Oxide diluted gadolinium oxide combustible absorber for controlling excess reactivity of innovative small modular reactors and method for designing same

Abstract

An oxide diluted gadolinium oxide combustible absorber related technique for controlling excess reactivity of an innovative small modular reactor (i-SMR) is disclosed. The oxide-diluted gadolinium oxide combustible absorber includes: a cylindrical combustible pellet formed by mixing and sintering oxides such as gadolinium oxide (Gd2O3) and Al2O3; a cylindrical oxide pellet installed above the combustible pellet and formed of an oxide such as Al2O3; and a cladding layer in which a plurality of combustible pellets and oxide pellets are vertically stacked and mounted with respect to the cylindrical central axis. The inner diameter of the cladding layer is larger than the outer diameter of the combustible pellet and the oxide pellet, so that a gap is formed between the combustible pellet or the oxide pellet and the cladding layer. The excess reactivity of an innovative small modular reactor (i-SMR) can be effectively controlled by using a nuclear fuel assembly in which a high-strength dispersed gadolinium oxide-oxide combustible absorber (HIGA) having a high gadolinium oxide content and an integrated gadolinium oxide combustible absorber (IGD) having a low gadolinium oxide content are integrated.

Description

Technical Field

[0001] This invention relates to an oxide-diluted gadolinium oxide combustible absorber for controlling excess reactivity in innovative small modular reactors (i-SMRs) and its design method. Specifically, this invention proposes a technique for effectively controlling excess reactivity in innovative small modular reactors (i-SMRs) by using a nuclear fuel assembly that integrates a high-intensity dispersed gadolinium oxide combustible absorber (HIGA) and an integral gadolinium oxide combustible absorber (IGD). Background Technology

[0002] Small modular reactors (SMRs) have attracted considerable attention in recent years due to their unique safety features, cost-effectiveness, and zero-carbon energy production capabilities. South Korea has been actively involved in the development of innovative SMRs (i-SMRs) utilizing pressurized water reactor (PWR) technology.

[0003] The innovative small modular reactor (i-SMR) addresses key design goals of safety, cost-effectiveness, and adaptability by integrating essential design features such as soluble boron-free (SBF) operation, increased fuel thermal margin, extended operating cycles, and operational flexibility. The control system for SBF operation is simplified by eliminating the need to adjust and monitor the boron concentration in the reactor coolant, thereby reducing the complexity of the overall Chemical and Volume Control System (CVCS).

[0004] Significant challenges arise when employing borate-free (SBF) operation and long-cycle operation, particularly in its crucial role in controlling excess reactivity throughout the fuel cycle, replacing water-soluble boron. One approach to this is the use of control rods to manage excess reactivity within the core. However, the limited number of control rods in a Small Modular Reactor (SMR) core necessitates careful consideration. Over-reliance on control rods without further measures can constrain core design, potentially leading to subcritical conditions or excessively high positive reactivity in the event of control rod deviance or removal accidents. Therefore, developing an alternative method for controlling excess reactivity is essential to ensure the safe and efficient operation of SMRs.

[0005] Korean Patent Registration No. 10-1925189, published on December 4, 2018, relates to a "Nuclear Fuel Composite Sintered Body and Manufacturing Method Thereof," disclosing a nuclear fuel composite sintered body that can provide efficient use of nuclear fuel by preventing inhomogeneity and cracking of the combustible absorber. The combustible absorber is characterized in that, relative to the total volume of the nuclear fuel composite sintered body, it is inserted into the nuclear fuel at a rate of 2 wt% or more and less than 20 wt%, excluding nuclear fuel containing UO2, and is inserted into and located at the radial center of the nuclear fuel to distinguish it from the nuclear fuel. It includes one or more shapes selected from the group consisting of amorphous, cylindrical, disk, spherical, rod, thin film, and polygonal column shapes.

[0006] However, further research is needed on combustible absorbers that are simple to prepare and can be effectively used, as well as nuclear fuel assemblies that use them.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent document 1: KR10-1925189B1 (2018.12.04). Summary of the Invention

[0010] Technical issues

[0011] One object of the present invention is to provide an oxide-diluted gadolinium oxide combustible absorber for controlling excess reactivity in an innovative small modular reactor (i-SMR) and a method thereof for designing the absorber.

[0012] Another object of the present invention is to provide a high-intensity dispersed gadolinium oxide combustible absorber (HIGA) and its design method for controlling excess reactivity in innovative small modular reactors (i-SMR).

[0013] Another object of the present invention is to provide a nuclear fuel assembly that integrates a high-intensity dispersed gadolinium oxide-oxide combustible absorber (HIGA) and an integrated gadolinium oxide combustible absorber (IGD) for controlling excess reactivity in innovative small modular reactors (i-SMR).

[0014] The problems to be solved by the present invention are not limited to those mentioned above, and other problems not mentioned can also be clearly understood by those skilled in the art from the following description.

[0015] Problem-solving methods

[0016] According to one aspect of the proposed invention, an oxide-diluted gadolinium oxide combustible absorber for controlling excess reactivity in an innovative small modular reactor (i-SMR) comprises: a cylindrical combustible pellet formed by sintering gadolinium oxide (Gd2O3) and a mixture of oxides including at least one of Al, Si, Ti, and Zr; a cylindrical oxide pellet formed by sintering the oxide contained in the combustible pellet and having the same outer diameter as the combustible pellet; and a cladding layer in which a plurality of combustible pellets and oxide pellets are stacked and mounted perpendicularly relative to the central axis of the cylinder.

[0017] The inner diameter of the coating layer is larger than the outer diameter of the combustible or oxide pellet, thereby creating a gap between the combustible or oxide pellet and the coating layer. The oxide pellet is installed above the combustible pellet.

[0018] According to other aspects, the gadolinium oxide content in the combustible pellet is 10 mol% to 20 mol% of the total combustible pellet.

[0019] According to other aspects, the oxide of the combustible cartridge is Al2O3.

[0020] According to another aspect of the proposed invention, a nuclear fuel assembly comprising an oxide-diluted gadolinium oxide combustor for controlling excess reactivity in an innovative small modular reactor (i-SMR) includes: fuel rods containing nuclear fuel; cooling water channels for circulating cooling water that carries away heat generated by nuclear fission; and a high-intensity dispersed gadolinium oxide-oxide combustor (HIGA).

[0021] The high-strength dispersed gadolinium oxide-oxide combustible absorber (HIGA) comprises: a cylindrical combustible core block formed by sintering a mixture of gadolinium oxide (Gd2O3) and oxides including at least one of Al, Si, Ti, and Zr; a cylindrical oxide core block formed by sintering the oxide contained in the combustible core block and having the same outer diameter as the combustible core block; and a coating layer in which a plurality of combustible core blocks and oxide core blocks are stacked and mounted perpendicularly to the central axis of the cylinder.

[0022] The inner diameter of the coating layer is larger than the outer diameter of the combustible or oxide pellet, thereby creating a gap between the combustible or oxide pellet and the coating layer. The oxide pellet is installed above the combustible pellet.

