Method for inhibiting excess reactivity of a nuclear reactor fuel assembly

Abstract

This invention discloses a method for suppressing excess reactivity in reactor fuel assemblies, relating to the field of nuclear power technology. The reactor includes multiple fuel assemblies, and the control method includes: after each reactor cycle, identifying a target fuel assembly, which is the fuel assembly with excess remaining reactivity from the previous cycle; and before the reactor enters the next cycle, inserting a neutron absorber into the target fuel assembly. This application, by identifying the target fuel assembly with excess remaining reactivity among the fuel assemblies after each cycle's discharge and inserting a neutron absorber into the target fuel assembly, directly suppresses the reactivity of the target fuel assembly, effectively reducing the relative power at the location of the target fuel assembly at the beginning of its lifespan, which is beneficial for power leveling. Compared to related technologies that add control rods to the reactor, this method eliminates the need for additional control rod drive structures and pressure vessel head openings, thus improving the economic efficiency of reactor operation.

Description

Technical Field

[0001] This invention relates to the field of nuclear power technology, and in particular to a method for suppressing excess reactivity in reactor fuel assemblies. Background Technology

[0002] In the design of nuclear reactor unit operation and refueling, adopting various practical and effective control methods to control the reactor's residual reactivity while ensuring safety is essential for the long-term stable operation of the reactor. When the overall residual reactivity of the reactor is insufficient to maintain the core's full-power criticality, refueling is necessary. In related technologies, during each refueling, typically only the older assemblies with deeper burn-out are removed from the core, while the remaining older assemblies with shallower burn-out, possessing sufficient residual reactivity, are reinserted and enter the next cycle of operation along with the new assemblies. However, when the residual reactivity of these older assemblies is too high, the excessive residual reactivity results in an excessively high relative power at the location of these older assemblies at the beginning of the reactor's lifespan. This leads to insufficient subcriticality during core shutdown, difficulty in flattening the radial power distribution, and challenges in ensuring the stability and safety of reactor operation. Summary of the Invention

[0003] This invention aims to at least solve one of the technical problems existing in the prior art. To this end, this invention proposes a method for suppressing excess reactivity of reactor fuel assemblies.

[0004] A method for suppressing excess reactivity of reactor fuel assemblies according to an embodiment of the present invention, wherein the reactor includes multiple fuel assemblies, and the suppression method includes: After each reactor cycle, a target fuel assembly is identified, which is the fuel assembly with excess reactivity remaining in the previous cycle. Before the next reactor cycle, a neutron absorber is inserted into the target fuel assembly.

[0005] The method for suppressing excess reactivity of reactor fuel assemblies according to embodiments of the present invention has at least the following beneficial effects: This application's embodiments involve inserting neutron absorbers into fuel assemblies with excess reactivity. The neutron absorbers directly suppress the reactivity of these fuel assemblies, effectively reducing the relative power at the location of these older assemblies at the beginning of their lifespan, which is beneficial for power leveling. Compared to related technologies that add control rods to the reactor, this method eliminates the need for additional control rod drive structures and pressure vessel head openings, thus improving the economic efficiency of reactor operation.

[0006] According to some embodiments of the present invention, after each reactor cycle, determining the target fuel assembly includes: If among the multiple fuel assemblies, there is a fuel assembly whose infinite multiplication factor exceeds a preset value, then the fuel assembly whose infinite multiplication factor exceeds the preset value is identified as the target fuel assembly.

[0007] According to some embodiments of the present invention, the neutron absorber includes at least one of a combustible poison rod and a fuel rod containing a combustible poison.

[0008] According to some embodiments of the present invention, the number of neutron absorbers inserted into the target fuel assembly is a multiple of 4 and less than or equal to 24.

[0009] According to some embodiments of the present invention, the fuel assembly includes fuel rods and a plurality of control rod guides disposed between the plurality of fuel rods; inserting a neutron absorber into the target fuel assembly prior to the next reactor cycle includes: Remove a portion of the fuel rod from the target fuel assembly and insert an unburned fuel rod containing flammable poison; And / or, insert a combustible poison rod into at least a portion of the control rod guide tube in the target fuel assembly.

