High burnup nuclear fuel pellet design

Abstract

A fuel design for high burnup in water-cooled reactors is disclosed. A nuclear fuel rod comprising an outer zone of annular fuel pellets composed of actinides and fissile material and an interior comprising a low density, high open porosity ceramic foam of zirconia or magnesia is used. Use of annular pellets lowers fuel temperature, reduces fission gas release and fuel swelling. Foam with low crushing strength is used to accommodate fuel swelling. Fresh fuel reactivity is controlled by burnable absorbers such as gadolinia, incorporated as sintered bodies within the foam or as part of the fuel composition or both. The placement of burnable absorber in the foam reduces the contact area with surrounding media, mitigating diffusional transport and preserving its self-shielding characteristic. A novel two-piece foam holder is proposed to insert the ceramic foam.

Description

[0001]TITLE OF THE INVENTION: High Burnup Nuclear Fuel Pellet Design DESCRIPTIONTECHNICAL FIELD: IPC ClassificationG21C 3 / 00 G21C 3 / 02 G21C 3 / 04 G21C 3 / 16 G21C 3 / 18 G21C 3 / 32 G21C 3 / 326 G21C 3 / 58 G21C 3 / 62 G21C 7 / 04 G21C 7 / 24 This invention relates to nuclear fuel design for achieving high burnup in water moderated reactors. BACKGROUND ART Currently, nuclear fuel rods for water-cooled reactors contain solid cylindrical fuel pellets enriched with up to 5% fissile material. To maintain the chain reaction, usually a third of the core is replaced with fresh fuel every 18 months. A high- burnup fuel rod could extend this interval, reducing spent fuel assemblies and costs, and increasing capacity factor. Such a design will require a higher initial fissile enrichment. However, achieving high burnup raises two key challenges. First, higher burnup increases released fission gas and fuel swelling, stressing the cladding. Conventional solid pellets lack space to accommodate additional swelling arising from higher burnup. Similarly, while a plenum exists for fission gas, increasing its size to accommodate the higher fission gas release at higher burnup will reduce the fuel column in existing reactors, limiting energy production. This design challenge leads to higher cladding stress at high burnup. Second, increased fissile loading necessitates more Burnable Absorber (BA) to control reactivity. Gadolinia, a common BA, is often uniformly distributed in fuel pellets, but this approach reduces thermal conductivity and melting temperature of the fuel. Additionally, when gadolinia is uniformly distributed in the fuel in a large number of fuel rods in an assembly, it can lead to a positive Moderator Void Coefficient (MVC). It is current practice to limit gadolinia to 8 wt.% in the fuel and deploy such fuel in a limited number of rods, for example, in less than 24 rods in a Pressurized Water Reactor (PWR) assembly containing 264 fuel rods. It should also be noted that when gadolinia is incorporated as part of the fuel composition, it leads to rapid burnout of the BA, incompatible with long-life high burnup designs. A centrally placed lumped gadolinia BA, as proposed in Reference [1], offers an alternative by separating the absorber from the fuel. This configuration utilizes gadolinia's self-shielding behavior, leading to a longer burnout time. However, the effectiveness of this approach depends heavily on the geometry and surface area of the BA. Operating at high temperatures, diffusional transport from its surface to the surrounding media can compromise its effectiveness. This invention addresses limitations of prior art by significantly reducing fuel temperature, up to 1000 K, at peak power zones through the use of annular fuel pellets. While annular pellets have been explored in the past, none address the challenge of high burnup. Reference [2] is primarily concerned with fuel fragment relocation from the inner surface of the annular pellet and proposes a central rope of refractory fiber or a plug to prevent fuel fragment movement and does not address concerns regarding high burnup nor does it indicate placement of burnable absorbers. Reference [3] utilizes annular pellets with internal boron BA but does not discuss other BA materials or their placement. The proposed invention incorporates a low-density, high open-porosity, low crushing strength ceramic foam within each annular pellet. This foam serves multiple purposes. It accommodates expanded fission gas release and due to its low crushing strength, readily accommodates high