[0023] According to other aspects, the gadolinium oxide content in the combustible pellet is 10 mol% to 20 mol% of the total combustible pellet.

[0024] According to other aspects, the oxide of the combustible cartridge is Al2O3.

[0025] According to other aspects, the nuclear fuel assembly also includes a first monolithic gadolinium oxide combustor (IGD) in which uranium oxide (UO2) and gadolinium oxide (Gd2O3) are uniformly mixed.

[0026] The uranium content of the first integrated gadolinium oxide combustible absorber (IGD) is less than that of the fuel rod. The gadolinium oxide content of the first integrated gadolinium oxide combustible absorber (IGD) is less than that of the high-strength dispersed gadolinium oxide-oxide combustible absorber (HIGA).

[0027] According to other aspects, the nuclear fuel assembly also includes a second monolithic gadolinium oxide combustor (IGD) in which uranium oxide (UO2) and gadolinium oxide (Gd2O3) are uniformly mixed.

[0028] The uranium content of the second integrated gadolinium oxide combustible absorber (IGD) is less than that of the first integrated gadolinium oxide combustible absorber (IGD). The gadolinium oxide content of the second integrated gadolinium oxide combustible absorber (IGD) is less than that of the high-intensity dispersed gadolinium oxide-oxide combustible absorber (HIGA). The number of the second integrated gadolinium oxide combustible absorbers (IGD) is less than the number of the high-intensity dispersed gadolinium oxide-oxide combustible absorbers (HIGA), but equal to or greater than the number of the first integrated gadolinium oxide combustible absorbers (IGD).

[0029] According to other aspects, the nuclear fuel assembly also includes a second monolithic gadolinium oxide combustor (IGD) in which uranium oxide (UO2) and gadolinium oxide (Gd2O3) are uniformly mixed.

[0030] The second integrated gadolinium oxide combustible absorber (IGD) has a greater uranium content than the first integrated gadolinium oxide combustible absorber (IGD). The second integrated gadolinium oxide combustible absorber (IGD) has a less gadolinium oxide content than the high-intensity dispersed gadolinium oxide-oxide combustible absorber (HIGA). The number of the second integrated gadolinium oxide combustible absorbers (IGD) is less than the number of the high-intensity dispersed gadolinium oxide-oxide combustible absorbers (HIGA), and equal to or less than the number of the first integrated gadolinium oxide combustible absorbers (IGD).

[0031] According to other aspects, the nuclear fuel assembly also includes a third type of gadolinium oxide combustor (IGD) in which uranium oxide (UO2) and gadolinium oxide (Gd2O3) are uniformly mixed.

[0032] The uranium content of the third integrated gadolinium oxide combustible absorber (IGD) is greater than that of the first integrated gadolinium oxide combustible absorber (IGD). The gadolinium oxide content of the third integrated gadolinium oxide combustible absorber (IGD) is less than that of the high-intensity dispersed gadolinium oxide-oxide combustible absorber (HIGA). The number of the third integrated gadolinium oxide combustible absorbers (IGD) is less than the number of the high-intensity dispersed gadolinium oxide-oxide combustible absorbers (HIGA), but equal to the number of the second integrated gadolinium oxide combustible absorbers (IGD).

[0033] According to another aspect of the proposed invention, an innovative small modular reactor (i-SMR) comprising an oxide-diluted gadolinium oxide combustible absorber for controlling excess reactivity includes: a core composed of nuclear fuel assemblies; a stainless steel reflector cylindrically surrounding the exterior of the core composed of the nuclear fuel assemblies; and a reactor vessel having an inner diameter larger than the outer diameter of the reflector, such that a downward flow path for cooling water is formed outside the reflector.

[0034] Invention Effects

[0035] The high-strength dispersed gadolinium oxide-oxide combustible absorber (HIGA) according to the present invention can effectively control excess reactivity in innovative small modular reactors (i-SMR).

[0036] Furthermore, according to the present invention, by using a nuclear fuel assembly that integrates a high-strength dispersed gadolinium oxide combustible absorber (HIGA) and an integrated gadolinium oxide combustible absorber (IGD), excess reactivity in innovative small modular reactors (i-SMR) can be controlled more effectively. Attached Figure Description

[0037] Figure 1a and Figure 1b This is a conceptual diagram illustrating a centrally shielded combustible absorber (CSBA) and a combustible absorber (CIMBA) structure that is inserted in a cylindrical shape and mechanically separated, according to the prior art.

[0038] Figure 2 It is a phase diagram of the compound of gadolinium oxide (Gd2O3) and aluminum oxide (Al2O3).

[0039] Figures 3a to 3d This is a conceptual diagram of the structure of a high-strength dispersed gadolinium oxide-oxide combustible absorber (HIGA) according to one embodiment.

[0040] Figure 4This is a schematic diagram illustrating an example of arranging an integral gadolinium oxide combustor (IGD) and a high-intensity dispersed gadolinium oxide-oxide combustor (HIGA) in a nuclear fuel assembly.

[0041] Figure 5 It is a graph showing the reactivity of nuclear fuel assembly burnup as a function of the arrangement of the combustible absorber (BA).

[0042] Figure 6 This is a graph showing the gadolinium (Gd) density as a function of the burnup rate of nuclear fuel assemblies, based on the arrangement of the combustible absorber (BA).

[0043] Figure 7 This is a conceptual diagram illustrating the arrangement of a nuclear fuel assembly according to one embodiment, with varying numbers of high-intensity dispersed gadolinium oxide combustible absorber (HIGA) rods.

[0044] Figure 8 This is a graph showing the reactivity of nuclear fuel assembly burnup based on the number of high-intensity dispersed gadolinium oxide combustible absorber (HIGA) rods according to one embodiment.

[0045] Figure 9 This is a graph showing the gadolinium (Gd) density based on the nuclear fuel assembly burnup as a function of the number of high-intensity dispersed gadolinium oxide combustible absorber (HIGA) rods, according to one embodiment.

[0046] Figure 10a and Figure 10b This is a conceptual diagram showing the outline structure and control rod pattern of an innovative small modular reactor (i-SMR).

[0047] Figure 11 This is a conceptual diagram illustrating the arrangement of nuclear fuel assemblies used in the initial startup cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0048] Figure 12 This is a graph showing the reactivity of the nuclear fuel assemblies used in the initial cycle of startup of an innovative small modular reactor (i-SMR) according to one embodiment.

[0049] Figure 13a and Figure 13b This is a schematic diagram illustrating the loading pattern and vertical axis structure of the nuclear fuel assemblies used in the initial startup cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0050] Figure 14This is a graph showing the excess reactivity under ARO conditions relative to the effective full power days (EFPD) of an innovative small modular reactor (i-SMR) during the initial power phase, according to one embodiment.

[0051] Figure 15a and Figure 15b This is a graph showing the peak factor and critical control rod position relative to the effective full power days (EFPD) of an innovative small modular reactor (i-SMR) in the initial power phase, according to one embodiment.

[0052] Figure 16a and Figure 16b This is a graph showing the power distribution by nuclear fuel assembly location and by height at different time points during the power initial stage of an innovative small modular reactor (i-SMR) according to one embodiment.