[0010] According to some embodiments of the present invention, inserting a combustible poison rod into at least a portion of the control rod guide tube in the target fuel assembly comprises: If the control rod guide tube to be inserted into the combustible poison rod in the next cycle is in an empty state, the combustible poison rod is directly inserted. If the control rod guide tube to be inserted with the combustible poison rod in the next cycle has the combustible poison rod inserted in the previous cycle, the combustible poison rod that was burned in the previous cycle is removed, and the combustible poison rod that has not been burned is inserted.

[0011] According to some embodiments of the present invention, the neutron absorber comprises fuel rods containing combustible poison, and the fuel assembly comprises fuel rods and a plurality of control rod guides disposed between the plurality of fuel rods; inserting the neutron absorber into the target fuel assembly prior to the next reactor cycle comprises: Remove a portion of the fuel rods from the target fuel assembly and insert an unburned fuel rod containing a combustible poison; wherein each of the removed fuel rods is located adjacent to at least one of the control rod guide tubes, and the positions of all the removed fuel rods are arranged centrally symmetrically with respect to the center of the fuel assembly.

[0012] According to some embodiments of the present invention, the fuel assembly includes fuel rods and a plurality of control rod guides disposed between the plurality of fuel rods, the neutron absorber includes combustible poison rods, and inserting the neutron absorber into the target fuel assembly prior to the next reactor cycle includes: A combustible poison rod is inserted into at least a portion of the control rod guide tube in the target fuel assembly; wherein the position of the control rod guide tube into which the combustible poison rod is inserted is centrally symmetrical with respect to the center of the fuel assembly.

[0013] According to some embodiments of the present invention, in at least one cycle of the fuel assembly, all fuel rods of a portion of the fuel assembly are fuel rods that do not contain combustible poisons.

[0014] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0015] The present invention will be further described below with reference to the accompanying drawings and embodiments, wherein: Figure 1 A flowchart illustrating a method for suppressing excess reactivity of reactor fuel assemblies according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the structural composition of a fuel assembly according to an embodiment of the present invention; Figure 3 This is a schematic diagram of a fuel assembly containing a flammable poison inserted according to an embodiment of the present invention; Figure 4 This is a schematic diagram of a fuel assembly with a combustible poison rod inserted, according to an embodiment of the present invention. Figure 5 A flowchart illustrating the insertion of a fuel rod containing a combustible poison into a fuel assembly according to an embodiment of the present invention; Figure 6 This is a flowchart illustrating the insertion of a combustible poison rod into a fuel assembly according to an embodiment of the present invention. Figure 7 A graph showing the reactivity of a fuel assembly of an embodiment of the present invention as a function of burnup when different numbers of fuel rods containing combustible poison are inserted. Figure 8 A graph showing the reactivity of a fuel assembly as a function of burnup when different numbers of combustible poison rods are inserted into it, according to an embodiment of the present invention. Figure 9 This is a schematic diagram showing the insertion position of the combustible poison rod in a fuel assembly according to an embodiment of the present invention; Figure 10 This is a schematic diagram showing the insertion position of the combustible poison rod in a fuel assembly according to another embodiment of the present invention.

[0016] Icon labels: 10. Fuel assembly; 100. Fuel rod; 200. Control rod guide tube; 300. Instrument measuring tube; 400. Neutron absorber; 410. Fuel rod containing flammable poison; 420. Flammable poison rod. Detailed Implementation

[0017] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0018] In the description of this invention, it should be understood that the orientation descriptions, such as up, down, etc., are based on the orientation or positional relationship shown in the drawings and are only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0019] In the description of this invention, "multiple" refers to two or more. The use of "first" and "second" is for distinguishing technical features only and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features or their sequential relationship.

[0020] In the description of this invention, unless otherwise explicitly defined, terms such as "setting," "installing," and "connecting" should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of the above terms in this invention in conjunction with the specific content of the technical solution.

[0021] This application provides a method for suppressing excess reactivity in reactor fuel assemblies.

