A method for managing core fuel of a small pressurized water reactor

Abstract

A small pressurized water reactor (PWR) core fuel management method, pertaining to the field of nuclear power, is disclosed. This method, applied to a small PWR containing 57 fuel assemblies, includes the following steps: loading a first batch of fuel assemblies with three different enrichments in an initial cycle, with the initial cycle lasting 24 months; performing five or six transition cycles, in which 17 spent fuel assemblies are removed and 17 replacement fuel assemblies are loaded in each cycle, with the transition cycle lasting 21-24 months; and entering a balancing cycle after the transition cycle, in which 17 spent fuel assemblies are removed and 17 replacement fuel assemblies are loaded in each balancing cycle, with the balancing cycle lasting 24 months. This method can achieve a 24-month balancing cycle and four batches of refueling, effectively improving the economic efficiency of the small PWR.

Description

Technical Field

[0001] The present invention belongs to the field of nuclear power, and in particular relates to a method for managing core fuel of a small pressurized water reactor. Background Art

[0002] Compared with large commercial pressurized water reactors, small pressurized water reactors have a simple system structure, a small size, a small amount of fuel loaded in the core, and a significant burnup shadow effect. At present, small pressurized water reactors generally adopt a fuel management scheme that does not require refueling and is passed through once. During refueling, the entire reactor is loaded and unloaded. Due to the uneven burnup of the core fuel, the fuel economy performance is poor. Patent CN108364695B discloses a fuel management method that uses Gd-containing burnable poison to achieve fuel management for small pressurized water reactors. However, this method only uses burnable poison rods to compensate for backup reactivity and has limited adjustment capabilities. At the same time, Gd-containing burnable poison rods are relatively expensive, are rarely used in commercial pressurized water reactors, and lack versatility. Therefore, providing a simpler core fuel management method for small pressurized water reactors has positive significance for improving the economy of small pressurized water reactors. Summary of the Invention

[0003] The purpose of the present invention is to provide a small pressurized water reactor core fuel management method to extend the small pressurized water reactor balance circulation fuel cycle to 24 months.

[0004] According to an embodiment of the present invention, a method for managing core fuel of a small pressurized water reactor is provided, which is applicable to a small pressurized water reactor having a core including 57 groups of 17×17 fuel assemblies. The method comprises the following steps:

[0005] Step a): In the first cycle of the small pressurized water reactor core, a first batch of fuel assemblies with three different enrichments are loaded, and the length of the first cycle is 24 months;

[0006] Step b): After the initial cycle, five or six transition cycles are performed; in each transition cycle, 17 burned-up fuel assemblies are unloaded and 17 replacement fuel assemblies are loaded; each transition cycle lasts 21-24 months;

[0007] Step c): After all the transition cycles are completed, a balancing cycle is entered; in each balancing cycle, 17 burned-up fuel assemblies are unloaded and 17 replacement fuel assemblies are loaded; each balancing cycle lasts 24 months.

[0008] Using this method, the balanced refueling cycle of a small pressurized water reactor can be extended to 24 months based on the use of low-enrichment fuel below 5%, realizing a long-cycle fuel management strategy of multi-batch loading and unloading, improving fuel utilization, and improving the economic efficiency of the small pressurized water reactor.

[0009] Furthermore, in some embodiments, in step b) and step c), the 17 burned-up fuel assemblies discharged include the fuel assembly at the center of the small pressurized water reactor core and the 16 fuel assemblies with the highest degree of burn-up among the remaining fuel assemblies.

[0010] Furthermore, in some embodiments, in the balancing cycle, the average batch discharge fuel consumption is 35,000-37,000 MWd / tU, and the maximum rod fuel consumption is 42,000-45,000 MWd / tU.

[0011] Furthermore, in some embodiments, the fuel assemblies in the small pressurized water reactor core adopt a low-leakage loading method. In the step a), the first batch of fuel assemblies includes a first enrichment, a second enrichment and a third enrichment, wherein the first enrichment < the second enrichment < the third enrichment; the fuel assembly arranged in the center of the small pressurized water reactor core adopts the first enrichment; the fuel assemblies at the outermost edge of the small pressurized water reactor core adopt the second enrichment; the 3×3 fuel assemblies in the middle of the small pressurized water reactor core respectively adopt the first enrichment and the second enrichment and are arranged in a checkerboard pattern; the fuel assembly using the third enrichment is separated from the outer boundary of the small pressurized water reactor core by a fuel assembly using the second enrichment.