[0053] Figure 17 This is a schematic diagram illustrating the arrangement of nuclear fuel assemblies used in an innovative small modular reactor (i-SMR) according to one embodiment during a steady-state period (equilibrium state period).

[0054] Figure 18 It is a graph showing the reactivity of the nuclear fuel assemblies used in the equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0055] Figure 19a and Figure 19b This is a schematic diagram illustrating the loading mode and vertical axis structure of the nuclear fuel assemblies used in the equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0056] Figure 20 This is a graph showing the excess reactivity of the effective full-power days (EFPD) of the equilibrium state cycle relative to an innovative small modular reactor (i-SMR) under ARO conditions according to one embodiment.

[0057] Figure 21a and Figure 21b This is a graph showing the peak factor and critical control rod position relative to the effective full power days (EFPD) of the equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0058] Figure 22a and Figure 22bThis is a graph showing the power distribution by stage, by nuclear fuel assembly location, and by height of the equilibrium state cycle of an innovative small modular reactor (i-SMR) according to one embodiment. Detailed Implementation

[0059] The above and additional situations are embodied by referring to the embodiments described in the accompanying drawings. It should be understood that, provided there are no contradictions with other descriptions, the constituent elements of each embodiment can be combined in various ways in the embodiments or with the constituent elements of other embodiments. Based on the principle that inventors can appropriately define terms and concepts to best interpret their own inventions, the terms used in this specification and its claims should be interpreted as conforming to the meaning and concept of the described content or the proposed technical idea. In this specification, modules or parts can be implemented using a set of program instructions stored in memory, or electronic components or circuit sets such as ASICs and FPGAs, so that these instructions can be executed in a computer or processor. Furthermore, the operation of each module or part can be performed by one or more processors or devices. Constituent elements with the same / similar reference numerals perform the same / similar functions and their description may be omitted. For constituent elements with omitted descriptions in the accompanying drawings, reference can be made to the preceding description of constituent elements with the same or similar reference numerals.

[0060] Table 1 shows the key design features of an innovative small modular reactor (i-SMR) according to one embodiment.

[0061] [Table 1]

[0062]

[0063] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0064] Figure 1a and Figure 1b This is a schematic diagram illustrating the structure of a centrally shielded combustible absorber (CSBA) and a combustible absorber (CIMBA) that is inserted in a cylindrical shape and mechanically separated, according to the prior art.

[0065] Figure 1a This is a conceptual diagram illustrating the structure of a centrally-shielded burnable absorber (CSBA). Figure 1b This is a schematic diagram showing the structure of a cylindrically inserted and mechanically separated burnable absorber (CIMBA).

[0066] Reference Figure 1a The Central Shielded Burnable Absorber (CSBA) integrates spherical gadolinium oxide (Gd₂O₃) into uranium dioxide (UO₂) pellets, utilizing the spatial self-shielding effect to delay the consumption of the burnable absorber (BA). Through this structure, the CSBA can maintain a reactive oscillation of less than 1000 pcm throughout the cycle, ensuring uniform distribution of all fuel pellets and minimizing core power distortion.

[0067] Reference Figure 1b The cylindrically inserted and mechanically separated combustible absorber (CIMBA) consists of cylindrical gadolinium oxide inserted into an annular fuel pellet, utilizing a spatial self-shielding effect. To control the increase in reactivity with gadolinium oxide consumption, CIMBA can utilize a multilayer structure (ML-CIMBA) or a mixed loading strategy of two different types of CIMBA (MIX-CIMBA), thereby further enhancing the self-shielding effect with burnup. By using enriched gadolinium isotopes, particularly 155 and 157, known for their high neutron absorption capacity, the combustible absorber (BA) can be consumed efficiently and progressively.

[0068] However, practical manufacturing challenges associated with CSBA and CIMBA, such as manufacturing toroidal pellets with a central void and inserting thin gadolinium oxide pins, interfere with manufacturability and pose significant economic barriers due to the lack of established gadolinium (Gd) enrichment facilities and their associated high costs. Therefore, to enable borate-free (SBF) operation in innovative small modular reactors (i-SMRs), it is necessary to develop novel burnout absorbers (BAs) with improved manufacturability. The novel burnout absorbers (BAs) should meet the design requirements of innovative small modular reactors (i-SMRs) while also satisfying basic nuclear design evaluations.

[0069] Among existing burnable absorbers (BAs), the integrated gadolinia burnable absorber (IGD) comprises 2 to 8 wt% gadolinium oxide (Gd₂O₃) homogeneously mixed with uranium dioxide (UO₂). This integrated gadolinia burnable absorber (IGD) can effectively manage excess reactivity over a period of 10 to 15 GWD / MTU. However, this IGD burnable absorber cannot meet the 24-month operating cycle and borate-free (SBF) operation requirements of innovative small modular reactors (i-SMRs). For borate-free (SBF) operation, it is crucial to maintain near-critical levels of excess reactivity over a 24-month cycle, which equates to approximately 20 MWD / MTU cycle burnup or 40 MWD / MTU core burnup. A novel burnable absorber (BA) is needed to address these issues and improve manufacturability.

[0070] Traditionally, boron (B-10) and gadolinium oxide have been preferred materials for burnup absorbers (BAs) due to their strong neutron absorption properties. Boron is favored for its stable burnup characteristics, but the internal pressure of the rods may increase due to the consumption of helium and lithium. Therefore, gadolinium (Gd) is preferred. However, when gadolinium oxide (Gd₂O₃), as a rare earth oxide, is used as a standalone material in the reactor, there are two drawbacks.

[0071] The first drawback is that the irreversible phase transition from a cubic (C-type) structure to a monoclinic (B-type) structure complicates the maintenance of structural stability within the reactor environment. Gadolinium oxide (Gd₂O₃) exists in three main polymorphic forms: hexagonal (A-type), monoclinic (B-type), and cubic (C-type). The C-type is stable at low temperatures but transforms into the B-type at high temperatures. Gadolinium oxide (Gd₂O₃) is heated to approximately 1250°C during the manufacture of combustible absorber pellets, transforming into the B-type. Even after cooling, the B-type Gdolinium oxide (Gd₂O₃) will not revert to the C-type. However, due to the thermal energy absorbed by the reactor, neutron interactions, and under specific conditions such as internal defects, the B-type structure can transform back into the C-type. If the B-type monoclinic structure (density 7.618 g / cm³) is present... 3 It transforms into a C-shaped cubic structure (density 8.352 g / cm³). 3 If this occurs, the density will increase by approximately 10%. This can lead to significant compressive stress, thereby compromising the structural integrity of the core and causing degradation and pulverization.

[0072] The second drawback is that when using a pellet composed entirely of gadolinium oxide (Gd₂O₃) in a high-temperature neutron environment, the material's high basicity causes rapid water absorption. Generally, oxygen gaps in the Gd₂O₃ matrix are filled with hydroxyl groups, increasing weight and volume. As a result, when the metal rods surrounding the gadolinium oxide (Gd₂O₃) and isolating it from the external high-temperature, high-pressure cooling water are damaged, the Gd₂O₃ is hydrated to Gd(OH)₃, thus losing its effective shape for controlling reactivity. This transformation weakens the effectiveness of gadolinium oxide (Gd₂O₃) in reactor reactivity management. Therefore, to ensure the stable use of gadolinium oxide (Gd₂O₃) in the reactor, it is necessary to combine and modify Gd₂O₃ with other metal oxides to mitigate the irreversible deformation and hydration of rare earth oxides.