[0022] A reactor core consists of multiple fuel assemblies, which can have different compositions. During the first cycle, these fuel assemblies are loaded into the reactor according to a pre-defined layout. After the previous cycle, if the overall residual reactivity of the reactor is insufficient to maintain full-power criticality, the reactor core needs to be refueled. During refueling, the older, more burnt-out assemblies are removed from the core and replaced with new ones. The remaining, less burnt-out assemblies, which have higher residual reactivity, are loaded back into the core and enter the next cycle along with the new assemblies. In simple terms, residual reactivity refers to the "extra" reactivity of the reactor beyond what is needed to maintain its critical operation.

[0023] However, for some older assemblies with excessive residual reactivity, especially fuel assemblies without combustible poisons, the excessive residual reactivity after the previous cycle can lead to an excessively high relative power at the location of these assemblies at the beginning of their lifespan when they are reintroduced into the reactor in subsequent cycles. This can easily cause problems such as insufficient subcriticality during core shutdown and difficulty in flattening radial power distribution. Conventional methods include adding control rods for control, or adjusting the enrichment or combustible poison ratio of the newly added assemblies. The former requires increasing the number of control rods, which also necessitates adding control rod drive mechanisms and pressure vessel head openings, making the operation complex and costly. The latter method, adjusting the new assemblies, still has difficulty in having a beneficial effect on the residual reactivity of the older assemblies.

[0024] To solve the above problems, please refer to... Figures 1 to 6 The method for suppressing excess reactivity of reactor fuel assemblies in this application includes: 101: After each reactor cycle, the target fuel assembly is determined. The target fuel assembly is the fuel assembly 10 with excess reactivity remaining in the previous cycle.

[0025] For ease of explanation below, the fuel assembly 10 discharged from the reactor in each cycle is defined as the old assembly. Specifically, after each cycle of discharge, the old assemblies with excess reactivity are identified as target fuel assemblies, which are then subjected to subsequent reactivity suppression operations before being reintroduced into the reactor. For old assemblies with insufficient reactivity, the old assemblies are replaced with new assemblies before being reintroduced into the reactor.

[0026] 102: Before the next reactor cycle, a neutron absorber is inserted into the target fuel assembly.

[0027] Understandably, the next reactor cycle refers to any cycle after the first cycle, excluding the first cycle. Before the next cycle, the reactivity of the target fuel assembly can be suppressed by inserting a neutron absorber 400 into the target fuel assembly before refueling. It should be noted that the position of the target fuel assembly in the core during the next cycle can be the same as in the previous cycle, or it can be adjusted and installed in a new position.

[0028] The method for suppressing excess reactivity of reactor fuel assemblies in this application involves identifying a target fuel assembly from the remaining assemblies after each reactor cycle. A neutron absorber 400 is then inserted into the target fuel assembly to directly suppress its residual reactivity. This effectively reduces the relative power at the location of the target fuel assembly at the beginning of its lifespan, facilitating power leveling. Compared to related technologies that involve adding control rods to the reactor, this method eliminates the need for additional control rod drive structures and pressure vessel head openings, simplifying refueling operations and improving reactor operational economy.

[0029] Fuel assembly 10 includes fuel rods 100. In some embodiments of this application, in at least one cycle of fuel assembly 10, all fuel rods 100 of a portion of fuel assembly 10 are fuel rods 100 that do not contain combustible poisons. The suppression method provided in this application is particularly applicable to fuel assemblies 10 in which all fuel rods 100 do not contain combustible poisons. In some embodiments of this application, the fuel rods 100 that do not contain combustible poisons can be UO2 fuel rods, and the fuel rods 100 that contain combustible poisons can be UO2 fuel rods containing gadolinium combustible poison (Gd2O3).

[0030] In some embodiments of this application, determining the target fuel assembly after each reactor cycle in step 101 above specifically includes the following steps: If multiple fuel assemblies 10 contain an infinite multiplication factor k inf If fuel component 10 exceeds the preset value, the infinite multiplication factor k will be increased. inf Fuel assembly 10 exceeding the preset value is identified as the target fuel assembly. Infinite multiplication factor k inf Reflecting the inherent ability of the fuel assembly to produce neutrons, it is a key parameter for assessing the reactivity remaining of fuel assembly 10, based on the infinite multiplication factor k. inf The target fuel assembly can be quickly identified.

