Patent Publication Number: US-2018040385-A1

Title: Nuclear fuel containing a neutron absorber mixture

Description:
BACKGROUND 
     Nuclear reactors generate energy from a nuclear chain reaction (i.e., nuclear fission) in which a free neutron is absorbed by the nucleus of a fissile atom in a nuclear fuel, such as Uranium-235 ( 235 U). When the free neutron is absorbed, the fissile atom splits into lighter atoms, and releases more free neutrons to be absorbed by other fissile atoms, resulting in a nuclear chain reaction. Thermal energy released from the nuclear chain reaction is converted into electrical energy through a number of other processes well known to those skilled in the art. 
     The advent of nuclear power reactors adapted to burn nuclear fuel having low fissile content levels (e.g., as low as that of natural uranium) has generated many new sources of burnable nuclear fuel. These sources include waste or recycled uranium from other reactors. This is not only attractive from a cost savings standpoint, but also based upon the ability to essentially recycle spent uranium back into the fuel cycle. 
     SUMMARY 
     Such nuclear fuel is often packaged in fuel bundles that can be added and removed from a reactor core. To maintain power generation, fresh fuel bundles are inserted to replace spent fuel bundles that have burned up beyond their useful life. Localized spikes in power may occur when fresh reactor fuel bundles are inserted. It is desirable to lower these power spikes to maintain closer to even power generation throughout a power generation cycle. A neutron absorber (which may also be referred to herein as “poison”, “burnable poison”, “absorber”, “burnable absorber”, etc.) may be included along with fissile content in a fuel bundle to reduce the nuclear chain reaction by absorbing some of the free neutrons, thereby lowering these power spikes. However, it can be undesirable to lower reactivity in general as the goal of the nuclear reactor is to generate power. Thus, achieving relatively even power generation throughout a power generation cycle, even as fuel bundles become spent and fresh fuel bundles are added, is a constant balancing act. 
     It is therefore an object of the present disclosure to provide a nuclear fuel bundle having an arrangement and composition that achieves a low reactivity impact and extends fuel discharge burnup while maintaining a low power impact (and related parameters) during normal operation of a reactor core. Some embodiments of the fuel design according to the present invention are characterized by using unique combinations and distributions of neutron absorber materials in the inner region of Canadian Deuterium Uranium (CANDU) nuclear reactor fuels, which can include CANFLEX fuel, and in fuel elements of non-CANDU fuel assemblies. 
     In some embodiments of the present disclosure, a fuel bundle for a nuclear reactor is provided, and comprises a fuel element containing at least one fissile material selected from the group consisting of U-233, U-235, PU-239, and PU-241 and containing at least two neutron absorbers selected from the group consisting of Gd, Dy, Hf, Er, and Eu; wherein the at least one fissile material and the at least two neutron absorbers are homogeneously mixed in the fuel element. 
     Some embodiments of the present invention provide a fuel element for a nuclear reactor, the fuel element comprising at least one fissile material selected from the group consisting of U-233, U-235, PU-239, and PU-241 and containing at least two neutron absorbers selected from the group consisting of Gd, Dy, Hf, Er, and Eu; wherein the at least one fissile material and the at least two neutron absorbers are homogeneously mixed in the fuel element. 
     In some embodiments of the present invention, a fuel bundle for a nuclear reactor is provided, and comprises: a plurality of fuel elements including inner elements and outer elements; wherein at least one of the inner elements includes a homogeneous mixture of a fissile material and at least two neutron absorbers. 
     Other aspects of the present disclosure will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a nuclear reactor employing any of the fuel bundles of  FIGS. 2-6 . 
         FIG. 2  is a cross-sectional view of a first construction of a nuclear fuel bundle in accordance with the disclosure, showing a number of possible fuel and absorber arrangements in the fuel bundle. 
         FIG. 3  is a cross-sectional view of a second construction of a nuclear fuel bundle in accordance with the disclosure, also showing a number of possible fuel and absorber arrangements in the fuel bundle. 
         FIG. 4  is a cross-sectional view of a third construction of a nuclear fuel bundle in accordance with the disclosure, also showing a number of possible fuel and absorber arrangements in the fuel bundle. 
         FIG. 5  is a cross-sectional view of a fourth construction of a nuclear fuel bundle in accordance with the disclosure, also showing a number of possible fuel and absorber arrangements in the fuel bundle. 
         FIG. 6  is graph illustrating reactivity decay characteristics of different absorbers. 
         FIG. 7  is a graph illustrating gains in refueling impact and discharge burnup of various absorber mixtures. 
     
