Oxide-diluted gadolinia burnable absorber for controlling surplus reactivity of innovative small modular reactor and design method thereof

Abstract

Disclosed is a technology related to an oxide-diluted gadolinia burnable absorber for controlling surplus reactivity of an innovative small modular reactor (i-SMR). The oxide-diluted gadolinia burnable absorber comprises: cylindrical burnable pellets formed by mixing and sintering gadolinia (Gd2O3) and oxides such as Al2O3; cylindrical oxide pellets formed of oxides such as Al2O3 and mounted above the burnable pellets; and a cladding layer having a plurality of burnable pellets and oxide pellets vertically stacked and mounted therein with respect to a cylindrical central axis. The inner diameter of the cladding layer is larger than the outer diameters of the burnable pellets and the oxide pellets, so that a gap is formed between the burnable pellets or the oxide pellets and the cladding layer. By using a nuclear fuel assembly integrating a high-intensity dispersed gadolinia-oxide burnable absorber (HIGA) having a high gadolinia content and an integrated gadolinia burnable absorber (IGD) having a low gadolinia content, surplus reactivity of an innovative small modular reactor (i-SMR) can be effectively controlled.

Description

Oxide-diluted gadolinia combustible absorber and its design method for controlling excess reactivity in an innovative small modular reactor

[0001] The present invention relates to an oxide-diluted gadolinia burnable absorber for controlling the excess reactivity of an innovative small modular reactor (i-SMR) and a design method thereof, and more particularly, to a technology for efficiently controlling the excess reactivity of an innovative small modular reactor (i-SMR) using a nuclear fuel assembly integrating a high-strength dispersed gadolinia-oxide burnable absorber (HIGA) and an integral gadolinia burnable absorber (IGD).

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

[0003] The innovative small reactor (i-SMR) addresses key design objectives of safety, cost-effectiveness, and adaptability by incorporating essential design features such as soluble boron-free (SBF) operation, improved fuel thermal margin, extended operating cycles, and flexible operation. 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] The transition to boric acid-free (SBF) operation and long-cycle operation presents significant challenges, particularly in replacing the critical role of soluble boron in controlling excess reactivity throughout the fuel cycle. One approach to addressing this issue is to utilize control rods to manage excess reactivity within the core. However, the limited number of control rods in a small modular reactor (SMR) core requires careful use. Over-reliance on control rods without additional measures can limit core design and potentially lead to high excess reactivity, potentially leading to subcritical conditions not being met or excessive positive reactivity in the event of a control rod ejection or withdrawal accident. Therefore, developing alternative methods to control excess reactivity is essential to ensuring the safe and efficient operation of small modular reactors (SMRs).

[0005] Korean Patent No. 10-1925189, published on December 4, 2018, relates to "Nuclear fuel composite sintered body and manufacturing method thereof," and discloses a nuclear fuel composite sintered body capable of providing efficient operation of nuclear fuel by preventing non-uniformity and cracking of a combustible absorber. The molded body of the combustible absorber is inserted into nuclear fuel at 2% by volume or more and less than 20% by volume based on the total volume of the nuclear fuel composite sintered body, does not include nuclear fuel containing UO2, is inserted and positioned in the radial center of the nuclear fuel so as to be separated from the nuclear fuel, and includes at least one selected from the group consisting of an irregular shape, a cylinder, a disk, a sphere, a rod, a film, and a polygonal pillar shape.

[0006] However, further research is needed on combustible absorbers that are simple to manufacture and can be operated efficiently, and nuclear fuel assemblies using them.

[0007] [Prior Art Literature]

[0008] [Patent Document]

[0009] (Patent Document 1) KR 10-1925189 B1 (2018.12.04.)

[0010] One object of the present invention is to provide an oxide-diluted gadolinia combustible absorbent for controlling excess reactivity of an innovative small modular reactor (i-SMR) and a design method thereof.

[0011] Another object of the present invention is to provide a high-strength dispersed gadolinia-oxide burnable absorber (HIGA) and a design method thereof for controlling excess reactivity of an innovative small modular reactor (i-SMR).

[0012] Another object of the present invention is to provide a nuclear fuel assembly integrating a high-strength dispersed gadolinia-oxide burnable absorber (HIGA) and an integral gadolinia burnable absorber (IGD) for controlling excess reactivity of an innovative small modular reactor (i-SMR).

[0013] The problems to be solved by the present invention are not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

[0014] According to one aspect of the proposed invention, an oxide-diluted gadolinia combustible absorbent for controlling excess reactivity of an innovative small modular reactor (i-SMR) comprises: a cylindrical combustible pellet formed by sintering a mixture of gadolinia (Gd2O3) and an oxide containing any one of Al, Si, Ti, and Zr; a cylindrical oxide pellet having the same outer diameter as the combustible pellet formed by sintering an oxide included in the combustible pellet; and a covering layer in which a plurality of combustible pellets and oxide pellets are vertically stacked and mounted based on the central axis of the cylinder.

[0015] The inner diameter of the above-mentioned coating 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 coating layer. The oxide pellet is mounted on top of the combustible pellet.

[0016] According to an additional aspect, the combustible pellet is characterized in that the content of gadolinia is 10 to 20 mol% of the total combustible pellet.

[0017] According to an additional aspect, the oxide of the combustible pellet is characterized as being Al2O3.

[0018] According to another aspect of the proposed invention, a nuclear fuel assembly including an oxide-diluted gadolinia burnable absorber for controlling excess reactivity of an innovative small modular reactor (i-SMR) includes a fuel rod including nuclear fuel, a coolant passage through which coolant for transferring heat generated by nuclear fission moves, and a high-strength dispersed gadolinia-oxide burnable absorber (HIGA).

[0019] The above high-strength dispersed gadolinia-oxide combustible absorbent (HIGA) comprises: a cylindrical combustible pellet formed by sintering a mixture of gadolinia (Gd2O3) and an oxide containing one of Al, Si, Ti, and Zr; a cylindrical oxide pellet having the same outer diameter as the combustible pellet formed by sintering an oxide contained in the combustible pellet; and a covering layer in which a plurality of combustible pellets and oxide pellets are vertically stacked and mounted based on the central axis of the cylinder.

[0020] The inner diameter of the above coating layer is larger than the outer diameter of the combustible pellets and oxide pellets, so that a gap is formed between the combustible pellets or oxide pellets and the coating layer. The oxide pellets are mounted on top of the combustible pellets.

[0021] According to an additional aspect, the combustible pellet is characterized in that the content of gadolinia is 10 to 20 mol% of the total combustible pellet.

[0022] According to an additional aspect, the oxide of the combustible pellet is characterized as being Al2O3.

[0023] In an additional aspect, the nuclear fuel assembly further comprises a first integral gadolinia burnable absorber (IGD) in which uranium oxide (UO2) and gadolinia (Gd2O3) are homogeneously mixed.

[0024] The uranium content of the first integral gadolinia burnable absorber (IGD) is less than the uranium content of the fuel rod. The gadolinia content of the first integral gadolinia burnable absorber (IGD) is less than the gadolinia content of the high-strength dispersed gadolinia-oxide burnable absorber (HIGA).

