Supercritical carbon dioxide cooled pressure tube reactor and use thereof

Abstract

The application relates to the technical field of nuclear reactors, in particular to a supercritical carbon dioxide cooling pressure tube type reactor and application thereof. The reactor comprises: a moderator container containing heavy water, and a radial reflection layer covering the radial periphery of the moderator container; a plurality of tubes immersed in the heavy water in the moderator container; a plurality of pressure tubes coaxially arranged in the tubes one by one, and the pressure tubes have coolant channels for circulating supercritical carbon dioxide; a fuel assembly arranged in the pressure tube; and a reactivity control assembly arranged in the radial reflection layer. Through the matching of the heavy water moderation characteristics and the gadolinium-based poison absorption characteristics, the reactivity consumption of the reactor in a long operation period is smoothly compensated, so that the micro reactor can realize long-period operation without replacing the fuel under the condition of not more than 25% low enrichment.

Description

Technical Field

[0001] This application relates to the field of nuclear reactor technology, and in particular to a supercritical carbon dioxide cooled pressure tubular reactor and its applications. Background Technology

[0002] Unmanned underwater vehicles (UUVs) are key platforms for performing long-endurance monitoring, underwater search and rescue, and scientific research missions. Due to the unique nature of the underwater environment, the power system, as the core component of the UUV, directly determines the scope and duration of mission execution through its energy density, endurance, and operational reliability.

[0003] Currently, traditional underwater power sources mainly include electrochemical batteries, fuel cells, and thermal engines (such as closed-cycle diesel engines). However, electrochemical batteries generally suffer from low energy density, making it difficult to support long-distance, large-scale deep-sea missions; fuel cells are limited by fuel capacity and system complexity, and their self-sufficiency still has room for improvement; while traditional thermal engines often face constraints in the confined underwater environment regarding air supply, exhaust treatment, and stealth. In contrast, nuclear power sources, with their extremely high energy density and ability to continuously output power for extended periods, hold promise for alleviating the range limitations of UUVs and providing solutions for supporting megawatt-level power.

[0004] To meet the demands of miniaturized platforms such as UUVs, reactor devices must simultaneously satisfy long operational lifetimes and a high degree of inherent safety within limited space. In existing microreactor designs, to compensate for the significant reactivity losses resulting from long operational lifetimes within a compact volume, methods such as increasing nuclear fuel enrichment or increasing the amount of combustible poison are commonly employed. However, high-enrichment fuels often come with higher processing costs and nuclear nonproliferation risks; while excessive loading of combustible poison can lead to an imbalance in the energy spectrum distribution of the reactor during the initial and final stages of operation, making it difficult to smoothly compensate for reactivity throughout its entire lifespan.

[0005] Furthermore, in miniaturized pressurized water reactors or fast reactor systems, the neutron spectrum is often hard. Under hard spectrum conditions, common neutron-absorbing materials (such as gadolinium isotopes) do not respond sufficiently within a specific energy range, leading to limited reactivity compensation efficiency and making it difficult to achieve an optimal balance of neutron economy without significantly increasing core volume. Simultaneously, how to achieve efficient passive heat dissipation using the limited surface area of ​​a miniaturized core under conditions such as coolant loss is also a critical technical challenge that urgently needs to be addressed to ensure the safety of underwater power sources.

[0006] Therefore, developing a micro nuclear power plant that can maintain a compact structure, achieve long-life operation through efficient neutronics design at low enrichment levels, and possess good safety has become a technical problem that needs to be solved in this field. Summary of the Invention

[0007] To address the aforementioned issues, this application provides a supercritical carbon dioxide-cooled pressure tube reactor and its application. A safe and compact pressure tube core structure is constructed, utilizing the moderation effect of heavy water to shift the neutron energy spectrum towards the intermediate energy region. This shifted neutron energy spectrum is then matched with the absorption characteristics of gadolinium-based combustible poisons non-uniformly doped in high-density uranium nitride fuel. Thus, while flattening the radial power distribution of the core, stable operation without refueling is achieved over long periods under low enrichment conditions.

[0008] To achieve the above objectives, the technical solution adopted in this application is as follows: A supercritical carbon dioxide cooled pressure tube reactor includes: a moderator container filled with heavy water, the radial periphery of which is covered with a radial reflective layer; multiple pipes immersed in the heavy water within the moderator container; multiple pressure-bearing pipes coaxially arranged in a corresponding manner within the pipes, the pressure-bearing pipes having coolant channels for circulating supercritical carbon dioxide; fuel assemblies disposed within the pressure-bearing pipes; and a reactivity control assembly disposed within the radial reflective layer; wherein the fuel assembly includes uranium nitride-based fuel rods, the uranium nitride-based fuel rods being non-uniformly doped with gadolinium-based combustible poison, the concentration distribution of the gadolinium-based combustible poison being configured to match the reactor energy spectrum shifted towards the intermediate energy region by the heavy water.

[0009] This technique allows for efficient circulation of high-temperature, high-pressure coolant within the pressurized pipe, while external, low-temperature, atmospheric-pressure heavy water provides excellent neutron economy. More importantly, the moderating effect of the heavy water shifts the originally hard, fast neutron spectrum towards the hyperthermal neutron region, and the shifted spectrum precisely matches that of gadolinium isotopes (such as ¹). 55 Gd,¹ 57 The Gd resonant absorption cross-section peak in the mid-energy region is matched, thereby effectively compensating for the reactive consumption during long cycles through the combined action of the fluid medium and the combustible poison, and effectively reducing neutron leakage in conjunction with the radial reflector layer.