    
    
     DETAILED DESCRIPTION 
     Before any constructions of the disclosure are explained in detail, it is to be understood that the disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The disclosure is capable of other constructions and of being practiced or of being carried out in various ways. 
     A number of nuclear fuel designs according to various constructions of the present disclosure are described and illustrated herein. These fuels can be used in a variety of nuclear reactors, and are described herein primarily with reference to pressurized heavy water reactors. Heavy water reactors can have, for example, pressurized horizontal or vertical tubes within which fuel is positioned. An example of such a reactor is a Canadian Deuterium Uranium (CANDU) nuclear reactor, a portion of which is shown schematically in  FIG. 1 . Other types of reactors can have un-pressurized horizontal or vertical tubes, such as apertured horizontal or vertical tubes. 
     Pressurized heavy water nuclear reactors are only one type of nuclear reactor in which various nuclear fuels of the present disclosure can be burned. Accordingly, such reactors are described herein by way of example only, it being understood that the various fuels of the present disclosure can be burned in other types of nuclear reactors. For example, the nuclear fuel designs may also be employed with light water reactors (LWR) such as supercritical water reactors (SCWR), pressurized water reactors (PWR), and boiling water reactor (BWR), as will be described toward the end of this disclosure. 
     Similarly, the various fuels of the present disclosure described herein can be positioned in any form within a nuclear reactor for being burned. By way of example only, the fuel can be loaded into tubes or can be contained in other forms (each of which are commonly called “pins” or “elements”, referred to herein only as “elements” for sake of simplicity). Examples of elements used in some constructions of the present disclosure are indicated at  22  in  FIGS. 2-5  as having a round cross section, and are described in greater detail below. However, the elements  22  may have other cross sectional shapes, such as a rectangular or square cross-sectional shapes. In the case of fuel contained within tubes, the tubes can be made of or include zirconium, a zirconium alloy, or another suitable material or combination of materials that in some cases is characterized by low neutron absorption. 
     Together, a plurality of elements can define a fuel bundle within the nuclear reactor. Such fuel bundles are indicated schematically at  14  in  FIG. 1 . The fuel bundle(s) may have a cylindrically shaped bundle geometry (i.e., in cross-section), such as those shown in  FIGS. 2-5 , or can instead have a square or rectangular geometry, such as those that would be used with a non-CANDU reactor such as a light water reactor having n×n fuel elements. The elements of each bundle  14  can extend parallel to one another in the bundle. If the reactor includes a plurality of fuel bundles  14 , the bundles  14  can be placed end-to-end inside a pressure tube  18 . In other types of reactors, the fuel bundles  14  can be arranged in other manners as desired. The pressure tube  18 , the fuel bundle  14 , and/or the fuel elements  22  can be configured in various shapes and sizes. For example, the pressure tubes  18 , fuel bundles  14 , and fuel elements  22  can have any cross-sectional shapes (i.e., other than the round shapes shown in  FIGS. 2-5 ) and sizes desired. As another example, the fuel elements  22  within each fuel bundle  14  can have any relative sizes (i.e., other than the uniform size or two-size versions of the fuel elements  22  shown in  FIGS. 2-5 ). 
     With continued reference to  FIG. 1 , when the reactor  10  is in operation, a heavy water coolant  26  flows over the fuel bundles  14  to cool the fuel elements and remove heat from the fission process. As shown in  FIGS. 2-5 , the heavy water coolant  26  is contained within the pressure tube  18 , and occupies subchannels between the fuel elements  22  of the fuel bundle  14 . The nuclear fuels of the present disclosure are also applicable to pressure tube reactors with different combinations of liquids/gasses in their heat transport and moderator systems. In any case, coolant  26  absorbing heat from the nuclear fuel can transfer the heat to downstream equipment (e.g., a steam generator  30 ), to drive a prime mover (e.g., turbine  34 ) to produce electrical energy. 