[0073] Figure 2 It is a phase diagram of the compounds of gadolinium oxide (Gd2O3) and aluminum oxide (Al2O3).

[0074] When Al2O3 is added to gadolinium oxide (Gd2O3) at a ratio exceeding 50 mol%, such as Figure 2 As shown, a GdAlO3 phase with a perovskite structure is formed. This compound structure does not exhibit the rapid water absorption characteristics of rare earth oxides. Furthermore, due to the lack of a monoclinic crystal structure as an unstable high-temperature phase, it cannot suddenly shift to a cubic form even in a neutron-rich reactor environment. Therefore, to optimally control excess reactivity and mitigate the problems caused by Gd2O3 while maintaining an appropriate gadolinium oxide content, a combustible absorber (BA) can be manufactured by mixing an appropriate proportion of Al2O3.

[0075] Figures 3a to 3d This is a conceptual diagram of the structure of a high-strength dispersed gadolinium oxide-oxide combustible absorber (HIGA) according to one embodiment.

[0076] Figure 3a This is a frontal view of the exterior of the oxide-diluted gadolinium oxide combustible absorber used to control excess reactivity in innovative small modular reactors (i-SMR). Figure 3b It is Figure 3a The diagram shows a cross-sectional view of the combustible absorber cut longitudinally (AA) and viewed from the side. Figure 3c It is Figure 3a The diagram shows a cross-sectional view of the combustible absorber cut horizontally (BB) and viewed from above. Figure 3d It's enlarged. Figure 3b The enlarged view of section C in the cross-sectional diagram of the combustible absorber shown.

[0077] Figure 3aIn this design, the oxide-diluted gadolinium oxide combustor, used to control excess reactivity in the innovative small modular reactor (i-SMR), is surrounded by a cladding layer 370. The oxide-diluted gadolinium oxide combustor has a similar elongated columnar shape to existing fuel rods.

[0078] Reference Figure 3b The combustible absorber includes combustible pellets 360, oxide pellets 350, and a covering layer 370. The combustible absorber is enclosed by an upper cover 320 and a lower cover 380. Inside the covering layer 370, above the lower cover 380, multiple combustible pellets 360 and one or more oxide pellets 350 are stacked perpendicularly to the cylindrical central axis. A partition 340, a spring 330, and the upper cover 320 are located above the oxide pellets 350.

[0079] Reference Figure 3c In the combustible absorber, the cylindrical combustible core block 360, which is concentric with the coating layer, is located inside the outer coating layer 370.

[0080] Reference Figure 3d The inner diameter of the cladding layer 370 is larger than the outer diameter of the combustible pellet 360 and the oxide pellet 350. Therefore, an air-filled gap 365 is formed between the combustible pellet 360 or the oxide pellet 350 and the cladding layer 370.

[0081] The combustible cartridge 360 ​​has a uniformly distributed combustible material composed of gadolinium oxide and oxides. The combustible cartridge 360 ​​can be manufactured by mixing and sintering gadolinium oxide (Gd₂O₃) and oxides. The oxides preferably contain any one of Al, Si, Ti, and Zr. That is, oxides such as Al₂O₃, SiO₂, TiO₂, ZrO₂, and Zr₂O₃ can be used, which are sintered with gadolinium oxide to form a water-insoluble compound. (Refer to...) Figure 3c An example of using Al2O3 as the oxide for combustible cartridge 360 ​​is shown.

[0082] Oxide pellet 350 is mounted above combustible pellet 360. Oxide pellet 350 is configured to prevent axial power peaking at the top during control rod removal. Oxide pellet 350 is formed in the same cylindrical shape as combustible pellet 360 for ease of assembly. Oxide pellet 350 is preferably manufactured by sintering the oxide used in the combustible pellet. (Refer to...) Figure 3b An example of oxide core 350 made of Al2O3 is shown.

[0083] like Figure 3cAs shown, when the gadolinium oxide (Gd₂O₃) content of the combustible pellet 360 is 10 to 20 mol% of the total combustible pellet, the oxide-diluted gadolinium oxide combustible absorber can be used as a high-intensity dispersed gadolinium oxide-oxide combustible absorber (HIGA). A greater amount of gadolinium oxide can be integrated into the high-intensity dispersed gadolinium oxide-oxide combustible absorber (HIGA) to improve the self-shielding effect.

[0084] Highly Intensive and Discrete Gadolinia / Alumina Burnable Absorber (HIGA) – an oxide-diluted gadolinia burnable absorber with increased gadolinium oxide content within appropriate ranges – can optimize excess reactivity control in innovative small modular reactors (i-SMRs) and ensure high production efficiency.

[0085] According to another aspect of the proposed invention, a nuclear fuel assembly comprising an oxide-diluted gadolinium oxide combustor for controlling excess reactivity in an innovative small modular reactor (i-SMR) includes: fuel rods containing nuclear fuel; cooling water channels for moving cooling water that dissipates heat generated by nuclear fission; and a high-intensity dispersed gadolinium oxide-oxide combustor (HIGA).

[0086] The high-strength dispersed gadolinium oxide-oxide combustible absorber (HIGA) comprises: a cylindrical combustible core block formed by sintering a mixture of gadolinium oxide (Gd2O3) and oxides including at least one of Al, Si, Ti, and Zr; a cylindrical oxide core block formed by sintering the oxide contained in the combustible core block and having the same outer diameter as the combustible core block; and a coating layer in which a plurality of combustible core blocks and oxide core blocks are stacked and mounted perpendicularly to the central axis of the cylinder.

[0087] The inner diameter of the coating layer is larger than the outer diameter of the combustible or oxide pellet, thereby creating a gap between the combustible or oxide pellet and the coating layer. The oxide pellet is installed above the combustible pellet.

[0088] According to other aspects, the gadolinium oxide content in the combustible pellet is 10 to 20 mol in the total combustible pellet.

[0089] According to other aspects, the oxide of the combustible cartridge is Al2O3.

[0090] The performance of nuclear fuel assemblies containing high-strength dispersed gadolinium oxide combustible absorbers (HIGA) was compared and evaluated with those without combustible absorbers and those with integrated gadolinium oxide combustible absorbers (IGD).

[0091] Figure 4 This is a schematic diagram illustrating an example of arranging an integral gadolinium oxide combustor (IGD) and a high-intensity dispersed gadolinium oxide-oxide combustor (HIGA) in a nuclear fuel assembly.

[0092] Figure 4 The standard 2D shape of a 17×17 SMR nuclear fuel assembly is shown. Light gray circles represent fuel rods, and dark gray circles represent cooling water channels (water holes) that move the heat generated by nuclear fission.