[0031] In one embodiment of this application, the preset value is 1.14. The excess reactivity of the fuel assembly 10 must satisfy the infinite multiplication factor k. inf ≥1.14. When the infinite multiplication factor of some fuel assemblies 10 in the reactor core is greater than or equal to 1.14, it means that the remaining reactivity of that fuel assembly 10 is excessive.

[0032] Understandably, the infinite multiplication factor of each fuel assembly 10 can be calculated, and the fuel assemblies 10 after being removed from the reactor can be tested and confirmed based on the calculation results. This allows for the quick and accurate identification of which fuel assemblies 10 have excess reactivity.

[0033] The neutron absorber 400 includes at least one of a combustible poison rod 420 and a fuel rod 410 containing a combustible poison. It should be noted that the combustible poison rod 420 is a discrete combustible poison independent of the fuel rod 100, while the fuel rod 410 containing the combustible poison is an integrated fuel-combustible poison combining the combustible poison and fuel. The fuel rod 410 containing the combustible poison is self-consumable during reactor operation.

[0034] In some embodiments, when the neutron absorber 400 is a combustible poison rod 420, it can be a borosilicate glass tube or the like. The combustible poison rod 420 is not used as a fuel rod 100. To avoid the combustible poison rod 420 occupying space in the fuel rod 100 and reducing fuel, the combustible poison rod 420 can be inserted into the control rod guide tube 200.

[0035] In some embodiments, when the neutron absorber 400 is a fuel rod 410 containing a combustible poison, it can be a gadolinium rod, a tungsten rod, an erbium rod, etc. When adding a fuel rod 410 containing a combustible poison to the fuel assembly 10, some of the fuel rods 100 in the fuel assembly 10 (i.e., the old assembly) can be pulled out and replaced with fuel rods 410 containing combustible poison.

[0036] Please refer to the reference. Figure 2 , Figure 3 and Figure 5 In some embodiments of this application, before the next reactor cycle in step 102 above, inserting a neutron absorber into the target fuel assembly includes: removing a portion of the fuel rods 100 from the target fuel assembly and inserting unburned fuel rods 410 containing combustible poison.

[0037] In this embodiment, by removing a portion of the fuel rods 100 from the target fuel assembly and inserting new, unburned fuel rods 410 containing combustible toxins into the vacated positions, the target fuel assembly, along with other newly replaced components, will absorb a large number of neutrons in the initial stage after the fuel rods 410 containing combustible toxins absorb neutrons, effectively suppressing the excessive reactivity of the target fuel assembly and effectively reducing the relative power at the location of the target fuel assembly.

[0038] It should be understood that when fuel assembly 10 has already inserted fuel rods 410 containing combustible poison in the previous cycle to suppress excessive residual reactivity, and the reactivity is still too high after the previous cycle is removed from the reactor, the fuel rods 100 to be removed before the next cycle is inserted can be the combustible poison fuel rods 410 that were inserted in the previous cycle and have been burned out, and new unburned combustible poison fuel rods 410 are inserted.

[0039] Please refer to Figure 2 Each fuel assembly 10 includes a control rod guide tube 200, which is arranged among multiple fuel rods 100. The control rod guide tube 200 is used for inserting control rods. By operating the insertion or lifting of the control rods through a drive mechanism, the neutron absorption in the reactor can be changed, thereby controlling the reactivity. The fuel assembly 10 also includes an instrumentation tube 300, which is located at the center of the fuel assembly 10.

[0040] Please refer to the reference. Figure 2 , Figure 4 and Figure 6In other embodiments of this application, before the next reactor cycle in step 102 above, inserting a neutron absorber into the target fuel assembly may further include: inserting a combustible poison rod 420 into at least a portion of the control rod guide tube 200 in the target fuel assembly. By inserting the combustible poison rod 420 into the control rod guide tube 200, the combustible poison rod 420 absorbs a large number of neutrons in the initial stage, effectively suppressing the excessive reactivity of the fuel assembly 10 and effectively reducing the relative power at the location of the fuel assembly 10.

[0041] The combustible poison rod 420 can be a borosilicate glass tube. It should be understood that after the target fuel assembly with the combustible poison rod 420 inserted is reinserted into the reactor, no control rods need to be inserted during the cycle; the combustible poison rod 420 alone is used to suppress the reactivity of the fuel assembly 10.