     With reference to  FIGS. 1-4  by way of example, the fuel elements  22  can include a central element(s)  38  (which could also include one or more central rings of elements or other grouping of central elements), a first plurality of elements  42  positioned radially outward from the central element  38 , a second ring or plurality of elements  46  positioned radially outward from the first plurality of elements  42 , and a third ring or plurality of elements  50  positioned radially outward from the second plurality of elements  46 . In the construction of  FIG. 5 , the fuel elements  22  also include a fourth ring or plurality of elements  52  positioned radially outward from the third plurality of elements  50 . The central element(s)  38  may be generally referred to herein as an inner element or elements, and the first, second, third, and/or fourth (or more) plurality of elements  42 ,  46 ,  50 ,  54  may be generally referred to herein as outer elements. For example,  FIGS. 2-5  illustrate a 37-element fuel bundle for CANDU designs, a 43-element fuel bundle for CANFLEX designs, a 43-element CANFLEX variant, and a 61-element CANFLEX variant, respectively. It should be understood that in other constructions, the fuel bundle  14  can include fewer or more elements  22 , and can include elements  22  in configurations other than those illustrated in  FIGS. 2-5 , such as a square lattice assembly for non-CANDU applications. The fuel elements  22  can be also positioned parallel to one another in one or more planes, elements arranged in a matrix or array having a block shape or any other cross-sectional shape, and elements in any other patterned or patternless configuration. 
     The various nuclear fuels of the present disclosure can include fissile materials that are used (e.g., blended) in conjunction with one or more other materials, as well as neutron absorbers as will be described in greater detail below. The nuclear fuel can be in pellet form, powder form, or in another suitable form or combination of forms. In some constructions, fuels of the present disclosure take the form of a rod, such as a rod of the fuel pressed into a desired form, a rod of the fuel contained within a matrix of other material, and the like. Also, fuel elements made of the materials according to the present disclosure can include a combination of tubes and rods and/or other types of elements. 
     The fuel elements  22  include fissile materials and/or a combination of fissile material(s) and neutron absorbers, some of which elements  22  may have different compositions from other elements  22 , as will be described in the various constructions below. Canadian Patent Application No. 2,174,983, filed on Apr. 25, 1996, describes examples of fuel bundles for a nuclear reactor. The fissile materials described herein can comprise any of the nuclear fuels in Canadian Patent Application No. 2,174,983, the contents of which are incorporated herein by reference. For example, the nuclear fuel includes any one or more of various uranium isotopes and/or plutonium isotopes, such as U-233, U-235, PU-239, and/or PU-241, and can include Thorium. In some constructions, the one or more of U-233, U-235, PU-239 and/or PU-241 have more than 0.9 wt % enrichment. More specifically, in some constructions the one or more of U-233, U-235, PU-239 and/or PU-241 have enrichment between about 0.9 wt % and about 20 wt %. In other constructions, the one or more of U-233, U-235, PU-239 and/or PU-241 have enrichment between about 0.9 wt % and about 5.0 wt %. For light water reactor applications by way of example only, the one or more of U-233, U-235, PU-239 and/or PU-241 may have between about 5.0 wt % and about 20 wt % enrichment. The nuclear fuel may include one or more ceramic fuel types of uranium-, plutonium-, and/or thorium-oxides. The nuclear fuel may also include mixed oxide (“MOX”) fuel containing a mixture of more than one oxide of fissile material. As an example, the nuclear fuel can include a mixture of plutonium oxides and uranium oxides, and in some embodiments can also include Thorium. 
