In-core detection-based passive start-up method

Abstract

The present invention belongs to the technical field of nuclear reactor design, and particularly relates to an in-core detection-based passive start-up method. A neutron detector is provided inside a reactor core fuel assembly. In an initial state of reactor start-up, a coolant is at room temperature, all control rods are inserted into a reactor core and a boron concentration is a shutdown boron concentration, the neutron detector obtains an effective count rate signal of not less than 0.5cps. During subsequent start-up, as the temperature of the coolant increases, the control rods are withdrawn from the reactor core or the boron concentration decreases, the neutron detector count rate exhibits a continuous increase, no detection blind zone is present, and real-time monitoring of inadvertent reactivity insertion is achieved, thereby enabling a reactor to reach a critical state safely and effectively. Additionally, by taking into account variations of the neutron detector count rate during the start-up, reactivity insertion processes of rod withdrawal and boron dilution are guided. The present invention uses the neutron detector arranged in the reactor core fuel assembly to realize blind-zone-free monitoring during the entire start-up process, thereby ensuring safe and efficient passive stat-up of the reactor.

Description

A passive boot method based on in-heap probing Technical Field

[0001] This invention belongs to the field of nuclear reactor design technology, specifically relating to a passive start-up method based on in-reactor detection. Background Technology

[0002] During the physical startup of a reactor, it is essential to strictly control the effective core multiplication factor (k). eff The growth rate of neutron flux is monitored to prevent the reactor from reaching an uncontrollable instantaneous critical state. Currently, neutron detectors in the nuclear instrumentation system located outside the pressure vessel are typically used to monitor changes in neutron flux during startup, thereby providing timely and effective criticality safety supervision of the reactor. At the same time, the physical startup of the reactor is achieved by extrapolating the reciprocal count rate of the detectors.

[0003] Typically, reactors are equipped with start-up neutron sources, including primary or secondary neutron sources. Their core function is to increase the neutron flux levels inside and outside the reactor core, ensuring the reactor startup process remains within the detection range of the detectors and eliminating detection "blind spots." Reactors generally deploy two primary neutron source assemblies (usually californium sources) in the first cycle. From the second cycle onwards, primary neutron source assemblies are no longer used; only secondary neutron source assemblies (usually antimony-beryllium sources) are employed. Primary and secondary neutron sources require substantial investment. Primary neutron sources are highly radioactive, necessitating stringent transportation and storage requirements, increasing human and material costs. Using primary neutron sources generates additional radioactive waste, increasing the amount of radioactive solid waste. Furthermore, the neutron intensity of both primary and secondary neutron sources decays over time. If a primary neutron source is purchased in advance or if the reactor shutdown period is too long, resulting in a secondary neutron source with insufficient intensity, it will be unusable during reactor startup, posing an additional risk.

[0004] To address the limitations of primary and secondary neutron sources, passive start-up technology emerged. Passive start-up refers to the safe completion of reactor fuel loading, maintenance of shutdown conditions, and criticality attainment by relying on neutrons spontaneously released from the fuel, without the need for an external neutron source. The reactor start-up neutron source commonly referred to is actually an additional neutron source assembly installed within the reactor; it is an "external neutron source." The nuclear fuel loaded in the reactor core can also release neutrons through spontaneous fission of uranium nuclides. In contrast, neutrons spontaneously released from the fuel are an "inherent" neutron source. In passive start-up, the spontaneously released neutrons from the fuel serve as the "start-up neutron source." However, for newly built reactor cores, the spontaneous neutron source intensity of new fuel assemblies is typically several orders of magnitude lower than that of externally added neutron sources. During the initial fuel loading and startup of the reactor, the core neutron flux induced by these neutrons is significantly lower than that of active startup, resulting in a large detection "blind zone" (defined as a detector count rate below 0.5 cps) for external nuclear monitoring instruments. This makes explicit criticality safety monitoring impossible and hinders effective guidance for reactive introduction processes such as control rod lifting or boron dilution, which is detrimental to the safety and efficiency of passive startup. Once the fuel assemblies have been irradiated within the reactor core to a certain burnup level, their spontaneous neutron source intensity will significantly increase (due to factors such as increased transuranic element fission reactions resulting from burnup), becoming comparable to that of externally added neutron sources. Startup using irradiated fuel assemblies can avoid the detection blind zone and ensure safe startup. Startup using irradiated fuel assemblies has been applied in several nuclear power plants, but this approach is not suitable for the first cycle startup of newly built reactors.