[0042] In some embodiments of this application, the insertion of a combustible poison rod 420 into at least a portion of the control rod guide tube 200 in the target fuel assembly by the above-described suppression method includes the following: If the control rod guide tube 200 of the combustible poison rod 420 to be inserted in the next cycle is in an empty state, the combustible poison rod 420 is directly inserted. If the control rod guide tube 200 to be inserted with a combustible poison rod 420 in the previous cycle has a combustible poison rod 420 inserted in the previous cycle, the combustible poison rod 420 that was burned in the previous cycle is removed, and an unburned combustible poison rod 420 is inserted.

[0043] As is easily understood, if the control rod guide tube 200 to be inserted with a flammable poison rod 420 in the next cycle is in an empty state, it means that this part of the control rod guide tube 200 has not been inserted with a flammable poison in the previous cycle, and only a new flammable poison rod 420 needs to be inserted. Conversely, if the control rod guide tube 200 to be inserted with a flammable poison rod 420 in the next cycle is not in an empty state, it means that this part of the control rod guide tube 200 has been inserted with a flammable poison rod 420 in the previous cycle. At this time, it is necessary to pull out these flammable poison rods 420 that have been burned in the previous cycle and replace them with new flammable poison rods 420 that have not been burned and reinsert them.

[0044] In some embodiments of this application, the number of neutron absorbers 400 inserted into the target fuel assembly is a multiple of 4, and less than or equal to 24. That is, the number of neutron absorbers 400 inserted when the target fuel assembly is re-entered into the reactor can be 4, 8, 12, 16, 20, or 24.

[0045] It is important to understand that the number of neutron absorbers 400 inserted into the target fuel assembly in multiples of four is to ensure that the inserted neutron absorbers 400 are rotationally symmetric with respect to the center of the target fuel assembly. For example... Figure 2 As shown, a typical fuel assembly 10 is generally configured as a square (the accompanying drawings of this application only illustrate a 17*17 array arranged in a square grid). By setting the number of neutron absorbers 400 inserted into the old assembly to a multiple of 4, these neutron absorbers 400 can be symmetrically inserted into the fuel assembly 10. In this way, the same number of neutron absorbers 400 can be inserted into each of the four quadrants of the fuel assembly 10, enabling the fuel assembly 10 to achieve rotational symmetry at a specific angle (e.g., 90°, 180°, 270°, etc.). On the one hand, this ensures that the reactivity suppression effect of the fuel assembly 10 is the same (or approximately the same) in all directions, rather than being excessively suppressed in a certain local area, which is beneficial for flattening the power distribution. On the other hand, it is precisely because of the above configuration that the fuel assembly 10 achieves rotational symmetry that the layout of the fuel assembly 10 after rotating around its center by a certain angle is the same as before the rotation. This allows the fuel assembly 10 to be quickly loaded into the reactor without distinguishing its orientation, facilitating the loading process of the fuel assembly 10.

[0046] The applicant of this application used an old module with an enrichment degree of 3.30% and a burnup of 8000 MWd / tU as an example, and tested the reactivity of the old module by inserting different numbers of gadolinium rods and borosilicate glass tubes. Figure 7 and Figure 8 The curve graph, in which Figure 7 The graph shows the reactivity of the fuel assembly as a function of burnup when different numbers of fuel rods containing combustible toxins are inserted into the fuel assembly of this application embodiment (nGd represents n gadolinium rods). As can be seen from the graph, as the number of fuel rods 410 (gadolinium rods) containing combustible toxins inserted increases, the reactivity suppression effect of the target fuel assembly is enhanced, and the release of reactivity during the burnup life tends to be gradual. Figure 8 The graphs showing the reactivity of the fuel assembly as a function of burnup when different numbers of combustible poison rods are inserted into it (nbp represents n borosilicate glass tubes) in embodiments of this application show that as the number of combustible poison rods 420 (borosilicate glass tubes) increases, the reactivity suppression effect of the target fuel assembly is enhanced, and the release of reactivity tends to level off during the burnup life. In other words, generally, the more neutron absorbers 400 inserted into the target fuel assembly, the stronger the reactivity suppression effect.

