjacent portions of the first lithium foil  230  and the second lithium foil  240 . These gas conduits can be configured to receive tritons generated by the first lithium foil  230  and the second lithium foil  240  and to allow carrier gas to flow therethrough (e.g., across the lithium-containing material; across a surface of the lithium foil). 
     In certain such embodiments in which only Li metal is used, the target  120  can comprise a stainless steel container which is filled with inert gas (e.g., Ar gas) before adding the lithium metal. Certain such embodiments advantageously use such inert glove box preparation techniques to substantially exclude tritiated water (TOH) from the target  120 . 
     In certain embodiments, the resulting lithium foil target  120  can have a total surface area that is at least four or five orders of magnitude greater than that of a single conventional target. In certain embodiments, the resulting lithium foil target  120  is configured to allow the tritium produced by neutron irradiation of the target  120  to easily diffuse, migrate, and escape from the surfaces of the target  120 , thereby allowing in-situ continual production and collection of tritium gas. 
     Example Collection Structure 
     In certain embodiments, the collection structure  130  comprises at least one gas conduit  132  having an input  134  configured to receive a carrier gas  136  and an output  138  configured to allow the carrier gas  136  and the received tritium to flow out of the at least one gas conduit  132  after the carrier gas  136  has flowed along the at least one target  120  (e.g., along the lithium-containing material; along a surface of lithium foil; along a surface of the lithium metal or lithium oxide). In certain embodiments, the carrier gas  136  can comprise argon gas. In certain embodiments, the target  120  is contained within the at least one gas conduit  132 .  FIG. 7A-7D  schematically illustrate example collection structures  130  configured to receive at least some of the tritium from the at least one target  120  in accordance with certain embodiments described herein. 
     In certain embodiments, the collection structure  130  can comprise low-carbon stainless steel and is formed in a manner to reduce or minimize connections (e.g., using welding to join portions of the collection structure  130  together). In certain embodiments, as schematically illustrated in  FIGS. 7A-7C , the collection structure  130  can comprise two flanges configured to be bolted together with a compression metallic gasket sandwiched between the two flanges to form a seal between the two flanges. The two flanges can be unbolted from one another to provide access to the interior of the collection structure  130 . In certain embodiments, as schematically illustrated by  FIGS. 7A and 7B , the collection structure  130  comprises valves (e.g., stainless steel ball valves) on the input  134  and the output  138  to control the flow of the carrier gas  136  into and out of the at least one gas conduit  132 . 
     In certain embodiments, the carrier gas  136  flows out of the input  134  in proximity to a first end of the target  120  (shown schematically in  FIG. 7A  as a plurality of lithium-containing pieces) and flows into the output  138  in proximity to a second end of the target  120  (see, e.g.,  FIGS. 7A and 7C ). In this way, the output  138  can be configured to receive the carrier gas and the received tritium generated by the target  120 . 
     In certain embodiments, the collection structure  130  further comprises at least one heating structure  160  configured to heat at least one of the carrier gas  136  flowing through the at least one gas conduit  132  and the at least one target  120 . In certain embodiments, the at least one of the carrier gas and the at least one target is heated to a temperature below the melting point of lithium (e.g., 180° C.), e.g., in a range between 130° C. and 150° C. For example, the at least one heating structure  160  can comprise a plurality of heating coils  162 , a heating plate  164 , or both a plurality of heating coils  162  and a heating plate  164 . As schematically illustrated by  FIG. 7B , the heating coils  162  can be positioned around a perimeter of the collection structure  130  and the heating plate  164  can be positioned at an end of the collection structure  130 . 
     In certain embodiments, the at least one heating structure  160  can be configured to heat the target  120 . Certain such embodiments can advantageously facilitate in situ recovery of tritium from the lithium-containing material of the target  120  by applying thermal energy (e.g., below the melting temperature of the lithium-containing material) to drive tritons out of the lithium (e.g., out of the lithium metal matrix). For example, the plurality of heating coils  162  can be positioned in proximity to the target  120  (shown schematically in  FIG. 7B  as a lithium-containing material which upon neutron irradiation produces tritons). 