     The fuel bundle  14  is characterized by using in some of its fuel elements  22  (such as specifically its inner element(s)) fissile material(s) with a mixture of neutron absorber materials (or neutron absorber mixture). The fissile material(s) may include one or more of the fissile materials described above. The mixture of neutron absorber materials (or neutron absorber mixture) includes two or more neutron absorbers. The two or more neutron absorbers may include two or more of gadolinium (Gd), dysprosium (Dy), hafnium (Hf), erbium (Er), and europium (Eu). In some embodiments, a neutron absorber mixture including gadolinium as the first neutron absorber and one or more of dysprosium, hafnium, erbium, and/or europium as the second or more neutron absorber(s) is particularly effective in various applications. In some preferred embodiments, the neutron absorber mixture includes gadolinium and dysprosium. 
     Various constructions of the fuel bundles  14  having fissile material(s) with the neutron absorber mixture in accordance with the present disclosure are presented in Table 1, Table 2, and Table 3. In some constructions, the neutron absorber mixture comprises between about 1 wt % and about 30 wt % of the fuel meat at the fresh fuel condition (Table 1). In some more specific constructions, the neutron absorber mixture comprises between about 1 wt % and about 20 wt % of the fuel meat at the fresh fuel condition (Tables 2 and 3). In some light water reactor applications, the neutron absorber mixture can comprise between about 10 wt % and about 40 wt % of the fuel meat at the fresh fuel condition (Table 2). The quantity of inner element(s) containing the fissile material(s) with the neutron absorber mixture may be between about 1 and about 11 elements for 37-61 element CANDU/CANFLEX fuel bundles or about 1 to about 10 wt % in multiple fuel elements in a non-CANDU fuel assembly (Table 1). More specifically, the quantity of inner element(s) may be between about 1 and about 7 elements for the 37-element bundles ( FIG. 2 ), between about 1 and about 8 elements for the 43-element bundle ( FIGS. 3 and 4 ), and between about 1 and about 11 elements for the 61-element bundle ( FIG. 5 ) (Table 2). The remaining outer elements include one or more of the fissile materials described above, preferably any of U-233, U-235, PU-239, PU-241, and Thorium. 
     For light water reactor applications, some or all of the elements may include the combination of the fissile material(s) with the neutron absorber mixture described above (Table 2). Alternatively, for light water reactor applications having pellets in the elements, some or all of the pellets in each element may have the combination of the fissile material(s) with the neutron absorber mixture described above. 
     The combination of fissile material(s) with the neutron absorber mixture described above is preferably a homogeneous combination or mixture having a generally even distribution of fissile material(s) and neutron absorber mixture throughout each whole element  22  (or pellet for those reactors employing fuel in pellet form). 
     With reference to the construction of  FIG. 2 , a 37-element fuel bundle for CANDU designs is shown. In one preferred construction, the central ring  38  includes the homogeneous mixture of absorbers and any one or more of the fissile materials described above, and the first, second, and third rings  42 ,  46 ,  50  include any one or more of the fissile materials as described above. 
     Turning to the construction of  FIG. 3 , a 43-element fuel bundle for CANFLEX designs is shown. In one preferred construction, the central ring  38  and the first ring  42  include one or more elements  22  having the homogeneous mixture of absorbers and any one or more of the fissile materials described above, and the second and third rings  46 ,  50  include any one or more of the fissile materials as described above. 
     Referring now to  FIG. 4 , a 43-element CANFLEX variant is shown. In one preferred construction, the central ring  38  includes the homogeneous mixture of absorbers and any one or more of the fissile materials described above, and the first, second, and third rings  42 ,  46 ,  50  include any one or more of the fissile materials as described above. 
     Finally, with reference to  FIG. 5 , a 61-element CANFLEX variant is shown. In one preferred construction, the central ring  38  and the first ring  42  include one or more elements  22  having the homogeneous mixture of absorbers and any one or more of the fissile materials described above, and the second, third, and fourth rings  46 ,  50 ,  52  include any one or more of the fissile materials as described above. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Major Parameters 
                 Application Range 
               
