Fuel assembly for axial segmentation of control rod, reactor core structure and operation method

Abstract

The invention relates to the technical field of reactor core structures of nuclear reactors, and discloses a fuel assembly with axially segmented control rods, a reactor core structure and an operation method.The fuel assembly with the axially segmented control rods comprises a plurality of fuel rods and a compensation rod, the fuel rods are arranged around the compensation rod, the compensation rod is divided into five sections in the axial direction, and the compensation rod is connected with the reactor core structure. A strong neutron absorber section, a weak neutron absorber section, a neutron absorber-free section, a moderator section and a fuel section are sequentially arranged from top to bottom. The control rod is axially and finely segmented to reduce the reactivity control difficulty of the reactor core at the beginning of the service life, effectively prolong the service life of the reactor core and improve the fuel utilization rate of the reactor core, and according to the operation method, the performance of the reactor core can be effectively improved.

Description

Technical Field

[0001] This invention relates to the field of nuclear reactor technology, and more specifically, to a fuel assembly with axially segmented control rods, a reactor core structure, and an operating method. Background Technology

[0002] From a safety perspective, reactor cores require a reactivity control system, which is a device that can introduce or reduce the reactivity of the core. At the same time, since the reactivity of the core is often lost due to energy output during operation, the design of reactivity introduction devices in long-life cores is particularly important.

[0003] Existing reactor core reactivity devices employ a spectral-shift control strategy in their control core. Spectral-shift control is a type of control method that achieves performance regulation or stability by altering the spectral distribution of particles, energy, or signals within a system (or device). Its core idea is to shift the system's "spectral shape" (i.e., characteristic frequencies, energy spectrum, or pole / zero distribution) in a desired direction, thereby influencing the system's gain, phase, power distribution, or response speed, achieving precise regulation of power, reactivity, steady-state, or transient behavior.

[0004] However, existing reactor core structures, by dividing the control rods into segments of neutron-absorbing material, non-neutron-absorbing material, and moderator, can control core reactivity to a certain extent. Even with the addition of spectral shift strategies, the problems of high difficulty in controlling core reactivity at the beginning of the reactor's lifespan and short core lifespan still exist. Summary of the Invention

[0005] This invention addresses the problems of high difficulty in core reactivity control and short core lifespan in existing technologies by providing a fuel assembly with axially segmented control rods, a core structure, and an operating method. The precise axial segmentation of the control rods reduces the difficulty of core reactivity control at the beginning of the core lifespan, effectively extends the core lifespan, and improves core fuel utilization. The operating method of this invention can effectively improve reactor core performance.

[0006] This invention is achieved through the following technical solution: A fuel assembly with axially segmented control rods includes fuel rods and compensation rods. Several fuel rods are arranged around the compensation rods. The compensation rods are divided into five segments along the axial direction, which are, from top to bottom, a strong neutron absorber segment, a weak neutron absorber segment, a neutron-free absorber segment, a moderator segment, and a fuel segment.

[0007] Optionally, the lengths of the strong neutron absorber segment and the weak neutron absorber segment are each half the length of the active region, while the lengths of the neutron-free absorber segment, the moderator segment, and the fuel segment are all equal to the length of the active region.

[0008] A reactor core structure based on the fuel assemblies includes 7 low-enrichment fuel assemblies with control rods arranged in the central region of the core, 36 medium-enrichment fuel assemblies with compensation rods and 12 medium-enrichment fuel assemblies with safety rods arranged in sequence on the periphery, and 30 high-enrichment fuel assemblies with compensation rods and control rods arranged on the outermost periphery of the core. Both the medium-enrichment fuel assembly with compensating rod and the high-enrichment fuel assembly with compensating rod and regulating rod include the fuel assembly with axially segmented control rod as described in claim 1.

[0009] Optionally, the 12 safety rods in the 12 medium-enriched fuel assemblies are symmetrically divided into 4 groups according to 1 / 3 of the core, and each group of safety rods is controlled by a drive mechanism to raise or insert the control rod absorber into the core.

[0010] Optionally, the seven low-enrichment fuel assemblies with regulating rods have seven regulating rods symmetrically divided into three groups according to one-third of the core. The central regulating rod is in one group, and the six peripheral regulating rods are in two groups. Each group of regulating rods is controlled by a drive mechanism to raise or insert the regulating rod absorber into the core.