[0025] In an additional aspect, the nuclear fuel assembly further comprises a second integral gadolinia burnable absorber (IGD) in which uranium oxide (UO2) and gadolinia (Gd2O3) are homogeneously mixed.

[0026] The uranium content of the second integral gadolinia combustible absorbent (IGD) is less than the uranium content of the first integral gadolinia combustible absorbent (IGD). The gadolinia content of the second integral gadolinia combustible absorbent (IGD) is less than the gadolinia content of the high-strength dispersed gadolinia-oxide combustible absorbent (HIGA). The number of the second integral gadolinia combustible absorbents (IGD) is less than the number of the high-strength dispersed gadolinia-oxide combustible absorbents (HIGA) and is equal to or greater than the number of the first integral gadolinia combustible absorbents (IGD).

[0027] In an additional aspect, the nuclear fuel assembly further comprises a second integral gadolinia burnable absorber (IGD) in which uranium oxide (UO2) and gadolinia (Gd2O3) are homogeneously mixed.

[0028] The uranium content of the second integral gadolinia combustible absorbent (IGD) is greater than the uranium content of the first integral gadolinia combustible absorbent (IGD). The gadolinia content of the second integral gadolinia combustible absorbent (IGD) is less than the gadolinia content of the high-strength dispersed gadolinia-oxide combustible absorbent (HIGA). The number of the second integral gadolinia combustible absorbents (IGD) is less than the number of the high-strength dispersed gadolinia-oxide combustible absorbents (HIGA) and is equal to or less than the number of the first integral gadolinia combustible absorbents (IGD).

[0029] In an additional aspect, the nuclear fuel assembly further includes a third integral gadolinia burnable absorber (IGD) in which uranium oxide (UO2) and gadolinia (Gd2O3) are homogeneously mixed.

[0030] The uranium content of the third integral gadolinia combustible absorbent (IGD) is greater than the uranium content of the first integral gadolinia combustible absorbent (IGD). The gadolinia content of the third integral gadolinia combustible absorbent (IGD) is less than the gadolinia content of the high-strength dispersed gadolinia-oxide combustible absorbent (HIGA). The number of the third integral gadolinia combustible absorbents (IGD) is less than the number of the high-strength dispersed gadolinia-oxide combustible absorbents (HIGA) and is equal to the number of the second integral gadolinia combustible absorbents (IGD).

[0031] According to another aspect of the proposed invention, an innovative small modular reactor (i-SMR) including an oxide-diluted gadolinia burnable absorber for controlling excess reactivity includes a core composed of nuclear fuel assemblies, a stainless steel reflector cylindrically surrounding the outside of the core composed of the nuclear fuel assemblies, and a reactor reactor vessel having an inner diameter larger than an outer diameter of the reflector to form a downward flow path through which coolant passes on the outside of the reflector.

[0032] The high-strength dispersed gadolinia-oxide combustible absorber (HIGA) according to the present invention can effectively control the excess reactivity of an innovative small modular reactor (i-SMR).

[0033] Furthermore, according to the present invention, the excess reactivity of an innovative small modular reactor (i-SMR) can be more effectively controlled by using a nuclear fuel assembly that integrates a high-strength dispersed gadolinia-oxide burnable absorber (HIGA) and an integral gadolinia burnable absorber (IGD).

[0034] Figures 1a and 1b are conceptual diagrams showing a central shielding combustible absorber (CSBA) and a cylindrically inserted and mechanically separated combustible absorber (CIMBA) structure according to the prior art.

[0035] Figure 2 is a phase diagram of a compound of gadolinia (Gd2O3) and alumina (Al2O3).

[0036] FIGS. 3A to 3D are conceptual diagrams showing the structure of a high-strength dispersed gadolinia-oxide combustible absorbent (HIGA) according to one embodiment.

[0037] Figure 4 is an exemplary drawing showing an example of an integrated gadolinia burnable absorber (IGD) and a high-strength dispersed gadolinia-oxide burnable absorber (HIGA) being arranged in a nuclear fuel assembly.

[0038] Figure 5 is a graph showing the reactivity according to the burnup degree of a nuclear fuel assembly depending on the arrangement of a combustible absorber (BA).

[0039] Figure 6 is a graph showing the gadolinium (Gd) density according to the burnup degree of a nuclear fuel assembly depending on the arrangement of a burnable absorber (BA).

[0040] FIG. 7 is a conceptual diagram showing the arrangement of a nuclear fuel assembly according to the number of high-strength dispersed gadolinia-oxide burnable absorber (HIGA) rods according to one embodiment.

[0041] FIG. 8 is a graph showing the reactivity according to the burnup of a nuclear fuel assembly according to the number of high-strength dispersed gadolinia-oxide burnable absorber (HIGA) rods according to one embodiment.

[0042] FIG. 9 is a graph showing the gadolinium (Gd) density as a function of the burnup of a nuclear fuel assembly according to the number of high-intensity dispersed gadolinia-oxide burnable absorber (HIGA) rods according to one embodiment.

[0043] Figures 10a and 10b are conceptual diagrams showing a schematic structure and control rod pattern of an innovative small modular reactor (i-SMR).

[0044] Figure 11 is a conceptual diagram showing the arrangement of nuclear fuel assemblies used in the initial operation cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0045] Figure 12 is a graph showing the reactivity according to the burnup of nuclear fuel assemblies used in the initial operation cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0046] FIGS. 13a and 13b are conceptual diagrams showing the loading pattern and vertical axis configuration of nuclear fuel assemblies used in the initial operation cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0047] FIG. 14 is a graph showing excess reactivity under ARO conditions for effective maximum power days (EFPD) of the initial cycle of operation of an innovative small modular reactor (i-SMR) according to one embodiment.

[0048] FIGS. 15a and 15b are graphs showing peak factors and critical control rod positions for effective maximum power days (EFPD) in the initial operation cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0049] Figures 16a and 16b are graphs showing the power distribution by time, location of nuclear fuel assembly, and height during the initial operation cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0050] Figure 17 is a conceptual diagram showing the arrangement of nuclear fuel assemblies used in an equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0051] FIG. 18 is a graph showing the reactivity according to the burnup of nuclear fuel assemblies used in the equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0052] FIGS. 19a and 19b are conceptual diagrams showing the loading pattern and vertical axis configuration of nuclear fuel assemblies used in an equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0053] FIG. 20 is a graph showing excess reactivity under ARO conditions for effective maximum power days (EFPD) of an equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0054] FIGS. 21a and 21b are graphs showing peak factors and critical control rod positions for effective maximum power days (EFPD) of an equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0055] Figures 22a and 22b are graphs showing the power distribution by time period, position of nuclear fuel assembly, and height of an equilibrium state cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0056] The aforementioned and additional aspects are embodied in embodiments described with reference to the attached drawings. It is understood that the components of each embodiment can be combined in various ways within the embodiment or with components of other embodiments, as long as there is no other mention or inconsistency between them. Based on the principle that an inventor can appropriately define the concept of a term to best describe his or her invention, the terms used in this specification and claims should be interpreted as meanings and concepts consistent with the described content or proposed technical idea. A module or part in this specification may be a set of program instructions stored in a memory so that it can be executed by a computer or processor, or may be implemented using a set of electronic components or circuits such as an ASIC or FPGA so that such instructions can be executed. In addition, the operation of each module or part may be performed by one or more processors or devices. Components denoted by the same or similar symbols perform the same or similar functions, and thus, their descriptions may be omitted. For components with drawing symbols whose descriptions are omitted, reference may be made to the previously described description of the components with the same or similar symbols.

