Reactor core of thorium-based lead-cooled small modular reactor and design method

Abstract

The invention provides a reactor core of a thorium-based lead-cooled small modular reactor and a design method. The whole reactor core is of a cylinder structure; the reactor core adopts thorium-based fuel, takes martensite heat-resistant steel as a reactor core structure material, and takes a lead-bismuth coolant as a reactor coolant; the fuel rods and the control rods are arranged in the hexagonal grid so as to be arranged in the active reactor core area; the reactor core control system adopts boron carbide as an absorbing material; outside the active reactor core area, lead is used as a reflecting layer, and martensite heat-resistant steel is used as a shielding layer. Efficient and safe combination of thorium-uranium fuel circulation and a lead-bismuth coolant can be realized, so that an inherent safe compact reactor core is formed, and the possibility of melt-down of the reactor core is fundamentally eradicated; through the optimized physical design of the reactor core, autonomous operation with a long life cycle is realized, the number of refueling times is reduced, and operation and maintenance are simplified.

Description

Technical Field

[0001] This invention relates to the field of nuclear reactor engineering, specifically to a thorium-based lead-cooled small modular reactor core and its design method. Background Technology

[0002] With the continued growth of global energy demand, the need for low-carbon and sustainable energy is increasing. Nuclear energy, as a high-energy-density baseload energy source, is becoming increasingly important. However, traditional large light water reactors (LWRs) face numerous challenges, including high investment costs, long construction periods, nuclear proliferation risks, spent fuel disposal, and public concerns about safety. To address these challenges, small modular reactors (SMRs), with their higher safety, more flexible plant deployment, lower initial investment, and better modular construction capabilities, have become an important development direction for next-generation nuclear energy systems. Regarding reactor coolant selection, liquid lead-bismuth eutectic alloy (LBE) has a lower melting point than pure lead, enabling high-temperature operation at atmospheric pressure, thus significantly improving thermal efficiency. Furthermore, LBE possesses excellent thermo-hydraulic properties, high heat capacity, and a high boiling point, providing strong passive heat dissipation capabilities, which can significantly enhance the inherent safety of the reactor. In addition, LBE has a weak neutron moderation effect, making it very suitable as a coolant for fast neutron spectrum reactors. In the field of nuclear fuel, the application of thorium-based fuels is considered one of the key pathways to achieving sustainable nuclear energy development. Thorium is far more abundant in nature than uranium, and 232 Th can transform into a fissile form after absorbing a neutron. 233 U, thus achieving efficient energy output. In addition, the thorium fuel cycle produces far fewer actinide nuclides than the uranium-plutonium fuel cycle, which can significantly reduce the toxicity and stockpile of long-lived radioactive waste. Summary of the Invention

[0003] Against this backdrop, there is an urgent need in the field for a novel small modular reactor system that can fully leverage the dual advantages of thorium fuel and lead-bismuth coolant while effectively addressing the challenges associated with their combination. Therefore, this invention provides a thorium-based lead-cooled small modular reactor core and its design method, enabling efficient and safe integration of the thorium-uranium fuel cycle with the lead-bismuth coolant to form an inherently safe and compact core, fundamentally eliminating the possibility of core meltdown. Through optimized core physics design, it achieves autonomous operation over a long lifespan, reducing refueling frequency and simplifying maintenance. Furthermore, the integrated core structure is suitable for modular manufacturing, transportation, and installation.

[0004] A core and design method for a thorium-based lead-cooled small modular reactor with inherent safety and long service life.

[0005] To achieve the above-mentioned technical features, the present invention aims to provide a core for a thorium-based lead-cooled small modular reactor, wherein the core is cylindrical in shape; the core uses thorium-based fuel, martensitic heat-resistant steel as the core structural material, and lead-bismuth coolant as the reactor coolant; fuel rods and control rods are arranged in a hexagonal grid within the active core region; the core control system uses boron carbide as the absorber material; outside the active core region, lead is used as a reflective layer, and martensitic heat-resistant steel is used as a shielding layer.

[0006] Preferably, the thorium-based fuel is a mixture of plutonium nitride and thorium nitride; wherein plutonium nitride is a fission fuel and thorium nitride is a breeder fuel.

[0007] Preferably, the enrichment degree of the blended fuel is defined as: ; In the formula, The enrichment level of the blended fuel. The total mass of fissile nuclides. The total mass of the fissile nuclei.

[0008] Preferably, the lead-bismuth coolant is a liquid lead-bismuth eutectic alloy.