       FIG. 7E  schematically illustrates an example target  120  compatible to be used with the collection structures  130  of  FIGS. 7A-7D . In certain embodiments, the target  120  can be configured to hold the lithium-containing material so as to allow the carrier gas  136  to flow through regions which receive the tritium generated by the lithium-containing material (e.g., along the surfaces of the lithium-containing material). For example, the target  120  can comprise slots which contain a plurality of strips of lithium metal spaced from one another to allow the carrier gas to flow across the surfaces of the lithium metal strips. In an example configuration, the target  120  can comprise 9 strips of lithium metal, with a surface area per strip of 80 cm 2 , a strip thickness of 0.07 cm, thereby providing a total lithium metal surface area of 716 cm 2  and a total lithium metal mass of 12.1 g. Other configurations of the target  120  are also compatible with producing tritium from lithium in accordance with certain embodiments described herein. 
     As a result of neutron bombardment,  6 Li nuclei in the target  120  are converted to tritium (T) and helium (He) gases. In certain embodiments, the collection structure  130  comprises a getter material (e.g., reversible metallic hydrides; depleted uranium; Zr) configured to trap the T gas while rejecting the He gas (e.g., Zr+xT→ZrT x ). In certain embodiments, a membrane (e.g., an inorganic membrane, such as those developed by Oak Ridge National Laboratory) may be used for the separation and collection of tritium that is produced in the form of tritiated water (TOH). In certain embodiments, one or both of the target  120  and the collection structure  130  can comprise a monitoring system which utilizes a getter material (e.g., reversible metallic hydrides; depleted uranium; Zr) to provide in-line, real-time continual measurements to assess the tritium production as functions of neutron intensity (e.g., fluence), natural lithium mass (e.g., surface area), temperature of the lithium mass, and/or irradiation period. 
       FIG. 8  schematically illustrates an example separation structure in accordance with certain embodiments described herein. In certain embodiments the separation structure comprises a housing, a plurality of gas conduits (e.g., hollow fiber tubes) within the housing, an input, one or more tritium outputs, and one or more carrier gas outputs. The plurality of gas conduits comprises a plurality of membranes (e.g., walls) that selectively allow tritium to pass through the membranes while preventing the carrier gas (e.g., argon) from passing through the membranes. In certain embodiments, the carrier gas and tritium mixture (e.g., argon and tritium mixture) flows out of the output  138  of the target  120  and into the input of the separation structure. The carrier gas and tritium mixture is directed to flow through the plurality of gas conduits (e.g., hollow fiber tubes) such that the tritium passes through the membranes while the argon does not pass through the membranes. After having flowed through the plurality of gas conduits, the carrier gas is directed to flow out of the housing via the one or more carrier gas outputs. After having passed through the plurality of membranes, the tritium is directed to flow out of the housing via the one or more tritium outputs to a storage structure (e.g., comprising metallic hydride) where the tritium can be stored. Other configurations of a separation structure are also compatible with certain embodiments described herein. 
     Example Neutron Multipliers and Neutron Reflectors 
     In certain embodiments, the system  100  further comprises at least one neutron multiplier  140  configured to generate neutrons in response to being irradiated by neutrons. Example neutron multipliers  140  can comprise one or more of the following: Be(n, 2n); Pb(n, 2n);  7 Li(n, n′t); natural uranium; depleted uranium; reactor fuel. In certain embodiments, the system  100  further comprises at least one neutron reflector  150  configured to redirect at least some neutrons impinging the at least one neutron reflector  150 . Example neutron reflector  150  in accordance with certain embodiments described herein can comprise graphite. The at least one target  120  is configured to be irradiated by at least some of the neutrons from the at least one neutron multiplier  140  and at least some of the neutrons redirected by the at least one neutron reflector  150 . 
     Example System Configurations 
     In certain embodiments, various example system configurations can be used for the tritium production system  100 . In certain embodiments, the system  100  includes at least one neutron multiplier  140  (e.g., at least one depleted uranium (DU) reflector block, at least one DU blanket boxes), at least one neutron reflector  150 , and at least one neutron-absorbing elements (e.g., comprising polyethylene materials). These components can be compiled together in various geometrical configurations to produce different neutron spectra (e.g., to produce optimal tritium production using the natural lithium metal targets  120 ). In certain embodiments, the purpose of optimization of the neutron spectrum is to enable every neutron of all energies to interact with the lithium to enhance tritium production. 