               
                   
               
             
            
               
                 Fuel geometry 
                 For CANDU fuels: 37-Element, 43-Element CANFLEX 
               
               
                   
                 fuel geometry and its variants. 61-Element CANFLEX fuel 
               
               
                   
                 geometry and its variants, any fuel geometries with fuel 
               
               
                   
                 pins between 43 and 61. 
               
               
                   
                 For Non-CANDU fuels: Any square lattice assembly. 
               
               
                 Fuel isotopic composition 
                 Ceramic fuel types of UO2, PUO2 and ThO2 
               
               
                 Neutron absorber materials 
                 Combination of Gd with any of Dy, Hf, Er and Eu 
               
               
                 Neutron absorber amount 
                 1 wt %~30 wt % of the fuel meat at fresh state 
               
               
                 Fissile materials to be combined 
                 Any of U-233, U-235, PU-239 and PU-241 
               
               
                 with absorber materials 
                   
               
               
                 Fissile enrichment with the neutron 
                 0.9 wt %~20 wt % 
               
               
                 absorber materials 
                   
               
               
                 Number of fuel elements with the 
                 1~11 element(s) for 37-61 element CANDU fuel 
               
               
                 mixture of above neutron absorber 
                 bundle, or 1~10 wt % in multiple fuel elements in a non- 
               
               
                 and fissile materials 
                 CANDU fuel assembly. 
               
               
                 Averaged coolant void reactivity 
                 −15 mk~+3 mk 
               
               
                 (CVR) at Nominal Condition 
                   
               
               
                 Average fuel discharge burnup  
                 7,000 MWD/T~60,000 MWD/T 
               
               
                 (at the fuel exit condition) 
                   
               
               
                 Coolant type 
                 Heavy water or light water 
               
               
                 Moderator type 
                 Heavy water or light water 
               
               
                 Reactor Type 
                 Thermal reactors: CANDU (and its variants such as 
               
               
                   
                 SCWR), PWR and BWR 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Application Range for 
                 Application Range for 
               
               
                 Major parameters 
                 CANDU 
                 LWR 
               
               
                   
               
             
            
               
                 Fuel geometry 
                 CANDU bundle: 37-, 43- 
                 LWR assembly consisting 
               
               
                   
                 and 61-Elements CANDU 
                 of n × n fuel pins in a 
               
               
                   
                 or CANFLEX designs and 
                 rectangular geometry. 
               
               
                   
                 its variants. 
                   
               
               
                   
                 *ex): 37-Element bundle 
                   
               
               
                   
                 design consists of 37 
                   
               
               
                   
                 elements (or pin or rod) in a 
                   
               
               
                   
                 cylindrically shaped bundle 
                   
               
               
                   
                 geometry. 
                   
               
               
                 Fissionable isotopic  
                 Ceramic fuel types of UO 2 , 
                 Ceramic fuel types of UO 2 , 
               
               
                 materials (1) 
                 PUO 2  or THO 2   
                 PUO 2  or THO 2   
               
               
                 Neutron absorber  
                 Gd + Dy, 
                 Gd + Dy, 
               
               
                 materials (2 or 3) 
                 Gd + Er, and 
                 Gd + Er, 
               
               
                   
                 Gd + Dy + Er, 
                 Gd + Hf, 
               
               
                   
                   
                 Gd + Dy + Er, 
               
               
                   
                   
                 Gd + Dy + Hf, and 
               
               
                   
                   
                 Gd + Er + Hf 
               
               
                 Final form of composite  
                 Element (or rod or pin) type 
                 Pin (or rod) type mixture 
               
               
                 burnable absorber  
                 mixture combine with 
                 combined with Neutron 
               
               
                 mixture (3) 
                 Neutron absorber materials 
                 absorber materials (2) + 
               
               
                   
                 (2) + Fuel isotopic  
                 Fuel isotopic materials (1) 
               
               
                   
                 materials (1) 
                 * Note:  
               
               
                   
                 * Note:  
                 The mixture is a 
               
               
                   
                 The mixture is a 
                 homogenized form of 
               
               
                   
                 homogenized form of 
                 absorber and fuel isotopes. 
               
               
                   
                 absorber and fuel isotopes. 
                   
               
               
                 Location of composite  
                 Center element (Total 1 
                 Full or partial usage in the 
               
               
                 burnable absorber  
                 element) 
                 pins of a fuel assembly. 
               
               
                 mixture element 
                 Center element + inner 
                 * Note:  
               
               
                   
                 ring. (Total 7 elements for 
                 Partial usage includes  
               
               
                   
                 37-Element bundle, 8 
                 partial number of pins  
               
               
                   
                 elements for 43-Element 
                 in an assembly and 
               
               
                   
                 Bundle and 11 elements for 
                 partial usage of mixture 
               
               
                   
                 61-Element Bundle) 
                 element pellets in a pin. 
               