[0011] Optionally, the 66 compensating rods in the 36 medium-enriched fuel assemblies with compensating rods and the 30 high-enriched fuel assemblies with compensating rods and regulating rods are symmetrically divided into 22 groups at 1 / 3 ratio. Each group of compensating rods is controlled by a drive mechanism to raise or insert the control rod absorber into the reactor core.

[0012] Optionally, the fuel enrichment of the high-enrichment fuel assembly with compensating rod and regulating rod is 21%; the fuel enrichment of the medium-enrichment fuel assembly with compensating rod and the medium-enrichment fuel assembly with safety rod is 19%; and the fuel enrichment of the low-enrichment fuel assembly with regulating rod is 16%.

[0013] Optionally, the B4C B-10 content in 12 medium-enriched fuel assemblies containing safety rods is 90%, the B4C B-10 content in 7 low-enriched fuel assemblies containing regulating rods is 30%, and the B4C B-10 content in 36 medium-enriched fuel assemblies containing compensating rods and 30 high-enriched fuel assemblies containing compensating rods and regulating rods is 40%.

[0014] Optionally, in low-enrichment fuel assemblies containing control rods, the length of the control rods is the same as the length of the active region in the core, and the material is a weak neutron absorber; in medium-enrichment fuel assemblies containing safety rods, the length of the safety rods is the same as the length of the active region in the core, and the material is a strong neutron absorber.

[0015] An operating method based on the aforementioned core structure includes the following steps: During reactor shutdown, the height of the compensator rods is adjusted so that the strong neutron absorber section and the weak neutron absorber section of the compensator rods are flush with the active region, allowing the control rod absorbers within the core to absorb neutrons and ensuring the core meets the shutdown depth requirements. After reactor startup, the compensator rods are raised so that the strong neutron absorber section and the weak neutron absorber section of the compensator rods are gradually removed from the core until no neutron absorber section remains flush with the active region of the core. At the end of the reactor's lifespan, the compensator rods continue to be removed from the core, and the moderator section and fuel section are gradually introduced into the core to soften the core energy spectrum and improve core reactivity until the end of the reactor's lifespan is reached.

[0016] The technical solution of the present invention has at least the following beneficial effects: The fuel assembly with axially segmented control rods of this invention employs an axial segmentation design, selecting five types of materials—strong neutron absorber, weak neutron absorber, structural material without neutron absorption, moderator, and fuel—for axial combination. During the rod lifting process, a gradual change from strong to weak to no neutron absorption allows for a smooth increase in core reactivity. Compared to structures that transition from neutron-absorbing structural materials to those without, this design offers better stability and can flatten the axial power distribution as much as possible. Furthermore, the gradual introduction of a moderator segment throughout the reactor's lifespan compensates for burnup losses in reactivity. By the end of the lifespan, both moderator and fuel are fully introduced into the core, resulting in a core energy spectrum that is either hyperthermal or thermal, thus extending the core's lifespan.

[0017] The reactor core structure and operation method of this invention comprises 85 fuel assemblies. The safety rods and control rods are designed without axial segmentation, while the compensating rods are designed with five axial segments, with an axial length four times that of the active core region. The core lifting procedure involves fully lifting the safety rods, raising the control rods to half the core height, and raising the compensating rods to core criticality. During burnup, the compensating rods are raised to compensate for burnup reactivity losses, while the control rods do not need to compensate for reactivity demands during power rises and falls or for short-term reaction disturbances. Ultimately, this achieves flattening of the core's power distribution throughout its lifespan, extends the core's lifespan, and improves the overall performance of the core. Attached Figure Description

[0018] Figure 1 This is a schematic cross-sectional view of the fuel assembly containing the regulating rod and the compensating rod of the present invention. Figure 2 This is a schematic cross-sectional view of the fuel assembly with safety rods according to the present invention; Figure 3 This is a schematic diagram of the axial structure of the safety bar of the present invention; Figure 4 This is a schematic diagram of the axial structure of the compensation rod of the present invention; Figure 5 This is a schematic diagram of the axial structure of the adjusting rod of the present invention; Figure 6This is a schematic diagram of the cross-sectional structure of the core structure of the present invention.