[0005] To address the startup issues of new reactors, a reliance on high-sensitivity external detectors (with a sensitivity of 30 cps / (n / cm)) has emerged. 2 Passive start-up technology using detectors with a sensitivity of 30 cps / (n / cm²) or higher. Its core is to install higher-sensitivity neutron detectors outside the reactor core, enabling neutron measurements under lower neutron flux density conditions, thereby reducing or even eliminating detection blind zones. my country's Tianwan VVER units 1-4 implemented first-cycle start-up without an external neutron source, using high-sensitivity neutron detectors, "dry" fuel loading to verify detector effectiveness, and strict control of the reactivity introduction rate during the criticality process to ensure reactor safety during the start-up without an external neutron source. In the VVER unit's start-up without an external neutron source, although detectors with a sensitivity of 30 cps / (n / cm²) were deployed... 2A high-sensitivity neutron detector (HNMD) is used, but the reactor remains in a detection blind zone until the effective core multiplication factor reaches approximately 0.99. Only when the effective core multiplication factor reaches approximately 0.99 can the high-sensitivity neutron detector escape the blind zone and perform explicit criticality safety monitoring during subsequent startup. Therefore, additional measures are needed to control the boron dilution pathway to prevent accidental criticality, and boron dilution operations must be performed more cautiously during the criticality process, which increases startup time to some extent. Therefore, reducing or eliminating the blind zone during passive startup of nuclear power reactors is a critical technical problem that urgently needs to be solved.

[0006] Under the same core conditions, the neutron flux inside the reactor vessel is much higher than that outside the reactor. Placing neutron detectors inside the reactor vessel can effectively increase the neutron flux rate received by the detectors, which is beneficial for improving the detector count rate signal and reducing or eliminating detection blind spots. However, on the other hand, the space available for neutron detectors within the core fuel assemblies is limited, and the geometry of the sensitive area of ​​the usable neutron detectors must be miniaturized to allow for their placement.

[0007] Currently, for general nuclear power reactors, neutron detectors deployed within the reactor core can be divided into two types: the first type is a neutron detector temporarily placed in the core during fuel loading for criticality safety monitoring; the second type is a neutron detector installed in the fuel assembly instrumentation tubes for neutron flux monitoring. The former type has higher sensitivity, reaching ~30 cps / (n / cm²). 2 The first type (approximately 1.5 neutron flux density per second) is suitable for neutron measurement under low neutron flux density conditions during fuel loading, but it is bulky and can only be used under low-temperature and low-pressure conditions. Therefore, it must be removed from the core after fuel loading and cannot be used for subsequent reactor startup. The second type can withstand the high temperature and strong radiation environment during normal core operation, but its sensitivity is very low. It can only be effectively measured under conditions of high core power level and high neutron flux density, and cannot be used for monitoring under low neutron flux density conditions during reactor startup. Summary of the Invention

[0008] The purpose of this invention is to provide a passive start-up method based on in-core detection. This method utilizes neutron detectors deployed within the reactor core fuel assemblies to achieve blind-zone-free monitoring throughout the start-up process, ensuring safe and efficient passive reactor start-up. This method avoids dependence on external start-up neutron sources, reduces construction costs, and eliminates the generation of neutron-related radioactive solid waste. Furthermore, it eliminates the detection blind zones of existing passive start-up technologies, effectively improving start-up safety and efficiency.

[0009] The passive start-up method based on in-pile detection proposed in this invention relies on a neutron detector that differs significantly from the two types mentioned above in terms of sensitivity and neutron measurement range: firstly, the detector has high sensitivity and can be used for low neutron flux density measurement under passive start-up; secondly, the detector is small in size and can be arranged in the detector channel set in the fuel assembly; and thirdly, it can operate normally in a high-temperature environment of ~300℃.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0011] A passive startup method based on in-core detection involves placing neutron detectors within the fuel assemblies of the reactor core. In the initial startup state, when the coolant is at room temperature, all control rods are inserted into the core, and the boron concentration is the same as during shutdown, the neutron detectors obtain an effective count rate signal of no less than 0.5 cps. During subsequent startup, as the coolant temperature rises, control rods are removed from the core, or the boron concentration decreases, the neutron detector count rate continuously increases, eliminating detection blind spots and enabling real-time monitoring of the introduction of unexpected reactivity. This allows the reactor to safely and effectively reach criticality. Simultaneously, the changes in the neutron detector count rate during startup guide the rod removal and boron dilution reactivity introduction processes.