[0093] Figure 4 The first arrangement (NOBA) indicates the absence of a flammable absorber (BA). Figure 4 The second arrangement (20IGD) represents the arrangement of 20 (=4×5) monolithic gadolinium oxide (Gd2O3) combustible absorbers (IGD8w / o) with a concentration of 8%. Figure 4 The third arrangement (20HIGA) indicates the arrangement of 20 (=4×5) high-strength dispersed gadolinium oxide-oxide combustible absorbers (HIGA) with a gadolinium oxide (Gd2O3) concentration of 20%.

[0094] Neutron analysis (including static and burnup calculations) is performed using the two-dimensional transport lattice code KARMA.

[0095] Figure 5 It is a graph showing the reactivity of nuclear fuel assembly burnup as a function of the arrangement of the combustible absorber (BA).

[0096] No markings indicate that no combustible absorbent material was used. Figure 4 The first layout (NOBA) graph, with the triangle markings indicating the use of integrated gadolinium oxide combustible absorber (IGD). Figure 4 The second configuration (20IGD) is plotted, with the × line indicating the use of High Strength Dispersed Gadolinium Oxide Combustible Absorber (HIGA). Figure 4 The graph of the third arrangement (20HIGA).

[0097] The IDG assembly (triangle) with 8 wt% combustible absorber (BA) rods exhibits lower reactivity compared to the assembly including HIGA rods (×). However, the reactivity retention period of the IGD assembly is limited to less than 20 GWD / MTU due to rapid gadolinium oxide consumption, followed by a gradual decrease in reactivity after gadolinium oxide burnup. In contrast, the HIGA assembly exhibits an extended reactivity profile that can maintain up to 40 GWD / MTU due to its enhanced steric self-shielding effect.

[0098] Figure 6 This is a graph showing the gadolinium (Gd) density as a function of the burnup rate of nuclear fuel assemblies, based on the arrangement of the combustible absorber (BA).

[0099] Figure 6 This indicates the gadolinium oxide consumption pattern between the IGD and HIGA assemblies. The × line indicates the use of an integrated gadolinium oxide combustible absorber (IGD). Figure 4 The second configuration (20IGD) curve, * indicates the use of high-strength dispersed gadolinium oxide-oxide combustible absorber (HIGA). Figure 4 The graph of the third arrangement (20HIGA).

[0100] The HIGA assembly exhibits a longer and more progressive gadolinium oxide consumption than the IGD assembly.

[0101] On the other hand, the gradual and consistent decrease in excess reactivity from the start of the HIGA component suggests that integrating both IGD and HIGA rods into the fuel assembly simultaneously is more effective in controlling excess reactivity.

[0102] Figure 7 This is a schematic diagram illustrating the arrangement of a nuclear fuel assembly according to one embodiment, with varying numbers of high-intensity dispersed gadolinium oxide combustible absorber (HIGA) rods.

[0103] By exploring changes in reactivity using different amounts of HIGA rods (20% Gd2O3), fundamental data for core design can be collected, thereby effectively managing long-term overreactivity.

[0104] Figure 7 The HIGA rods are represented by double circles. Four, eight, twelve, sixteen, and twenty HIGA rods can be placed in the nuclear fuel assembly in sequence.

[0105] Figure 8 This is a graph showing the reactivity of nuclear fuel assembly burnup based on the number of high-intensity dispersed gadolinium oxide combustible absorber (HIGA) rods according to one embodiment.

[0106] Reference Figure 8It can be seen that the reactive retention period is extended to approximately 40 GWD / MTU. Furthermore, analysis of reactive behavior related to the number of HIGA rods confirms that the efficacy of reactive blocking increases with the number of HIGA rods.

[0107] Figure 9 This is a graph showing the gadolinium (Gd) density based on the nuclear fuel assembly burnup as a function of the number of high-intensity dispersed gadolinium oxide combustible absorber (HIGA) rods, according to one embodiment.

[0108] Reference Figure 9 Due to the significant self-shielding effect of each HIGA rod, gadolinium oxide is slowly consumed up to 35 GWD / MTU regardless of the number of HIGA rods. If this point is exceeded, rapid burn-out of up to about 40 GWD / MTU occurs, resulting in the near-complete depletion of gadolinium oxide (Gd2O3).

[0109] Figure 10a and Figure 10b This is a conceptual diagram showing the outline structure and control rod pattern of an innovative small modular reactor (i-SMR).

[0110] Figure 10a and Figure 10b This diagram illustrates the core design and control rod configuration of an innovative small modular reactor (i-SMR). To enhance the stability of the i-SMR, an integrated control instrumentation (ICI) system is integrated at the top to prevent seepage through the reactor vessel bottom. This system consists of 16 strategically deployed ICIs to monitor the core status. However, with an ICI system at the top, the number of available control rods is reduced to 53, potentially compromising subcriticality under ARI (All Rod-In) and N-1 (Stuck Rod-Out) conditions.

[0111] The control rod consists of four regulating bank groups (R1, R2, R3, R4) optimized based on factors such as excess reactivity, power distribution, and subcritical considerations, and one shutdown bank group (SB). Using Ag-In-Cd in the control bank adjustments minimizes the impact of control rod burnup, while the shutdown bank utilizes B4C at a concentration of 95% B-10 to increase shutdown margin and ensure subcriticality.

[0112] The integration of highly intensive and discrete gadolinia / oxide burnable absorbers (HIGAs) requires careful uranium loading evaluation. This is because including HIGA rods proportionally reduces the number of fuel rods, and the average nominal power density of the fuel may increase. To mitigate this, the number of HIGA rods can be limited to 16 to balance the 24-month cycle target and compliance with peak power limits. With this configuration, the nominal linear power density of the fuel rods is set at 3.86 kW / ft, which is 30% lower than existing PWRs such as the OPR1000 and APR1400, which have a linear power density of approximately 5.5 kW / ft.

[0113] Figure 11 This is a schematic diagram showing the arrangement of nuclear fuel assemblies used in the initial startup cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0114] According to other aspects, the nuclear fuel assembly also includes a first monolithic gadolinium oxide combustor (IGD) in which uranium oxide (UO2) and gadolinium oxide (Gd2O3) are uniformly mixed.

[0115] The uranium content of the first integrated gadolinia burnable absorber (IGD) is less than that of the fuel rod. The gadolinium oxide content of the first integrated gadolinia burnable absorber (IGD) is less than that of the high-strength dispersed gadolinium oxide-oxide burnable absorber (HIGA).

[0116] According to other aspects, the nuclear fuel assembly also includes a second monolithic gadolinium oxide combustor (IGD) in which uranium oxide (UO2) and gadolinium oxide (Gd2O3) are uniformly mixed.

[0117] The uranium content of the second integrated gadolinium oxide combustible absorber (IGD) is less than that of the first integrated gadolinium oxide combustible absorber (IGD). The gadolinium oxide content of the second integrated gadolinium oxide combustible absorber (IGD) is less than that of the high-intensity dispersed gadolinium oxide-oxide combustible absorber (HIGA). The number of the second integrated gadolinium oxide combustible absorbers (IGD) is less than the number of the high-intensity dispersed gadolinium oxide-oxide combustible absorbers (HIGA), but equal to or greater than the number of the first integrated gadolinium oxide combustible absorbers (IGD).