[0047] Understandably, the specific number of neutron absorbers 400 to be inserted can be determined based on the current residual reactivity of the old assembly. For example, in a cycle, when a target fuel assembly with very high residual reactivity needs to be placed near the center of the reactor core, due to the high thermal neutron density at the core center, it may be necessary to insert 24 neutron absorbers 400 into the fuel assembly 10 to effectively reduce the relative power at the target fuel assembly's entry point. As another example, if 16 neutron absorbers 400 were inserted into the target fuel assembly in the previous cycle, after the target fuel assembly exits the reactor, its reactivity has been consumed during the previous cycle, resulting in a significantly reduced residual reactivity compared to before the previous cycle. If the target fuel assembly is not placed near the center of the reactor core in the next cycle, then the number of neutron absorbers 400 inserted during the next re-entry can be reduced; 8 neutron absorbers 400 can be inserted in the next cycle.

[0048] In some embodiments of this application, when replacing some fuel rods 100 by inserting fuel rods 410 containing flammable poison into the target fuel assembly, the removal of some fuel rods 100 from the target fuel assembly must satisfy the following relationship: the position of each removed fuel rod 100 is adjacent to at least one control rod guide tube 200, and the positions of the removed fuel rods 100 are centrally symmetrical with respect to the center of the fuel assembly 10. It can be understood that the position of the removed fuel rod 100 is the insertion position of the fuel rod 410 containing flammable poison; that is, when inserting fuel rods 410 containing flammable poison into the target fuel assembly to replace some fuel rods 100, the insertion position of the fuel rods 410 containing flammable poison satisfies the above relationship.

[0049] In the above embodiments, based on the fact that the number of fuel rods 410 containing combustible poison inserted is a multiple of 4, by making the insertion positions of the fuel rods 410 containing combustible poison centrally symmetrical with respect to the center of the fuel assembly 10, the same number of fuel rods 410 containing combustible poison can be distributed in all four quadrants of the fuel assembly 10. This ensures that the reactivity suppression effect of the fuel assembly 10 is the same (or approximately the same) in all directions, and avoids excessive suppression of reactivity in a certain local area, which is beneficial for flattening the power distribution. Moreover, the above embodiments also insert the fuel rods 410 containing combustible poison into a position adjacent to the control rod guide tube 200, so that the fuel rods 410 containing combustible poison are close to the control rod guide tube 200. Since the control rod guide tube 200 is mostly in an idle state during the reactor cycle, the thermal neutron density at the control rod guide tube 200 is generally high. Placing the fuel rods 410 containing combustible poison close to the control rod guide tube 200 can effectively absorb neutrons near the control rod guide tube 200, effectively reducing the high power at this position, which is further beneficial for power flattening.

[0050] It should be noted that the position of the removed fuel rod 100 (i.e., the insertion position of the fuel rod 410 containing flammable poison) is adjacent to the control rod guide tube 200. It can be adjacent to the control rod guide tube 200 along the side length direction of the square grid (including the horizontal and vertical directions), or it can be adjacent to the control rod guide tube 200 in an inclined direction (parallel to the diagonal direction of the square grid). The accompanying drawings of the embodiments of this application are only for illustrative purposes by way of the latter example and should not be regarded as a limitation of this application.

[0051] Understandably, in order to achieve both the requirement that the insertion position of the fuel rod 410 containing flammable poison is adjacent to the control rod guide tube 200 and centrally symmetrical, the position of the control rod guide tube 200 in the layout of the fuel assembly 10 must also be centrally symmetrical with respect to the center of the fuel assembly 10.