[0057] Table 1 presents the main design features of an innovative small modular reactor (i-SMR) according to one embodiment.

[0058] Characteristics Reactor type Integral PWR Plant capacity (number of reactors) 680 MWe (4) Thermal / electrical capacity per reactor 520 MWth / 170 MWe Reactor coolant pump Vertical canned motor type NSSS Operating pressure 15 MPa Core inlet / outlet coolant temperature 295.5℃ / 320.0℃ Fuel type / assembly arrangement UO2 / 17 × 17 square pitch Number of fuel assemblies in the core 69 Fuel enrichment < 5 w / o Core burnup < 62,000 MWD / MTU Refueling interval 24 months Batches Two batches Reactivity control (boric acid-free operation) Control rod, burnable absorber rods or moderator temperature Steam generator Helical once-through type Safety system Fully passive Design life 80 years Seismic design (SSE) 0.5 g

[0059] Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the attached drawings.

[0060] Figures 1a and 1b are conceptual diagrams showing a central shielding combustible absorber (CSBA) and a cylindrically inserted and mechanically separated combustible absorber (CIMBA) structure according to the prior art.

[0061] Figure 1a is a conceptual diagram showing the structure of a centrally-shielded burnable absorber (CSBA), and Figure 1b is a conceptual diagram showing the structure of a cylindrically inserted and mechanically separated burnable absorber (CIMBA).

[0062] Referring to Figure 1a, the central shielding burnable absorber (CSBA) utilizes the spatial self-shielding effect to delay burnable absorber (BA) depletion by incorporating spherical gadolinia (Gd2O3) into uranium dioxide (UO2) pellets. This structure allows the CSBA to maintain a reactivity swing of less than 1000 pcm throughout the cycle, ensuring uniform distribution across all fuel pellets and minimizing core power distortion.

[0063] Referring to Fig. 1b, the cylindrically inserted and mechanically separated combustible absorber (CIMBA) consists of cylindrical gadolinium embedded in annular fuel pellets, utilizing the spatial self-shielding effect. To control the reactivity increase due to gadolinium depletion, the CIMBA can be further enhanced to achieve a burnup-dependent self-shielding effect by utilizing a multi-layer structure (ML-CIMBA) or a mixed loading strategy using two different types of CIMBA (MIX-CIMBA). Enriched gadolinium isotopes known for their high neutron absorption capabilities, particularly 155 and 157, can be used to efficiently and progressively deplete the combustible absorber (BA).

[0064] However, practical manufacturing challenges associated with CSBA and CIMBA, such as creating annular pellets with a central void and inserting thin gadolinium pins, hinder manufacturability due to the lack of established gadolinium (Gd) enrichment facilities and the associated high costs, posing significant economic challenges. Therefore, there is a need to develop novel combustible absorbers (BAs) with improved manufacturability for boric acid-free (SBF) operation in innovative small modular reactors (i-SMRs). These new BAs must meet the design requirements of the i-SMR while also satisfying fundamental nuclear design assessments.

[0065] Among the existing burnable absorbers (BA), the integral gadolinia burnable absorber (IGD) contains 2–8 wt% gadolinia (Gd2O3) homogeneously mixed with uranium dioxide (UO2). This integral gadolinia burnable absorber (IGD) is effective in managing excess reactivity for a period of 10–15 GWD / MTU. However, this IGD burnable absorber does not meet the 24-month operating cycle and boron-free (SBF) operation requirements of the innovative small modular reactor (i-SMR). For SBF operation, it is important to maintain the excess reactivity at a near-critical level for a 24-month cycle, which corresponds to a cycle burnup of approximately 20 megawatts di (MWD) / MTU or a core burnup of 40 MWD / MTU. To address these challenges and improve manufacturability, new flammable absorbers (BAs) are needed.

[0066] Boron (B-10) and gadolinia have traditionally been preferred materials for burnable absorbers (BAs) due to their strong neutron absorption properties. While boron is preferred for its stable combustion properties, its depletion by helium and lithium can increase the internal pressure of the rod. Therefore, gadolinium (Gd) is preferred. However, gadolinia (Gd2O3), a rare earth oxide, has two properties that complicate its application as a standalone material in nuclear reactors.

[0067] First, the irreversible phase transformation from the cubic (C-type) structure to the monoclinic (B-type) structure complicates the maintenance of structural stability in the reactor environment. Gadolinia (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 and converts to the B-type at high temperatures. Gadolinia (Gd2O3) is heated to approximately 1250°C during the manufacture of combustible absorbent pellets and converts to the B-type. Gadolinia (Gd2O3) converted to the B-type does not revert back to the C-type even when cooled. However, under certain conditions, such as internal defects due to thermal energy absorbed in the reactor and neutron interaction, the B-type structure can be converted back to the C-type. The monoclinic structure of the B-type (density 7.618 g / cm) 3 ) has a cubic structure of type C (density 8.352 g / cm 3 ) when converted to a solid, the density increases by about 10%. This causes significant compressive stresses, which can compromise the structural integrity of the pellets and lead to disintegration and powdering.

[0068] Second, when utilizing pellets composed entirely of gadolinia (Gd2O3) in a high-temperature neutron environment, rapid moisture absorption due to the material's high basicity becomes a problem. Typically, oxygen vacancies within the Gd2O3 matrix are filled with hydroxyl groups, resulting in increased weight and volume. Consequently, if the metal rod that surrounds the gadolinia (Gd2O3) and isolates it from the external high-temperature, high-pressure coolant becomes damaged, the Gd2O3 will hydrate to Gd(OH)3, losing its effective form for reactivity control. This transformation reduces the utility of gadolinia (Gd2O3) in reactor reactivity management. Therefore, to ensure the stable use of gadolinia (Gd2O3) within the reactor, it is necessary to modify Gd2O3 by combining it with other metal oxides to mitigate the irreversible transformation and hydration of rare earth oxides.

[0069] Figure 2 is a phase diagram of a compound of gadolinia (Gd2O3) and alumina (Al2O3).

[0070] When Al2O3 is added to gadolinia (Gd2O3) in a ratio exceeding 50 mol.%, a GdAlO3 phase with a perovskite structure is formed, as shown in Fig. 2. This phase does not exhibit the rapid moisture absorption characteristic of rare earth oxides. In addition, since it does not have a monoclinic structure, which is an unstable high-temperature phase, an abrupt transition to a cubic phase is impossible even in the neutron-rich environment of a nuclear reactor. Therefore, a combustible absorbent (BA) can be manufactured by mixing an appropriate ratio of Al2O3 to alleviate the problems caused by Gd2O3 while maintaining an appropriate gadolinia content to optimally control excess reactivity.

