Patent Publication Number: US-2023147092-A1

Title: System and method for energy conversion using an aneutronic nuclear fuel

Description:
FIELD 
     The present disclosure relates generally to an aneutronic nuclear fuel for direct conversion of heat to electric energy. 
     BACKGROUND 
     This section provides background information related to the present disclosure and is not necessarily prior art. 
     The potential low operational costs and overall economic competitiveness of nuclear power is marginalized by the use of traditional, dynamic heat to electric energy conversion methods. The use of these auxiliary systems causes traditional power plants to suffer significant operational and maintenance costs, which lowers the economic effectiveness and efficiency of such plants. One alternative to cumbersome dynamic heat to electric energy conversion systems is Thermionic Energy Conversion (TEC)—a direct heat to electric energy conversion process which generates electricity from thermionic emission. TEC provides an opportunity for low maintenance, autonomous electrical power generation and illustrates the potential for economically competitive advanced nuclear reactors. 
     Some examples of TEC systems utilize an interelectrode plasma to conduct thermionically-emitted electrons from a “hot” side (i.e., an emitter) of the system to a “cold” side (i.e., a collector) of the system. Though all demonstrable TEC systems use emitted electrons to ionize a plasma of low ionic potential (e.g., cesium), this method limits device efficiency. Previous efforts explored using fission fragments from an unclad fuel element to ionize the interelectrode plasma. However, these previous efforts required enough fuel to sustain a chain reaction, i.e. for a critical system. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     One aspect of the disclosure provides an aneutronic nuclear fuel. The nuclear fuel includes a net neutron-producing material, a neutron-consuming material, and a neutron moderating material. Upon exposure of the net neutron-producing material, the neutron-moderating material, and the neutron-consuming material to a neutron source, a ratio of the net neutron-producing material to (i) the neutron-consuming material and (ii) the neutron-moderating material is operable to convert neutrons into charged particles without producing net neutrons. 
     Implementations of this aspect of the disclosure may include one or more of the following optional features. In some implementations, the net neutron producing material is fissile. In some examples, the net neutron-producing material is fertile. Optionally, the net neutron-producing material may undergo fission. In some implementations, the neutron-consuming material undergoes neutron activation. In some examples, the neutron-consuming material undergoes neutron activation in a beta decay process. In some implementations, the ratio of the neutron-producing material to the neutron-consuming material is able to produce charged particles to ionize a plasma. 
     Another aspect of the disclosure provides a method of generating electricity. The method includes, producing a plurality of neutrons with a first material, and consuming at least one of the plurality of neutrons with a second material. The method also includes moderating a quantity of the plurality of neutrons with a third material, and exposing the first material, the second material, and the third material to a neutron source. 
     This aspect may include one or more of the following optional features. In some implementations, the first material is fissile. In some examples, the first material is fertile. Optionally, the first material may undergo fission. In some implementations, the second material undergoes neutron activation. In some examples, the second material undergoes neutron activation in a beta decay process. In some implementations, the method also includes, ionizing a plasma with the charged particle. 
     Another aspect of the disclosure provides a nuclear fuel cell. The nuclear fuel cell includes, a net neutron-producing material defining a thickness T 1 , a neutron-consuming material, and a neutron-moderating material. The nuclear fuel also includes a cladding surrounding the net neutron-producing material and defining a second thickness T 2 , where a ratio of the thickness T 1  to the thickness T 2  is operable to increase a rate of transmission of electrons from an emitter to a collector. 
     This aspect may include one or more of the following optional features. In some implementations, the net neutron-producing material is fissile. In some examples, the net neutron-producing material is fertile. Optionally, the net neutron-producing material may undergo fission. In some implementations, the neutron-consuming material undergoes neutron activation. In some examples, the neutron-consuming material undergoes neutron activation in a beta decay process. 
     Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected configurations and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
         FIG.  1    is a functional block diagram of a prior art thermionic energy conversion system. 
         FIG.  2    is a functional block diagram of a thermionic energy conversion system in accordance with the principles of the present disclosure. 
         FIG.  3    is a functional block diagram of a thermionic energy conversion system in accordance with the principles of the present disclosure. 
         FIG.  4    is a schematic view of a fuel cell utilizing a thermionic energy conversion system in accordance with the principles of the present disclosure. 
         FIG.  5    is a flow diagram of a method of operating the fuel cell of  FIG.  4    in accordance with the principles of the present disclosure. 