[0052] Combination Figure 2 As shown, Figure 2 The diagram illustrates the positional distribution of control rod guide tubes 200 in a fuel assembly 10 according to an embodiment of this application. The fuel assembly 10 includes 17*17 grid positions and 24 control rod guide tubes 200. The 24 control rod guide tubes 200 are arranged symmetrically with respect to the center of the fuel assembly 10. The positions of the control rod guide tubes 200 in the square grid are (6,3), (9,3), (12,3), (4,4), (14,4), (3,6), (9,6), (6,6), (12,6), (15,6), (3,9), (6,9), (12,9), (15,9), (3,12), (6,12), (9,12), (12,12), (15,12), (4,14), (14,14), (6,15), (9,15), (12,15). Please refer to... Figure 3In one embodiment of this application, after the previous cycle of decommissioning, 16 fuel rods 410 (gadolinium rods) containing combustible poison are inserted into the fuel assembly 10 to suppress excessive reactivity. The positions of the 16 gadolinium rods can be (3,3), (15,3), (7,4), (11,4), (4,7), (7,7), (11,7), (14,7), (4,11), (7,11), (11,11), (14,11), (7,14), (11,14), (3,15), (15,15). At this point, the fuel assembly 10 can be considered as comprising four equal parts along the horizontal centerline and the vertical centerline, wherein the gadolinium rods located at (3,3) and (7,7), (15,3) and (11,7), (3,15) and (7,11), (15,15) and (11,11) are symmetrical within these four parts; the fuel assembly 10 can also be considered as comprising eight equal parts along the horizontal centerline, the vertical centerline and the two diagonals, wherein the gadolinium rods located at (7,4), (11,4), (4,7), (14,7), (4,11), (14,11), (7,14), (11,14) are symmetrical within these eight parts.

[0053] In some other embodiments of this application, when a combustible poison rod 420 is inserted into the control rod guide tube 200 of the target fuel assembly, the insertion of the combustible poison rod 420 into at least a portion of the control rod guide tube 200 of the target fuel assembly satisfies the following relationship: the position of the control rod guide tube 200 of the combustible poison rod 420 to be inserted is centrally symmetrical with respect to the center of the fuel assembly 10.

[0054] Similarly to the above embodiments, based on the number of flammable poison rods 420 inserted being a multiple of 4, by setting the insertion position of the flammable poison rods 420 to be centrally symmetrical with respect to the center of the fuel assembly 10, the same number of flammable poison rods 420 can be distributed in all four quadrants of the fuel assembly 10. This can make the reactivity suppression effect of the fuel assembly 10 the same (or approximately the same) in all directions, and will not limit the reactivity of a certain local area to be excessively suppressed, which is beneficial to flattening the power distribution.

[0055] The following example, using the insertion of a borosilicate glass tube into the control rod guide tube 200, illustrates the positional distribution of the flammable poison rod 420. Figure 2 Taking the positional distribution of the control rod guide tube 200 of the fuel assembly 10 as an example, when eight borosilicate glass tubes are inserted into the fuel assembly 10, in one embodiment of this application, please refer to... Figure 9Nine borosilicate glass tubes can be inserted into the control rod guide tubes 200 located at (4,4), (14,4), (9,6), (6,9), (12,9), (9,12), (4,14), and (14,14), respectively. In this case, the fuel assembly 10 can be considered as comprising four borosilicate glass tubes located at (4,4), (14,4), (4,14), and (14,14), divided equally along the horizontal and vertical center lines, achieving symmetry within each of these four divisions. Alternatively, the fuel assembly 10 can be considered as comprising four borosilicate glass tubes located at (9,6), (6,9), (12,9), and (9,12), divided equally along the two diagonals, achieving symmetry within each of these four divisions. For another embodiment of this application, please refer to... Figure 10 Eight borosilicate glass tubes are inserted into the control rod guide tubes 200 located at (6, 3), (9, 3), (3, 6), (15, 6), (3, 12), (15, 12), (9, 15), and (12, 15), respectively. The fuel assembly 10 can be considered as comprising eight equal parts divided along the horizontal centerline, the vertical centerline, and the two diagonals. The eight borosilicate glass tubes located at (6, 3), (9, 3), (3, 6), (15, 6), (3, 12), (15, 12), (9, 15), and (12, 15) are symmetrically positioned within these eight parts. It is understood that when this application uses the insertion of a combustible poison rod 420 into the control rod guide tube 200 to suppress the residual reactivity of the fuel assembly 10, the insertion position of the combustible poison rod 420 includes, but is not limited to, the two embodiments described above. The accompanying drawings are only illustrative of the two embodiments and should not be construed as limiting the scope of this application.