[0071] FIGS. 3A to 3D are conceptual diagrams showing the structure of a high-strength dispersed gadolinia-oxide combustible absorbent (HIGA) according to one embodiment.

[0072] FIG. 3a is a front view of the exterior of an oxide-diluted gadolinia combustible absorbent for controlling excess reactivity of an innovative small modular reactor (i-SMR), FIG. 3b is a cross-sectional view taken vertically (AA) through the combustible absorbent illustrated in FIG. 3a and viewed from the side, FIG. 3c is a cross-sectional view taken horizontally (BB) through the combustible absorbent illustrated in FIG. 3a and viewed from above, and FIG. 3d is an enlarged view of a portion C in the cross-sectional view of the combustible absorbent illustrated in FIG. 3b.

[0073] In Fig. 3a, the oxide-diluted gadolinia combustible absorbent for controlling excess reactivity of the innovative small modular reactor (i-SMR) is externally wrapped with a cladding layer (370). The oxide-diluted gadolinia combustible absorbent is formed in a streamlined shape similar to a conventional fuel rod.

[0074] Referring to FIG. 3b, the combustible absorbent includes combustible pellets (360), oxide pellets (350), and a covering layer (370). The combustible absorbent is closed with an upper cap (320) and a lower cap (380). Inside the covering layer (370), a plurality of combustible pellets (360) and one or more oxide pellets (350) are mounted, which are vertically stacked based on a cylindrical central axis above the lower cap (380). A spacer (340), a spring (330), and an upper cap (320) are positioned above the oxide pellets (350).

[0075] Referring to FIG. 3c, the combustible absorbent has cylindrical combustible pellets (360) positioned concentrically with the outer covering layer (370) inside the outer covering layer.

[0076] Referring to FIG. 3d, the inner diameter of the covering 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 covering layer (370).

[0077] The combustible pellet (360) has a combustible material composed of gadolinia and an oxide uniformly distributed therein. The combustible pellet (360) can be manufactured by mixing gadolinia (Gd2O3) and an oxide and sintering them. The oxide preferably includes any one of Al, Si, Ti, and Zr. That is, oxides such as Al2O3, SiO2, TiO2, ZrO2, and Zr2O3, which are sintered together with gadolinia to form a compound that does not dissolve in water, can be used. Referring to Fig. 3c, an example in which Al2O3 is used as the oxide of the combustible pellet (360) is illustrated.

[0078] The oxide pellet (350) is mounted on top of the combustible pellet (360). The oxide pellet (350) is included to prevent an axial profile that is tilted upward during control rod withdrawal. The oxide pellet (350) is formed into the same cylindrical shape as the combustible pellet (360) to facilitate assembly. The oxide pellet (350) is preferably manufactured by sintering the oxide used in the combustible pellet. Referring to FIG. 3b, an example of the oxide pellet (350) manufactured from Al2O3 is illustrated.

[0079] When the content of gadolinia (Gd2O3) of the combustible pellet (360) is 10 to 20 mol% of the total combustible pellet, as shown in Fig. 3c, the oxide-diluted gadolinia combustible absorbent can be used as a high-strength dispersed gadolinia-oxide combustible absorbent (HIGA). A greater amount of gadolinia can be incorporated into the high-strength dispersed gadolinia-oxide combustible absorbent (HIGA) to enhance the self-shielding effect.

[0080] Highly Intensive and Discrete Gadolinia / Alumina Burnable Absorber (HIGA), an oxide-diluted gadolinia burnable absorber with an appropriately high gadolinia content, can optimize the control of excess reactivity and ensure high productivity in innovative small modular reactors (i-SMRs).

[0081] According to another aspect of the proposed invention, a nuclear fuel assembly including an oxide-diluted gadolinia burnable absorber for controlling excess reactivity of an innovative small modular reactor (i-SMR) includes a fuel rod including nuclear fuel, a coolant passage through which coolant for transferring heat generated by nuclear fission moves, and a high-strength dispersed gadolinia-oxide burnable absorber (HIGA).

[0082] The above high-strength dispersed gadolinia-oxide combustible absorbent (HIGA) comprises: a cylindrical combustible pellet formed by sintering a mixture of gadolinia (Gd2O3) and an oxide containing one of Al, Si, Ti, and Zr; a cylindrical oxide pellet having the same outer diameter as the combustible pellet formed by sintering an oxide contained in the combustible pellet; and a covering layer in which a plurality of combustible pellets and oxide pellets are vertically stacked and mounted based on the central axis of the cylinder.

[0083] The inner diameter of the above coating layer is larger than the outer diameter of the combustible pellets and oxide pellets, so that a gap is formed between the combustible pellets or oxide pellets and the coating layer. The oxide pellets are mounted on top of the combustible pellets.

[0084] According to an additional aspect, the combustible pellet is characterized in that the content of gadolinia is 10 to 20 mol% of the total combustible pellet.

[0085] According to an additional aspect, the oxide of the combustible pellet is characterized as being Al2O3.

[0086] The performance of nuclear fuel assemblies with high-strength dispersed gadolinia-oxide burnable absorbers (HIGA) was compared with those without burnable absorbers and those with integral gadolinia burnable absorbers (IGDs).

[0087] Figure 4 is an exemplary drawing showing an example of an integrated gadolinia burnable absorber (IGD) and a high-strength dispersed gadolinia-oxide burnable absorber (HIGA) being arranged in a nuclear fuel assembly.

[0088] Figure 4 shows the standard 2D geometry of a 17 × 17 SMR fuel assembly. Light gray circles represent fuel rods, and dark gray circles represent water holes that transport heat generated by nuclear fission.

[0089] The first batch (NOBA) in Fig. 4 represents a case without a combustible absorbent (BA), the second batch (20 IGD) in Fig. 4 represents a case in which 20 (= 4 × 5) integral gadolinia combustible absorbents (IGD 8 w / o) having a gadolinia (Gd2O3) concentration of 8% are placed. The third batch (20 HIGA) in Fig. 4 represents a case in which 20 (= 4 × 5) high-strength dispersed gadolinia-oxide combustible absorbents (HIGA) having a gadolinia (Gd2O3) concentration of 20% are placed.

[0090] Neutron analyses, including static and depletion calculations, were performed using the 2D transfer grid code KAMRA.

[0091] Figure 5 is a graph showing the reactivity according to the burnup degree of a nuclear fuel assembly depending on the arrangement of a combustible absorber (BA).

[0092] The unmarked line is the graph for the first batch (NOBA) of FIG. 4 without using a combustible absorber, the triangle-marked line is the graph for the second batch (20 IGD) of FIG. 4 using an integral gadolinia combustible absorber (IGD), and the ×-marked line is the graph for the third batch (20 HIGA) of FIG. 4 using a high-strength dispersed gadolinia-oxide combustible absorber (HIGA).

[0093] The IDG assembly (triangle) equipped with 8 wt% combustible absorbent (BA) rods exhibits lower reactivity than the assembly containing HIGA rods (marked with an X). However, the reactivity duration of the IGD assembly is limited to less than 20 GWD / MTU due to rapid gadolinia depletion, after which the reactivity gradually decreases after gadolinia combustion. In contrast, the HIGA assembly exhibits extended reactivity, maintaining up to 40 GWD / MTU, due to enhanced spatial self-shielding.