     
    
    
     Corresponding reference numerals indicate corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION 
     Example configurations will now be described more fully with reference to the accompanying drawings. Example configurations are provided so that this disclosure will be thorough, and will fully convey the scope of the disclosure to those of ordinary skill in the art. Specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of configurations of the present disclosure. It will be apparent to those of ordinary skill in the art that specific details need not be employed, that example configurations may be embodied in many different forms, and that the specific details and the example configurations should not be construed to limit the scope of the disclosure. 
     As shown in  FIG.  1   , a thermionic energy conversion system  100  may include an emitter  102 , a medium  104 , a collector  106 , and a heat source  108 . As will be described in more detail below, the thermionic energy conversion system  100  converts heat from the heat source  108  directly into electrical energy from thermionic emissions, which can then be used to drive an electrical load  110  by placing a bias voltage across the emitter  102  and collector  106 . The bias voltage is proportional to the difference between the respective work functions of the emitter  102  and collector  106 . In doing so, the system  100  allows for the elimination of various parts (e.g., a turbine) that would otherwise be required to produce electrical energy in conventional power generation. 
     As illustrated in  FIG.  1   , the emitter  102  contains electrons  112 . When the emitter  102  is heated by the heat source  108 , the emitter  102  emits the electrons  112 . The emitted electrons  112  enter the medium  104  between the emitter  102  and the collector  106 . If the medium  104  between the emitter  102  and the collector  106  is conductive, an electric current, capable of driving the load  110 , is produced. 
     As the emitter  102  is heated by the heat source  108 , the emitter  102  emits electrons  112 . The electrons  112  emitted by the emitter  102  enter the medium  104  between the emitter  102  and the collector  106 . The negative charge of the electron  112  emitted by the emitter  102  repels other negatively-charged electrons  112 . Thus, when the electrons  112  emitted from the emitter  102  enter the medium  104 , the negative charge of the electrons  112  repels additional electrons  112  and inhibits and/or prevents such additional electrons  112  from leaving the emitter  102  and reaching the collector  106 , creating a space charge which reduces the efficiency of the system  100 . In some implementations, a plasma acts as the medium  104  between the emitter  102  and the collector  106 . In this regard, the medium  104  may be referred to herein as the “plasma  104 .” The plasma  104  may mitigate the space charge of the medium  104  allowing electrons  112  to leave the emitter  102  without inhibiting other electrons to also leave the emitter. The plasma  104  may take different forms. For example, in some implementations, the plasma  104  includes a vapor such as cesium vapor. 
     The efficiency of the system  100  can be increased by reducing the negative space charge with the plasma  104 . As the negative space charge is neutralized by the plasma  104 , additional electrons  112  are more freely emitted from the emitter  102 , thus increasing the current flow through the plasma  104  and, in turn, improving efficiency of the system  100 . In this regard, when the plasma  104  is in a natural, pre-ionized state (i.e., a rarified vapor or gas), it may not conduct electrons  112 . The plasma  104  may be ionized by coming into contact with the emitter  102 , allowing the emitter  102  to transmit the electrons  112  across the plasma  104 . In another implementation, the plasma  104  may be ionized by the emitted electron  112  striking a neutral atom of the plasma  104  and ionizing the neutral atom into an additional electron and an ion. The plasma  104  may conduct electrons  112  after the plasma  104  is ionized. When plasma  104  is ionized, electrons  112  are able to conduct from the emitter  102  through the plasma  104  to the collector  106  thereby generating an electrical current. The flow of electrons  112  from the heated emitter  102  to the collector  106  generates electrical energy which may be used to drive the load  110 . 
     Utilizing the electrons  112  to ionize the plasma  104  reduces the total efficiency of the thermionic energy conversion system  100 . For example, by ionizing the plasma  104  to allow additional electrons  112  to emit from the emitter  102 , electrons  112  expend their energy on ionizing the plasma  104  rather than producing electrical energy, thus decreasing the electrical efficiency of the system  100 . 
     A heavy ion thermionic energy conversion (HITEC) system  200  is illustrated in  FIG.  2   . The HITEC system  200  may be substantially similar to the thermionic energy conversion system  100 , except as otherwise shown or described herein. An example method of ionizing plasma  104  in the HITEC system  200  utilizes fission fragments  208 . For example, a neutron source  202  produces neutrons  204 . The HITEC system  200  may also include a net neutron-producing material  206  that can either be fissile (e.g., U-235)—that is, capable of a fission reaction after absorbing a neutron—or fertile (e.g., U-238)—that is, not capable of undergoing a fission reaction after absorbing a neutron. 