       FIG. 9A  is a schematic side view of an example system  100  in accordance with certain embodiments described herein.  FIG. 9B  is a schematic top view of the example system  100  of  FIG. 9A  in accordance with certain embodiments described herein. The example system  100  of  FIGS. 9A and 9B  comprises a neutron generator  110  (e.g., a limitless-life neutron generator) configured to direct a beam of D +  ions to irradiate a neutron source (e.g., D or T) and configured to emit neutrons upon being irradiated by the beam of D +  ions. At least some of the generated neutrons propagate outwardly from the neutron generator  110  to impinge the at least one target  120 . 
     In the example system  100  of  FIGS. 9A and 9B , the neutron generator  110  has a generally cylindrical structure and is surrounded by a plurality of neutron multipliers  140  and a plurality of targets  120 . The neutron multipliers  140  can comprise a plurality of structures  140   a  (e.g., cylinders) comprising beryllium and/or a plurality of structures  140   b  (e.g., cylinders) comprising natural uranium. While  FIGS. 9A and 9B  show the plurality of Be-containing structures  140   a  alternating in the radial direction with the plurality of U-containing structures  140   b , other configurations are also compatible with certain embodiments described herein. In certain embodiments, the objective of the neutron multiplier is to enhance and increase the production of tritium from lithium. 
     In the example system of  FIGS. 9A and 9B , the targets  120  comprise elongate structures (e.g., tubes; rods) containing a lithium-containing material (e.g., lithium metal or lithium oxide), and are oriented generally parallel to the beam of D +  ions of the neutron generator  110 . These lithium-containing elongate structures (e.g., rods comprising LiO 2 ) are packed between the neutron multipliers  140  (e.g., between the Be-containing structures  140   a  and the U-containing structures  140   b ), and these regions containing the packed lithium-containing elongate structures can be termed “lithium blankets.” In the example system  100  of  FIGS. 9A and 9B , the neutron generator  110 , the targets  120 , and the neutron multipliers  140  are generally surrounded by a neutron reflector  150  (e.g., graphite) configured to reflect at least a portion of the neutrons towards the targets  120 . 
       FIG. 10  is a schematic view of a plurality of lithium-containing elongate structures to be used as targets  120  in accordance with certain embodiments described herein. These targets  120  can be configured to facilitate tritium breeding and collection and can comprise a lithium-containing material (e.g., LiO 2  pellets or powder; Li metal) and at least one collection structure  130 . For example, each elongate structure of the target  120  can comprise an input  134  configured to receive a carrier gas  136  (e.g., argon), a gas conduit  132  (e.g., steel tube) which contains the lithium-containing material, and an output  138  configured to allow the carrier gas  136  and the received tritium to flow out of the elongate structure into a manifold  172  for collection. The gas conduit  132  (e.g., tube) of the elongate structure can comprise reduced activation martensitic steel (e.g., EUROFER steel tube; see, e.g., A-A. F. Tavossoli et al., “Materials design data for reduced activation martensitic steel type,” J. Nuclear Mat&#39;l, Vols. 329-333, Proc. of 11 th  Int&#39;l Conf. on Fusion Reactor Mat&#39;ls, pp. 257-262 (2004)) in which the lithium-containing material is contained. The gas conduit  132  can be configured to allow the carrier gas  136  to propagate along (e.g., through) the lithium-containing material from the input  134  to the output  138 . The carrier gas  136 , along with any tritium gas picked up by the carrier gas  136  while flowing through the elongate structure, can be collected at the manifold  172  and directed to flow through a separation structure (e.g., as shown in  FIG. 8 ). The tritium from the separation structure can be directed towards a tritium storage structure comprising one or more materials (e.g., foam; Ti getter; cryogenic collector; metallic sponges; metallic hydrides) configured to store the tritium. 
