Patent Publication Number: US-2009238321-A1

Title: Nuclear power plant with actinide burner reactor

Description:
BACKGROUND 
     The present invention relates generally to nuclear power plants, and more specifically to a nuclear power plant capable of processing actinides produced as byproducts from uranium based nuclear reactors. 
     U.S. Publication No. 2004/0022342 A1 discloses a method of incineration of minor actinides in nuclear reactors. The minor actinides to be incinerated, are embedded in at least one finite region of a core of a thermal reactor, wherein the finite region is isolated from the rest of the core by means of a barrier layer that absorbs thermal neutrons but is transparent to fast neutrons. This publication requires a common barrier layer of fissible material that isolates the finite “fast island” from the rest of a core. The publication discloses that the thermal reactor may be a pressurized water reactor, or a high-temperature-gas-cooled reactor. U.S. Publication No. 2004/0022342 A1 is hereby incorporated by reference herein. 
     “A Liquid-Metal Reactor for Burning Minor Actinides of Spent Light Water Reactor Fuel—I: Neutronics Design Study” by Hangbok Choi and Thomas J. Downar discloses a decoupled core with two zones: a minor actinide zone and a plutonium-enriched zone. The minor actinide zone was used to burn the minor actinides effectively using a hard spectrum, while the plutonium zone was introduced to compensate for the deteriorating safety performance due to heavy minor actinide loading. 
     Advanced gas-cooled reactors (hereinafter AGRs) typically use uranium as the fuel, graphite as the neutron moderator and carbon dioxide as coolant. Several Generation III reactors are also in development, including sodium-cooled fast reactors, gas-cooled fast reactors, lead-cooled fast reactors and molten salt reactors. These are typically envisioned as fast reactors. 
     Byproduct actinides are transuranic byproducts created from the use of the U235 thermal spectrum fuel cycle in currently deployed light water reactors (LWRs), and include americium 241 and neptunium 237. 
     SUMMARY OF THE INVENTION 
     In accordance with an embodiment of the present invention, a thermal nuclear reactor includes a coolant and a core. The core includes a moderator material and a plurality of fuel assemblies arranged within the moderator material. Each fuel assembly includes an inner region and an outer region surrounding the inner region. The inner region includes at least one first fuel structure containing at least one byproduct actinide, and the outer region includes a plurality of second fuel structures containing thermal spectrum driver fuel, the second fuel structures being less than one thermal neutron mean free path apart. 
     By having the plurality of second fuel structures at less than one thermal neutron mean free path apart, the second fuel structures can shield the inner region from thermal neutrons and thereby suppress the thermal neutron flux density in the inner region. The resultant high fast neutron density in the inner region can aid in burning the byproduct actinides in the first fuel structure, while the plurality of second fuel structures permits easy provision of the thermal driver fuel, for example in the form of fuel rods containing uranium. 
     In accordance with a second embodiment of the present invention, a thermal nuclear reactor includes a coolant including a molten salt or metal, and a core. The core includes a moderator material and a plurality of fuel assemblies arranged within the moderator material. Each fuel assembly includes an inner region and an outer region surrounding the inner region. The inner region includes at least one first fuel structure containing at least one byproduct actinide, and the outer region includes at least one second fuel structure containing thermal spectrum driver fuel shielding the first fuel structure from thermal neutrons. 
     By using a molten salt or metal, which have high thermal performance, as the coolant, the second thermal spectrum driver fuel can provide good thermal neutron shielding. For example, the at least one second fuel structure may include a plurality of fuel structures less than one thermal neutron mean free path apart, or be fashioned as a single plate or cylinder surrounding the inner region. By using the molten salt or metal, the core can operate at a high power density with a plurality of the fuel assemblies while retaining the advantageous safety properties of a thermal reactor. 
     According to a third embodiment of the present invention, a thermal nuclear reactor includes a coolant and a core. The core includes a moderator material and a plurality of fuel assemblies arranged within the moderator material, and separated solely from each other by the moderator material. Each fuel assembly includes an inner region and an outer region surrounding the inner region. The inner region includes at least one first fuel structure containing at least one byproduct actinide, and the outer region includes at least one second fuel structure containing thermal spectrum driver fuel shielding the first fuel structure from thermal neutrons. 
     By having the plurality of fuel assemblies located next to each other and separated solely by the moderator material, a new type of reactor can be created which permits substantial burning of byproduct actinides in the inner regions while still operating with the safety characteristics of a thermal nuclear reactor. Preferably, at least nine of the fuel assemblies are provided. 