[0012] Furthermore, in some embodiments, in step a), the number of the fuel assemblies using the first enrichment level is 5, the number of the fuel assemblies using the second enrichment level is 40, and the number of the fuel assemblies using the third enrichment level is 12.

[0013] Furthermore, in some embodiments, in step a), the first enrichment is 0.92%, the second enrichment is 1.90%, and the third enrichment is 3.00%.

[0014] Furthermore, in some embodiments, in step b), the replacement fuel assembly includes the fuel assembly with the highest remaining reserve reactivity in the spent fuel pool and 16 new fuel assemblies with an enrichment of 3.40%-3.50%; and in step c), the replacement fuel assembly includes the fuel assembly with the highest remaining reserve reactivity in the spent fuel pool and 16 new fuel assemblies with an enrichment of 3.50%-3.60%.

[0015] Furthermore, in some embodiments, the fuel assemblies in the small pressurized water reactor core adopt a high leakage loading method. In the step a), the first batch of fuel assemblies includes a first enrichment, a second enrichment and a third enrichment, wherein the first enrichment < the second enrichment < the third enrichment; the fuel assemblies arranged in the center of the small pressurized water reactor core adopt the first enrichment; the fuel assemblies at the outermost edge of the small pressurized water reactor core adopt the third enrichment; the 3×3 fuel assemblies in the middle of the small pressurized water reactor core respectively adopt the first enrichment and the second enrichment and are arranged in a checkerboard pattern.

[0016] Furthermore, in some embodiments, in step a), the number of the fuel assemblies using the first enrichment level is 5, the number of the fuel assemblies using the second enrichment level is 32, and the number of the fuel assemblies using the third enrichment level is 20.

[0017] Furthermore, in some embodiments, in step a), the first enrichment is 1.40%, the second enrichment is 1.90%, and the third enrichment is 2.70%.

[0018] Furthermore, in some embodiments, in step b), the replacement fuel assembly includes a new fuel assembly with an enrichment of 2.40%-2.60% and 16 new fuel assemblies with an enrichment of 3.55%-3.65% arranged in the center of the small pressurized water reactor core; in step c), the replacement fuel assembly includes a new fuel assembly with an enrichment of 2.40%-2.60% and 16 new fuel assemblies with an enrichment of 3.40-3.60% arranged in the center of the small pressurized water reactor core.

[0019] Furthermore, in some embodiments, when refueling is performed, the core loading maintains 1 / 4 rotational symmetry. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of loading the first batch of fuel assemblies in one embodiment;

[0021] Figure 2 A schematic diagram of loading balanced cycle fuel assemblies in one embodiment;

[0022] Figure 3 A schematic diagram of a balanced cycle fuel assembly switching strategy in one embodiment;

[0023] Figure 4 This is a schematic diagram of loading the first batch of fuel assemblies in another embodiment;

[0024] Figure 5 A schematic diagram of loading balanced cycle fuel assemblies in another embodiment;

[0025] Figure 6 A schematic diagram of a balanced cycle fuel assembly switching strategy in another embodiment;

[0026] Figure 7 Schematic diagram of the rotational symmetry relationship in a fuel assembly in one embodiment.

[0027] It should be understood that in the above-mentioned figures, each square represents a fuel assembly, and the numbers in the squares only indicate the ranking of the residual reserve reactivity of the fuel assembly in the illustrated state among all the fuel assemblies in the core, with 1 being the lowest and 4 being the highest, and * not being counted in the ranking; the residual reserve reactivity of fuel assemblies with the same number in the same figure is roughly at the same level, but not strictly equal; the residual reserve reactivity of fuel assemblies in different figures cannot be directly compared based on the numbers.

[0028] The purpose of the above-mentioned drawings is to provide a detailed explanation of the present invention so that those skilled in the art can understand the technical concepts of the present invention, and is not intended to limit the present invention. For the sake of simplicity, the above-mentioned drawings only schematically depict structures related to the technical features of the present invention and do not strictly depict the complete structure and all details according to actual scale. DETAILED DESCRIPTION

[0029] The present invention will be further described in detail below through specific embodiments with reference to the accompanying drawings.

[0030] References to "embodiments" herein mean that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment herein. The appearance of such phrases in various places in the specification does not necessarily refer to the same embodiment, nor does it limit mutually exclusive independent or alternative embodiments. Those skilled in the art will appreciate that an embodiment herein may be combined with other embodiments as long as no structural conflicts arise.

[0031] In the description herein, terms such as "first" and "second" are used only to distinguish different objects and should not be understood to indicate relative importance or to limit the quantity, specific order, or primary and secondary relationship of the described technical features. In the description herein, "plurality" means at least two.