burnup fuel swelling. Additionally, it allows for Burnable Absorber (BA) incorporation, through emplacement of sintered bodies of BA within the foam structure, physically separated from the fuel, or as part of the annular fuel composition, or by a combination of these methods. Unlike prior art where incorporation of a central BA body or ceramic fiber plug has required special fabrication processes / methods, the use of ceramic foam allows a simple, reliable, and repeatable method for its insertion in the interior of the annular pellet with or without BA bodies, described in this patent. SUMMARY OF THE INVENTION This invention relates to a novel nuclear fuel rod designed for high burnup. The key innovation lies in its unique structure and preferred options for incorporating Burnable Absorbers (BA). Annular fuel pellets: Made of uranium oxide enriched to a maximum of 20% in U-235 or mixed oxide fuel containing U-235 and plutonium isotopes. This annular design significantly reduces fuel temperature. Ceramic foam interior: Composed of low-density, high open-porosity zirconia or magnesia to accommodate fission gas and fuel swelling. The foam has low crushing strength and will offer little resistance to fuel swelling into the interior space as a result of fission creep of fuel. A method for inserting the foam into the interior is described in this patent. Burnable Absorber (BA) Incorporation: Three flexible methods are proposed to control fresh fuel reactivity. (1) Discrete BA bodies emplaced in the foam: Sintered bodies of BA materials, comprised of one or more oxides of Gadolinium, Erbium, Dysprosium, Samarium, Europium or Hafnium, offering targeted reactivity control. The density of the sintered BA bodies preferably higher than 95% of its theoretical density to mitigate in-reactor densification and maintain its geometry. (2) Uniform distribution of BA in the annular fuel pellet: One or more oxides of Gadolinium, Erbium, Dysprosium, Samarium, Europium or Hafnium directly mixed into the fuel and sintered to fabricate annular pellets. (3) A combination of the two methods namely, BA bodies emplaced in the foam as well as uniform distribution of the BA in the annular pellet fuel. Fuel Rod and Fuel Assembly: The annular pellets and foam, with or without BA, are loaded into a cladding tube, held by an axial spring, filled with helium, and sealed with end plugs to form a complete fuel rod. The process steps for assembly of the fuel rod follow the same steps as that for solid fuel pellets currently in use. Additionally, the BA can be strategically incorporated in specific axial segments of the rod for further optimization. Nuclear fuel rods fabricated as noted herein, with and / or without BA incorporated, are assembled with other appurtenances to form a nuclear fuel assembly for insertion in a power reactor. Insertion of ceramic foam in the annular pellet: One method for inserting the foam into the center hole of the annular fuel pellet is described here using a specially designed two-piece annular foam holder for ease of use and consistent results. The foam holder comprises two identical interlocking half-cylinders with a stepped central hole that matches the inner diameter of the annular pellet in its upper portion and the annular pellet's outer diameter in its lower portion. A cylindrical ceramic foam insert, slightly larger than the annular pellet's hole, is placed between the two halves of the holder, and compressed as they are brought together. The foam is positioned directly above the annular pellet that is placed in the lower portion of the foam holder. A plunger pushes the foam insert into the pellet's interior space. The method can be adapted to incorporate Burnable Absorber (BA) bodies within the foam. Burnable absorbers in the form of sintered bodies, comprised of one or more oxides of Gadolinium, Erbium, Dysprosium, Samarium, Europium or Hafnium are used to control fresh fuel reactivity. The principal requirement for a BA body to be emplaced in the foam is it be physically separated from the annular pellet by the intervening foam. One of two methods can be used for emplacing the BA body in the foam. In the first method, the foam insert itself is comprised of two halves with a recess matching the BA body. The foam assembled with the BA body in the recess is then compressed as the two halves of the holder body are brought together. In a second method, the BA body could be pressed into the foam after the foam insert has been inserted into the annular