[0009] Preferably, the reactor core has a negative reactivity coefficient.

[0010] Another aspect of the present invention provides a core design method for a thorium-based lead-cooled small modular reactor, comprising the following steps: S1: Design the overall structure and materials of the reactor core; S2: Design the layout scheme of the reactor core; S3: Analyze the control function of the core control system to ensure safety and reliability; S4: Analyze the hot zero-power (HZP) state of the reactor core; S5: Analyze the burnup characteristics of the reactor core, including its lifespan, power distribution, and effective share of delayed neutrons, to ensure that the reactor core has a long lifespan while maintaining a uniform and reasonable power distribution and an effective share of delayed neutrons within a reasonable range. S6: Analyze the fuel temperature coefficient, coolant temperature coefficient, and voiding coefficient of the reactor core to ensure that the reactor core has a negative reactivity coefficient.

[0011] Preferably, in the process of designing the overall structure and materials of the reactor core in S1, the Monte Carlo program OpenMC is used for analysis to perform full three-dimensional high-precision modeling of the designed reactor core. While ensuring the high fidelity of the reactor core design, safety, economy and environmental requirements are considered to ensure that the nuclear reactor can operate efficiently, safely and reliably. Preferably, in S1, martensitic heat-resistant steel is used as the core support structure and the reflector layer is isolated from the internal coolant; the arrangement of the reflector layer improves neutron utilization and flattens the radial power distribution of the outermost fuel assembly to improve safety; the core has a total of five grid frames, and the grid frames of the second ring of control rod assemblies extend vertically to form core support columns to ensure the structural stability of the core center region.

[0012] Preferably, in S2, the reactor core adopts a three-zone fuel distribution method to ensure a flattened core power distribution. The fuel mixture enrichment is 12.0% to 15.0%, with the enrichment gradually increasing from the inside to the outside. The core control system (RCS) includes core control rod assemblies arranged on the innermost side of the core, using moderately enriched natural boron carbide absorbers; the core shutdown system (RSS) includes core shutdown rod assemblies with the same geometry as the RCS, arranged at the center and outer sides of the core, using enriched boron carbide absorbers. Preferably, in step S3, when analyzing the control action of the control system, the effective increment coefficient is calculated when all control rods are inserted at the beginning of the lifespan. k eff Calculated values ​​between 0.95000 and 0.97000 are sufficient to keep the reactor core in a subcritical state, conforming to the core control rod design criteria; the effective increment factor is [value missing] when all control rods and shutdown rods are inserted at the beginning of the reactor's lifespan. k eff The calculated values ​​are between 0.85000 and 0.90000, indicating that the core has sufficient shutdown depth and meets the core shutdown rod design criteria.

[0013] Preferably, in the S4 step analysis of the HZP state of the reactor core, the control rod step division is as follows: step 0 is when the control rod is fully inserted, and step 240 is when the counterweight block with a certain neutron absorption cross section of the control rod leaves the active region of the reactor core. The single step size is 0.75 cm. Multiple simulations are performed for all control rod steps, and the effective increment coefficient is... k eff When the calculated value is between 0.99950 and 1.00050, the core is considered to be in the HZP state, and the number of steps for all control rods in the corresponding HZP state is 59. Preferably, in step S5, when analyzing the core burnup characteristics, under hot full-power conditions, the core lifespan exceeds 20 years, and the effective value-added coefficient is calculated at 23 years. k effBetween 0.99950 and 1.00050, the reactor core is in a state of readiness for shutdown. The radial power factor of the core throughout its lifespan is between 0.72 and 1.29, and the axial power factor of the core, which is further subdivided into nine layers, is between 0.5 and 1.6. The power distribution of the core is uniform and reasonable, and the effective share of delayed neutrons is between 0.28% and 0.30%.

[0014] Preferably, when S6 analyzes the reactivity coefficient of the reactor core, both the fuel temperature coefficient and the coolant temperature coefficient are negative, exhibiting negative temperature coefficients; the cavitation coefficient increases from -0.011617 to -0.098594 with increasing cavitation fraction, exhibiting a negative cavitation coefficient; when the cavitation fraction in the reactor core coolant increases, the reactivity of the reactor core decreases; and the decrease in reactor core reactivity becomes increasingly significant with increasing cavitation fraction; when the cavitation fraction is 100%, a scenario of complete coolant loss is simulated, and the decrease in reactivity is greater than 4000 pcm; in the case of complete coolant loss, even without relying on the control rods and shutdown rods, the reactor core can reach a subcritical state by utilizing the residual heat removal system, thereby achieving a safe shutdown.