[0118] In the innovative small modular reactor (i-SMR), two loading modes were used to facilitate a 24-month operating cycle. The initial cycle nuclear design included multiple fuel assembly structures with various loading modes for HIGA and IGD rods that conform to the characteristics of the burnt-out absorber (BA).

[0119] Given that the first cycle only loads new fuel, an assembly with a flat reactivity profile is needed to manage excess reactivity in the core.

[0120] As a result, eight types of fuel assemblies with flat reactivity profiles were selected based on the number of IGD rods (4 to 12 rods, containing 1 wt% to 8 wt% Gd2O3) and 16 HIGA rods (with Gd2O3 content ranging from 10 wt% to 18 wt%). The specific structures of the selected fuel assemblies are as follows: Figure 11 As shown in Table 2. In the figures, the IGD numbers represent the gadolinium oxide (Gd2O3) content. For example, IGD 4w / o indicates an IGD rod containing 4wt% gadolinium oxide.

[0121] Table 2 shows the specific structure of the nuclear fuel assemblies used in the initial start-up cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0122] [Table 2]

[0123]

[0124] The enrichment of the standard fuel rods is standardized to 4 wt% across all assembly types. In each assembly, especially when the control rod assembly is primarily extracted, an upper cut-back region of approximately 10 cm is provided to control the axial power distribution shifting upwards towards the top at the end of the cycle.

[0125] Figure 11 The design shown, including the HIGA nuclear fuel assembly pattern, represents the design of nuclear fuel assemblies for the initial core (one cycle) design of a borate-free reactor. Since borate is absent in a borate-free core, the design can be achieved by controlling the excess reactivity of each assembly. Eight types of assemblies are used for core design in one cycle, each type contributing to a gradual increase in excess reactivity at its designated loading position within the core loading model throughout the cycle.

[0126] To control excessive reactivity, HIGA and IGD combustible absorbent rods, such as... Figure 11The arrangement is as shown. Combustible absorber rods are typically placed near the conduit containing a large amount of moderator (water) to adequately absorb neutrons. The aim is to control excess reactivity by utilizing IGD in the early stages of the cycle and the self-shielding effect of the combustible absorber material of HIGA in the middle and later stages of the cycle. Furthermore, the use of HIGA is set to 16 rods to minimize the impact on line power density (kW / ft), and the rods are selected as shown... Figure 11 The arrangement shown is used to stabilize the radial power distribution within the component.

[0127] Furthermore, for IGD 2w / o, placing it in the area with the highest concentration of moderators allows for fine-tuning of excess reactivity at the beginning of the cycle. This reduces the frequency of control rod usage during core operation at the start of the cycle. For IGD 8w / o, placing it near the moderators on the periphery of the assembly allows for design with... Figure 12 The corresponding excess reactivity curve.

[0128] Figure 12 This is a graph showing the reactivity of the nuclear fuel assemblies used in the initial cycle of startup of an innovative small modular reactor (i-SMR) according to one embodiment.

[0129] Integrating HIGA and IGD into nuclear fuel assemblies, such as Figure 12 As shown, combining the slow consumption effect of HIGA with the effective reactivity control of IGD can achieve good reactivity smoothing.

[0130] Figure 13a and Figure 13b This is a conceptual diagram illustrating the loading pattern and vertical axis structure of the nuclear fuel assemblies used in the initial startup cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0131] According to other aspects of the proposed invention, an innovative small modular reactor (i-SMR) including an oxide-diluted gadolinium oxide combustible absorber for controlling excess reactivity comprises: a core composed of nuclear fuel assemblies; a stainless steel reflector cylindrically surrounding the exterior of the core composed of the nuclear fuel assemblies; and a reactor vessel having an inner diameter larger than the outer diameter of the reflector, such that a downward flow path for cooling water is formed outside the reflector.

[0132] Fuel loading mode such as Figure 13a and Figure 13b As shown, in order to optimize the radiative power distribution, components with higher responsiveness are strategically deployed in the surrounding area.

[0133] Figure 14This is a graph showing the excess reactivity of the effective full-power days (EFPD) of the initial startup cycle relative to an innovative small modular reactor (i-SMR) under ARO conditions according to one embodiment.

[0134] Reactor core calculations were performed using the KNF KARMA / ASTRA code package. Based on the existing two-stage procedure, two sets of fragments were generated using 2D group transport calculations with KARMA, and 3D node calculations with 3D thermal / pressure feedback were performed using ASTRA.

[0135] In this boric acid-free (SBF) operation, unlike typical PWR designs where criticality is determined by boron concentration, the control rod position is adjusted to achieve criticality. Excess responsiveness under ARO (All Rod-Out) conditions, expressed in terms of Effective Full Power Days (EFPD), is as follows: Figure 14 As shown.

[0136] The expected cycle length is approximately 840 EFPD, with peak reactivity reaching approximately 1200 pcm, and a difference of 800 pcm between minimum and maximum reactivity. This indicates that BA HIGA and IGD effectively modulate excessive reactivity.

[0137] Figure 15a and Figure 15b This is a graph showing the peak factor and critical control rod position corresponding to the effective full power days (EFPD) during the initial operating cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0138] Figure 15a and Figure 15b The highest point factor and critical position of the control bar group are shown, with the control library repeated 50%. Considering a 20% margin in the control bar position, the calculated cycle length exceeds 780 EFPD corresponding to 20711 MWD / MT, thus satisfying the required cycle period of 730 days.

[0139] Figure 16a and Figure 16b This is a schematic diagram illustrating the power distribution of nuclear fuel assemblies by location and by height at different operating moments during the initial operating cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0140] The maximum radial pin pickup factor (Fr) was recorded as 1.351. As can be seen from Figure 16a, the radial power distribution between components is quite uniform, and the maximum three-dimensional peaking factor (Fq) was observed to be an appropriate 2.080. Figure 16b An axial one-dimensional peak factor (Fz) of approximately 1.5 was observed.

[0141] With a nominal linear power density of 3.86 kW / ft, the radiating integrated 2D linear power density is 5.215 kW / ft, and the peak 3D linear power density is 8.029 kW / ft. Both are below the typical targets for commercial PWRs (7.5 kW / ft and 12 kW / ft).

[0142] Table 3 shows the reactivity coefficients (HFP, HZP) of an innovative small modular reactor (i-SMR) according to one embodiment at different stages of the initial operating cycle.

[0143] [Table 3]

[0144]

[0145] Table 3 details the fuel temperature coefficient (FTC), MTC, and subcriticality at different burnout stages. The FTC remains in the range of -4 to -3 pcm / K, similar to the existing PWR values, while the MTC remains below -65 pcm / K (i.e., negative) due to the absence of water-soluble boron.