pellet. This is feasible as the crushing strength of the foam is low. Those skilled in the art could be expected, based on the considerations outlined in this patent, to develop modifications to the method described here for insertion of foam and Burnable Absorber body in the annular fuel pellet and perform such operations in an automated facility. ADVANTAGES OF THE PROPOSED INVENTION: Enhanced Fuel Performance: Reduced fuel temperature: Annular fuel pellets lower peak temperatures by up to 1,000 K, minimizing fission gas release and fuel swelling, thereby enhancing cladding integrity. Improved accommodation of fuel swelling: The high open porosity foam with its low crushing strength, < 2 MPa, readily accommodates fuel swelling accommodation through fuel fission creep to the interior and reduces cladding stress from fuel swelling. Optimized neutron absorption: Magnesia and Zirconia have extremely low cross-section for absorption of thermal neutrons. The low-density of the foam further minimizes parasitic neutron absorption. Loss-of-coolant (LOCA) response: The initial increase in cladding temperature during a LOCA when the fuel and cladding are in contact is dependent on the stored heat of the fuel pellet just prior to LOCA. The smaller mass of the fuel pellet and its lower average temperature lead to a smaller increase in cladding temperature. Advantages of BA bodies emplaced in foam: Placing BA (e.g., sintered gadolinia) in the foam isolates it from the fuel, preventing adverse thermal impacts in the fuel. Targeted reactivity control: Tailoring BA surface area and mass within the foam allows for targeted adjustments in fresh fuel reactivity reduction and burnout time. Mitigate diffusional transport: The foam mitigates diffusion of BA into the surrounding media by minimizing the contact area thereby preserving the geometry and self-shielding properties of the BA. Improved Moderator Void Coefficient (MVC) is achieved compared to uniformly mixed-in-fuel BA designs. With foam-based BA, a larger fraction of fuel rods in a fuel assembly can incorporate BA without incurring a positive Moderator Void Coefficient. Potential elimination of in-fuel BA: If the fuel assembly nuclear design allows, all BA can be emplaced in the foam, it simplifies fuel fabrication and eliminates BA-related fuel sintering steps. Additionally, BA bodies and ceramic foam can be sourced externally, further streamlining the fabrication process. Flexible BA / foam placement: The BA bodies do not require exact radial centering within the foam. Axial and azimuthal contiguity of the foam is not necessary. Advantages of Annular Pellets with Intrinsic BA in Fuel: Increased Actinide Loading: Additional space within the annular design allows incorporating BA directly into the fuel without sacrificing actinide material, unlike solid pellets. Mitigated Thermal Impact: Lower operating temperatures in annular pellets make them less susceptible to the adverse thermal effects (decreased conductivity, melting temperature) associated with BA inclusion in fuel. Improved Gap Conductivity: Reduced fission gas release due to lower fuel temperature maintains gap conductivity between cladding and pellet, preventing thermal performance degradation. Advantage of novel method for emplacing foam: Two-piece holder assembly, identical to each other, proposed for easier and repeatable emplacement of foam. The proposed method ensures easy alignment and insertion of the foam, guaranteeing consistent results in production. Precise foam placement minimizes material waste and ensures optimal foam volume within the pellet. The method readily accommodates the incorporation of burnable absorbers for targeted fresh fuel reactivity reduction, offering a complete solution for high-burnup fuel pellet fabrication. This novel design allows for higher discharge burnup compared to conventional designs, is technically feasible with existing materials and processes, and is readily adaptable to existing water-cooled reactors like Pressurized Water Reactors. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a radial cross-section of the proposed fuel rod of the invention. Figure 2 illustrates an elevation view of the half-cylinder of the foam-holder. Figure 3 illustrates a plan view of the half-cylinder of the foam-holder. Figure 4A illustrates the pin-cell geometry for nuclear analysis of solid fuel pellet design. Figure 4B illustrates the pin-cell geometry for nuclear analysis of proposed design with no Burnable Absorber. Figure 4C illustrates the pin-cell geometry for nuclear analysis of proposed design with cylindrical gadolinia absorber in the foam. Figure 5 shows the variation of infinite multiplication factor with Effective Full Power Days (EFPD) of operation for four cases. DETAILED DESCRIPTION OF THE INVENTION AND EMBODIMENTS Figure 1 illustrates a radial cross-section of the proposed fuel rod invention. Annular nuclear fuel pellets 225 enriched in fissile material, either UO2 enriched in U-235 or Mixed Oxide (MOX) with U-235 and Pu isotopes, are positioned closest to the cladding 210 with a gap 215 for ease of loading. High open porosity (>80%), ultra-light (less than 1 gram per cubic centimeter) ceramic foam 230, comprised of either zirconia or magnesia, fills the space interior to the pellets. For purposes of the detailed description that follows, it is assumed that the ceramic foam is comprised of zirconia. A single cylindrical Burnable Absorber (BA) body 240, assumed here to be sintered gadolinia, is shown in the center of the zirconia foam. Embodiments of BA body: The shape of the BA body, particularly in the case of gadolinia, has a major impact on its BOL reactivity worth and will be specified based on nuclear analysis. While Figure 1 depicts a cylindrical BA, any well- defined shape which can be emplaced in the foam, physically separated from the fuel is admissible, and shapes like prisms, spheres, or plates can be used. Transport codes considering BA geometry and mass loading should be used to determine the impact on reactivity and burnout for the chosen BA geometry. The BA body does not require contiguous axial placement or a uniform shape along the fuel rod length. Additionally, while Figure 1 shows the BA centered in the foam, as long as it is physically separated from the fuel pellets, its placement is flexible. In another embodiment, the emplaced BA geometry and its mass may be varied along the axis of the fuel rod. Such an arrangement could be used to tailor the axial reactivity profile and thereby the axial power profile of fresh fuel. Embodiments for foam and its support of emplaced BA body: Although an axially contiguous foam has been shown in Fig.1, it can be discontinuous axially or azimuthally. The functional requirements of the ceramic foam are to separate the BA bodies from the fuel pellets, hold the BA bodies in place, and mitigate movement of pellet fragments. Frictional forces between the foam and the BA body can secure the BA body axially. In another embodiment, the foam by being present in an axial space below the BA body can provide axial support. In some embodiments, the BA body is present only in specific axial sections for optimized control and power profile. In such cases, or when there is no BA body in the foam, the foam fills the entire space within the annular pellets. In other embodiments, fuel rods without BA may be used in some of the fuel rods in a fuel assembly to increase the discharge exposure of the assembly if sufficient reduction in fresh fuel reactivity has been achieved with the rest of the fuel rods in the assembly with BA. Embodiments in respect to annular fuel: In an embodiment of the fuel rod, the fissile loading and geometry of the annular fuel pellets may be varied along the axis of the fuel rod by varying the inner radius and / or fissile enrichment along the axis. Figures 2 and 3 illustrate the elevation and plan views respectively of one of the two half-cylinders of the foam-holder. These two half-cylinders are identical to each other. The elevation, shown in Fig.2, is drawn as viewed normal to the vertical stepped hole, where the body of the half-cylinder is labeled 102. The upper part of the stepped hole 104 has a diameter that matches the inner diameter of the annular nuclear fuel pellet and the height to which the hole extends is about twice the annular fuel pellet height. The lower part of the stepped hole 106 has a diameter slightly larger than the outer diameter of the annular nuclear fuel pellet so that the annular pellet can be placed here in a slide fit without interference and extends over a height slightly less than the pellet height. The plan view, shown in Fig.3, is drawn when viewed from the top-down, showing the holder body 102 and the hole for holding the foam in the form of a semi-circle 104 The hole in the lower portion of the holder that will accommodate the annular pellet, not visible from the top, is seen as the semi-circular area 106 bounded by the dotted semi-circle. Foam 230 is initially