[0012] Neutron detectors are placed within the reactor core fuel assemblies, including available space inside guide tubes, instrument tubes, and other components.

[0013] The neutron detector is positioned at the very center of the reactor core's active region.

[0014] The neutron detector is positioned in the water gap of the core's central assembly.

[0015] When the neutron detector is placed in the active region of the reactor core, its radial dimension is less than 2 cm and smaller than the inner diameter of the guide tube or instrument tube.

[0016] The neutron detector is a boron trifluoride detector, with a shell made of 302 stainless steel and filled with boron trifluoride gas.

[0017] The neutron detector is cylindrical, with an outer diameter of 12 mm and a length of 79 cm.

[0018] The length of the neutron detector does not exceed the height of the fuel assembly.

[0019] The material of the neutron detector sensor contains one or more isotopes of uranium, helium, or boron.

[0020] The reactor vessel houses the neutron-sensitive area of ​​the neutron detector, the neutron detector signal lead-out lines are arranged above the reactor core, and other structures around the reactor core are used to lead out the detector signal lines to an external display.

[0021] Neutron detectors can operate normally at temperatures below 300°C.

[0022] A type of neutron detector, which is a cylindrical boron trifluoride detector.

[0023] Its outer shell is made of 302 stainless steel and filled with boron trifluoride gas.

[0024] Its outer diameter is 12mm and its length is 79cm.

[0025] Its sensor material contains one or more isotopes of uranium, helium, or boron.

[0026] Its length does not exceed the height of the fuel assembly.

[0027] An application of a neutron detector, which is placed inside the fuel assembly of a reactor core, for the safe and efficient passive startup of the reactor to reach criticality.

[0028] In the initial stage of reactor startup, when the coolant is at room temperature, all control rods are inserted into the core, and the boron concentration is the same as that during shutdown, the neutron detector obtains an effective count rate signal of not less than 0.5 cps. During subsequent startup, as the coolant temperature rises, control rods are removed from the core, or the boron concentration decreases, the neutron detector's count rate continuously increases, eliminating detection blind spots and enabling real-time monitoring of the introduction of unexpected reactivity. This allows the reactor to safely and effectively reach criticality. Simultaneously, by combining the changes in the neutron detector's count rate during startup, the process of rod removal and boron dilution reactivity introduction is guided.

[0029] Neutron detectors are placed within the reactor core fuel assemblies, including available space inside guide tubes, instrument tubes, and other components.

[0030] The neutron detector is positioned at the very center of the reactor core's active region.

[0031] The neutron detector is positioned in the water gap of the core's central assembly.

[0032] The reactor vessel houses the neutron-sensitive area of ​​the neutron detector, the neutron detector signal lead-out lines are arranged above the reactor core, and other structures around the reactor core are used to lead out the detector signal lines to an external display.

[0033] The beneficial effects achieved by this invention are as follows:

[0034] This invention proposes a method for achieving safe reactor startup without an external neutron source by arranging appropriately sized neutron detectors within the available space inside and outside the reactor core fuel assemblies, including guide tubes, instrument tubes, and other components. By addressing the difference in neutron flux inside and outside the reactor, and through the rational arrangement of detectors, the reactor can safely and effectively reach criticality without the use of an external neutron source. This saves significant neutron source procurement costs, eliminates potential risks associated with using an external neutron source, reduces the generation of radioactive solid waste, and eliminates detection blind zones, effectively improving the safety and efficiency of passive startup. Attached Figure Description

[0035] Figure 1 is a schematic diagram of neutron detectors arranged in the active zone of the reactor core or the reflector layer adjacent to the core: blue represents the reactor vessel boundary, purple represents the reflector layer, and red represents the detectors; Figure 2 is a characteristic curve of the criticality process of a reactor without an external neutron source. Detailed Implementation

[0036] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0037] A passive startup method based on in-core detection involves arranging neutron detectors within the reactor core fuel assemblies, including available space inside guide tubes, instrument tubes, and other components. The core does not load or use primary or secondary neutron sources; startup is initiated using neutrons spontaneously released from the fuel assemblies. The geometry of the neutron detectors placed within the reactor vessel meets the arrangement requirements. The reactor vessel can accommodate the neutron-sensitive area of ​​the neutron detectors, there is sufficient space above the core for arranging neutron detector signal leads, and other structures around the core provide space for the detector signal lines to be routed to an external signal display.