[0010] Furthermore, multiple uranium nitride-based fuel rods are arranged in a concentric ring array inside the pressure tube; and along the inner ring to the outer ring of the concentric ring array, the mass fraction of gadolinium-based combustible poison shows a discrete stepwise decreasing distribution.

[0011] This technique utilizes a radially discrete, decreasing distribution of combustible poisons to strongly suppress the high reactivity of the inner ring during the initial stages of core operation, while providing relatively weaker suppression of the outer ring. This macroscopic geometric arrangement, combined with the burnup characteristics of gadolinium poisons, effectively reduces the central power peak and flattens the radial power distribution, thereby ensuring the thermal-hydraulic safety margin of the fuel assemblies throughout the operating cycle.

[0012] Furthermore, the pipelines are arranged in a regular polygonal array within the moderator container; and the number of uranium nitride-based fuel rods inside the pressure-bearing pipes is 19, consisting of one central fuel rod and inner and outer ring fuel rods arranged around the central fuel rod, forming a double-layered ring array.

[0013] This technology optimizes the cell volume ratio of moderator to fuel within a limited micro-volume. Compared to traditional large pressure tube reactor grids, the 19-bar double-layer annular fuel assembly appropriately reduces the fuel rod density within a single tube, thus preserving sufficient convective heat transfer channel cross-sectional area for supercritical carbon dioxide fluid, reducing fluid resistance, and meeting the requirements of overall miniaturization and compact assembly of nuclear power plants.

[0014] Furthermore, the mass fraction of gadolinium-based combustible poison in the central fuel rod is 1.8% to 2.2%; the mass fraction of gadolinium-based combustible poison in the inner and outer fuel rods is 0.8% to 1.2%.

[0015] This technique provides specific quantitative physical parameters. The 2% and 1% graded concentration configuration can control the radial power peak factor within the designed safety range while taking into account the overall fissile material content of uranium nitride fuel, thus avoiding excessively high local heat flux density.

[0016] Furthermore, at both ends of the uranium nitride-based fuel rod, which are far from the active region, there are axially arranged neutron reflector plugs made of non-fissile material; wherein, the material of the neutron reflector plugs includes beryllium oxide.

[0017] This technology addresses the axial neutron leakage problem in microreactors caused by the increased surface area to volume ratio. By utilizing the excellent neutron scattering cross-section and low absorption characteristics of beryllium oxide, neutrons attempting to escape axially are reflected back into the active fuel region without increasing the external geometry of the fuel rods. This improves the overall neutron economy of the reactor core and forms the design basis for supporting long-life cycle operation.

[0018] Furthermore, an annular cavity is formed between the pipe and the pressure-bearing pipe, and the annular cavity is used to fill inert gas; the annular cavity is connected to a gas composition analysis device, which is configured to detect changes in the concentration of carbonaceous compounds in the inert gas.

[0019] Through this technical means, the inert gas in the annular cavity first acts as a thermal barrier, effectively reducing the radial heat transfer from the high-temperature pressure pipe to the low-temperature heavy water pool during steady-state operation, thus ensuring heat conversion efficiency. Secondly, an online status monitoring mechanism is introduced. If a micro-leak occurs at the boundary of the pressure pipe, leading to the leakage of supercritical carbon dioxide, the gas composition analysis equipment can detect the abnormal concentration of newly added carbonaceous compounds in the background gas, thereby achieving an early safety warning of the pressure boundary failure.

[0020] Furthermore, the reactivity control assembly includes multiple control drums circumferentially distributed within the radial reflector layer; each control drum comprises a beryllium oxide matrix and a boron carbide neutron absorber eccentrically disposed within the beryllium oxide matrix, and the control drum is configured to adjust the reactivity of the reactor by rotating about its own axis.

[0021] This technology integrates the rotating drum into the external radial reflector layer, and by mechanically rotating it, changes the effective area and distance of the boron carbide neutron absorber towards the fuel active region, thereby regulating the reactor core reactivity. The fixed motion stroke and highly enclosed structure enhance the overall operational reliability of the system.

[0022] Furthermore, the multiple control drums include a first drum and a second drum arranged alternately in a ring; the first drum is a regulating drum for normal power regulation, and the second drum is a safety drum for emergency shutdown.

[0023] This technology allows the adjustment drum to be specifically designed to smooth out slow reactive fluctuations and load following caused by burnup, while the safety drum is normally kept at the position that introduces maximum positive reactivity and deflects rapidly only when it receives an abnormal operating condition signal, enabling the reactor to reliably achieve emergency shutdown.

[0024] Furthermore, the pressure tube reactor has a height of 0.5 meters to 1 meter and a diameter of 0.6 meters to 1.2 meters; and the uranium isotope enrichment in the uranium nitride-based fuel rods (41) is 15% to 25%.