[0146] Table 4 shows the staged subcriticality (CZP) of the initial startup cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0147] [Table 4]

[0148]

[0149] Table 4 briefly describes the subcriticality under CZP (cold zero power) conditions. The maximum subcritical k-eff under the ARO scenario is 0.93191 at cycle start (BOC), without considering uncertainties. Conservatively including uncertainties such as 10% of the total rod value and 500 pcm of reactivity, the adjusted subcriticality is 0.94852, which is below the subcritical standard of 0.95. Similarly, under N-1 conditions, the maximum subcritical k-eff is 0.95843, and the conservative value of 0.97330 for rod value and reactivity uncertainties is still below the N-1 subcritical standard of 0.99. These subcriticality calculations confirm that the reactor core can be safely shut down during operation.

[0150] Figure 17 This is a conceptual diagram illustrating the arrangement of nuclear fuel assemblies used in the equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0151] According to other aspects, the nuclear fuel assembly also includes a first monolithic gadolinium oxide combustor (IGD) in which uranium oxide (UO2) and gadolinium oxide (Gd2O3) are uniformly mixed.

[0152] The uranium content of the first integrated gadolinium oxide combustible absorber (IGD) is less than that of the fuel rod. The gadolinium oxide content of the first integrated gadolinium oxide combustible absorber (IGD) is less than that of the high-strength dispersed gadolinium oxide-oxide combustible absorber (HIGA).

[0153] According to other aspects, the nuclear fuel assembly also includes a second monolithic gadolinium oxide combustor (IGD) in which uranium oxide (UO2) and gadolinium oxide (Gd2O3) are uniformly mixed.

[0154] The second integrated gadolinium oxide combustible absorber (IGD) has a greater uranium content than the first integrated gadolinium oxide combustible absorber (IGD). The second integrated gadolinium oxide combustible absorber (IGD) has a less gadolinium oxide content than the high-intensity dispersed gadolinium oxide-oxide combustible absorber (HIGA). The number of the second integrated gadolinium oxide combustible absorbers (IGD) is less than the number of the high-intensity dispersed gadolinium oxide-oxide combustible absorbers (HIGA), and equal to or less than the number of the first integrated gadolinium oxide combustible absorbers (IGD).

[0155] According to other aspects, the nuclear fuel assembly also includes a third type of gadolinium oxide combustor (IGD) in which uranium oxide (UO2) and gadolinium oxide (Gd2O3) are uniformly mixed.

[0156] The uranium content of the third integrated gadolinium oxide combustible absorber (IGD) is greater than that of the first integrated gadolinium oxide combustible absorber (IGD). The gadolinium oxide content of the third integrated gadolinium oxide combustible absorber (IGD) is less than that of the high-intensity dispersed gadolinium oxide-oxide combustible absorber (HIGA). The number of the third integrated gadolinium oxide combustible absorbers (IGD) is less than the number of the high-intensity dispersed gadolinium oxide-oxide combustible absorbers (HIGA), but equal to the number of the second integrated gadolinium oxide combustible absorbers (IGD).

[0157] In a balanced cycle, such as Figure 17 As shown, the reactor core is equipped with four types of nuclear fuel assemblies, including 16 HIGA rods with a gadolinium (Gd) content of 9% to 15% and IGD rods with a content of 4% to 8 wt%. Detailed composition is shown in Table 5.

[0158] Table 5 shows the specific structure of the nuclear fuel assemblies used in the equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0159] [Table 5]

[0160]

[0161] Figure 17 The design shown, including the HIGA, is for a reloaded core (after 2 cycles) design in a borate-free reactor. For reloaded cores, the combustible absorber material in the assemblies burned out in the previous cycle has already been consumed, and excess reactivity is out of control. Therefore, the combustible absorber material in the reloaded assemblies can be considered to play a greater role.

[0162] For X01, use up to 20 IGD 8w / o units, such as Figure 18 As shown in curve X01, this is a means of controlling excess reactivity of the new fuel loaded onto the replacement core, which is slowly increased from a sub-critical state (k-inf value below 1) at the beginning of the cycle to the middle of the cycle. For X03, in X02, IGD 3w / o is arranged in the duct-dense region, thus easily controlling excess reactivity at the beginning of the cycle.

[0163] Figure 18 It is a graph showing the reactivity of the nuclear fuel assemblies used in the equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0164] Figure 18 The reactivity curves of the components described in the paper show that fuel assemblies X01 and X02 achieve a consistent excess reactivity curve throughout the consumption process, while X03 and X04 are designed to have gradually increasing curves to offset the decrease in reactivity of the fuel assemblies during a single burnout.

[0165] Figure 19a and Figure 19b This is a conceptual diagram illustrating the loading pattern and vertical axis structure of the nuclear fuel assemblies used in the equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0166] According to another aspect of the proposed invention, an innovative small modular reactor (i-SMR) comprising an oxide-diluted gadolinium oxide combustible absorber for controlling excess reactivity includes: a core composed of nuclear fuel assemblies; a stainless steel reflector cylindrically surrounding the exterior of the core composed of said nuclear fuel assemblies; and a reactor vessel having an inner diameter larger than the outer diameter of said reflector, such that a downward flow path for cooling water is formed outside said reflector.

[0167] Figure 19a and Figure 19bThe core load pattern and shaft structure relative to the balancing cycle are roughly represented. A 10cm scaled-down assembly X01 is positioned at the center to influence the radial power distribution towards the core, thereby improving subcriticality. Another assembly, 20cm scaled down, is positioned around the perimeter to achieve a smoother power distribution.

[0168] Figure 20 This is a graph showing the excess reactivity of the effective full-power days (EFPD) of the equilibrium state cycle relative to an innovative small modular reactor (i-SMR) under ARO conditions according to one embodiment.

[0169] Figure 20 The excess reactivity curve is shown in the ARO state, which represents a reactivity oscillation of approximately 800 pcm and a cycle length of 780 EFPD.

[0170] Figure 21a and Figure 21b This is a graph showing the peak factor and critical control rod position relative to the effective full power days (EFPD) of the equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0171] Peak factor, axial shape index, and critical control bar position relative to the fuel consumption trajectory, such as Figure 21a and Figure 21b As shown, the maximum Fr and Fq reach 1.517 and 2.265 respectively at the end of the loop (EOC).

[0172] Figure 22a and Figure 22b This is a graph showing the power distribution by stage, by nuclear fuel assembly location, and by height of the equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0173] Compared to the initial loop value, Fr is higher because the power distribution of each assembly in the EOC is biased towards the H01 assembly side, but as Figure 22a and Figure 22b As shown, their axial shape is more uniform, therefore their Fq is lower.

[0174] However, the nominal linear power density remains at 3.86 kW / ft, indicating a 2D linear power density of 5.856 kW / ft for radial integration and a maximum 3D linear power density of 8.743 kW / ft. Both of these are below the standard targets of 7.5 kW / ft and 12 kW / ft for commercial PWRs.

[0175] Table 6 shows the stage-wise reactivity coefficients (HFP, HZP) of the equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0176] [Table 6]

[0177]

[0178] The reactivity coefficients at the start of the cycle (BOC), mid-cycle (MOC), and end of the cycle (EOC) are summarized in Table 6. Negative FTC is consistent with the general PWR value, and MTC is less than -60 pcm / ℃, which should be appropriately maintained in a negative state.