[0038] The sensitivity of the neutron detectors placed inside the reactor vessel meets the design requirements for reactor physical startup. In the initial startup state, such as when the coolant is at room temperature, all control rods are inserted into the core, and the boron concentration is the same as during shutdown, the neutron detectors inside the reactor can obtain an effective count rate signal of no less than 0.5 cps. During subsequent startup, as the coolant temperature rises, control rods are removed from the core, or the boron concentration decreases, the count rate of the neutron detectors inside the core continuously increases, with no detection blind spots. This allows for real-time monitoring of the introduction of unexpected reactivity, enabling the reactor to safely and effectively reach criticality. Furthermore, by combining the count rate changes of these neutron detectors during startup, the process of introducing reactivity, such as rod removal and boron dilution, can be guided, improving passive startup efficiency.

[0039] In this embodiment, the reactor of a typical pressurized water reactor corresponding to Figure 1 is used as the object of description to introduce the method. In Figure 1, the light blue area represents the reactor vessel, the black area is the basket, the purple area represents the reflector layer, and the purple area is the core active region where fuel assemblies are arranged. In this embodiment, the detector is arranged in the exact center of the core active region, while the other arrangements of the core remain unchanged. This invention utilizes only the fuel assembly... 235 U and 238 The reactor is started using neutrons generated by spontaneous nuclear fission. The specific implementation method is as follows:

[0040] Figure 1 shows a schematic diagram of the detector arrangement within the reactor core and reactor vessel. The original design required a neutron source to be placed in the active region of the core to enable the external source range detectors to obtain effective counts before reactor criticality. According to the technical solution provided by this invention, a small-sized boron trifluoride detector is placed in the center of the core for neutron monitoring before reactor criticality. The detector has an outer diameter of 12 mm, a length of 79 cm, a shell made of 302 stainless steel, and is filled with boron trifluoride gas. The small-sized boron trifluoride detector can be placed in the water gap of the core center assembly.

[0041] When the reactor is in a cold shutdown state, a small-sized detector is placed at the very center of the core and connected to the external electrical system. The change in the count rate of the detector within the core during the criticality process is shown in Figure 2. In the initial stage of reactor startup, the detector within the core provides an effective count rate greater than 0.5 cps, and it can effectively monitor the reactor throughout the subsequent startup process without any detection blind spots.

[0042] A passive startup method based on in-reactor detection replaces an external neutron source for reactor startup. No external neutron source is required during startup. There are no external primary or secondary neutron sources. The startup relies solely on neutrons spontaneously released from the fuel. Neutron detectors for monitoring neutron flux during startup are located within the reactor fuel assemblies, including in the guide tubes, instrument tubes, and other available space inside and outside the assemblies. The neutron flux inside the reactor vessel is significantly higher than outside, requiring lower sensitivity from the neutron detectors. In the initial startup state, such as when the coolant is at room temperature, all control rods are inserted into the core, and the boron concentration is the same as during shutdown, the neutron detectors can obtain an effective count rate signal of at least 0.5 cps. The neutron detectors can operate normally at high temperatures up to 300°C. The detectors are cylindrical, and when placed in the active region of the reactor core, their radial dimension is less than 2 cm and smaller than the inner diameter of the guide tubes or instrument tubes. The length of the neutron detectors does not exceed the height of the fuel assemblies. The material of the neutron detector sensor contains one or more isotopes of uranium, helium, or boron.

[0043] This invention proposes a passive startup method that eliminates detection blind zones by arranging neutron detectors within reactor fuel assemblies. The neutron flux inside the reactor vessel is significantly higher than outside, requiring lower detector sensitivity, but necessitating the use of appropriately sized neutron detectors. Compared to external neutron source startup methods, passive startup eliminates the risks associated with neutron source use, reduces the generation of solid radioactive waste, and significantly saves costs. Compared to existing passive startup methods that rely on high-sensitivity external neutron detectors, the method proposed in this invention eliminates detection blind zones, effectively improving the safety and efficiency of passive startup.