[0032] Compared with conventional large commercial pressurized water reactors, small pressurized water reactors have lower power, smaller scale, use less fuel in the reactor, and have more flexible application scenarios. For example, they can take on part of the peak load when the heating load is large in winter, and meet the application needs of residential heating, industrial gas supply, seawater desalination, etc.

[0033] Currently, small pressurized water reactor cores typically use 57 groups of 17×17 fuel assemblies with an active section height of approximately 190 cm. The existing mainstream fuel management scheme is a one-time pass without refueling, which leads to uneven fuel consumption and reduced fuel economy. Patent CN108364695B discloses a fuel management method for a 100,000-kilowatt reactor core that can achieve a 24-month refueling cycle. However, this method requires the use of burnable poison rods containing Gd with various concentrations, making the fuel management scheme complex. At the same time, the balance cycle of this scheme still requires separate management of odd and even cycles, which in fact does not fully achieve a stable and balanced fuel cycle. Furthermore, this method does not have soluble boron in the first loop to compensate for backup reactivity, and the power regulation flexibility is insufficient.

[0034] To overcome the shortcomings of the prior art, an embodiment of the present invention provides a small pressurized water reactor core fuel management method that can stably achieve a 24-month refueling cycle for a small pressurized water reactor comprising 57 sets of 17×17 fuel assemblies. Furthermore, in this method, the fuel assemblies use conventional integral burnable poisons, eliminating the need for redesigning the burnable poison or fuel assemblies. Specifically, this method includes a first cycle, five or six transition cycles, and a subsequent balancing cycle. The method comprises the following steps:

[0035] Step a): In the first cycle, the first batches of fuel assemblies with three different enrichments are loaded into the reactor core using three loading batches. The first cycle length is 24 months.

[0036] Step b): Starting with the second cycle, the transition cycle begins. At the beginning of each transition cycle, 17 spent fuel assemblies are removed from the reactor core and 17 transition fuel assemblies are replaced. Each transition cycle lasts 21-22 months. The 17 removed fuel assemblies include the fuel assembly located at the center of the reactor core and the 16 remaining fuel assemblies with the highest burnup (lowest residual reserve reactivity).

[0037] Step c): After the transition cycle, the reactor enters the balancing cycle. At the beginning of each balancing cycle, 17 spent fuel assemblies are unloaded from the reactor core and 17 balancing fuel assemblies are loaded. Each balancing cycle lasts 24 months. During the balancing cycle, the average batch discharge burnup is 35,000-37,000 MWd / tU, and the maximum rod burnup is 42,000-45,000 MWd / tU.

[0038] In a preferred embodiment, the small pressurized water reactor has a rated power of 200 MWt and is loaded with 57 square 17×17 fuel assemblies, each fuel assembly including 264 fuel rods, 24 guide tubes and 1 measuring tube, and the active region height is 190 cm.

[0039] The core's fuel assemblies are loaded using a low-leakage method (remaining backup fuel assemblies with low reactivity are placed around the core to reduce radioactive leakage outside the core). The first batch of fuel assemblies has a first enrichment of 0.92%, a second enrichment of 1.90%, and a third enrichment of 3.00%. There are five 0.92% enrichment fuel assemblies in the first batch, 40 1.90% enrichment fuel assemblies in the first batch, and 12 3.00% enrichment fuel assemblies in the first batch. Each first batch fuel assembly contains 0, 44, or 88 integral burnable poison rods.

[0040] When loading, if Figure 1 As shown, the numbers in the boxes represent the residual reserve reactivity of the corresponding fuel assembly. The reactor core is radially divided into three zones. Fuel assembly E5 is located at the center of the reactor core and uses fuel assemblies with the lowest residual reserve reactivity (residual reserve reactivity denoted as 1) and an enrichment of 0.92%. The outermost fuel assemblies use fuel assemblies with an intermediate residual reserve reactivity (residual reserve reactivity denoted as 2) and an enrichment of 1.90%. These fuel assemblies contain zero burnable poison rods. Nine fuel assemblies within a 3×3 area in the center of the reactor (coordinate range FD, 4-6) use fuel assemblies with 0.92% and 1.90% enrichment arranged in a checkerboard pattern. Fuel assemblies with the highest residual reserve reactivity (residual reserve reactivity denoted as 3) and an enrichment of 3.00% are arranged in the second outermost layer of the reactor core, separated from the outer core boundary by one 1.90% fuel assembly. The initial cycle length is 24 months.