[0019] Figure label: 53-Compensation rod or regulating rod, 51-Fuel rod, 52-Component box, 4-Lead-bismuth coolant layer, 54-Safety rod, 61-Strong neutron absorber section, 62-Weak neutron absorber section, 63-Neutral absorber-free section, 64-Moderator section, 65-Fuel section, 101-Fuel assembly with regulating rod, 102-Fuel assembly with compensation rod, 103-Fuel assembly with compensation rod and regulating rod, 104-Fuel assembly with safety rod. Detailed Implementation

[0020] The present invention provides a specific embodiment of a reactor core structure with a core power of 100MWt and a lifespan of 3000EFPD.

[0021] The reactor core structure comprises 85 fuel assemblies, including 7 low-enrichment fuel assemblies with conditioning rods (101), 36 medium-enrichment fuel assemblies with compensating rods (102), 12 medium-enrichment fuel assemblies with safety rods (104), and 30 high-enrichment fuel assemblies with compensating rods and conditioning rods (103). The equivalent diameter of the active region is 126 cm, the height of the active region is 100 cm, the core height-to-diameter ratio is 0.79, and the core power density is 80 W / cm³. 3 The initial uranium loading at the end of the reactor core's lifespan is 1.29 tons, and the average burnup depth at the end of its lifespan is 232 GWd / tU.

[0022] Seven low-enrichment fuel assemblies 101 with control rods are arranged in the central region of the reactor core. Thirty-six medium-enrichment fuel assemblies 102 with compensation rods and twelve medium-enrichment fuel assemblies 104 with safety rods are arranged sequentially on the periphery. The outermost layer of the core consists of thirty high-enrichment fuel assemblies 103 with compensation rods and control rods. The medium-enrichment fuel assemblies 102 and the high-enrichment fuel assemblies 103 are categorized into two arrangements based on their location: inside the core (medium-enrichment fuel assemblies 102) and outside the core (high-enrichment fuel assemblies 103). The control rods inside the core are relatively more valuable due to their location in areas with higher neutron flux. The control rods outside the core are located in the core reflector layer region and do not occupy the core active area for control. They are axially segmented, and the neutron reflection effects of different materials are used to influence the reactivity of the core. Thirty high-enrichment fuel assemblies 103 with compensating and regulating rods are divided into two groups: 15 fuel assemblies with compensating rods and 15 fuel assemblies with regulating rods, with the two types of fuel assemblies arranged alternately. The compensating or regulating rods 53 adopt the same structural design in cross-section, differing only in the axial direction.

[0023] The 12 medium-enrichment fuel assemblies 104, each containing safety rods, have 12 safety rods 54 arranged symmetrically in one-third of the core, forming 4 groups. Each group of safety rods 54 is controlled by a drive mechanism to raise or lower the control rod absorber into the core. The 7 low-enrichment fuel assemblies 101, each containing conditioning rods, have 7 conditioning rods arranged symmetrically in one-third of the core, forming 3 groups. The central conditioning rod forms one group, and the outer 6 conditioning rods form 2 groups. Each group of conditioning rods is controlled by a drive mechanism to raise or lower the control rod absorber into the core. The 36 medium-enrichment fuel assemblies 102, each containing compensating rods, and the 30 high-enrichment fuel assemblies 103, each containing compensating rods and conditioning rods, have a total of 66 compensating rods arranged symmetrically in one-third of the core, forming 22 groups. Each group of compensating rods is controlled by a drive mechanism to raise or lower the control rod absorber into the core.

[0024] The fuel enrichment of high-enrichment fuel assembly 103 with compensating rods and regulating rods is 21%; the fuel enrichment of medium-enrichment fuel assembly 102 with compensating rods and medium-enrichment fuel assembly 104 with safety rods is 19%; and the fuel enrichment of low-enrichment fuel assembly 101 with regulating rods is 16%. The core is divided into radial and axial zones based on fuel enrichment to flatten the core power distribution.