     When the neutron source  202  produces a neutron that is absorbed by the net neutron-producing material  206  (e.g., U-235) the neutron-producing material  206  becomes unstable splitting into fission fragments  208  and releases several new neutrons in the process. The new neutrons released from the fission process may themselves undergo fission according to the following equation to create additional fission fragments  208  and release neutrons resulting in a chain reaction, where “n” is a neutron  204 . 
         U -235+ n   th →( U -236)→(fission fragment-1)+(fission fragment-2)+2.5 n   fast  
 
     The fission fragments  208 , generated from the fission process, enter the plasma  104  between the emitter  102  and collector  106  and ionize the plasma  104 . The use of fission fragments  208  to ionize the plasma  104  in the HITEC system  200  allows more electrons  112  to flow from the emitter  102  through the plasma  104  to the collector  106 . In particular, electrons  112  can solely flow from the emitter  102  to the collector  106  to generate electrical energy, rather than being used to ionize the plasma  104 , thereby increasing efficiency of the HITEC system  200 . 
     Utilizing the fission fragments  208  to ionize the plasma  104  causes a build-up of heavy metals in the plasma  104  to occur. In particular, the fission fragments  208 , after ionizing the plasma  104 , become neutral heavy metal particles within the plasma  104 . The build-up of the fission fragments  208  as neutral heavy metal particles in the plasma  104  increases the likelihood that the electrons  112  emitted from the emitter  102  will collide with a neutral heavy metal particle. Electrons  112  that collide with the neutral heavy metal particle from the fission fragment  208  may lose energy due to the collisions, therefore, the electrons  112  produce less electrical energy, reducing the efficiency of the HITEC system  200 . In some examples, fission fragments  208  are deposited onto the surface of the emitter  102 , causing the emitter to emit fewer electrons  112 , and further reducing the overall efficiency of the HITEC system  200  by reducing the amount of electrical energy produced. 
     As illustrated in  FIG.  2   , the HITEC system  200  may contain a neutron-consuming material  210  and a neutron-moderating material  212 . The neutron-moderating material  212  reduces the velocity of the neutrons  204  released from the neutron-producing material  206 . For example, as the neutron source  202  produces neutrons  204  that are absorbed by the neutron-producing material  206  or neutron-consuming material  210 , the neutron of the neutron-producing material  206  becomes unstable splitting into fission fragments  208  and may release several new neutrons to stabilize. The released neutrons  204  from fission may travel at a high velocity, resulting in a low likelihood of absorption by the neutron-producing material  206  or neutron consuming material  210 . 
     The neutron-moderating material  212  (e.g., graphite, water, or Zirconium Hydride) reduces the velocity of the fast neutrons  204  produced by fission, thus increasing the likelihood that the released neutrons are absorbed by the neutron-producing material  206  and neutron consuming material  210 , which, in turn, can result in the production of more fission fragments  208 . As the velocity of more neutrons  204  produced by the fission process is reduced by the neutron-moderating material  212 , resulting in the absorption of more neutrons  204  by the neutron-producing material  206  and the production of more fission fragments  208 , the HITEC system  200  becomes less dependent on the neutron source  202  to start the fission chain reaction, thus increasing the efficiency of the HITEC system  200 . 
     As illustrated in  FIG.  2   , the neutron-consuming material  210  absorbs neutrons  204  generated from fission to regulate the reproduction of neutrons in the HITEC system  200 . The quantity of neutrons  204  produced by the HITEC system  200  without the neutron-consuming material  210  may cause the system to enter a supercritical state where the number of neutrons  204  produced accelerates at an uncontrolled rate, causing the number of net neutrons in the HITEC system  200  to be higher than is desirable for long term sustainability. 
     Another exemplary HITEC system  300  is illustrated in  FIG.  3   . The HITEC system  300  may be substantially similar to the HITEC system  200 , except as otherwise shown or described herein. The HITEC system  300  may include a neutron-producing material  306 , a neutron-consuming material  210  (e.g., a fission-capable material  306 ), a plasma  104 , and beta decay particles  310 . The neutron-producing material  306  can either be fissile (e.g., U-235) or fertile (e.g., U-238). In this regard, a fissile neutron-producing material  306  may be capable of a fission reaction after absorbing a neutron  204 , while a fertile neutron-producing material  306  may be incapable of undergoing a fission reaction after absorbing a neutron  204 . 