[0025] Thanks to the combined synergistic effect of heavy water moderation, beryllium oxide reflection, and high-density uranium nitride fuel, the reactor can maintain criticality and operate for extended periods under low enrichment conditions of no more than 25%. This not only controls fuel enrichment costs and meets nuclear nonproliferation security requirements, but also ensures that the reactor's geometric envelope can be adapted to space-constrained applications.

[0026] In this regard, this application also provides the application of the supercritical carbon dioxide cooled pressure tubular reactor as described above as a nuclear power source for underwater unmanned equipment or deep-sea closed facilities.

[0027] In summary, this application has at least one of the following improvements: 1. This application isolates the pressurized, high-temperature supercritical carbon dioxide coolant from the external, low-temperature, atmospheric-pressure heavy water moderator. Heavy water shifts the neutron energy spectrum towards the intermediate energy region, significantly improving the neutron absorption efficiency of the gadolinium-based combustible poison. Through the direct matching of the heavy water moderation characteristics and the gadolinium-based poison absorption characteristics, the reactivity consumption of the reactor during long operating cycles is smoothly compensated, enabling the microreactor to achieve long-term operation without refueling under low enrichment conditions of no more than 25%.

[0028] 2. This application employs a stepped distribution of gadolinium-based combustible poison concentration from the inside out, addressing the issue of premature poison depletion in the central region under heavy water energy spectrum conditions. The moderating effect of heavy water shifts the energy spectrum towards the mid-energy region. While this mid-energy spectrum improves gadolinium absorption efficiency, it also leads to excessively rapid depletion of gadolinium poisons in the high-flux region of the core. The non-uniform concentration configuration, with higher concentrations at the inside and lower at the outside, directly compensates for the faster depletion rate in the central region, making the depletion time of gadolinium poisons more consistent across all regions of the core. This not only effectively reduces the central power peak in the early stages of operation but also ensures that the radial power distribution remains flat in the later stages of long-cycle operation (e.g., controlling the radial power peak factor to approximately 1.044), guaranteeing the reactor's thermal-hydraulic safety margin.

[0029] 3. This application employs a double-layered casing structure consisting of a manifold and a pressure-bearing pipe, using the external heavy water pool as a passive heat sink. In the event of a loss-of-coolant accident (LOCA), without external power supply or equipment intervention, the decay heat generated by the reactor core can be directly dissipated into the atmospheric pressure heavy water pool through the thermal radiation of the fuel rods and structural heat conduction, passing through the manifold and pressure-bearing pipe. This ensures that the maximum temperature of the fuel pellets and cladding remains within safe limits under accident conditions, achieving the inherent safety of the unit.

[0030] 4. An annular cavity filled with inert gas exists between the manifold and the pressure-bearing pipe, serving a dual function of thermal insulation and leak monitoring. During normal steady-state operation, this cavity acts as an insulation layer, reducing heat loss from the high-temperature pressure-bearing pipe to the low-temperature heavy water and ensuring the thermoelectric conversion efficiency of the secondary loop system. Simultaneously, this cavity connects to a gas composition analysis device. If a minor rupture occurs in the pressure-bearing pipe, leading to supercritical carbon dioxide leakage, the device can detect the increased concentration of carbonaceous compounds within the cavity, providing an early warning of damage to the pressure-bearing boundary. Attached Figure Description

[0031] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a cross-sectional structural diagram of an optional pressure tube in an embodiment of this application; Figure 2 This is a schematic diagram of the structure of an optional uranium nitride-based fuel rod according to an embodiment of this application; Figure 3 This is a schematic diagram of the radial cross-section of an optional reactor according to an embodiment of this application; exist Figures 1 to 3In the middle, 1. Moderator container; 2. Piping; 3. Pressure pipe; 31. Coolant channel; 4. Fuel assembly; 41. Uranium nitride-based fuel rod; 411. Fuel pellet; 412. Cladding; 413. Neutron reflector plug pellet; 51. Control drum; 511. First drum; 512. Second drum; 6. Annular cavity; Figure 4 The curve showing the change of the effective proliferation factor keff over running time is provided in the embodiments of this application; Figure 5 Neutron energy spectrum provided for embodiments of this application; Figure 6 This is a diagram showing the axial power distribution of the reactor core provided in an embodiment of this application. Figure 7 The core radial power distribution diagram provided in the embodiments of this application; Figure 8 A cloud map of steady-state axial temperature distribution of the pressure tube provided in an embodiment of this application; Figure 9 The curves showing the temperature variation over time of uranium nitride-based fuel rod cladding under accident conditions provided in this application embodiment; Figure 10 The curve showing the temperature change over time of uranium nitride-based fuel pellets under accident conditions provided in this application embodiment. Detailed Implementation

[0032] To enable those skilled in the art to better understand the technical solutions of this application, the technical solutions of this application will be further described below in conjunction with the accompanying drawings and embodiments.

[0033] Reference Figures 1 to 3 A supercritical carbon dioxide cooled pressure tube reactor includes a moderator container 1, multiple pressure tubes, fuel assemblies 4, and a reactivity control assembly. The moderator container 1 contains heavy water, and its radial outer periphery is covered with a radial reflective layer. The pressure tubes include manifolds 2 and pressure-bearing tubes 3, with multiple manifolds 2 and multiple pressure-bearing tubes 3 corresponding one-to-one. The multiple manifolds 2 are immersed in the heavy water in the moderator container 1. The multiple pressure-bearing tubes 3 are coaxially arranged in the manifolds 2, and the pressure-bearing tubes 3 have coolant channels 31 for circulating supercritical carbon dioxide. The fuel assemblies 4 are disposed in the pressure-bearing tubes 3. The reactivity control assembly is disposed within the radial reflective layer. The fuel assemblies 4 include uranium nitride-based fuel rods 41, which are non-uniformly doped with gadolinium-based combustible poison. The concentration distribution of the gadolinium-based combustible poison is configured to match the reactor energy spectrum shifted towards the intermediate energy region by the heavy water.