[0015] The present invention has the following beneficial effects: 1. This invention targets fourth-generation reactor systems and proposes a core design scheme for a thorium-based lead-cooled small modular reactor. This core utilizes abundant thorium resources while possessing inherent good safety and a long service life.

[0016] 2. The reactor core of this invention adopts a modular design, which significantly simplifies the system structure, reduces the number of welds and interfaces, and thus greatly improves reliability. This feature makes it highly compatible with the concept of factory manufacturing, overall transportation, and rapid on-site installation of small modular reactors (SMRs). Attached Figure Description

[0017] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0018] Figure 1 This is a radial cross-sectional view of the reactor core of the present invention.

[0019] Figure 2 This is an axial cross-sectional view of the reactor core of the present invention.

[0020] Figure 3 This is a core layout diagram of the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0022] Example 1: See Figure 1-2 This paper presents radial and axial cross-sectional views of the reactor core, as well as its layout. The core of a thorium-based lead-cooled small modular reactor is cylindrical in shape. It uses thorium-based fuel, martensitic heat-resistant steel as the core structural material, and lead-bismuth coolant as the reactor coolant. Fuel rods and control rods are arranged in a hexagonal grid within the active core region. The core control system uses boron carbide as the absorber material. Outside the active core region, lead is used as a reflector layer, and martensitic heat-resistant steel as a shielding layer. This core achieves an efficient and safe combination of the thorium-uranium fuel cycle and lead-bismuth coolant, resulting in an inherently safe and compact core that fundamentally eliminates the possibility of core meltdown. Through optimized core physics design, it achieves autonomous operation over a long lifespan of more than 20 years, reducing refueling frequency and simplifying operation and maintenance. Furthermore, its integrated core structure is suitable for modular manufacturing, transportation, and installation.

[0023] Furthermore, the thorium-based fuel is a mixture of plutonium nitride and thorium nitride; wherein plutonium nitride is a fission fuel and thorium nitride is a breeder fuel.

[0024] Furthermore, the enrichment degree of the blended fuel is defined as: ; In the formula, The enrichment level of the blended fuel. The total mass of fissile nuclides. The total mass of the fissile nuclei.

[0025] Furthermore, the lead-bismuth coolant is a liquid lead-bismuth eutectic alloy.

[0026] Furthermore, the reactor core has a negative reactivity coefficient.

[0027] Example 2: See Figure 1-3 This embodiment provides a core design method for a thorium-based lead-cooled small modular reactor, including the following steps: S1: Design the overall structure and materials of the reactor core; In the core design, the open-source Monte Carlo program OpenMC was used for analysis. The program's high modeling flexibility and computational accuracy were fully utilized to create a high-precision, full-3D model of the designed core. This ensured high fidelity in the core design while also considering safety, economic, and environmental requirements, guaranteeing the efficient, safe, and reliable operation of the nuclear reactor.

[0028] The reactor core is cylindrical in shape, using a PuN-ThN hybrid fuel system with plutonium nitride (PuN) as fission fuel and thorium nitride (ThN) as breeder fuel. The core structure is constructed from highly corrosion-resistant martensitic heat-resistant steel (T91), and the reactor coolant is lead-bismuth, a low-melting-point lead-based material. The fuel rods and control rods are arranged in a hexagonal grid within a near-circular active core region. Outside the active core region, lead, which is more economical than LBE, serves as a reflector. A 1.45 cm thick T91 layer, providing core support, isolates the reflector from the internal coolant. This reflector arrangement significantly improves neutron utilization, flattens the radial power distribution of the outermost fuel assemblies, and provides a degree of safety. The core has five grid layers; the grid for the second ring of control rod assemblies extends vertically to form core support columns, ensuring structural stability in the central core region.

[0029] S2: Design the layout scheme of the reactor core; The reactor core employs a three-zone fuel distribution, which facilitates the flattening of the core power distribution. The innermost fuel layer has a mixed fuel enrichment of 12.0%, the next innermost layer 13.5%, and the outermost layer 15.0%. The core control system (RCS) includes core control rod assemblies located on the next innermost side of the core, using moderately enriched natural boron carbide (40% abundance) absorber material. The RSS includes core shutdown rod assemblies with the same geometry as the RCS, located at the core center and outer sides, using enriched boron carbide (85% abundance) absorber material. Lead serves as the reflector layer on the outer side of the active core, and T91 serves as the shielding layer.