     During operation, the HITEC system  300  uses the neutron source  202  to produce neutrons  204  that may be absorbed by a neutron-producing material  306 . After absorbing the neutrons  204 , the neutron-producing material  306  may undergo fission. In particular, when the neutron-producing material  306  absorbs the neutron  204  of the neutron-producing material  306  may become unstable, splitting into fission fragments  208  and releasing several new neutrons  204  in the process. In some examples, when the neutron-producing material  306  absorbs the neutron  204 , the neutron-producing material  306  produces heavy and/or light fission fragments  208  and several neutrons  204 . The released neutrons  204  from the fission process may, in turn, absorb into the neutron-producing material  306  creating additional fission fragments  208  and releasing additional neutrons  204 . 
     As the HITEC system  300  operates, the fission fragments  208  may undergo a beta decay process. In particular, the fission fragment  208  generated by the fission process, undergoes beta decay where the fission fragment  208  converts one of its neutrons  204  into a proton  312  by releasing an additional electron referred to herein as a “beta decay particle  310 .” The beta decay reaction of the fission fragment  208  can be described by the following equation: 
     
       
      
       Z 
       A 
       XN→ 
       Z+1 
       A 
       X′ 
       N-1 
       +e 
       − 
       + v   
      
     
     Where X is a parent nucleus, X′ is a daughter nucleus, Z is a proton number, N is a neutron number, A is the sum of the proton number and neutron number, e −  is an electron, and  v  is an antineutrino. 
     The beta decay particle  310 , released from the beta decay process, enters the plasma  104  between the emitter  102  and the collector  106  and ionizes the plasma  104 . The beta decay particle  310  contains a much higher energy than electrons  112  emitted from the emitter  102 . The higher energy of the beta decay particle  310  allows the beta decay particle  310  to ionize magnitudes more plasma  104  atoms compared to electrons  112  emitted from the emitter  102 , thereby increasing the amount of electricity produced by, and the overall efficiency of, the HITEC system  300 . For example, the beta decay particle  310  ionizes the plasma  104  by the process described above, thereby (i) allowing electrons  112  to flow from the emitter  102  through the plasma  104  to the collector  106  and (ii) increasing the efficiency of the HITEC system  300 . In this regard, the beta decay particle  310  can ionize the plasma  104  without creating a build-up of neutral heavy metal particles in the plasma  104 . In particular, the use of the beta decay particle  310  for ionization from the beta decay process can result in an indirect use of the fission fragments  208  that does not result in the neutral heavy metal particle build up that occurs upon ionization of the plasma  104  using fission fragments  208  during operation of the HITEC system  200  described above relative to  FIG.  2   . 
     In some implementations, neutron activation may occur when the neutron  204  is absorbed by the neutron-consuming material  210 , causing radioactivity (e.g., alpha decay, beta decay, gamma decay) to occur in the neutron-consuming material  210 . In particular, the neutron-consuming material  210  may undergo neutron activation in a beta decay process. When the neutron-consuming material  210  absorbs the neutron  204  the neutron-consuming material  210  undergoes beta decay. Thus, the neutron-consuming material  210 , by neutron activation, may produce beta decay particles  310  that enter the plasma  104 . 
       FIG.  4    illustrates a fuel cell  400  using a HITEC system (e.g., HITEC system  300 ). The fuel cell  400  may include an emitter  102 , a plasma  104 , a collector  106 , a neutron moderating material  408 , and fuel  410 . The fuel  410  may include the neutron producing material  306 , neutron consuming material  210 , and neutron moderating material  408  in a ratio that is capable of converting neutrons into beta decay particles  310  without producing net neutrons. The fuel  410  may include a thin cladding  412  that retains the fission fragments  208  within the fuel  410 , while allowing the beta decay particles  310  to escape the fuel  410  and to enter the plasma  104  for ionization. By retaining the fission fragments  208  within the fuel  410 , the thin cladding  412  prevents fission fragments  208  from entering the plasma  104  for ionization, thus preventing the previously-described build-up of neutral heavy metal particles, while still allowing beta decay particles  310  to ionize the plasma  104 . In some examples, the thin cladding  412  provides an additional safety mechanism to prevent radioactive material (i.e., fission fragments  208 ) from entering the environment. In some implementations, a thickness T 1  of the thin-cladding  412  is less than fifteen microns. In particular, the thickness T 1  of the thin cladding may be between ten microns and one hundred microns. In some implementations, the thickness T 1  of the thin cladding is substantially equal (+/−ten percent) to ten microns. In particular, the thickness T 1  of the cladding  412  may be between 0.25% and 1.25% of a thickness T 2  of the fuel  410 . In some examples, the thickness T 1  of the cladding  412  is between 0.5% and 1% of the thickness T 2  of the fuel  410 . In this regard, the range of the beta particles  310  may be two to three orders of magnitude greater than the range of the fission fragments  208 , such that thin cladding  412  and, in particular, the ratio of the thickness T 1  to the thickness T 2 , allows for the retention of the fission fragments  208  within the fuel element  410  and the release of the beta particles  310  to the plasma  104  for ionization thereof, which, in turn increases the efficiency of the fuel cell  400  and the system  300  relative to the system  200 . 