[0094] Figure 6 is a graph showing the gadolinium (Gd) density according to the burnup degree of a nuclear fuel assembly depending on the arrangement of a combustible absorber (BA).

[0095] Figure 6 shows the gadolinia depletion pattern between IGD and HIGA assemblies. The line marked × is the graph for the second batch (20 IGD) of Figure 4 using the integral gadolinia combustible absorber (IGD), and the line marked * is the graph for the third batch (20 HIGA) of Figure 4 using the high-strength dispersed gadolinia-oxide combustible absorber (HIGA).

[0096] HIGA assemblies exhibit longer and more gradual gadolinium depletion than IGD assemblies.

[0097] Meanwhile, the gradual and consistent decrease in overreactivity from the start of the HIGA assembly suggests that incorporating both IGD and HIGA rods into the fuel assembly is more effective for effective overreactivity control.

[0098] FIG. 7 is a conceptual diagram showing the arrangement of a nuclear fuel assembly according to the number of high-strength dispersed gadolinia-oxide burnable absorber (HIGA) rods according to one embodiment.

[0099] By exploring the reactivity changes using different amounts of HIGA rods (20% Gd2O3), we can gather basic data for core design that can efficiently manage excessive reactivity over long periods.

[0100] In Figure 7, HIGA rods are shown as double circles. 4, 8, 12, 16, and 20 HIGA rods can be arranged in sequence in a nuclear fuel assembly.

[0101] FIG. 8 is a graph showing the reactivity according to the burnup of a nuclear fuel assembly according to the number of high-strength dispersed gadolinia-oxide burnable absorber (HIGA) rods according to one embodiment.

[0102] Referring to Figure 8, it can be seen that the reactivity retention period extends up to approximately 40 GWD / MTU. Furthermore, an analysis of the reactivity behavior in relation to the number of HIGA rods confirms that the effectiveness of reactivity blocking increases with the number of HIGA rods.

[0103] FIG. 9 is a graph showing the gadolinium (Gd) density as a function of the burnup of a nuclear fuel assembly according to the number of high-intensity dispersed gadolinia-oxide burnable absorber (HIGA) rods according to one embodiment.

[0104] Referring to Figure 9, it is shown that due to the significant self-shielding effect of each HIGA rod, gadolinia is slowly depleted up to a maximum of 35 GWD / MTU, regardless of the number of HIGA rods. Beyond this point, rapid combustion occurs up to a maximum of approximately 40 GWD / MTU, resulting in almost complete depletion of gadolinia (Gd2O3).

[0105] Figures 10a and 10b are conceptual diagrams showing a schematic structure and control rod pattern of an innovative small modular reactor (i-SMR).

[0106] Figures 10a and 10b illustrate the core design and control rod pattern of the Innovative Small Modular Reactor (i-SMR). To enhance the stability of the i-SMR, a top-mounted Integral Control Instrumentation (ICI) system is incorporated to prevent penetration through the bottom of the reactor vessel. This system consists of 16 strategically placed ICIs to monitor the core condition. However, the adoption of a top-mounted ICI reduces the number of available control rods to 53, potentially compromising subcriticality under All Rod-In (ARI) and Stuck Rod-Out (N-1) conditions.

[0107] The control rods consist of four control bank groups (R1, R2, R3, and R4) optimized for factors such as overreactivity, power distribution, and subcriticality considerations, and one shutdown bank group (SB). The control bank uses Ag-In-Cd to minimize the effects of control rod burnout, while the shutdown bank utilizes B4C with a 95% B-10 concentration to increase the shutdown margin and ensure subcriticality.

[0108] The integration of high-strength dispersed gadolinia-oxide burnable absorbers (HIGA) requires careful uranium loading evaluation. This is because the inclusion of HIGA rods proportionally reduces the number of fuel rods and potentially increases the average nominal power density of the fuel. To mitigate this, the number of HIGA rods is limited to 16, balancing the 24-month cycle target with peak power constraints. This configuration sets the nominal linear power density of the fuel rods at 3.86 kW / ft, which is 30% lower than that of existing PWRs such as the OPR1000 and APR1400, which have linear power densities of approximately 5.5 kW / ft.

[0109] Figure 11 is a conceptual diagram showing the arrangement of nuclear fuel assemblies used in the initial operation cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0110] In an additional aspect, the nuclear fuel assembly further comprises a first integral gadolinia burnable absorber (IGD) in which uranium oxide (UO2) and gadolinia (Gd2O3) are homogeneously mixed.

[0111] The uranium content of the first integral gadolinia burnable absorber (IGD) is less than the uranium content of the fuel rod. The gadolinia content of the first integral gadolinia burnable absorber (IGD) is less than the gadolinia content of the high-strength dispersed gadolinia-oxide burnable absorber (HIGA).

[0112] In an additional aspect, the nuclear fuel assembly further comprises a second integral gadolinia burnable absorber (IGD) in which uranium oxide (UO2) and gadolinia (Gd2O3) are homogeneously mixed.

[0113] The uranium content of the second integral gadolinia combustible absorbent (IGD) is less than the uranium content of the first integral gadolinia combustible absorbent (IGD). The gadolinia content of the second integral gadolinia combustible absorbent (IGD) is less than the gadolinia content of the high-strength dispersed gadolinia-oxide combustible absorbent (HIGA). The number of the second integral gadolinia combustible absorbents (IGD) is less than the number of the high-strength dispersed gadolinia-oxide combustible absorbents (HIGA) and is equal to or greater than the number of the first integral gadolinia combustible absorbents (IGD).

[0114] The innovative small modular reactor (i-SMR) utilizes a two-batch loading pattern to facilitate a 24-month operating cycle. The initial cycle nuclear design includes a variety of fuel assembly configurations with varying loading patterns of HIGA and IGD rods tailored to the characteristics of the burnable absorber (BA).

[0115] Considering that only fresh fuel is loaded in the first cycle, it is necessary to use an assembly with a flat reactivity curve to manage excess reactivity within the core.

[0116] As a result, eight types of fuel assemblies with flat reactivity curves were selected depending on the number of IGD rods (4 to 12 rods, containing 1 to 8 wt% Gd2O3) and 16 HIGA rods (containing Gd2O3 contents ranging from 10% to 18). The specific compositions of the selected fuel assemblies are shown in Fig. 11 and Table 2. The number of IGD in the figure represents the content of gadolinia (Gd2O3). For example, IGD 4 w / o represents IGD rods containing 4 wt% gadolinia.

[0117] Table 2 shows the specific configuration of nuclear fuel assemblies used in the initial operation cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0118] Type Fuel rod Combustible absorber rod HIGA1st IGD2nd IGDU-235 (%)Gd2O3(%)U-235 (%)Gd2O3(%)Quantity U-235 (%)Gd2O3(%)Quantity A014.0010------A024.00103.5044---A034.00123.5044---A044.00143.50442.5084A054.00153.50442.5084A064.00163.50442.5084A074.00183.95142.5088A084.00183.75242.5088

[0119] The enrichment of the common fuel rod is standardized at 4 wt% across all assembly types. Each assembly has a 10 cm top cutback area to control shaft power distribution toward the end of the cycle, particularly when the control bank is primarily withdrawn.