     A nuclear power plant having a reactor of the types described above is also provided, and may include a heat exchange system removing heat from the coolant to power a generator. 
     The present invention also provides a fuel assembly including an inner region and an outer region surrounding the inner region. The inner region includes at least one first fuel structures containing at least one byproduct actinide, and the outer region includes a plurality of second fuel structures containing thermal spectrum driver fuel, the second fuel structures being less than one thermal neutron mean free path apart. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention will be described with respect to the drawings in which: 
         FIG. 1  shows schematically a nuclear power plant according to a preferred embodiment of the present invention; 
         FIG. 2  shows a top view of a core of a nuclear reactor of the nuclear power plant shown in  FIG. 1 ; 
         FIG. 3  shows a three-dimensional cutaway view of the core shown in  FIG. 2 ; 
         FIG. 4  shows a portion of a fuel assembly in the core shown in  FIG. 2 ; 
         FIG. 5  shows a three-dimensional cutaway view of the fuel assembly shown in  FIG. 4 ; 
         FIG. 6  shows a Table 1: Various Coolants and their Properties Relative to Graphite; and 
         FIG. 7  shows another preferred embodiment of a fuel assembly according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows schematically a nuclear power plant  100  according to one embodiment of the present invention. Nuclear power plant  100  includes a thermal nuclear reactor  10 , a heat exchange system  106 , a turbine  102  and a generator  104 . Heat exchange system  106  includes a hot leg  108  and a cold leg  110 . Hot leg  108  is downstream of reactor  10  and upstream of turbine  102 . Cold leg  110  is downstream of turbine  102  but upstream of reactor  10 . In hot leg  108 , steam travels from nuclear reactor  10  to turbine  102 . In cold leg  110 , water travels from turbine  102  to nuclear reactor  10 . 
     Nuclear reactor  10  includes a containment wall  12 , a core  20 , circulators  14 ,  16 , supports  22 ,  24  and coolant  18 . Coolant  18  preferably is a high thermal performance coolant such as a liquid salt or metal. Heat is transferred from coolant  18  to heat exchange system  106  at a heat area  107  to turn water in cool leg  110  into steam. Circulators  14 ,  16  move coolant  18  through core  20 . 
     Core  20  includes a moderator  30  and a plurality of fuel assemblies  40  located in a plurality of coolant channels  32  in moderator  30 . Each fuel assembly  40  is placed inside a coolant channel  32  in moderator  30 . Moderator  30  may be, for example, graphite, for example, in single block form or several pieces fitted together, for example, hollow hexagonals of graphite material. Coolant  18  is circulated through core  20  and moves through coolant channels  32  past fuel assemblies  40 . 
       FIGS. 2 and 3  show fuel assemblies  40  arranged in channels  32  of core  20  in a square lattice arrangement. Each fuel assembly  40  is spaced apart by a fuel assembly pitch, p. The moderator thickness, t, is also shown.  FIG. 3  shows a three dimensional cutaway view of core  20 . Each fuel assembly  40  includes a plurality of fuel structures  50 , which may be, for example, a plurality of rods supported at one end for insertion into coolant channel  32 . 
       FIGS. 4 and 5  show a portion of a fuel assembly  40  surrounded by moderator  30 . Fuel assembly  40  includes a thermal driver fuel region  130  and a fast target fuel region  140 . Coolant channel  32  is cut, for example, in a block of moderator  30 . The moderator may be, for example, graphite. Thermal driver fuel region  130  and fast target fuel region  140  may be configured as a series of concentric annuli  60 ,  62 ,  64 ,  66 ,  68  of fuel containing structures  50  placed in coolant channel  32 . Fuel containing structures  50  may be, for example, pins, cylinders, plates or other fuel containing devices. 
     In  FIG. 4 , thermal driver fuel region  130  includes outermost concentric annuli  66 ,  68  and fast target fuel region  140  includes innermost concentric annuli  60 ,  62 ,  64 . Annuli  66 ,  68  include fuel structures  50  containing thermal spectrum driver fuel  52 , for example, pelletized 5 w/o UO 2 . Annuli  60 ,  62 ,  64  include target fuel  54 , for example, byproduct actinide oxide pellets, a canister of molten target fuel, or another arrangement that places target material within the central portion of fuel assembly  40  may be used. Target material may include, for example, americum 241 and neptunium 237. Fuel structures  50  in annuli  60 ,  62 ,  64 ,  66 ,  68  are arranged in a way so the fuel content of each fuel structure  50  in each annulus  60 ,  62 ,  64 ,  66 ,  68  may be varied so that the thermal driver fuel region  130  surrounds the fast target fuel region  140 . 