shaped as a cylinder with a diameter slightly greater than the inner diameter of the annular pellet and is of a height equal to the pellet height. Zirconia and magnesia ceramic foam with an open porosity of about 80%, density < 1 g / cc and crushing strength < 2 MPa, are available in the form of sheets or boards and can be ordered for specified thickness. Cylindrical shapes can be punched out of the sheets readily as its shear strength is also low. Initially, the two halves of the foam holder 102 are slightly separated and the foam is held between the two halves in the upper portion 104 of the foam holder. When they are brought together, the foam will be held in this part of the foam holder. The holder can now be placed over the annular pellet 225 and a plunger above the foam can push the foam to insert it into the annular pellet. The travel of the plunger should be limited so it stops as it reaches the level of the step in the hole in the foam holder. Other appurtenances can be added as required to hold the pieces and to automate the process. Foam properties could differ based on the composition and process used in its fabrication. Potential expansion of the foam in the direction normal to compression direction, if any, should be factored into the initial shape of the foam so that it does not expand into the interspace between the two halves of the holder. Insertion of Burnable Absorber (BA) bodies 240 in the foam follow similar steps. In one method, the cylindrical foam is split into two equal halves radially, with a recess in its cross-section to accommodate the BA body. The foam with the BA body is held between the two halves of the holder which are then brought together and inserted into the annular pellet with the use of the plunger. In view of the low crushing strength of the foam, an initial recess in the foam may not be needed. Alternatively, depending upon the foam characteristics and the shape of the BA body, it can be inserted directly into the foam after it has been assembled into the annular pellet as the crushing strength of the foam is exceptionally low. The foam is comprised of zirconia or magnesia. Zirconia (ZrO2) and Magnesia (MgO) were chosen due to their excellent properties for nuclear applications. These ceramics offer high melting points well above the expected operating temperature of the foam, ensuring their structural integrity during operation. Additionally, they exhibit minimal neutron absorption, minimizing their impact on reactor performance. Readily available zirconia and magnesia foams, used in many commercial applications, may contain impurities and their presence should be evaluated to assess if the functional and operational requirements are fully met even in the presence of such impurities. Among the requirements, it should be noted that there should be no off-gassing from the foam over the expected operating temperature range and the foam melting temperature should be higher than the peak expected operating temperature. Any parasitic neutron absorption arising from the impurities should be evaluated to determine its impact on potential reduction in Effective Full Power Operating (EFPD) days. Hafnium, for instance, is always present with zirconium in commercial zirconia foam products and while it can be removed, it will increase the cost. This may or may not be advisable depending upon its effect on EFPD as the amount of hafnium present may be too small to be a significant factor. Fabrication of uranium oxide annular pellets and BA bodies such as a sintered gadolinia cylinder to required dimensions is established art. Low density, highly porous ceramic foam material is readily available and its fabrication is also an established art. The process steps for assembly of a nuclear fuel rod with solid fuel pellets, for placing a hold-down spring on top of the fuel pellets, for helium fill, and for sealing the cladding by welding with end caps are well established. In the present instance, where solid pellets are replaced by annular fuel pellets with ceramic foam emplaced in the interior of the annular fuel pellets with or without BA bodies emplaced in the foam, the process steps for fabrication of a nuclear fuel rod would be similar. In embodiments where BA is incorporated as part of the annular fuel pellet composition, the fabrication of such fuel is also accomplished art and does not entail new technology. INDUSTRIAL APPLICABILITY: EXAMPLE OF HOW IT WILL BE USED The invention is illustrated with an example of a 