[0120] The design of the fuel assembly pattern, including the HIGA shown in Fig. 11, is for the initial core (cycle 1) of a boron-free reactor. Since boron-free cores lack boric acid, the design of a boron-free core is possible by controlling the excess reactivity of each assembly. Cycle 1 used eight types of assemblies for core design, each type playing a role in achieving a flat excess reactivity throughout the cycle at its loading position within the core loading model.

[0121] To control the excess reactivity, the HIGA and IGD combustible absorber rods were arranged as shown in Fig. 11. The combustible absorber rods are generally arranged near the guide tube with a lot of moderator (water) to properly absorb neutrons. The purpose is to control the excess reactivity by utilizing the self-shielding effect of the combustible absorber material of IGD at the beginning of the cycle and HIGA after the middle of the cycle. In addition, the use of HIGA was set to 16 to minimize the influence of the linear power density (kW / ft), and the arrangement as shown in Fig. 11 was selected to stabilize the radial power distribution within the assembly.

[0122] In addition, for IGD 2 w / o, the moderator is placed in the most densely packed location, enabling fine adjustment of the initial surplus reactivity. As a result, the frequency of control rod use during initial core operation can be reduced. For IGD 8 w / o, the moderator is placed near the outer periphery of the assembly, and is designed to have the surplus reactivity curve corresponding to Fig. 12.

[0123] Figure 12 is a graph showing the reactivity according to the burnup of nuclear fuel assemblies used in the initial operation cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0124] By integrating HIGA and IGD into a nuclear fuel assembly, excellent reactivity flattening can be achieved by combining the slow depletion effect of HIGA and the effective reactivity control of IGD, as shown in Fig. 12.

[0125] FIGS. 13a and 13b are conceptual diagrams showing the loading pattern and vertical axis configuration of nuclear fuel assemblies used in the initial operation cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0126] According to another aspect of the proposed invention, an innovative small modular reactor (i-SMR) including an oxide-diluted gadolinia burnable absorber for controlling excess reactivity includes a core composed of nuclear fuel assemblies, a stainless steel reflector cylindrically surrounding the outside of the core composed of the nuclear fuel assemblies, and a reactor reactor vessel having an inner diameter larger than an outer diameter of the reflector to form a downward flow path through which coolant passes on the outside of the reflector.

[0127] The fuel loading pattern is illustrated in Figures 13a and 13b, strategically placing assemblies with higher reactivity in the peripheral region to optimize radial power distribution.

[0128] FIG. 14 is a graph showing excess reactivity under ARO conditions for effective maximum power days (EFPD) of the initial cycle of operation of an innovative small modular reactor (i-SMR) according to one embodiment.

[0129] The reactor core calculations were performed using the KNF KARMA / ASTRA code package. Following a conventional two-step procedure, 2D assembly-by-assembly transport calculations using KARMA were performed to generate two-group cross-sections, and 3D nodal calculations were performed using ASTRA, incorporating 3D thermal / hydraulic feedback.

[0130] In this boron-free (SBF) operation, unlike typical PWR designs where boron concentration determines criticality, the control rod position is adjusted to achieve criticality. The excess reactivity for the All Rod-Out (ARO) condition, expressed in effective maximum power days (EFPD), is shown in Figure 14.

[0131] The predicted cycle length is approximately 840 EFPD, the peak excess reactivity reaches approximately 1200 pcm, and the difference between the minimum and maximum reactivity is 800 pcm. This demonstrates that BA HIGA and IGD effectively control excess reactivity.

[0132] FIGS. 15a and 15b are graphs showing peak factors and critical control rod positions for effective maximum power days (EFPD) in the initial operation cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0133] Figures 15a and 15b illustrate the peak factors and critical positions of the control bank with 50% overlap with the regulatory bank. The calculated cycle length exceeds 780 EFPD, corresponding to 20,711 MWD / MTU, considering a 20% margin in control rod positioning, thus meeting the required cycle period of 730 days.

[0134] Figures 16a and 16b are graphs showing the power distribution by time, location of nuclear fuel assembly, and height during the initial operation cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0135] The maximum radial pin picking factor (Fr) was recorded as 1.351, with a fairly flat assembly-wise radial power distribution as shown in Figure 16a, and the maximum 3D pin picking factor (Fq) was observed to be a reasonable value of 2.080. Figure 16b shows an axial 1D pin picking factor (Fz) of approximately 1.5.

[0136] With a nominal linear power density of 3.86 kW / ft, the radial integrated 2D linear power density is 5.215 kW / ft and the peak 3D linear power density is 8.029 kW / ft. Both are lower than typical targets for commercial PWRs (7.5 kW / ft and 12 kW / ft, respectively).

[0137] Table 3 shows the reactivity coefficients (HFP, HZP) of the initial operation cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0138] Combustion periodCore powerITC (pcm / ℃)FTC (pcm / ℃)MTC (pcm / ℃)BOCHFP-65.96-2.97-62.99HZP-70.86-3.47-67.39MOCHFP-65.96-3.28-62.67HZP-70.86-3.71-67.15EOCHFP-65.96-3.48-62.47HZP-70.86-3.90-66.96

[0139] Table 3 details the fuel temperature coefficient (FTC), MTC, and subcriticality at various combustion stages. The FTC remains in the range of -4 to -3 pcm / K, similar to conventional PWR values, while the MTC remains consistently negative, below -65 pcm / K, due to the absence of soluble boron.

[0140] Table 4 shows the subcriticality (CZP) at each stage of the initial operation cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0141] Combustion TimeARI (k-eff < 0.95)N-1 (k-eff < 0.99)RawConservativeRawConservativeBOC0.931910.948520.958430.97330MOC0.921630.938090.946060.96091EOC0.906930.923970.930950.94645

[0142] Table 4 briefly describes the subcriticality under cold zero power (CZP) conditions. The maximum subcritical k-eff for the ARO scenario is 0.93191 at the beginning of the cycle (BOC), without considering uncertainties. Conservatively incorporating uncertainties such as 10% of the total rod value and 500 pcm of reactivity, the adjusted subcriticality is 0.94852, which is lower than the subcriticality criterion of 0.95. Similarly, the maximum subcritical k-eff for the N-1 condition is 0.95843, a conservative value of 0.97330 that accounts for rod value and reactivity uncertainties, and remains lower than the N-1 subcriticality criterion of 0.99. These subcriticality calculations confirm that the reactor core can be safely shut down during operation.

[0143] Figure 17 is a conceptual diagram showing the arrangement of nuclear fuel assemblies used in an equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0144] In an additional aspect, the nuclear fuel assembly further comprises a first integral gadolinia burnable absorber (IGD) in which uranium oxide (UO2) and gadolinia (Gd2O3) are homogeneously mixed.