[0034] As a preferred embodiment, the reactor is designed with a thermal power of approximately 3.2 MW and a design life of up to 10 years, requiring no refueling during full-power operation. Its main coolant is supercritical carbon dioxide, with a design operating pressure of approximately 15 MPa, an inlet temperature of approximately 350 °C, and an outlet temperature of approximately 500 °C. This arrangement allows the high-temperature, high-pressure supercritical carbon dioxide coolant to circulate relatively independently within the pressure tube 3. Utilizing the high density and high specific heat capacity of supercritical fluids near the critical point, combined with the Brayton cycle, the volume of the secondary loop energy conversion system can be significantly reduced. Meanwhile, the external atmospheric-pressure, low-temperature heavy water provides a neutron moderation environment. This moderation effect causes the originally hard fast neutron energy spectrum to shift towards the ultrathermal neutron region (intermediate energy region), resulting in thermal and intermediate-energy neutrons accounting for approximately 65%. The shifted energy spectrum matches the large resonance absorption cross-section peak of gadolinium isotopes (such as 155Gd and 157Gd) in the mid-energy region, thus smoothly compensating for reactivity depletion through neutronics coupling between the fluid medium and the combustible poison over long operating periods. Furthermore, uranium nitride fuel possesses high thermal conductivity (approximately 20 W / m·K) and high uranium density (approximately 13.5 g / cm³). 3 Combined with the neutron reflection effect of the radial reflector layer, the overall neutron economy of the reactor core can be significantly improved.

[0035] In some embodiments, multiple uranium nitride-based fuel rods 41 are arranged in a multi-ring concentric array inside the pressure tube 3; and the mass fraction of gadolinium-based combustible poison exhibits a discrete, stepwise decreasing distribution from the inner to the outer ring of the concentric ring array. This radially stepwise decreasing arrangement of combustible poison effectively suppresses the high neutron flux and localized high reactivity in the central region during the initial stage of core operation, while providing relatively weaker suppression in the peripheral region. Gadolinium poison does not produce gaseous byproducts such as helium after absorbing neutrons, which helps maintain the stability of the fuel's macroscopic thermal conductivity. This geometric arrangement, combined with the burnup characteristics of gadolinium poison, helps to reduce the peak power at the core center, promoting a flatter radial power distribution, thereby controlling the radial power peak factor to approximately 1.044, and preserving sufficient thermal-hydraulic safety margin for the fuel assembly 4 throughout its long operational life.

[0036] Specifically, heavy water, acting as a moderator, shifts the neutron spectrum towards the mid-energy region, significantly increasing the absorption cross-section of gadolinium isotopes. However, this mid-energy spectrum also leads to a significantly faster consumption rate of gadolinium poisons in the high-flux region at the core center. If the combustible poisons are uniformly distributed, the gadolinium poisons in the core center region will be depleted prematurely in the later stages of operation, resulting in a local power surge. Therefore, this embodiment employs a discrete, stepped concentration distribution. This non-uniform design is specifically designed to counteract the rapid consumption of poisons in the central region caused by the mid-energy spectrum, ensuring that the gadolinium poisons in each region are consumed synchronously throughout the entire 10-year operational lifespan, thereby ensuring continuous suppression of central power and maintaining a stable radial power distribution.

[0037] Furthermore, the moderator tubes 2 are arranged in a regular polygonal array within the moderator container 1; and the number of uranium nitride-based fuel rods 41 inside the pressure tubes 3 is 19 in total, consisting of one central fuel rod and inner and outer ring fuel rods arranged around the central fuel rod, forming a double-layer annular array. The entire reactor core can be equipped with 19 pressure tubes 3, with the inner diameter of the pressure tubes 3 preferably being approximately 52 mm and the outer diameter approximately 58 mm; the inner diameter of the moderator tubes 2 is preferably approximately 66 mm and the outer diameter approximately 68 mm. This array arrangement optimizes the grid volume ratio of moderator to fuel within a limited micro-volume. The double-layer annular fuel assembly 4 with 19 rods appropriately reduces the fuel rod density within a single tube. This not only preserves the required convective heat transfer channel cross-sectional area for supercritical carbon dioxide fluid but also meets the design requirements for overall miniaturization and compact structure of the nuclear power plant.

[0038] Furthermore, the mass fraction of gadolinium-based combustible poison in the central fuel rod is 1.8% to 2.2%; The gadolinium-based combustible poison has a mass fraction of 0.8% to 1.2% in both the inner and outer fuel rods. This graded concentration configuration provides specific quantitative physical control parameters, which can limit the radial power peak factor within the safe design baseline while taking into account the total initial fissile material content of the uranium nitride fuel. This effectively avoids the potential risk of excessive temperature rise in the cladding 412 due to excessively high local heat flux density.

