NUCLEAR FUEL CONTAINING A NEUTRON ABSORBER MIXTURE

Abstract

Fuel bundles for nuclear reactors are provided, and can include a fuel element containing U-233, U-235, PU-239, and/or PU-241 fissile material, along with at least two neutron absorbers consisting of Gd, Dy, Hf, Er, and/or Eu, wherein the fissile material(s) and the at least two neutron absorbers are homogeneously mixed in the fuel element. Fuel bundles for nuclear reactors are also provided that include fuel elements having 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. Fuel elements for nuclear reactors are also provided, and can include U-233, U-235, PU-239, and/or PU-241 fissile material, along with at least two neutron absorbers consisting of Gd, Dy, Hf, Er, and/or Eu, wherein the fissile material(s) and the at least two neutron absorbers are homogeneously mixed in the fuel element.

Claims

1. A fuel bundle for a nuclear reactor, the fuel bundle comprising: 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.

2. The fuel bundle of claim 1, wherein the fuel element is a first fuel element, the fuel bundle further comprising a second fuel element containing a fissile material, wherein the first fuel element is disposed in an inner portion of the fuel bundle and the second fuel element is disposed in an outer portion of the fuel bundle.

3. The fuel bundle of claim 2, further comprising a plurality of fuel elements containing a fissile material disposed in the outer portion of the fuel bundle.

4. The fuel bundle of claim 1, wherein the at least one fissile material is more than 0.9 wt % enrichment.

5. The fuel bundle of claim 1, wherein the at least one fissile material is between about 0.9 wt % and about 20 wt % enrichment.

6. The fuel bundle of claim 1, wherein the at least two neutron absorbers include Gd and Dy.

7. The fuel bundle of claim 1, wherein the at least two neutron absorbers provide between about 1 wt % and about 30 wt % in the fuel element at a fresh state.

8. The fuel bundle of claim 1, wherein the at least two neutron absorbers provide between about 1 wt % and about 20 wt % in the fuel element at a fresh state.

9. The fuel bundle of claim 1, wherein the at least two neutron absorbers include Gd and Dy providing between about 1 wt % and about 20 wt % in the fuel element at a fresh state.

10. 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.

11. The fuel element of claim 10, wherein the at least one fissile material is more than 0.9 wt % enrichment.

12. The fuel element of claim 10, wherein the at least one fissile material is between about 0.9 wt % and about 20 wt % enrichment.

13. The fuel element of claim 10, wherein the at least two neutron absorbers include Gd and Dy.

14. The fuel element of claim 10, wherein the at least two neutron absorbers provide between about 1 wt % and about 30 wt % in the fuel element at a fresh state.

15. The fuel element of claim 10, wherein the at least two neutron absorbers provide between about 1 wt % and about 20 wt % in the fuel element at a fresh state.

16. The fuel bundle of claim 10, wherein the at least two neutron absorbers include Gd and Dy providing between about 1 wt % and about 20 wt % in the fuel element at a fresh state.

17. A fuel bundle for a nuclear reactor, the fuel bundle comprising: 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.

18. The fuel bundle of claim 17, wherein the fissile material includes at least one material selected from the group consisting of U-233, U-235, PU-239, and PU-241, and wherein the at least two neutron absorbers are selected from the group consisting of Gd, Dy, Hf, Er, and Eu.

19. The fuel bundle of claim 17, wherein the fissile material includes at least one material selected from the group consisting of U-233, U-235, PU-239, and PU-241, and wherein the at least two neutron absorbers include Gd and Dy providing between about 1 wt % and about 30 wt % in the at least one of the inner elements at a fresh state.

20. The fuel bundle of claim 17, wherein the fuel bundle is a CANDU fuel bundle having a generally cylindrical geometry.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 is a schematic diagram of a nuclear reactor employing any of the fuel bundles of FIGS. 2-6.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] FIG. 6 is graph illustrating reactivity decay characteristics of different absorbers.

[0015] FIG. 7 is a graph illustrating gains in refueling impact and discharge burnup of various absorber mixtures.

DETAILED DESCRIPTION

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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 nn 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).

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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).

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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-US-00001 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-US-00002 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.sub.2, Ceramic fuel types of UO.sub.2, materials (1) PUO.sub.2 or THO.sub.2 PUO.sub.2 or THO.sub.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-US-00003 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.sub.2, PUO.sub.2 or materials (1) THO.sub.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

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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/T30,000 MWD/T for CANDU reactors, and/or 30,000 MWD/T60,000 MWD/T for LWR reactors.

[0037] 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.

[0038] 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.