[0145] The uranium content of the first integral gadolinia burnable absorber (IGD) is less than the uranium content of the fuel rod. The gadolinia content of the first integral gadolinia burnable absorber (IGD) is less than the gadolinia content of the high-strength dispersed gadolinia-oxide burnable absorber (HIGA).

[0146] In an additional aspect, the nuclear fuel assembly further comprises a second integral gadolinia burnable absorber (IGD) in which uranium oxide (UO2) and gadolinia (Gd2O3) are homogeneously mixed.

[0147] The uranium content of the second integral gadolinia combustible absorbent (IGD) is greater than the uranium content of the first integral gadolinia combustible absorbent (IGD). The gadolinia content of the second integral gadolinia combustible absorbent (IGD) is less than the gadolinia content of the high-strength dispersed gadolinia-oxide combustible absorbent (HIGA). The number of the second integral gadolinia combustible absorbents (IGD) is less than the number of the high-strength dispersed gadolinia-oxide combustible absorbents (HIGA) and is equal to or less than the number of the first integral gadolinia combustible absorbents (IGD).

[0148] In an additional aspect, the nuclear fuel assembly further includes a third integral gadolinia burnable absorber (IGD) in which uranium oxide (UO2) and gadolinia (Gd2O3) are homogeneously mixed.

[0149] The uranium content of the third integral gadolinia combustible absorbent (IGD) is greater than the uranium content of the first integral gadolinia combustible absorbent (IGD). The gadolinia content of the third integral gadolinia combustible absorbent (IGD) is less than the gadolinia content of the high-strength dispersed gadolinia-oxide combustible absorbent (HIGA). The number of the third integral gadolinia combustible absorbents (IGD) is less than the number of the high-strength dispersed gadolinia-oxide combustible absorbents (HIGA) and is equal to the number of the second integral gadolinia combustible absorbents (IGD).

[0150] In the equilibrium cycle, the core is equipped with four types of nuclear fuel assemblies, as shown in Figure 17, including 16 HIGA rods with a gadolinium (Gd) content of 9% to 15% and IGD rods with a gadolinium (Gd) content of 4 to 8 wt%. The detailed configuration is shown in Table 5.

[0151] Table 5 shows the specific configuration of nuclear fuel assemblies used in the equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0152] TypeFuel rodCombustible absorber rodHIGA1st IGD2nd IGD2nd IGDU-235 (%)Gd2O3(%)U-235 (%)Gd2O3(%)QuantityU-235 (%)Gd2O3(%)QuantityU-235 (%)Gd2O3(%)QuantityX014.95152.50820------X024.95122.50842.70743.3544X034.95122.50883.35443.5534X044.9593.5044------

[0153] The design of the nuclear fuel assembly pattern, including the HIGA shown in Fig. 17, is for the design of nuclear fuel assemblies for boric acid-free refueling reactors (cycle 2 and later). In the case of refueling reactors, the combustible absorber materials in the assemblies burned in the previous cycle have already been burned, so there is no residual reaction control capability. Therefore, the role of the combustible absorber materials in the refueling assemblies can be considered greater.

[0154] In the case of X01, the number of IGD 8 w / o used is up to 20, and as shown in the X01 curve in Fig. 18, it is used as a means of controlling the excess reactivity of the new fuel loaded into the replacement core by gradually increasing from a sub-critical state where the k-inf value at the beginning of the cycle is 1 or less to the middle of the cycle. In the case of X03, the IGD 3 w / o in X02 can be arranged in a dense area of ​​the guide tube to facilitate the control of the excess reactivity at the beginning of the cycle.

[0155] FIG. 18 is a graph showing the reactivity according to the burnup of nuclear fuel assemblies used in the equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0156] The reactivity curves of the assemblies depicted in Figure 18 indicate that fuel assemblies X01 and X02 achieve a consistent excess reactivity profile throughout depletion, while X03 and X04 are designed with progressively increasing curves to offset the decreasing reactivity of the single-burned fuel assemblies.

[0157] FIGS. 19a and 19b are conceptual diagrams showing the loading pattern and vertical axis configuration of nuclear fuel assemblies used in an equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0158] According to another aspect of the proposed invention, an innovative small modular reactor (i-SMR) including an oxide-diluted gadolinia burnable absorber for controlling excess reactivity includes a core composed of nuclear fuel assemblies, a stainless steel reflector cylindrically surrounding the outside of the core composed of the nuclear fuel assemblies, and a reactor reactor vessel having an inner diameter larger than an outer diameter of the reflector to form a downward flow path through which coolant passes on the outside of the reflector.

[0159] Figures 19a and 19b schematically illustrate the core load pattern and shaft configuration for the equilibrium cycle. The 10 cm scaled-down assembly X01 is centrally positioned to influence the radial power distribution toward the core periphery, thereby improving subcriticality. The other 20 cm scaled-down assemblies are positioned peripherally to achieve a flat power distribution.

[0160] FIG. 20 is a graph showing excess reactivity under ARO conditions for effective maximum power days (EFPD) of an equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0161] Figure 20 shows the overreactivity curve of the ARO state, which shows a reactivity swing of about 800 pcm and a cycle length of 780 EFPD.

[0162] FIGS. 21a and 21b are graphs showing peak factors and critical control rod positions for effective maximum power days (EFPD) of an equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0163] The peak factor, axial shape index, and critical control rod position for the combustion trajectory are shown in Figs. 21a and 21b, with maximum Fr and Fq reaching 1.517 and 2.265, respectively, at the end of cycle (EOC).

[0164] Figures 22a and 22b are graphs showing the power distribution by time period, position of nuclear fuel assembly, and height of an equilibrium state cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0165] Compared to the values ​​of the initial cycle, Fr is higher because the power distribution per assembly is biased towards the H01 assembly at EOC, but Fq is lower because the axial shape is more uniform as shown in Figs. 22a and 22b.

[0166] However, the nominal linear power density remained at 3.86 kW / ft, resulting in a radially integrated 2D linear power density of 5.856 kW / ft and a maximum 3D linear power density of 8.743 kW / ft, both of which are lower than the standard targets for commercial PWRs of 7.5 kW / ft and 12 kW / ft, respectively.

[0167] Table 6 shows the reactivity coefficients (HFP, HZP) of the equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0168] Combustion periodCore powerITC (pcm / ℃)FTC (pcm / ℃)MTC (pcm / ℃)BOCHFP-64.16-3.02-61.15HZP-69.54-3.60-65.94MOCHFP-63.70-3.24-60.47HZP-68.92-3.78-65.14EOCHFP-69.41-3.37-66.04HZP-74.29-3.95-70.34

[0169] The reactivity coefficients at the beginning of the cycle (BOC), middle of the cycle (MOC) and end of the cycle (EOC) are summarized in Table 6, with negative FTCs consistent with typical PWR values ​​and MTCs remaining suitably negative at less than -60 pcm / ℃.

[0170] Table 7 shows the subcriticality (CZP) of the equilibrium cycle of an innovative small modular reactor (i-SMR) according to one embodiment.