     Fuel structures  50  in thermal driver fuel region  130  are in very close proximity to each other. The pitch p s  of fuel structures  50  is shown in  FIG. 4 . The pitch p s  is less than one thermal neutron mean free path so fuel structures  50  are less than one thermal neutron mean free path apart. Annuli  60 ,  62 ,  64  are located several mean free paths into assembly  40 . Thus, thermal neutrons entering fuel assembly  40  from moderator  30  are absorbed into thermal driver fuel region  130  by annuli  66 ,  68  and do not reach annuli  60 ,  62 ,  64  of fast target fuel region  140 . This regional self-shielding effect causes the thermal neutron flux density to decrease significantly within a couple of mean free paths into fuel assembly  40  from moderator  30 . This shielding effect provides a neutron flux spectral shift within fuel assembly  40 . The inner region, fast target fuel region  140 , is dominated by high fast neutron flux density while the thermal neutron flux density is depressed. The outermost annuli  66 ,  68  in thermal driver fuel region  130  are well utilized due to their close proximity to moderator  30 . The neutron spectral shift phenomenon allows reactor  10  to transmutate the byproduct actinides in the fast target fuel region  140 . 
     The behavior of core  20  in reactor  10  can be driven by thermal driver fuel region  130 , thus providing the safer control and transient behavior characteristics of a thermal spectrum critical reactor system, as opposed to those of third generation fast reactors. The fuel assembly pitch p and the moderator thickness t can be selected so the temperature coefficient remains safely negative, even though a plurality of fuel assemblies can be located next to each other and separated solely by the moderator material. At the same time, byproduct actinides can be burned. The arrangement of fuel structures  50  into thermal driver fuel region  130  and fast target fuel region  140  allows for optimal transuranic burning capabilities of minor actinides in region  140  while maintaining the safety and controllability characteristics associated with thermal neutrons in region  130 . 
     Because the outer annuli of fuel structures in the  FIG. 1  embodiment must be within close proximity to each other in order to amplify self shielding, a high thermal performance coolant is needed or low power densities need to be employed. It is generally not preferred to use low power densities because they are not as cost efficient or effective. If coolant  18  has above average heat transfer and energy storage thermo-physical properties, localized hot spots and temperature peaks in thermal driver fuel region  130  advantageously can be minimized. Also, coolant  18  preferably has relatively low thermal absorption and scatter cross-sections. A high-performance coolant  18  also can allow high power density fuel assemblies to be engineered with pitch values comparable to one mean free path or less and coolant  18  also can have a relatively low impact on the neutron energy spectrum of the system.  FIG. 6  shows preferred coolants that may be considered for use in reactor  10  and the relevant thermal and nuclear property data. The coolant preferably is a molten salt or liquid such as, for example, molten lithium fluoride-beryllium fluoride, sodium fluoride-zirconium fluoride, sodium, lead, or lead-bismuth. 
     Neither coolant channel  32  nor annuli  60 ,  62 ,  64 ,  66 ,  68  need be cylindrical, for example, prismatic or other geometric shapes may be used, as long as the fuel is zoned radially from the center of fuel assembly  40  towards an outer peripheral region where moderator  30  is located.  FIG. 7  shows an alternative geometric configuration, where moderator  230  and coolant channel  232  are hexagonal. Annuli  268 ,  266 ,  264 ,  262  including fuel structures  250  are arranged in concentric hexagons in coolant channel  232 . Thermal driver region  330  includes annuli  268  and  266 , while fast target region  240  includes annuli  262  and  264 . Thus, thermal neutrons entering fuel assembly  240  from moderator  230  are absorbed into thermal driver fuel region  330  by annuli  266 ,  268  and do not reach annuli  260 ,  262 ,  264  of fast target fuel region  340 . The moderator hexagonals can then be placed against each other to provide the core modulator structure. 
     In addition, having the outer region of the fuel assembly be separate structures less than one thermal neutron mean free path apart is advantageous as it permits easier manufacture of the fuel assembly and safer characteristics, it is also possible to fashion the outer region as a single plate or cylinder of thermal spectrum driver fuel surrounding the inner region. 
     In the preceding specification, the invention has been described with reference to specific exemplary embodiments and examples thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative manner rather than a restrictive sense.