3-batch, two-year refueling cycle fuel design that uses uranium oxide fuel and zirconium-alloy cladding for application in PWRs. The burnup performance of a typical PWR fuel rod is simulated using four pin-cell models (Figures 4A-4C) analyzed by the MONTEBURNS code (Reference 4). All models have a single fuel rod 366 cm long, surrounded by a 1.25 cm square water channel 200, and reflective boundary conditions. The cladding 210 outer radius is 0.47 cm and inner radius is 0.4128 cm. The gas gap is not modeled as it closes early in operation. The four cases studied are summarized in Table 1. Case 1, following conventional design practice, uses a solid pellet 220 (Fig.2A); Cases 2-4 use the proposed invention with annular fuel pellets 225 and interior zirconia foam 230. Case 2 (Fig.2B) has no burnable absorber (BA), while Cases 3 and 4 (Fig 2C) include gadolinia BA cylinders 240 within the foam. Case 4 has a larger BA radius compared to Case 3. Table 1: Fuel Rod Designs Analyzed Case U-235 BA radius Gadolinia Fuel type Enrichment Number Loading, g (cm) Loading, g 1 Solid pellets - - 2 Annular pellets, no BA 10% 116 - - Annular 3 pellets, BA in 10% 116 0.06 29.3 foam Annular 4 pellets, BA in 10% 116 0.12 29.3 foam The results of nuclear analyses tracking the variation of infinite multiplication factor (k-inf) with days of operation for the four cases are presented in Figure 3. Specific results for the four cases are summarized in Table 2. Key observations from Table 2 are: (1) The Effective Full Power Days (EFPD) of Cases 2, 3, and 4 (annular fuel designs) are higher than Case 1 (solid fuel without BA); (2) Case 2 (without BA) achieves the highest EFPD; (3) Introducing a gadolinia burnable absorber (BA) in Cases 3 and 4 reduces the Beginning-of-Life (BOL) reactivity (k- inf), and that a larger BA surface area (Case 4) leads to a more significant reduction in k-inf but has a shorter burnout time compared to the smaller BA (Case 3); (4) Including gadolinia in the fuel itself (not shown) drastically reduces BOL k-inf to less than 1.0, highlighting the flexibility of the design for reactivity control. Results of Thermal performance: The differential fuel temperature between fuel pellet outer surface and peak fuel temperature (fuel center in the case of solid fuel pellet and fuel inner surface in the case of annular fuel pellet) was calculated for a peak power of 47.4 kW / m and a rod average power of 16.9 kW / m under the assumption of a constant fuel thermal conductivity of 1.8 W / m / K and are presented in Table 3. The dramatic reduction in fuel temperature will result in reduced fission gas release and reduced fuel swelling. Table 2: Performance Comparison of Analyzed Fuel Rod Designs Case 2 Case 3 Case 4 Case 1 (Solid Feature (Annular, No (Annular, (Annular, Pellet) BA) Small BA) Large BA) Effective Full Power Days (EFPD) 2135 2256 2158 2143 Change in EFPD from Case 1, days - Beginning-of-Life k- inf (BOL) 1.4427 1.5326 1.1868 1.0403 Burn Time to BA Burnout (days)- - 900640 Table 3: Thermal Performance Comparison of Analyzed Fuel Rod Designs Cases 2, 3, 4 Reduction in Case 1 (Solid Feature (Annular Fuel Peak Fuel Pellet), K Pellets), K Temperature, K Peak Power Location 2096 944 1152 Average Power Location 748 337 411 Gadolinia Burnable Absorber and Zirconia Foam Temperatures: Gadolinia and zirconia foam in a reactor core experience minimal heat generation compared to the fuel pellets where fission reactions occur. These materials reside within a thermal well – their temperature is primarily dictated by the surrounding hot fuel. Two minor heat sources contribute to a slight increase in their temperature: gamma radiation and energy deposited by fission products directly recoiling from the fuel. The resulting temperatures of the zirconia foam and gadolinia remain well below their respective melting points of 2988 K and 2690 K Better Moderator Void Coefficient (MVC) Response using Emplaced BA bodies in Ceramic Foam: High burnup fuel designs require a higher enrichment of fissile material. However, this enrichment can increase the risk of criticality during storage and transportation if not managed properly. Traditionally, burnable absorbers (BAs), such as gadolinia, uniformly distributed in the fuel, are added to a limited number of fuel rods in a fuel assembly. While effective, uniformly dispersed gadolinia in a larger fraction of the fuel rods (e.g., 25% of the fuel bundle) has a significant drawback: it can turn the MVC positive. Studies (Reference 5) have shown that uniformly dispersing gadolinia BA in fuel in 64 out of 264 fuel rods (around 25%) with gadolinia loadings between 0.1wt.