[0041] Starting with the second cycle, 17 spent fuel assemblies were removed from the core and placed into the spent fuel pool, where 17 replacement fuel assemblies were installed. The removed fuel assemblies included E5, the center fuel assembly, and the 16 remaining fuel assemblies with the lowest remaining reserve reactivity. Among the replacement fuel assemblies, the one with the highest remaining reserve reactivity was selected from the spent fuel pool and installed in E5, effectively flattening the power distribution near the center of the core. The remaining replacement fuel assemblies were new fuel assemblies with a 3.45% enrichment, each containing 88 burnable poison rods. The second cycle lasted 22 months, and the subsequent five transition cycles were 24 months each.

[0042] Starting from the 7th cycle, the balance cycle begins. At the beginning of each balance cycle, 17 spent fuel assemblies in the core are unloaded and put into the spent fuel pool, and 17 replacement fuel assemblies are loaded. Among them, the unloaded fuel assemblies include the fuel assembly E5 in the center of the reactor core and the remaining 16 fuel assemblies with the lowest reserve reactivity. The layout of the core fuel assemblies in the balance cycle is as follows: Figure 2As shown, among the replacement fuel assemblies, the fuel assembly with the highest residual reserve reactivity (marked with *) is selected from the spent fuel pool and installed at position E5 to effectively flatten the power distribution near the center of the core; the remaining 16 replacement fuel assemblies use an enrichment of 3.50% (the residual reserve reactivity of the new fuel assembly is denoted as 4); the fuel assemblies that have undergone two cycles and those that have undergone three cycles and contain integral burnable poison rods (residual reserve reactivity is denoted as 2 and 3 respectively) are arranged at the periphery of the core to reduce core leakage; the remaining fuel assemblies are arranged in a checkerboard pattern in the central area of ​​the core according to their different residual reserve reactivities.

[0043] Specifically, the core loading maintains 1 / 4 rotational symmetry, as Figure 7 As shown in , adjacent areas are mirror-symmetrical, and diagonal areas are centrally symmetrical. Figure 3 As shown. Taking 1 / 8 core area ( Figure 7 Take the loading of area VIII in the above as an example: the central assembly E5 is loaded with the used fuel assembly with the highest reserve reactivity in the spent fuel pool, the used fuel assembly at position C7 is used at position D5, the used fuel assembly at position D5 is used at position A5, the used fuel assembly at position A5 is used at position D6, the used fuel assembly at position A6 is used at position C6, the used fuel assembly at position H3 is used at position A6, the used fuel assembly at position H4 is used at position B7, and new fuel assemblies with a fuel enrichment of 3.50% are loaded at positions B5, B6, and C7; the balancing cycle length is 24 months. The remaining areas are loaded as follows: Figure 7 The symmetrical relationship shown above allows for the switching strategy to be implemented. The batch discharge burnup is 37,000 MWd / tU, and the maximum rod burnup does not exceed 42,000 MWd / tU.

[0044] In another preferred embodiment, the small pressurized water reactor has a rated power of 200 MWt and is loaded with 57 square 17×17 fuel assemblies, each fuel assembly including 264 fuel rods, 24 guide tubes and 1 measuring tube, and the active region height is 190 cm.

[0045] The core fuel assemblies are loaded using a high-leakage method. The first batch of fuel assemblies has a first enrichment of 1.40%, a second enrichment of 1.90%, and a third enrichment of 2.70%. There are five 1.40% enrichment fuel assemblies, 32 1.90% enrichment fuel assemblies, and 20 2.70% enrichment fuel assemblies. Each first batch of fuel assemblies contains 0, 44, or 64 integral burnable poison rods.

[0046] When loading, if Figure 4As shown, the numbers in the boxes represent the residual reserve reactivity of the corresponding fuel assembly. The reactor core is radially divided into three zones. Fuel assembly E5, located at the center of the reactor core, uses 1.40% enrichment fuel assemblies with the lowest residual reserve reactivity (residual reserve reactivity denoted as 1). The outermost fuel assemblies use 2.70% enrichment fuel assemblies with the highest residual reserve reactivity (residual reserve reactivity denoted as 3). Nine fuel assemblies within a 3×3 area in the center of the reactor (coordinate range FD, 4-6) use 1.40% and 1.90% enrichment fuel assemblies (residual reserve reactivity denoted as 2) arranged in a checkerboard pattern. The remaining positions are filled with the remaining 1.90% enrichment fuel assemblies. The initial cycle length is 24 months.