[0030] S3: Analyze the control function of the core control system to ensure safety and reliability; Among them, when all control rods are inserted at the beginning of the service life, the effective increment coefficient is... k eff The calculated value is 0.97571, which allows the core to be in a subcritical state, conforming to the core control rod design criteria; the effective increment factor is [value missing] when all control rods and shutdown rods are inserted at the beginning of the reactor's lifespan. k eff The calculated value is 0.85569, indicating that the core has sufficient shutdown depth and meets the core shutdown rod design criteria.

[0031] S4: Analyze the hot zero-power (HZP) state of the reactor core; Regarding the step division for control rods, step 0 is defined as the complete insertion of the control rod, and step 240 is defined as the removal of the counterweight block with a certain neutron absorption cross-section from the active core region of the control rod. The step size is 0.75 cm. Multiple simulations were performed for all control rod steps. k effWhen the calculated value is 1.00007, the core is considered to be in the HZP state, and the corresponding number of steps for all control rods in the HZP state is 59. The power on the outer side of the core is higher than that on the inner side. This phenomenon is caused by the combined effects of fuel enrichment zoning design, absorber rod arrangement, and neutron flux spatial regulation. Moreover, this power distribution design aims to avoid excessively high power peaks in the core center region.

[0032] S5: Analyze the burnup characteristics of the reactor core, including its lifespan, power distribution, and effective share of delayed neutrons, to ensure that the reactor core has a long lifespan while maintaining a uniform and reasonable power distribution and an effective share of delayed neutrons within a reasonable range. Under 75 MWth hot full-power conditions, the core lifespan exceeds 20 years. Calculations were made at 23 years. k eff The power factor is 1.00028, indicating the core is ready for shutdown at any time. Throughout its lifespan, the radial power factor of the core ranges from 0.72 to 1.29, and the axial power factor, when the rods are further subdivided into nine layers, ranges from 0.5 to 1.6, demonstrating a uniform and reasonable power distribution within the core. Furthermore, the effective fraction of delayed neutrons ranges from 0.28% to 0.30%.

[0033] S6: Analyze the fuel temperature coefficient, coolant temperature coefficient, and voiding coefficient of the reactor core to ensure that the reactor core has a negative reactivity coefficient.

[0034] The fuel temperature coefficient and the coolant temperature coefficient are -0.30284×10⁻⁵ K. -1 and -0.00846×10⁻⁵K -1 The core exhibits a negative temperature coefficient. The void fraction increases from -0.011617 to -0.098594, also exhibiting a negative void fraction. As the void fraction in the core coolant increases, the core reactivity decreases. Furthermore, the decrease in core reactivity becomes increasingly pronounced with increasing void fraction. A simulation of complete coolant loss at a void fraction of 100% resulted in a reactivity decrease greater than 4000 pcm. Even in the event of complete coolant loss, without relying on control rods and shutdown rods, the core can easily reach a subcritical state through the proper utilization of the core residual heat removal system, thereby achieving a safe shutdown.

[0035] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A core of a thorium-based lead-cooled small modular reactor, characterized in that, The reactor core has an overall cylindrical structure; the reactor core uses thorium-based fuel, martensitic heat-resistant steel as the core structural material, and lead-bismuth coolant as the reactor coolant; fuel rods and control rods are arranged in hexagonal grids within the active core region; the core control system uses boron carbide as the absorber material; outside the active core region, lead is used as a reflective layer and martensitic heat-resistant steel as a shielding layer.

2. The core of a thorium-based lead-cooled small modular reactor according to claim 1, characterized in that, The thorium-based fuel is a mixture of plutonium nitride and thorium nitride; wherein plutonium nitride is a fission fuel and thorium nitride is a breeder fuel.

3. The core of a thorium-based lead-cooled small modular reactor according to claim 2, characterized in that, The enrichment degree of the mixed fuel is defined as: ； In the formula, The enrichment level of the blended fuel. The total mass of fissile nuclides. The total mass of the fissile nuclei.

4. The core of a thorium-based lead-cooled small modular reactor according to claim 1, characterized in that, The lead-bismuth coolant is a liquid lead-bismuth eutectic alloy.

5. The core of a thorium-based lead-cooled small modular reactor according to claim 1, characterized in that, The reactor core has a negative reactivity coefficient.