[0025] The B4C B-10 content in the 12 medium-enriched fuel assemblies (104) containing safety rods is 90%, the B4C B-10 content in the 7 low-enriched fuel assemblies (101) containing regulator rods is 30%, and the B4C B-10 content in the 36 medium-enriched fuel assemblies (102) containing compensation rods and the 30 high-enriched fuel assemblies (103) containing compensation rods and regulator rods is 40%. The total value of the 54 safety rods is 6856 pcm, the 3 regulator rods are worth 1655 pcm, and the total value of the 22 compensation rods is 15867 pcm. The average value of a single bundle of compensation rods and regulator rods is 720 pcm lower than the core delayed neutron fraction. Meanwhile, the core reactivity control rods meet the design requirements for shutdown depth and sticking rods.

[0026] Among them, the medium-enrichment fuel assembly 102 with compensating rod and the high-enrichment fuel assembly 103 with compensating rod and regulating rod are fuel assemblies with axially segmented control rods (here referring to compensating rods). Each assembly includes fuel rods 51 and compensating rods. Several fuel rods 51 are arranged around the compensating rods and filled with a lead-bismuth coolant layer 4 to form the fuel assembly. The compensating rod is divided into five segments axially, from top to bottom: a strong neutron absorber segment 61, a weak neutron absorber segment 62, a neutron-free absorber segment 63, a moderator segment 64, and a fuel segment 65.

[0027] The strong neutron absorber section 61 is made of strong neutron absorber materials, such as B4C, Gd₂O₃, and silver-indium-cadmium. The weak neutron absorber section 62 is made of weak neutron absorber materials, such as Er₂O₃ and Hf. The neutron-free absorber section 63 uses structural materials that do not absorb neutrons, such as stainless steel, zirconium, and other alloys. The moderator section 64 uses moderator materials, such as zirconium hydride and yttrium hydride. The fuel section 65 can use fuel with the same enrichment level as the low-enrichment fuel in the core, maximizing core fuel utilization while ensuring core economics. The B-10 enrichment in B4C can use the natural enrichment level, or B-10 can be enriched to a certain concentration as needed, up to a maximum enrichment of 90%.

[0028] The compensating rod of this invention adopts an axially segmented design. During the lifting process, the axial segmentation of the compensating rod with different types of materials results in different core energy spectra, thereby adjusting the core reactivity, improving core fuel utilization, and extending core life. The gradual change in the rod lifting process—from strong to weak to no formation—allows for a smooth increase in core reactivity. Compared to structures that transition from materials with neutron-absorbing structures to those without, this design offers better stability and flattens the axial power distribution as much as possible. Simultaneously, a moderator segment 64 is gradually introduced throughout the core's life to compensate for burnup losses. By the end of the life, both the moderator and fuel are fully introduced into the core, resulting in a hyperthermal or thermal energy spectrum, thus extending the core's life.

[0029] The lengths of the strong neutron absorber section 61 and the weak neutron absorber section 62 are each half the length of the active region, while the lengths of the neutron-free absorber section 63, the moderator section 64, and the fuel section 65 are all equal to the length of the active region.

[0030] In the low-enrichment fuel assembly 101 containing a control rod, the control rod is designed without segments, with a length consistent with the length of the active region of the core. The material is a weak neutron absorber, forming a continuous weak neutron absorber segment 62. In the medium-enrichment fuel assembly 104 containing a safety rod, the safety rod 54 is designed without segments, with a length consistent with the length of the active region of the core. The material is a strong neutron absorber, forming a continuous strong neutron absorbing segment 61, which is used for emergency shutdown under accident conditions.

[0031] An operation method based on a reactor core structure includes the following steps: During reactor shutdown, the height of the compensator rods is adjusted so that the strong neutron absorber section 61 and the weak neutron absorber section 62 of the compensator rods are flush with the active region, allowing the control rod absorbers within the core to absorb neutrons and meet the shutdown depth requirements. After reactor startup, the compensator rods are raised so that the strong neutron absorber section 61 and the weak neutron absorber section 62 of the compensator rods are gradually removed from the core until the intermediate neutron absorber section 63 is flush with the active region of the core. At the end of the reactor's lifespan, the compensator rods are further removed from the core, and the moderator section 64 and the fuel section 65 are gradually introduced into the core to soften the core energy spectrum and improve core reactivity until the end of the reactor's lifespan is reached.