     The neutron moderating material  408  may include a lightweight material that is also non-absorbing of neutrons (e.g., graphite). As illustrated in  FIG.  4   , the fuel  410  may be disposed at a central portion of the fuel cell  400 . The emitter  102  may be disposed around (e.g., surrounding) the fuel  410 . The plasma  104  may be disposed around (e.g., surrounding) the emitter  102 , such that the emitter  102  is disposed between the fuel  410  and the plasma  104 . The collector  106  may be disposed around (e.g., surrounding) the plasma  104 , such that the plasma  104  is disposed between the collector  106  and the emitter  102 . The neutron moderating material  408  may be disposed around (e.g., surrounding) the collector  106 . The cladding  412  may be disposed around the fuel  410  and between the fuel  410  and the emitter  102 . 
     The fuel cell  400  may generate electrical energy by conducting electrons  112  from the emitter  102  to the collector  106 . The plasma  104  may reside between the emitter  102  and collector  106  to act as a conductive medium. In this regard, in order for the plasma  104  to conduct electrons  112 , the plasma  104  must be ionized, since plasma  104  in a non-ionized state does not conduct electrons  112  from the emitter  102  to the collector  106 . When ionized, the plasma  104  may allow electrons  112  to flow from the emitter  102  to the collector  106 . 
     With reference to  FIGS.  3  and  4   , the fuel cell  400  may use the HITEC system  300  to ionize the plasma  104 . For example, the neutron source  202  may produce neutrons  204  that undergo a fission process, as previously described. The neutron moderating material  408  may reduce the velocity at which the neutrons  204  born from the fission process travel to increase the likelihood that the neutrons  204  undergo additional fission processes, thereby causing a fission chain reaction. The fission process creates fission fragments  208  that further decay to create beta decay particles  310 . The beta decay process converts the neutron  204  into a proton  312  by releasing the beta decay particle  310 , as previously described. The beta decay particle  310  may then be used to ionize the plasma  104 , putting the plasma  104  in the ionized state and, thus, making the plasma  104  a conductive medium for electrons  112  to flow through. Additionally, the fission fragments  208  produce heat for thermionic emission. The heat produced by the fission fragments  208  heats the emitter  102  allowing more electrons  112  to emit from the emitter  102  to the collector  106 . 
     With reference to  FIG.  5   , a method  500  of HITEC is illustrated. At step  502 , the method  500  may include the heat source that heats the emitter  102  to release electrons  112  from the emitter  102  into the plasma  104 . In some examples, the heat source includes heat produced by the fission fragments  208 . The plasma  104  is in a pre-ionized state not conducting electrons  112  from the emitter  102  to the collector  106 . At step  504 , the neutron source  202  produces the neutron that is absorbed by the neutron-producing material  206 . At step  506 , once the neutron-producing material  206  absorbs the neutron, the neutron-producing material  206  becomes unstable splitting into fission fragments  208 . At step  508 , the fission fragment  208  produced by the fission, beta decays into the proton  312  by releasing the beta decay particle  310 . At step  510 , the beta decay particle  310  escapes the thin cladding  412  to enter and ionize the plasma  104 . The ionization of the plasma  104  by the beta decay particle  310  reduces the negative charge of the plasma  104  allowing additional electrons  112  to emit from the emitter  102  into the plasma  104 . At step  512 , the emitted electrons  112  from the emitter  102  conduct through the plasma  104  to the collector  106  to generate electrical energy capable of driving the load  110 . 
     The terminology used herein is for the purpose of describing particular exemplary configurations only and is not intended to be limiting. As used herein, the singular articles “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. Additional or alternative steps may be employed. 
     When an element or layer is referred to as being “on,” “engaged to,” “connected to,” “attached to,” or “coupled to” another element or layer, it may be directly on, engaged, connected, attached, or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” “directly attached to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     The terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections. These elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example configurations. 
     The foregoing description has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular configuration are generally not limited to that particular configuration, but, where applicable, are interchangeable and can be used in a selected configuration, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.