[0039] In some embodiments, uranium nitride-based fuel rods 41 are axially provided with neutron reflector plugs 413 made of non-fissile material at both ends away from the active region; wherein the neutron reflector plugs 413 are made of beryllium oxide. A single fuel rod has a diameter of approximately 9 mm and a total length of approximately 950 mm, with the active region height being approximately 650 mm. Addressing the axial neutron leakage problem caused by the increased specific surface area in microreactors, the high high-energy neutron scattering cross section and extremely low absorption characteristics of beryllium oxide material are utilized to reflect axially escaping neutrons back to the fuel active region without expanding the external geometric envelope of the fuel rod. This component feature improves the axial power distribution of the reactor core and reduces axial neutron leakage, providing a crucial physical basis for supporting long-cycle, low-enrichment operation of the reactor.

[0040] As a preferred embodiment, the pressure-bearing pipe 3 and the cladding 412 of the uranium nitride-based fuel rod 41 are made of oxide dispersion reinforced ferritic steel. Specifically, 12YWT type ODS steel can be used, which has high-density nano-scale Y-Ti-O clusters dispersed in its matrix. Utilizing the pinning effect of the nano-scale oxide clusters on dislocations, the pressure-bearing components and cladding 412 can be endowed with excellent high-temperature creep resistance (such as not fracturing under a load of 800 ℃ and 138 MPa for over 14,500 hours) and radiation swelling resistance. This feature enables it to maintain the relative integrity of its mechanical boundaries and prevent the escape of fission products when dealing with supercritical carbon dioxide corrosion environments and potential ultra-high temperature accident transient conditions.

[0041] In some embodiments, an annular cavity 6 is formed between the manifold 2 and the pressure-bearing pipe 3, and the annular cavity 6 is filled with an inert gas. The annular cavity 6 is connected to a gas composition analysis device configured to detect changes in the concentration of carbonaceous compounds in the inert gas. As an example, the inert gas is preferably argon. The inert gas in the annular cavity 6 acts as a physical thermal shielding layer, which can reduce the radial heat transfer rate from the high-temperature pressure-bearing pipe 3 to the low-temperature heavy water tank during steady-state operation (e.g., limiting heat leakage to about 0.642%), thus ensuring the thermoelectric conversion efficiency of the system. At the same time, a non-destructive online condition monitoring mechanism is introduced in conjunction with the gas composition analysis device. If a micro-leak occurs at the pressure-bearing boundary, causing the supercritical carbon dioxide working medium to seep out, the analysis device can detect the abnormal concentration of newly added carbonaceous compounds in the background gas, thereby achieving an early safety warning of pre-rupture leakage (LBB). In addition, in extreme conditions such as coolant loss, the double-layer tube structure combined with the moderator container 1 can use the atmospheric pressure heavy water pool as a passive final heat sink. It mainly relies on the thermal radiation and conduction across the annular cavity 6 to dissipate the core decay heat, ensuring that the highest cladding temperature 412 during an accident is within a controlled range, thus improving the safety of the device.

[0042] In some embodiments, the reactivity control assembly includes a plurality of control drums 51 circumferentially distributed within a radial reflector layer. Each control drum 51 comprises a beryllium oxide matrix and a boron carbide neutron absorber eccentrically disposed within the beryllium oxide matrix. The control drums 51 are configured to adjust the reactor reactivity by rotating about their own axis. Integrating the control assembly in the form of drums within the external radial reflector layer replaces the traditional direct-insertion control rod drive mechanism, avoiding the potential for dynamic seal leakage caused by mechanical penetration of the reactor pressure-bearing boundary. By mechanically rotating to change the effective absorption distance and projected area of ​​the boron carbide neutron absorber towards the fuel active region, the reactivity of the reactor core can be smoothly adjusted. The mechanism has a fixed stroke and a sealed overall structure, which helps improve the system reliability under long-term maintenance-free operation conditions.

[0043] In some embodiments, the plurality of control drums 51 include a first drum 511 and a second drum 512 arranged alternately in a ring; the first drum 511 is a regulating drum for normal power regulation, and the second drum 512 is a safety drum for emergency shutdown. The system may have six regulating drums and six safety drums with larger geometric dimensions evenly distributed in a ring. This configuration introduces a functional heterogeneity and redundancy distribution mechanism into the reactivity control system. The regulating drums are specifically designed to smooth out slow reactivity fluctuations caused by burnup and to perform load following, while the safety drums are normally positioned to introduce the greatest positive reactivity, deflecting rapidly only upon receiving an abnormal operating condition command. This function allows the control system to provide an independent and sufficient negative reactivity insertion margin (e.g., keff < 0.95) under accident conditions, ensuring the reliable execution of emergency shutdown actions.

[0044] To verify the feasibility and technical effectiveness of the technical solution of this application, the following data is provided for obtaining various performance verification data of this application. Figures 4 to 10 The specific implementation parameters and calculation model boundary conditions are as follows. Those skilled in the art can construct a complete physical and thermal model and reproduce the results of this application based on the following defined parameters: (1) Core thermal and fluid boundary: The reactor is designed to have a thermal power of 3.2 MW; the main coolant is supercritical carbon dioxide (S-CO2), the working pressure is set to 15 MPa, the core inlet temperature is 350 ℃, the outlet temperature is 500 ℃, and the single-channel inlet velocity is set to 7.9 m / s; the moderator is atmospheric pressure heavy water (D2O), and in the accident transient calculation, it is conservatively assumed that the moderator temperature remains constant at 350 K.