6. The core design method for a thorium-based lead-cooled small modular reactor according to any one of claims 1-5, characterized in that, Includes the following steps: S1: Design the overall structure and materials of the reactor core; S2: Design the layout scheme of the reactor core; S3: Analyze the control function of the core control system to ensure safety and reliability; S4: Analyze the hot zero-power (HZP) state of the reactor core; S5: Analyze the burnup characteristics of the reactor core, including its lifespan, power distribution, and effective share of delayed neutrons, to ensure that the reactor core has a long lifespan while maintaining a uniform and reasonable power distribution and an effective share of delayed neutrons within a reasonable range. S6: Analyze the fuel temperature coefficient, coolant temperature coefficient, and voiding coefficient of the reactor core to ensure that the reactor core has a negative reactivity coefficient.

7. The core design method for a thorium-based lead-cooled small modular reactor according to claim 6, characterized in that, In the process of designing the overall structure and materials of the reactor core in S1, the Monte Carlo program OpenMC is used for analysis to perform full three-dimensional high-precision modeling of the designed reactor core. While ensuring the high fidelity of the reactor core design, safety, economy and environmental requirements are considered to ensure that the nuclear reactor can operate efficiently, safely and reliably. In S1, martensitic heat-resistant steel is used as the core support structure and the reflector layer is isolated from the internal coolant. The arrangement of the reflector layer improves neutron utilization and flattens the radial power distribution of the outermost fuel assembly to improve safety. The core has a total of five grid frames. The grid frames of the second ring of control rod assemblies extend vertically to form core support columns to ensure the structural stability of the core center area.

8. The core design method for a thorium-based lead-cooled small modular reactor according to claim 6, characterized in that, The S2 core adopts a three-zone fuel distribution method to ensure a flattened core power distribution. The fuel mixture enrichment is 12.0% to 15.0%, with the enrichment gradually increasing from the inside to the outside. The core control system (RCS) includes core control rod assemblies arranged on the innermost side of the core, using moderately enriched natural boron carbide absorbers; the core shutdown system (RSS) includes core shutdown rod assemblies with the same geometry as the RCS, arranged at the center and outer sides of the core, using enriched boron carbide absorbers. In S3, when analyzing the control action of the control system, the effective increment coefficient is calculated when all control rods are inserted at the beginning of the lifespan. k eff Calculated values ​​between 0.95 and 0.97 are sufficient to keep the core in a subcritical state, conforming to the core control rod design criteria; when all control rods and shutdown rods are inserted at the beginning of the reactor's lifespan, the effective increment factor is [missing value]. k eff The calculated values ​​are between 0.85 and 0.90, indicating that the core has sufficient shutdown depth and meets the core shutdown rod design criteria.

9. The core design method for a thorium-based lead-cooled small modular reactor according to claim 6, characterized in that, In the S4 analysis of the HZP state of the reactor core, regarding the step division of the control rods, step 0 is defined as the complete insertion of the control rods, and step 240 is defined as the departure of the counterweight block with a certain neutron absorption cross-section from the active region of the core. The single step size is 0.75 cm. Multiple simulations are performed for all control rod steps, and the effective increment coefficient is... k eff When the calculated value is between 0.99950 and 1.00050, the core is considered to be in the HZP state, and the number of steps for all control rods in the corresponding HZP state is 59. In the analysis of the core burnup characteristics in S5, under hot full-power conditions, the core lifespan exceeds 20 years, and the effective value-added coefficient is calculated at 23 years. k eff Between 0.99950 and 1.00050, the reactor core is in a state of readiness for shutdown. The radial power factor of the core throughout its lifespan is between 0.72 and 1.29, and the axial power factor of the core, which is further subdivided into nine layers, is between 0.5 and 1.

6. The power distribution of the core is uniform and reasonable, and the effective share of delayed neutrons is between 0.28% and 0.30%.

10. The core design method for a thorium-based lead-cooled small modular reactor according to claim 6, characterized in that, When S6 analyzes the reactivity coefficient of the reactor core, both the fuel temperature coefficient and the coolant temperature coefficient are negative, exhibiting negative temperature coefficients. The void fraction increases from -0.011617 to -0.098594, also exhibiting negative void fractions. As the void fraction in the core coolant increases, the reactivity of the reactor core decreases. Furthermore, the decrease in core reactivity becomes increasingly pronounced with the increase in void fraction. A simulation of complete coolant loss was performed at a void fraction of 100%, resulting in a reactivity decrease of more than 4000 pcm. In the event of complete coolant loss, even without relying on control rods and shutdown rods, the reactor core can reach a subcritical state by utilizing the core residual heat removal system, thereby achieving a safe shutdown.

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