[0032] The core lifting procedure is as follows: safety rod 54 is fully lifted, regulating rod is raised to half core height, and compensating rod is raised to core criticality. During burnup, the compensating rod is raised to compensate for burnup reactivity loss. The regulating rod does not need to compensate for the reactivity demand of power rise and fall, or for short-term reaction disturbance compensation.

Claims

1. A fuel assembly with axially segmented control rods, comprising fuel rods and compensating rods, wherein a plurality of fuel rods are arranged around the compensating rods, characterized in that, The compensation rod is divided into five sections along the axial direction, which are, from top to bottom, a strong neutron absorber section, a weak neutron absorber section, a neutron-free absorber section, a moderator section, and a fuel section.

2. The fuel assembly with axially segmented control rod according to claim 1, characterized in that, The lengths of the strong neutron absorber section and the weak neutron absorber section are each half the length of the active region, while the lengths of the neutron-free absorber section, the moderator section, and the fuel section are all equal to the length of the active region.

3. A core structure based on the fuel assembly of claim 1 or 2, characterized in that, The reactor core consists of 7 low-enrichment fuel assemblies with control rods arranged in the central region, 36 medium-enrichment fuel assemblies with compensation rods and 12 medium-enrichment fuel assemblies with safety rods arranged in sequence on the periphery, and 30 high-enrichment fuel assemblies with compensation rods and control rods arranged on the outermost periphery. Both the medium-enrichment fuel assembly with compensating rod and the high-enrichment fuel assembly with compensating rod and regulating rod include the fuel assembly with axially segmented control rod as described in claim 1.

4. The core structure according to claim 3, characterized in that, The 12 medium-enriched fuel assemblies with safety rods consist of 12 safety rods that are symmetrically divided into 4 groups according to 1 / 3 of the core. Each group of safety rods is controlled by a drive mechanism to raise or insert the control rod absorber into the core.

5. The core structure according to claim 3, characterized in that, The seven low-enrichment fuel assemblies with regulating rods consist of seven regulating rods that are symmetrically divided into three groups according to one-third of the core. The central regulating rod is in one group, and the six regulating rods on the periphery are in two groups. Each group of regulating rods is controlled by a drive mechanism to raise or insert the regulating rod absorber into the core.

6. The core structure according to claim 3, characterized in that, The 36 medium-enriched fuel assemblies with compensating rods and the 30 high-enriched fuel assemblies with compensating rods and regulating rods comprise 66 compensating rods, which are symmetrically divided into 22 groups in a 1 / 3 ratio. Each group of compensating rods is controlled by a drive mechanism to raise or insert the control rod absorber into the reactor core.

7. The core structure according to claim 3, characterized in that, The fuel enrichment of the high-enrichment fuel assembly with compensating rod and regulating rod is 21%; the fuel enrichment of the medium-enrichment fuel assembly with compensating rod and the medium-enrichment fuel assembly with safety rod are both 19%; and the fuel enrichment of the low-enrichment fuel assembly with regulating rod is 16%.

8. The core structure according to claim 3, characterized in that, The B4C B-10 content in 12 medium-enriched fuel assemblies containing safety rods was 90%, the B4C B-10 content in 7 low-enriched fuel assemblies containing regulator rods was 30%, and the B4C B-10 content in 36 medium-enriched fuel assemblies containing compensation rods and 30 high-enriched fuel assemblies containing compensation rods and regulator rods was 40%.

9. The core structure according to claim 3, characterized in that, In low-enrichment fuel assemblies containing control rods, the length of the control rods is the same as the length of the active core region, and the material is a weak neutron absorber; in medium-enrichment fuel assemblies containing safety rods, the length of the safety rods is the same as the length of the active core region, and the material is a strong neutron absorber.

10. An operation method based on the core structure according to any one of claims 3-9, characterized in that, During reactor shutdown, the height of the compensator rods is adjusted so that the strong and weak neutron absorber sections are aligned with the active region, allowing the control rod absorbers within the core to absorb neutrons and meet the shutdown depth requirements. After reactor restart, the compensator rods are raised, gradually removing the strong and weak neutron absorber sections from the core until no neutron absorber section remains aligned with the active region. At the end of the reactor's lifespan, the compensator rods continue to be removed from the core, and the moderator and fuel sections are gradually introduced into the core to soften the core energy spectrum and improve core reactivity until the end of the reactor's lifespan is reached.

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