[0045] (2) Geometric and structural dimension constraints: A total of 19 pressure tubes are installed in the entire core, arranged in a regular polygonal array; among them, the inner diameter of the pressure tube 3 is 52 mm and the outer diameter is 58 mm; the inner diameter of the tube 2 sleeved on the outside is 66 mm and the outer diameter is 68 mm. Each pressure tube 3 contains 19 fuel rods (including 1 central rod and 18 fuel rods arranged in two rings around it). The outer diameter of a single fuel rod is 9 mm, and the total length is 950 mm, of which the height of the central active zone is 650 mm. Based on the above component dimensions, the effective calculated height envelope of the entire core is within 1 m, and the overall diameter is within 1.2 m.

[0046] (3) Materials physics and nuclide configuration: Fuel pellet 411 uses uranium nitride (UN) enriched at 25%, with non-fissile material BeO embedded at both ends of the fuel rod (150 mm area each) as end reflector layers; cladding 412, pressure tube and manifold 2 are all made of 12YWT type oxide dispersion strengthened (ODS) steel; the annular gap between the pressure tube and manifold 2 is filled with inert argon (Ar). The combustible poison is Gd2O3, which is non-uniformly doped into the UN fuel: the doping mass fraction in the central fuel rod is 2%, and the doping mass fraction in the two outer rings of fuel rods is 1%. A BeO reflector layer is set radially on the periphery, and 12 control drums 51 containing B4C absorbers and BeO matrix are evenly distributed circumferentially inside it.

[0047] This application Figures 4 to 10 The neutron physics properties (based on the OpenMC program) and the three-dimensional thermal-hydraulic properties (based on the Fluent program, using a 1 / 6 symmetric model, approximately 3.26 million grids, and the SST k-ω turbulence model and DO radiation model) shown are all verification results calculated based on the specific models constructed with the above-mentioned geometric boundaries and physical configurations.

[0048] Figure 4 The graph shows the effective multiplication factor (keff) over operating time. Neutron physics analysis based on the open-source Monte Carlo program (OpenMC) indicates that with beryllium oxide neutron reflector end plugs at both ends of the fuel rods, using uranium nitride fuel enriched to 25%, the keff value (black line in the graph) remains above the critical limit (1.0) throughout a 10-year continuous full-power operation period, and shows a steady decreasing trend with increasing burnup. For comparison, if the end beryllium oxide reflector structure is removed, to maintain the same thermal power and the keff requirement for a 10-year operating life, the fuel enrichment needs to be significantly increased to approximately 37% (red line in the graph). This comparative data confirms the objective physical role of the end beryllium oxide reflector in reducing axial neutron leakage and improving core neutron economy, and is a key feature supporting the reactor's long-life, compact design at a lower enrichment level (25%).

[0049] Figure 5The figure shows the neutron energy spectrum distribution in the reactor core under full-power operation. As shown in the logarithmic coordinate system, the horizontal axis represents neutron energy (unit: MeV), and the vertical axis represents normalized neutron flux. The distribution curves in the figure show that after using heavy water as a moderator, the neutron energy spectrum in the reactor core softened significantly, with the flux peak shifting noticeably towards the medium-energy region. Energy spectrum integration reveals that the total proportion of thermal neutrons and medium-energy neutrons (energy 0.625 eV ~ 0.1 MeV) in the core reached approximately 65.04%, while the proportion of fast neutrons (energy > 0.1 MeV) was limited to approximately 34.96%. This energy spectrum distribution data objectively confirms the efficient moderation effect of heavy water. It is precisely based on this neutron energy spectrum shift towards the medium-energy region shown in the figure that the large resonance absorption cross-section peak of gadolinium-based combustible poison in the medium-energy region can be accurately matched, thus supporting the long-life reactivity control of this embodiment under low fuel enrichment (25%) conditions.

[0050] Figure 6 The figure shows the axial power distribution of the reactor core under full-power operation. As indicated by the scatter plot and fitted curve, thanks to the heavy water moderation mechanism and the confinement of neutrons by the end reflectors, the axial power in the active region of the core (approximately 650 mm in height) exhibits an approximately symmetrical cosine distribution. Numerical extraction reveals that the axial power peak factor is approximately 1.27. This distribution pattern indicates that the axial neutron flux within the reactor is reasonably controlled, avoiding severe local power distortion in the axial direction, thus providing a relatively gentle heat flux density boundary for the heat transfer analysis of the fuel rod cladding 412.

[0051] Figure 7 This diagram compares the radial power distribution of the reactor core under different arrangements of combustible poisons. It shows the relative power distribution across the 19 pressure tubes and 3 channels of the entire core. In the uniform arrangement without graded doping (marked as black values ​​at the bottom of the diagram), the radial power peak factor of the central channel is 1.062. However, in the partitioned arrangement using a discrete, stepped decrease in the mass fraction of gadolinium-based combustible poison from the inside out (i.e., 2% doping in the inner ring and 1% in the outer ring) (marked as red values ​​at the top of the diagram), the power peak in the core center is effectively suppressed, reducing the maximum radial power peak factor of the entire core to 1.044. This comparison quantitatively confirms the actual effect of the stepped poison arrangement on flattening the radial power of the core, ensuring that the core power peak factor meets the "less than 1.4" requirement in conventional nuclear safety design.