               
                   
                 Partial usage of absorber 
                   
               
               
                   
                 mixture elements in the 
                   
               
               
                   
                 ‘Center element + inner 
                   
               
               
                   
                 ring’ case. 
                   
               
               
                 Neutron absorber amount 
                 1 wt %~20 wt % of absorber 
                 10 wt %~40 wt % of 
               
               
                   
                 materials (2) in any 
                 absorber materials (2) in any 
               
               
                   
                 composite mixture (3) at the 
                 composite mixture (3) at the 
               
               
                   
                 fresh fuel condition 
                 fresh fuel condition 
               
               
                 Fissile materials to  
                 Any of U-233, U-235,  
                 Any of U-233, U-235,  
               
               
                 be combined with  
                 PU-239 and PU-241 
                 PU-239 and PU-241 
               
               
                 absorber materials 
                   
                   
               
               
                 Fissile enrichment with  
                 0.9 wt %~5.0 wt % 
                 5.0 wt %~20.0 wt % 
               
               
                 the neutron 
                   
                   
               
               
                 absorber materials 
                   
                   
               
               
                 Averaged coolant void  
                 −15 mk~+3 mk 
                 irrelevant (negative 
               
               
                 reactivity (CVR) at  
                   
                 inherently) 
               
               
                 Nominal Condition 
                   
                   
               
               
                 Average fuel discharge  
                 10,000 MWD/T~35,000 
                 35,000 MWD/T~65,000 
               
               
                 burnup (at the fuel exit  
                 MWD/T 
                 MWD/T 
               
               
                 condition) 
                   
                   
               
               
                 Coolant type 
                 Heavy water or light water 
                 Light water 
               
               
                 Moderator type 
                 Heavy water 
                 Light water 
               
               
                 Reactor Type 
                 CANDU or Pressurized 
                 Pressurized Water Reactor 
               
               
                   
                 Heavy Water Reactor 
                 and 
               
               
                   
                   
                 Boiling Water Reactor 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Major parameters 
                 Application Range 
               
               
                   
               
             
            
               
                 Fuel geometry 
                 CANDU bundle: 37-, 43- and 61- 
               
               
                   
                 Elements CANDU or CANFLEX 
               
               
                   
                 designs and its variants. 
               
               
                   
                 *ex): 37-Element bundle design consists 
               
               
                   
                 of 37 elements (or pin or rod) in a 
               
               
                   
                 cylindrically shaped bundle geometry. 
               
               
                 Fissionable isotopic  
                 Ceramic fuel types of UO 2 , PUO 2  or 
               
               
                 materials (1) 
                 THO 2   
               
               
                 Neutron absorber materials  
                 Gd + Dy, 
               
               
                 (2 or 3) 
                 Gd + Er, 
               
               
                   
                 Gd + Dy + Er 
               
               
                 Final form of composite  
                 Element (or rod or pin) type mixture 
               
               
                 burnable absorber 
                 combined with Neutron absorber 
               
               
                 mixture (3) 
                 materials (2) + Fuel isotopic materials 
               
               
                   
                 (1) 
               
               
                 Location of composite  
                 Center element (Total 1 element) 
               
               
                 burnable absorber mixture 
                 Center element + inner ring. (Total 7 
               
               
                 element 
                 elements for 37-Element bundle, 8 
               
               
                   
                 elements for 43-Element Bundle and 11 
               
               
                   
                 elements for 61-Element Bundle) 
               
               
                   
                 Partial usage of absorber mixture 
               
               
                   
                 elements in the ‘Center element + inner 
               
               
                   
                 ring’ case. 
               