Claims

1. A passive booting method based on in-heap probing, characterized in that: Neutron detectors are placed inside the fuel assemblies of the reactor core. In the initial state of reactor startup, when the coolant is at room temperature, all control rods are inserted into the core, and the boron concentration is the same as the shutdown boron concentration, the neutron detectors obtain an effective count rate signal of not less than 0.5 cps. During subsequent startup, as the coolant temperature rises, control rods are removed from the core, or the boron concentration decreases, the count rate of the neutron detectors continuously increases, eliminating detection blind spots and enabling real-time monitoring of the introduction of unexpected reactivity. This allows the reactor to safely and effectively reach criticality. Simultaneously, the changes in the neutron detector count rate during startup guide the rod removal and boron dilution reactivity introduction processes.

2. The passive boot method based on in-pile detection according to claim 1, characterized in that: Neutron detectors are placed within the reactor core fuel assemblies, including available space inside guide tubes, instrument tubes, and other components.

3. The passive boot method based on in-pile detection according to claim 1, characterized in that: The neutron detector is positioned at the very center of the reactor core's active region.

4. The passive boot method based on in-pile detection according to claim 3, characterized in that: The neutron detector is positioned in the water gap of the core's central assembly.

5. The passive boot method based on in-pile detection according to claim 3, characterized in that: When the neutron detector is placed in the active region of the reactor core, its radial dimension is less than 2 cm and smaller than the inner diameter of the guide tube or instrument tube.

6. The passive boot method based on in-heap detection according to claim 1, characterized in that: The neutron detector is a boron trifluoride detector, with a shell made of 302 stainless steel and filled with boron trifluoride gas.

7. The passive boot method based on in-pile detection according to claim 1, characterized in that: The neutron detector is cylindrical, with an outer diameter of 12 mm and a length of 79 cm.

8. The passive boot method based on in-heap detection according to claim 1, characterized in that: The length of the neutron detector does not exceed the height of the fuel assembly.

9. The passive boot method based on in-pile detection according to claim 1, characterized in that: The material of the neutron detector sensor contains one or more isotopes of uranium, helium, or boron.

10. The passive boot method based on in-pile detection according to claim 1, characterized in that: The reactor vessel houses the neutron-sensitive area of ​​the neutron detector, the neutron detector signal lead-out lines are arranged above the reactor core, and other structures around the reactor core are used to lead out the detector signal lines to an external display.

11. The passive boot method based on in-pile detection according to claim 1, characterized in that: Neutron detectors can operate normally at temperatures below 300°C.

12. A neutron detector, characterized in that: It is a cylindrical boron trifluoride detector.

13. The neutron detector according to claim 12, characterized in that: Its outer shell is made of 302 stainless steel and filled with boron trifluoride gas.

14. The neutron detector according to claim 12, characterized in that: Its outer diameter is 12mm and its length is 79cm.

15. The neutron detector according to claim 12, characterized in that: Its sensor material contains one or more isotopes of uranium, helium, or boron.

16. The neutron detector according to claim 12, characterized in that: Its length does not exceed the height of the fuel assembly.

17. An application of a neutron detector, characterized in that: It is placed inside the reactor core fuel assembly for safe and efficient passive startup of the reactor to reach criticality.

18. The application of the neutron detector according to claim 17, characterized in that: In the initial stage of reactor startup, when the coolant is at room temperature, all control rods are inserted into the core, and the boron concentration is the same as that during shutdown, the neutron detector obtains an effective count rate signal of not less than 0.5 cps. During subsequent startup, as the coolant temperature rises, control rods are removed from the core, or the boron concentration decreases, the neutron detector's count rate continuously increases, eliminating detection blind spots and enabling real-time monitoring of the introduction of unexpected reactivity. This allows the reactor to safely and effectively reach criticality. Simultaneously, by combining the changes in the neutron detector's count rate during startup, the process of rod removal and boron dilution reactivity introduction is guided.

19. The application of the neutron detector according to claim 17, characterized in that: Neutron detectors are placed within the reactor core fuel assemblies, including available space inside guide tubes, instrument tubes, and other components.

20. The application of the neutron detector according to claim 17, characterized in that: The neutron detector is positioned at the very center of the reactor core's active region.

21. The application of the neutron detector according to claim 20, characterized in that: The neutron detector is positioned in the water gap of the core's central assembly.

22. The application of the neutron detector according to claim 17, characterized in that: The reactor vessel houses the neutron-sensitive area of ​​the neutron detector, the neutron detector signal lead-out lines are arranged above the reactor core, and other structures around the reactor core are used to lead out the detector signal lines to an external display.

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