[0052] To verify the thermo-hydraulic characteristics and thermal safety margin of the reactor structure in this embodiment, a three-dimensional thermo-hydraulic coupling analysis of the full-power steady-state condition was performed based on computational fluid dynamics (CFD) methods (as an example, the SST k-ω turbulence model and discrete coordinate radiation model can be used in the calculation). Considering that the pressure tube geometry model has good circumferential symmetry, Figure 8 The static temperature calculation results based on the 1 / 6 symmetric model are shown.

[0053] like Figure 8 The temperature gradient distribution and numerical extraction results show that, under steady-state operation, the highest steady-state temperature of fuel pellet 411 is approximately 1769 K, and the highest steady-state temperature of fuel rod cladding 412 is approximately 1063 K. These temperature peaks are controlled within the safe limits of the core design under normal operating conditions (i.e., cladding 412 temperature below 1073 K and pellet temperature below 1873 K). The maximum temperature difference between the outlets of each cooling channel is 111 K, demonstrating the rationality of the structural design of fuel assembly 4 and coolant channel 31.

[0054] Furthermore, the analysis data further indicates that the heat leakage into the heavy water moderator across the annular cavity 6 during steady-state operation accounts for only about 0.642% of the total heat released by the fuel rods in the entire reactor; the steady-state temperature of the outer surface of the manifold 2 is approximately 355 K, only slightly higher than the ambient temperature of the incoming moderator (350 K). These calculations objectively confirm that the argon annular cavity 6 between the manifold 2 and the pressure pipe 3, combined with the physical properties of the supercritical carbon dioxide fluid itself, constructs an effective thermal barrier inside the reactor, significantly limiting the ineffective heat dissipation from the high-temperature region of the reactor core to the ambient-temperature moderator, thus ensuring the overall high-efficiency thermoelectric conversion capability of the system in the miniaturized design.

[0055] For the large breach accident scenario, the decay heat removal capacity analysis is conducted based on the following conservative assumptions: the moderator cooling system remains intact, and the moderator temperature is maintained at 350 K during the transient process; the entire pressure pipe remains intact, and the convective heat transfer coefficient with the moderator is 1500 W / (m²·K); natural convection of the coolant is not considered, and decay heat is removed solely through radiation and conduction. The decay heat power is derived from the fuel consumption calculation results.

[0056] Reference Figure 9 Transient calculations show that in the initial stage after reactor shutdown (within the first 4 seconds), due to the deterioration of convective heat transfer caused by coolant loss, the temperature of the cladding 412 of each fuel rod ring exhibits a sharp upward trend. Under this accident condition, radiative heat transfer across the annular cavity 6 becomes the main pathway for dissipating decay heat (radiative heat dissipation accounts for more than 80% of the total heat dissipation of the cladding 412 after 4 seconds of shutdown). Under this physical mechanism, the radiative heat dissipation capacity is stronger because the temperature difference between the outer ring fuel rods and the pressure tubes and their equivalent heat transfer area are significantly larger than those of the central and inner ring fuel rods. Figure 9As can be clearly seen, the outer fuel rod cladding 412 reaches its peak temperature and begins to decline at approximately 3 seconds; while the cladding 412 of the central and inner fuel rods reaches its highest temperature successively between 110 and 120 seconds. Throughout the accident process, thanks to the passive heat absorption effect of the heavy water moderator as the final heat sink, the highest cladding 412 temperature is effectively limited to approximately 1578 K (corresponding to an accident safety limit below 1400 °C). This pressure tube geometry and heat transfer characteristics constitute one of the key factors contributing to the reactor's good natural safety characteristics under accident conditions.

[0057] Reference Figure 10 Corresponding to the physical process of the temperature response of the cladding 412, within a very short time after the reactor shutdown is triggered and the fission reaction terminates, the heat accumulated inside the fuel pellet 411 is rapidly transferred to the cladding 412 because it no longer generates fission heat. Figure 10 As can be seen from the magnified view, after the initial drop in temperature of fuel pellet 411, a slight rebound occurred due to the decay heat power remaining at a high level for a short period while the heat dissipation path was limited by radiative heat transfer efficiency. Subsequently, it entered a monotonic decline phase. The peak pellet temperature during the accident was approximately 1590 K, far below the accident safety limit. Throughout the entire accident, the highest pellet temperature remained at approximately 1590 K, strictly meeting the safety design limit of not exceeding 1800℃ (i.e., 2073.15 K) under accident conditions. This data verifies the inherent safety of this microreactor design.

[0058] In this embodiment, the height of the pressure tubular reactor is preferably 0.5 meters to 1 meter, and the diameter is preferably 0.6 meters to 1.2 meters; the uranium isotope enrichment in the uranium nitride-based fuel rods 41 is preferably 15% to 25% (preferably 20% to 25% for a lifespan of 10 years or more).