% and 0.5wt.% of the fuel can result in a positive MVC, ranging from +2.2x10-3to +4.4x10-3. These reported values were calculated under conditions of 5 wt.% of dissolved boron in the coolant. This patent proposes a novel solution using BA such as gadolinia emplaced in ceramic foam to achieve a more negative MVC. Instead of uniformly mixing gadolinia with the fuel, the design incorporates it within the ceramic foam structure present in all the fuel rods in a fuel bundle. Perturbation analysis has shown that this approach (Case 3) wherein the BA are emplaced in foam in all the fuel rods of a fuel assembly can achieve a significantly negative MVC of -5.0x10-3, compared to the positive values observed with uniformly dispersed gadolinia in fuel in only about a fourth of the rods as found in Reference 5. Surface Area and Mass Effect of Burnable Absorber Body Emplaced in the Foam: The nuclear evaluations of Cases 3 and 4 confirm the importance of BA geometry and loading in determining reactivity worth and burnout time. There is therefore a need to maintain the integrity of the BA geometry to realize the desired nuclear response in practice. It requires the minimization of diffusional transport of gadolinia to the surrounding material with which it is in contact, as otherwise the surface geometry of the BA body will be lost and the reduction in reactivity and the burnout time become less predictable. In the present invention, the contact area between the sintered gadolinia body and its surroundings is minimized by emplacing it in zirconia foam thereby mitigating diffusional transport. In addition, the BA could be expected to undergo in-reactor densification if the as-fabricated density of BA were significantly lower than its theoretical density which will lead to shrinkage and alteration of its geometry. For this reason, the relative density of the BA, which is the as-fabricated density compared to its theoretical density, should be as high as possible, preferably higher than 95%. If it is necessary to increase the surface area of the BA body without increasing the mass loading of the BA, two approaches are available: one is to configure a BA shape which increases the surface area for the same mass loading such as a triangular cross-section, and a second option would be to combine the BA with other oxides with a high melting temperature and low parasitic absorption such as zirconium oxide. Effective Full Power Days of Operation The zirconia foam in and of itself does not reduce the potential discharge EFPD significantly both because its cross-section for neutron absorption is low and as a result of its low density. The nuclear analysis results show that the discharge EFPD is a function of BA loading and decreases with increasing loading as a result of its residual parasitic neutron absorption. Discharge EFPD can be enhanced by enriching selected gadolinium isotopes to minimize residual neutron absorption penalty as indicated in Reference [5]. It could also be enhanced by optimizing the mass of BA by appropriate selection of its incorporation method and its spatial distribution in the fuel rods. In the present invention, three embodiments for fresh fuel reactivity control using BA are available. The proposed invention for fuel rod design described in this patent could be deployed in existing plants for longer refueling intervals in place of current designs. Those skilled in the art could be expected, based on the considerations outlined in this patent, to develop designs specific to the reactor parameters and other refueling intervals similar to the example cited here. REFERENCES CITED (1) U.S. Patent No.11,049,625 B2, “Nuclear Fuel Pellet with Central Burnable Absorber” (2021), Assigned to Korea Advanced Institute of Science and Technology. (2) U.S. Patent No.11,049,625 B2, “Void Plug for Annular Fuel Pellets” (1989), Assigned to The Babcock and Wilcox Company. (3) U.S. Patent No.11367537 “Annular Nuclear Fuel Pellets with Central Burnable Absorber” (2022), Assigned to Westinghouse Electric Company LLC. (4) “User’s Manual, Version 2.0 for MONTEBURNS, Version 5B,” by R. Trellue and D.I. Poston, LA-UR-99-4999, Los Alamos National Laboratory (1999) (5) “Development of Improved Burnable Poisons for Commercial Power Reactors,” by Groesbeck, M.L., Renier, J.-P.A., and Bigelow, Final Report on NERI Project Number 99-0074, September 2003.