               
                 Neutron absorber amount 
                 1 wt %~20 wt % of absorber materials 
               
               
                   
                 (2) in any composite mixture (3) at the 
               
               
                   
                 fresh fuel condition 
               
               
                 Fissile materials to be  
                 Any of U-233, U-235, PU-239 and  
               
               
                 combined with absorber  
                 PU-241 
               
               
                 materials 
                   
               
               
                 Fissile enrichment with the  
                 0.9 wt %~5.0 wt % 
               
               
                 neutron absorber materials 
                   
               
               
                 Averaged coolant void  
                 −15 mk~+3 mk 
               
               
                 reactivity (CVR) at Nominal 
                   
               
               
                 Condition, including for  
                   
               
               
                 CANDU reactors 
                   
               
               
                 Average fuel discharge  
                 10,000 MWD/T~30,000 MWD/T 
               
               
                 burnup (at the fuel exit  
                   
               
               
                 condition) 
                   
               
               
                 Coolant type 
                 Heavy water 
               
               
                 Moderator type 
                 Heavy water 
               
               
                 Reactor Type 
                 CANDU 
               
               
                   
               
            
           
         
       
     
     The purpose of the neutron absorber mixture is primarily to effectively control simultaneously the following design parameters: coolant void reactivity, linear element rating, fueling impact and fuel burnup. Different neutron absorbers have different depletion characteristics. By using more than one neutron absorber, these depletion characteristics are combined such that the absorbers can work during different phases of the fuel depletion period. The first neutron absorber, such as the gadolinium, helps control reactivity by providing extra reactivity of the fuel while the fuel burns out around mid-burnup. The second (or more) neutron absorber helps reduce coolant void reactivity until the end of fuel discharge burnup. Gadolinium has been known as an effective absorber for short-term reactivity control purposes; however, it has been discovered in accordance with the present disclosure that in a specific environment as in a CANDU type reactor (and some non-CANDU reactors as discussed above) having a more hardened neutron spectrum than that of natural uranium, gadolidium can be used for longer-term reactivity control purposes. 
     As illustrated in  FIGS. 6 and 7 , the fuel designs disclosed herein achieve low reactivity impact and thus extend the fuel discharge burnup while maintaining a low power impact and related parameters during normal operation of the reactor core. From the aspect of reactivity decay of a fuel, the decay curve ( FIG. 6 ) is smoothed with the use of the combined fissile material(s) and mixture of neutron absorber materials compared with Dy+Gd and Dy alone. 
     Furthermore, it is desirable to decrease coolant void reactivity (CVR), and even provide a negative CVR, in a pressurized heavy water nuclear reactor such as the CANDU reactor. Canadian Patent No. 2,097,412, the entire contents of which are incorporated by reference herein, provides a useful background on the science of reducing coolant void reactivity, in particular in CANDU reactors. With this invention, CVR could also be maintained negative without a significant impact on fuel discharge burnup. Prior art designs using a single burnable poison to limit CVR would decrease fuel discharge burnup. 
     Previously, CANDU fuels could typically not achieve higher burnup than around 10,000 MWd/T. This is mainly due to the high refueling impact (such as power peaking or high channel and bundle powers) during online refueling because higher burnup can only be achieved based on enriched fuel designs. Thus, high-burnup and low reactivity impact are two competing design features. The fuels disclosed herein are intended to resolve this issue and can extend fuel burnup up to 35,000 MWd/T in CANDU reactors and up to 70,000 MWd/T in LWR reactors. By way of example only, in some embodiments the fuels disclosed herein can extend fuel burnup to ˜7,000 MWD/T˜30,000 MWD/T for CANDU reactors, and/or ˜30,000 MWD/T˜60,000 MWD/T for LWR reactors. 
     As described in detail above, the fuels disclosed herein can also be applied to non-CANDU reactors such as PWR to achieve a fuel designs with reduced power peaking or extended fuel burnup. High burnup fuel enables deeper burning of fissile materials and thus enables more neutron economy. The main economic benefits in reaching high burnup fuel are high fuel resident time in the reactor (less amount of fuel fabrication, i.e., it takes three times less fuel than in CANDU NU), less waste to disposition (less storage area is needed), and reduced propensity for proliferation. 
     Thus, the disclosure provides, in some embodiments, a fuel design characterized by using a mixture of neutron absorber materials in an inner region of CANDU fuel, and in some fuel elements of a non-CANDU fuel assembly. The neutron absorber mixture suppresses reactivity of the core, controls local power peak and/or controls coolant void reactivity. Various features and advantages of the disclosure are set forth in the following claims.