[0056] In one embodiment of this application, in at least one fuel assembly 10 that has been fed into the reactor, some fuel assemblies 10 have all fuel rods 100 that do not contain combustible poisons. For those fuel assemblies 10 with all fuel rods 100 that do not contain combustible poisons, there is excessive residual reactivity after the current cycle is discharged from the reactor. By adding a neutron absorber 400 to the fuel assembly 10, the neutron absorber 400 effectively suppresses the reactivity of the fuel assembly 10 and effectively reduces the relative power at the location of the old assembly at the beginning of its service life.

[0057] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for suppressing excess reactivity of reactor fuel assemblies, characterized in that, The reactor comprises multiple fuel assemblies, and suppression methods include: After each reactor cycle, a target fuel assembly is identified, which is the fuel assembly with excess reactivity remaining in the previous cycle. Before the next reactor cycle, a neutron absorber is inserted into the target fuel assembly.

2. The method for suppressing excess reactivity of reactor fuel assemblies according to claim 1, characterized in that, After each reactor cycle, the target fuel assembly is identified, including: If among the multiple fuel assemblies, there is a fuel assembly whose infinite multiplication factor exceeds a preset value, then the fuel assembly whose infinite multiplication factor exceeds the preset value is identified as the target fuel assembly.

3. The method for suppressing excess reactivity of reactor fuel assemblies according to claim 1, characterized in that, The neutron absorber includes at least one of a combustible poison rod and a fuel rod containing a combustible poison.

4. The method for suppressing excess reactivity of reactor fuel assemblies according to claim 1, characterized in that, The number of neutron absorbers inserted into the target fuel assembly is a multiple of 4 and less than or equal to 24.

5. The method for suppressing excess reactivity of reactor fuel assemblies according to any one of claims 1 to 4, characterized in that, The fuel assembly includes fuel rods and a plurality of control rod guide tubes disposed among the fuel rods; the insertion of a neutron absorber into the target fuel assembly prior to the next reactor cycle includes: Remove a portion of the fuel rod from the target fuel assembly and insert an unburned fuel rod containing flammable poison; And / or, insert a combustible poison rod into at least a portion of the control rod guide tube in the target fuel assembly.

6. The method for suppressing excess reactivity of reactor fuel assemblies according to claim 5, characterized in that, The insertion of a combustible poison rod into at least a portion of the control rod guide tube in the target fuel assembly includes: If the control rod guide tube to be inserted into the combustible poison rod in the next cycle is in an empty state, the combustible poison rod is directly inserted. If the control rod guide tube to be inserted with the combustible poison rod in the next cycle has the combustible poison rod inserted in the previous cycle, the combustible poison rod that was burned in the previous cycle is removed, and the combustible poison rod that has not been burned is inserted.

7. The method for suppressing excess reactivity of reactor fuel assemblies according to any one of claims 1 to 4, characterized in that, The neutron absorber includes fuel rods containing combustible poison, and the fuel assembly includes fuel rods and a plurality of control rod guides disposed between the plurality of fuel rods; inserting the neutron absorber into the target fuel assembly before the next reactor cycle includes: Remove a portion of the fuel rods from the target fuel assembly and insert an unburned fuel rod containing a combustible poison; wherein each of the removed fuel rods is located adjacent to at least one of the control rod guide tubes, and the positions of all the removed fuel rods are arranged centrally symmetrically with respect to the center of the fuel assembly.

8. The method for suppressing excess reactivity of reactor fuel assemblies according to any one of claims 1 to 4, characterized in that, The fuel assembly includes fuel rods and a plurality of control rod guide tubes disposed among the fuel rods, the neutron absorber includes combustible poison rods, and the insertion of the neutron absorber into the target fuel assembly prior to the next reactor cycle includes: A combustible poison rod is inserted into at least a portion of the control rod guide tube in the target fuel assembly; wherein the position of the control rod guide tube into which the combustible poison rod is inserted is centrally symmetrical with respect to the center of the fuel assembly.

9. The method for suppressing excess reactivity of reactor fuel assemblies according to any one of claims 1 to 4, characterized in that, In at least one cycle of the fuel assembly, all fuel rods of a portion of the fuel assembly are fuel rods that do not contain flammable poisons.

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