[0171] Combustion TimeARI (k-eff < 0.95)N-1 (k-eff < 0.99)RawConservativeRawConservativeBOC0.930110.946760.954640.96969MOC0.923800.939960.943520.95839EOC0.922890.939580.941860.95732

[0172] Table 7 also verifies the reactor's subcriticality under ARO and N-1 CZP conditions by incorporating conservative uncertainty margins for control rod values ​​(10%) and reactivity (500 pcm), thereby confirming the reactor's ability to be safely shut down during operation.

[0173] While the present invention has been described above through embodiments illustrated with reference to the attached drawings, it is not limited thereto and should be construed to encompass various modifications that would be readily apparent to those skilled in the art. The scope of the patent claims is intended to encompass such modifications.

[0174] [Explanation of symbols]

[0175] 350: Oxide pellet (350) 360: Combustible pellet (360)

[0176] 365: Gap (365) 370: Covering layer (370)

Claims

1. In the oxide diluted gadolinia combustible absorber for controlling the excess reactivity of the innovative small modular reactor (i-SMR), Cylindrical combustible pellets formed by sintering a mixture of gadolinia (Gd2O3) and an oxide containing any one of Al, Si, Ti, and Zr; Cylindrical oxide pellets having the same outer diameter as the combustible pellets formed by sintering the oxide contained in the combustible pellets; and A coating layer comprising a plurality of combustible pellets and oxide pellets vertically stacked and mounted based on a cylindrical central axis; The inner diameter of the above coating layer is larger than the outer diameter of the combustible pellets and oxide pellets, so that a gap is formed between the combustible pellets or oxide pellets and the coating layer, The above oxide pellets are mounted on top of the combustible pellets, Diluted oxide gadolinia combustible absorbent.

2. In paragraph 1, The above combustible pellets are, Characterized in that the content of gadolinia is 10 to 20 mol% of the total combustible pellets, Diluted oxide gadolinia combustible absorbent.

3. In paragraph 1, The oxide of the above combustible pellet is characterized by being Al2O3. Diluted oxide gadolinia combustible absorbent.

4. In a nuclear fuel assembly including an oxide-diluted gadolinia burnable absorber for controlling excess reactivity of an innovative small modular reactor (i-SMR), Fuel rods containing nuclear fuel; Coolant passages through which coolant moves to transfer heat generated by nuclear fission; and Contains high-strength dispersed gadolinia-oxide combustible absorbent (HIGA); The above high-strength dispersed gadolinia-oxide combustible absorbent (HIGA) is, Cylindrical combustible pellets formed by sintering a mixture of gadolinia (Gd2O3) and an oxide containing any one of Al, Si, Ti, and Zr; Cylindrical oxide pellets having the same outer diameter as the combustible pellets formed by sintering the oxide contained in the combustible pellets; and A coating layer comprising a plurality of combustible pellets and oxide pellets vertically stacked and mounted based on a cylindrical central axis; The inner diameter of the above coating layer is larger than the outer diameter of the combustible pellets and oxide pellets, so that a gap is formed between the combustible pellets or oxide pellets and the coating layer, The above oxide pellets are mounted on top of the combustible pellets, Nuclear fuel assembly.

5. In paragraph 4, The above combustible pellets are, Characterized in that the content of gadolinia is 10 to 20 mol% of the total combustible pellets, Nuclear fuel assembly.

6. In paragraph 4, The oxide of the above combustible pellet is characterized by being Al2O3. Nuclear fuel assembly.

7. In paragraph 4, Further comprising a first integral gadolinia combustible absorbent (IGD) in which uranium oxide (UO2) and gadolinia (Gd2O3) are homogeneously mixed; The uranium content of the first integral gadolinia burnable absorber (IGD) is less than the uranium content of the fuel rod, The gadolinia content of the above first integral gadolinia combustible absorbent (IGD) is less than the gadolinia content of the above high-strength dispersed gadolinia-oxide combustible absorbent (HIGA). Nuclear fuel assembly.

8. In paragraph 7, Further comprising a second integral gadolinia combustible absorbent (IGD) in which uranium oxide (UO2) and gadolinia (Gd2O3) are homogeneously mixed; The uranium content of the second integral gadolinia combustible absorbent (IGD) is less than the uranium content of the first integral gadolinia combustible absorbent (IGD), The gadolinia content of the second integral gadolinia combustible absorbent (IGD) is less than the gadolinia content of the high-strength dispersed gadolinia-oxide combustible absorbent (HIGA), The number of the second integral gadolinia combustible absorbents (IGDs) is less than the number of the high-strength dispersed gadolinia-oxide combustible absorbents (HIGAs) and is equal to or greater than the number of the first integral gadolinia combustible absorbents (IGDs). Nuclear fuel assembly.

9. In paragraph 7, Further comprising a second integral gadolinia combustible absorbent (IGD) in which uranium oxide (UO2) and gadolinia (Gd2O3) are homogeneously mixed; The uranium content of the second integral gadolinia combustible absorbent (IGD) is greater than the uranium content of the first integral gadolinia combustible absorbent (IGD), The gadolinia content of the second integral gadolinia combustible absorbent (IGD) is less than the gadolinia content of the high-strength dispersed gadolinia-oxide combustible absorbent (HIGA), The number of the second integral gadolinia combustible absorbents (IGDs) is less than the number of the high-strength dispersed gadolinia-oxide combustible absorbents (HIGAs) and is equal to or less than the number of the first integral gadolinia combustible absorbents (IGDs). Nuclear fuel assembly.

10. In paragraph 9, Further comprising a third integral gadolinia combustible absorber (IGD) in which uranium oxide (UO2) and gadolinia (Gd2O3) are homogeneously mixed; The uranium content of the third integral gadolinia combustible absorbent (IGD) is greater than the uranium content of the first integral gadolinia combustible absorbent (IGD), The gadolinia content of the third integral gadolinia combustible absorbent (IGD) is less than the gadolinia content of the high-strength dispersed gadolinia-oxide combustible absorbent (HIGA), The number of the third integral gadolinia combustible absorbent (IGD) is less than the number of the high-strength dispersed gadolinia-oxide combustible absorbent (HIGA) and is equal to the number of the second integral gadolinia combustible absorbent (IGD). Nuclear fuel assembly.

11. In an innovative small modular reactor (i-SMR) including an oxide diluted gadolinia combustible absorber for controlling excess reactivity, A core consisting of nuclear fuel assemblies of paragraph 7 or 8; A stainless steel reflector cylindrically surrounding the exterior of the core composed of the above nuclear fuel assemblies; and A reactor reactor vessel having an inner diameter larger than the outer diameter of the reflector to form a downward flow path through which coolant flows on the outside of the reflector; Innovative small modular reactor (i-SMR).

12. In an innovative small modular reactor (i-SMR) including an oxide-diluted gadolinia combustible absorber for controlling excess reactivity, A core comprising nuclear fuel assemblies according to any one of paragraphs 7, 9 and 10; A stainless steel reflector cylindrically surrounding the exterior of the core composed of the above nuclear fuel assemblies; and A reactor reactor vessel having an inner diameter larger than the outer diameter of the reflector to form a downward flow path through which coolant flows on the outside of the reflector; Innovative small modular reactor (i-SMR).

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