[0179] Table 7 shows the staged subcriticality (CZP) of the equilibrium state cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0180] [Table 7]

[0181]

[0182] Table 7 also integrates the conservative uncertainty allowable values ​​relative to the control rod value (10%) and reactivity (500 pcm), verifying the subcriticality of the reactor under ARO and N-1 CZP conditions, thereby confirming the reactor's ability to safely shut down during operation.

[0183] The invention has been described above with reference to the embodiments in the accompanying drawings, but is not limited thereto, and should be construed as including various modifications which will be apparent to those skilled in the art. The claims are intended to cover these modifications.

[0184] Explanation of reference numerals in the attached figures

[0185] 350: Oxide pellet (350); 360: Combustible pellet (360);

[0186] 365: gap (365); 370: covering layer (370).

Claims

1. An oxide-diluted gadolinium oxide burnable absorber for controlling excess reactivity of an innovative small modular reactor (i-SMR), the oxide-diluted gadolinium oxide burnable absorber comprising: a cylindrical burnable pellet formed by sintering a mixture of gadolinium oxide (Gd 2 O 3 ) and an oxide including at least one of Al, Si, Ti, and Zr; a cylindrical oxide pellet formed by sintering the oxide contained in the burnable pellet and having the same outer diameter as the burnable pellet; and a cladding layer having a plurality of the burnable pellet and the oxide pellet vertically stacked and installed inside thereof with respect to a central axis of the cylinder, an inner diameter of the cladding layer being greater than an outer diameter of the burnable pellet and the oxide pellet, such that a gap is formed between the burnable pellet or the oxide pellet and the cladding layer, the oxide pellet being installed above the burnable pellet. 2.The oxide-diluted gadolinium oxide burnable absorber of claim 1, wherein a content of the gadolinium oxide in the burnable pellet is 10 mol% to 20 mol% in the total burnable pellet. 3.The oxide-diluted gadolinium oxide burnable absorber of claim 1, wherein the oxide of the burnable pellet is Al 2 O 3. 4.A nuclear fuel assembly including an oxide-diluted gadolinium oxide burnable absorber for controlling excess reactivity of an innovative small modular reactor (i-SMR), the nuclear fuel assembly comprising: a fuel rod containing a nuclear fuel; a cooling water channel for circulating a flow of cooling water that takes away heat generated by nuclear fission; and a high-intensity dispersed gadolinium oxide-oxide burnable absorber (HIGA), wherein the high-intensity dispersed gadolinium oxide-oxide burnable absorber (HIGA) comprises: a cylindrical burnable pellet formed by sintering a mixture of gadolinium oxide (Gd 2 O 3 ) and an oxide including at least one of Al, Si, Ti, and Zr; a cylindrical oxide pellet formed by sintering the oxide contained in the burnable pellet and having the same outer diameter as the burnable pellet; and a cladding layer having a plurality of the burnable pellet and the oxide pellet vertically stacked and installed inside thereof with respect to a central axis of the cylinder, an inner diameter of the cladding layer being greater than an outer diameter of the burnable pellet and the oxide pellet, such that a gap is formed between the burnable pellet or the oxide pellet and the cladding layer, the oxide pellet being installed above the burnable pellet. 5.The nuclear fuel assembly of claim 4, wherein a content of the gadolinium oxide in the burnable pellet is 10 mol% to 20 mol% in the total burnable pellet. 6.The nuclear fuel assembly of claim 4, wherein the oxide of the burnable pellet is Al 2 O 3. 7.The nuclear fuel assembly of claim 4, further comprising: a first integrated gadolinium oxide burnable absorber (IGD) in which uranium oxide (UO 2 ) and gadolinium oxide (Gd 2 O 3 ) are uniformly mixed, a uranium content of the first integrated gadolinium oxide burnable absorber (IGD) being less than a uranium content of the fuel rod, ​ The first monolithic gadolinia combustible absorber (IGD) has a gadolinia content less than the high intensity dispersed gadolinia-oxide combustible absorber (HIGA).

8. The nuclear fuel assembly of claim 7, further comprising: a second monolithic gadolinia combustible absorber (IGD) having uranium oxide (U02) and gadolinia oxide (Gd203) uniformly mixed, the second monolithic gadolinia combustible absorber (IGD) has a uranium content greater than the first monolithic gadolinia combustible absorber (IGD), the second monolithic gadolinia combustible absorber (IGD) has a gadolinia content less than the high intensity dispersed gadolinia-oxide combustible absorber (HIGA), the second monolithic gadolinia combustible absorber (IGD) is less in number than the high intensity dispersed gadolinia-oxide combustible absorber (HIGA) and equal to or less in number than the first monolithic gadolinia combustible absorber (IGD).

9. The nuclear fuel assembly of claim 7, further comprising: a second monolithic gadolinia combustible absorber (IGD) having uranium oxide (U02) and gadolinia oxide (Gd203) uniformly mixed, the second monolithic gadolinia combustible absorber (IGD) has a uranium content greater than the first monolithic gadolinia combustible absorber (IGD), the second monolithic gadolinia combustible absorber (IGD) has a gadolinia content less than the high intensity dispersed gadolinia-oxide combustible absorber (HIGA), the second monolithic gadolinia combustible absorber (IGD) is less in number than the high intensity dispersed gadolinia-oxide combustible absorber (HIGA) and equal to or less in number than the first monolithic gadolinia combustible absorber (IGD).

10. The nuclear fuel assembly of claim 9, further comprising: a third monolithic gadolinia combustible absorber (IGD) having uranium oxide (U02) and gadolinia oxide (Gd203) uniformly mixed, the third monolithic gadolinia combustible absorber (IGD) has a uranium content greater than the first monolithic gadolinia combustible absorber (IGD), the third monolithic gadolinia combustible absorber (IGD) has a gadolinia content less than the high intensity dispersed gadolinia-oxide combustible absorber (HIGA), the third monolithic gadolinia combustible absorber (IGD) is less in number than the high intensity dispersed gadolinia-oxide combustible absorber (HIGA) and equal in number to the second monolithic gadolinia combustible absorber (IGD).

11. An innovative small modular reactor (i-SMR) comprising an oxide diluted gadolinia combustible absorber for controlling excess reactivity, the innovative small modular reactor (i-SMR) comprising: a core comprised of the nuclear fuel assembly of claim 7 or 8; a reflector of stainless steel material in the shape of a cylinder surrounding the outside of the core comprised of the nuclear fuel assembly; and a containment vessel. A reactor vessel having an inner diameter larger than an outer diameter of the reflector so that a downflow path for cooling water is formed outside the reflector.

12. An innovative small modular reactor (i-SMR) comprising an oxide dilution gadolinium oxide combustible absorber for controlling excess reactivity, the innovative small modular reactor (i-SMR) comprising: a core composed of the nuclear fuel assembly of any one of claims 7, 9, and 10; a reflector of stainless steel material surrounding the outside of the core composed of the nuclear fuel assembly in a cylindrical shape; and a reactor vessel having an inner diameter larger than an outer diameter of the reflector so that a downflow path for cooling water is formed outside the reflector. ​

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