       FIG. 11  is a schematic top view of another example system  100  in accordance with certain embodiments described herein. The example system  100  of  FIG. 11  uses a five-unit design which can maximize production efficiency. The example system  100  comprises five neutron generators  110 , each of which has a generally cylindrical structure, is positioned generally at the center of its respective unit, and is surrounded by a plurality of neutron multipliers  140  and a plurality of targets  120 . The neutron multipliers  140  can comprise a plurality of Be-containing structures  140   a  (e.g., cylinders) and the plurality of targets  120  can be positioned within multiple lithium-containing zones  182  surrounding the neutron generator  110  (e.g., between the neutron generator  110  and the Be-containing structures  140   a ; between the Be-containing structures  140   a ). In addition, the example system  100  of  FIG. 11  can comprise a plurality of other lithium-containing targets  120  (e.g., comprising natural Li 2 O) between the neutron generators  110  and other neutron multipliers  140   b  (e.g., comprising natural uranium; reactor fuel) positioned between the other lithium-containing targets  120  and the neutron generators  110 . In the example system  100  of  FIG. 11 , the neutron generators  110 , the lithium-containing targets  120 , and the neutron multipliers  140  are generally surrounded by a neutron reflector  150  (e.g., graphite) configured to reflect at least a portion of the neutrons towards the targets  120 . In addition, the example system  100  of  FIG. 11  can comprise a plurality of cooling channels  190  through which a coolant (e.g., air) can flow to remove heat from the system  100  (e.g., from each unit containing a neutron generator  110 ). While  FIG. 11  shows a particular configuration of the neutron generators  110 , lithium-containing targets  120 , neutron multipliers  140 , and neutron reflector  150 , other configurations are also compatible with certain embodiments described herein. 
     Example Approximations of Performance 
     The cross section for  6 Li shown in  FIG. 3A  can be approximated by an assumption that one neutron of all energies interacting with  6 Li will produce 1 triton. Furthermore, in certain embodiments, the following calculations and/or assumptions may be made to characterize the expected performance:
         1 Mole of  6 Li (6 grams)=&gt;1 Mole Tritium (3 grams)   1 Mole of T=&gt;6.023×10 23  nuclei   Neutron generator=&gt;3×10 9  neutrons/second   1 hour of operation=&gt;3×10 9 ×3600=&gt;˜1×10 13  neutrons   In one hour of operations, 10 13  neutrons=&gt;10 13  triton nuclei.   Tritium half-life=&gt;12.33 years   Activities=λN   N=10 13  triton nuclei   Activity after 1 hour of irradiation with 2 Moles of  6 Li=
           =10 13 ×0.693/(12.33×365×24×3600) dps   =10 7  dps (beta decay)   This activity can be detected by either with a typical beta liquid scintillation counter or with a RGA spectrometer.   
           If the neutron generator produces 10 13  neutrons/second, then in one hour, 10 17  nuclei of tritium will be produced.
           For 1000 hours (˜1.4 months) of non-stop irradiation, the system can produce 10 20  nuclei of tritium.   With one or more neutron multipliers and multiplicity (e.g., DU (fast fission), Natural U oxide (thermalized and fast fission), and Be (n,2n)), a multiplication of about &gt;100 can be achieved=&gt;˜10 22  nuclei of tritium.   A non-stop 14 months of operation can produce 10 23  nuclei of tritium (˜1 Mole).   A 10-unit cluster can produce &gt;10 24  nuclei of tritium=&gt;˜10 Moles/14-month cycle (30 grams).   Projected US Tritium needs=&gt;˜1000 Moles/18-month cycle (3000 grams).   One 10-unit cluster of such systems can provide 1/100 US tritium needs.   
           Cost estimates can be made: Setup Costs (one 10 13  n/s neutron generator=˜$250 K; material cost=˜$200 K/unit; setup=˜$3.0 M; 10-unit cluster setup and material cost=&gt;$7.5 M) and estimated yearly operational cost to produce 10 Moles of tritium with a 10-unit cluster is about $5.0 M. Other estimates will vary depending on various parameters, including the costs of materials such as the cost of lithium.       

     Although described above in connection with particular embodiments, it should be understood the descriptions of the embodiments are illustrative of the invention and are not intended to be limiting. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the claims.