[0059] The aforementioned size range is based on the physical requirements of adapting to the limited space of unmanned underwater vehicles (UUVs), while its lower limit ensures that the reactor has sufficient volume. The upper limit of uranium isotope enrichment is set below 25% to comply with international nuclear non-proliferation guidelines and the utilization standards of high-enriched low-enriched uranium (HALEU); the lower limit of the size range ensures that, under the constraint of miniaturized volume, combined with high-density uranium nitride fuel and heavy water moderation mechanisms, the reactor core can provide sufficient backup reactivity to meet the long-term operation requirements of more than 10 years. This parameter configuration achieves a balance between the reactor's geometry and nuclear physics performance, making long-term operation in a compact space physically feasible.

[0060] Based on this, on the other hand, this application also provides the application of the supercritical carbon dioxide cooled pressure tubular reactor, as described above, as a nuclear power source for unmanned underwater equipment or deep-sea confined facilities. Applying the aforementioned pressure tubular reactor, which combines high energy conversion density, long cycle time, and passive safety features, to deep-sea or underwater confined scenarios can effectively overcome the energy density bottleneck and self-sufficiency limitations faced by traditional power sources, thereby providing megawatt-level power support for unmanned underwater platforms with long endurance and high stealth characteristics.

[0061] The foregoing has shown and described the basic principles, main features, and advantages of this application. Those skilled in the art should understand that this application is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of this application. Various changes and modifications can be made to this application without departing from the spirit and scope thereof, and all such changes and modifications fall within the scope of this application as claimed. The scope of protection of this application is defined by the appended claims and their equivalents.

Claims

1. A supercritical carbon dioxide-cooled pressure tubular reactor, characterized in that, include: The moderator container (1) contains heavy water and is covered with a radial reflective layer on its radial outer periphery. Multiple pipes (2) are submerged in heavy water within the moderating agent container (1); Multiple pressure-bearing pipes (3) are installed in the pipe (2) in a corresponding manner, and the pressure-bearing pipes (3) have coolant channels (31) for circulating supercritical carbon dioxide inside. Fuel assembly (4), disposed within the pressure pipe (3); and A reactive control component is disposed within the radial reflective layer; The fuel assembly includes uranium nitride-based fuel rods (41) which are non-uniformly doped with gadolinium-based combustible poisons. The concentration distribution of the gadolinium-based combustible poisons is configured to match the reactor energy spectrum shifted toward the medium energy region by the heavy water.

2. The supercritical carbon dioxide cooled pressure tubular reactor according to claim 1, characterized in that: Multiple uranium nitride-based fuel rods (41) are arranged in a multi-ring array inside the pressure tube (3); Along the inner to outer ring of the annular array, the mass fraction of the gadolinium-based combustible poison exhibits a discrete decreasing distribution.

3. The supercritical carbon dioxide cooled pressure tubular reactor according to claim 2, characterized in that: The pipes (2) are arranged in a polygonal array inside the moderating agent container (1); Furthermore, the number of uranium nitride-based fuel rods (41) inside the pressure-bearing pipe (3) is 19, consisting of one central fuel rod and inner and outer ring fuel rods arranged around the central fuel rod, forming a double-layer ring array.

4. The supercritical carbon dioxide cooled pressure tubular reactor according to claim 3, characterized in that: The mass fraction of gadolinium-based combustible toxin in the central fuel rod is 1.8% to 2.2%. The mass fraction of gadolinium-based combustible toxin in the inner and outer fuel rods is 0.8% to 1.2%.

5. The supercritical carbon dioxide cooled pressure tubular reactor according to claim 2, characterized in that: The uranium nitride-based fuel rod (41) has neutron reflector plugs (413) made of non-fissile material at both ends. The neutron reflector plug (413) is made of beryllium oxide.

6. The supercritical carbon dioxide cooled pressure tubular reactor according to claim 1, characterized in that: An annular cavity (6) is formed between the pipe (2) and the pressure-bearing pipe (3), and the annular cavity (6) is used to fill inert gas; The annular cavity (6) is connected to a gas composition analysis device, which is configured to detect changes in the concentration of carbonaceous compounds in the inert gas.

7. The supercritical carbon dioxide cooled pressure tubular reactor according to claim 1, characterized in that: The reactive control assembly includes a plurality of control drums (51) disposed within the radial reflective layer; The control drum (51) includes a beryllium oxide matrix and a boron carbide neutron absorber eccentrically disposed inside the beryllium oxide matrix, and the control drum (51) is configured to adjust the reactivity of the reactor by rotating about its own axis.

8. The supercritical carbon dioxide cooled pressure tubular reactor according to claim 7, characterized in that: The plurality of control drums (51) include a first drum (511) and a second drum (512) arranged in annular alternation. The first drum (511) is an adjustment drum for normal power regulation, and the second drum (512) is a safety drum for emergency shutdown.

9. The supercritical carbon dioxide cooled pressure tubular reactor according to claim 1, characterized in that: The pressure tube reactor has a height of 0.5 meters to 1 meter and a diameter of 0.6 meters to 1.2 meters; Furthermore, the uranium isotope enrichment in the uranium nitride-based fuel rod (41) is 15% to 25%.

10. The application of the supercritical carbon dioxide cooled pressure tubular reactor as described in any one of claims 1-9 as a nuclear power source for underwater unmanned equipment or deep-sea enclosed facilities.

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