Claims

AMENDED CLAIMS received by the International Bureau on 22 September 2024 (22.09.2024)Claims

1. ^ [Cancelled]

2. [Amended] A nuclear fuel pellet comprising: an outer annular fuel pellet comprised of actinides and fissile material; a low-density, high openporosity, and low crushing strength interior of ceramic foam composed of zirconia or magnesia; and burnable neutron absorbers comprised of one or more oxides of Gadolinium, Erbium, Dysprosium, Samarium, Europium or Hafnium incorporated as solid bodies emplaced in the foam or as part of fuel composition or both.

3. [Amended] A nuclear fuel rod comprising a plurality of nuclear fuel pellets according to Claim 2.

4. [Amended] In one embodiment of Claim 3, the surface area and mass of the solid burnable neutron absorbers bodies emplaced in foam are varied as a function of its axial position in the rod.

5. [Cancelled]

6. [Amended] A method for inserting ceramic foam into the hole of the outer annular fuel pellet of Claim 2, comprising: a two-piece foam holder of identical halves designed to form a single annular body with a stepped central hole when brought together, employing a cylindrical piece of foam, and a plunger; placing the foam between the two halves of the foam holder in the smaller diameter hole space; bringing the two halves together; placing the annular fuel pellet in the larger diameter hole space; inserting the plunger and pushing the foam down into the pellet hole.

7. [Added] In one embodiment of Claim 6, the foam is pre-loaded with solid burnable neutron absorber bodies.

8. [Added] In another embodiment of Claim 6, solid burnable neutron absorber bodies are inserted into the foam after the foam has been inserted into the hole in the outer annular fuel pellet.

9. [Added] In another embodiment of Claim 3, the fraction of burnable neutron absorbers in the fuel is varied as a function of its axial position in the rod. IStatement under Article 19 (1)Statement regarding Amended Claim 2:Original Claim 2 describes the configuration of a nuclear fuel pellet comprising an outer annular fuel pellet and an interior of low-density, high open-porosity, and low crushing strength ceramic foam composed of zirconia or magnesia. Burnable neutron Absorbers (BA) are essential to high burnup design. While many high burnup fuel rods in a fuel assembly will incorporate BA, some may not. Original Claim 2 was written with the provision that the incorporation of BA was optional. The applicant was not aware that a prior patent existed where an annular fuel with interior zirconia foam had been described by Lang et.al., US Patent No. 3,285,826 of November 1966, which was discovered by the Examiner in the prior art search process. The Examiner correctly noted that the recitation of "optionally, burnable absorbers incorporated as solid bodies emplaced, or as part of the fuel composition or both," implied that this limitation, may or may not be included in Original Claim 2. When burnable absorbers are not incorporated, US Patent No. 3,285,826 would cover this configuration, and therefore Original Claim 2 would lack an Inventive Step (IS). In Amended Claim 2, this option has been deleted and only the configuration where BA is incorporated in the nuclear fuel pellet has been recited. Neither the patent of Lang et.al., nor any other prior art search by the applicant discloses a configuration where BA is incorporated, emplaced in interior foam or as part of the fuel composition, in conjunction with outer annular fuel pellets with interior ceramic foam. It therefore constitutes an Inventive Step. The flexibility in the design of the nuclear fuel pellet, incorporating BA by two different methods which have distinctly different burnout times, is important to tailoring fuel rod and assembly nuclear response in high burnup fuel design.Statement regarding Amended Claim 3:Amended Claim 3 is more concise and enumerates the use nuclear fuel pellets according to Amended Claim 2 in a fuel rod. Other appurtenances and the method of fabrication of a nuclear fuel rod are prior art and well known and therefore, are not recited in Amended Claim 3.Statement regarding Amended Claim 6, New Claim 7, and New Claim 8:Amended Claim 6, New Claim 7 and New Claim 8 are partitions of Original Claim 6.

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