[0047] Starting with the second cycle, 17 spent fuel assemblies were removed from the core and deposited into the spent fuel pool, where 17 replacement fuel assemblies were installed. These included the E5 fuel assembly at the center of the reactor core and the 16 remaining fuel assemblies with the lowest reserve reactivity. A new fuel assembly with a 2.50% enrichment was installed in the E5 position, while the remaining replacement fuel assemblies used new fuel assemblies with a 3.60% enrichment, each containing 44 burnable poison rods. The second cycle lasted 23 months, and the five subsequent transition cycles were each 24 months long.

[0048] Starting from the 7th cycle, the balance cycle begins. At the beginning of each balance cycle, 17 spent fuel assemblies in the core are unloaded and put into the spent fuel pool, and 17 replacement fuel assemblies are loaded. Among them, the unloaded fuel assemblies include the fuel assembly E5 in the center of the reactor core and the remaining 16 fuel assemblies with the lowest reserve reactivity. The layout of the core fuel assemblies in the balance cycle is as follows: Figure 5 As shown, among the replacement fuel assemblies, a new fuel assembly with an enrichment of 2.5% (marked with *) is installed at the E5 position. The fuel assembly includes 44 integral burnable poison rods. The remaining 16 replacement fuel assemblies use an enrichment of 3.45% (the remaining reserve reactivity of the new fuel assembly is 4) and are all arranged at the periphery of the core to flatten the radial power distribution of the core. The remaining fuel assemblies that have undergone one, two and three cycles are arranged in a checkerboard pattern in the central area of ​​the core.

[0049] Specifically, the core loading maintains 1 / 4 rotational symmetry, as Figure 7 As shown in , adjacent areas are mirror-symmetrical, and diagonal areas are centrally symmetrical. Figure 6 As shown. Taking 1 / 8 core area ( Figure 7Take the loading of area VIII in the figure as an example: the central assembly E5 is loaded with a fuel assembly with an enrichment of 2.50%, the D5 position continues to use the old assembly at position G3, the C5 position continues to use the old assembly at position J6, the B5 position continues to use the old assembly at position G5, the A5 position continues to use the old assembly at position A6, the D6 position continues to use the old assembly at position G6, the C6 position continues to use the old assembly at position F8, the B6 position continues to use the old assembly at position B7, and the A6 and B7 positions are loaded with new fuel assemblies with a fuel enrichment of 3.45%, each new fuel assembly containing 44 or 64 integral burnable poison rods; the remaining areas are loaded as follows Figure 7 The switching strategy is implemented using the symmetrical relationships shown. The equilibrium cycle length is 24 months. The batch discharge burnup is 35,000 MWd / tU, and the maximum rod burnup does not exceed 44,500 MWd / tU.

[0050] The small pressurized water reactor (SWR) core fuel management method provided in the above-mentioned embodiment can effectively improve the fuel utilization efficiency and economy of the SWR, enabling a four-batch refueling cycle with a 24-month period under a balanced cycle, thereby enhancing the reliability and flexibility of SWR operation. By implementing different fuel management strategies based on different deployment modes (high leakage and low leakage), it can achieve clean heating and serve as a backup heat source for winter heating projects, absorbing some of the peak load during winter heating periods. This meets the needs of various applications such as residential heating, industrial steam supply, and seawater desalination.

[0051] The purpose of the above embodiments is to provide a further detailed description of the present invention in conjunction with the accompanying drawings so that those skilled in the art can understand the technical concept of the present invention. Within the scope of the present invention, optimization or equivalent replacement of the technical solutions involved, as well as combination of implementation methods in different embodiments without conflict of structure and principle, all fall within the scope of protection of the present invention.

Claims

1. A method for managing fuel in a small pressurized water reactor core, wherein the small pressurized water reactor core comprises 57 groups of 17×17 fuel assemblies, characterized in that: The fuel assembly uses an integral burnable poison, and the small pressurized water reactor core fuel management method includes the following steps: Step a): In the first cycle of the small pressurized water reactor core, a first batch of fuel assemblies with three different enrichments are loaded, and the length of the first cycle is 24 months; the first batch of fuel assemblies includes a first enrichment, a second enrichment and a third enrichment, wherein the first enrichment < the second enrichment < the third enrichment; the fuel assembly arranged in the center of the small pressurized water reactor core adopts the first enrichment; the fuel assembly at the outermost periphery of the small pressurized water reactor core adopts the second enrichment, and the 3×3 fuel assemblies in the middle of the small pressurized water reactor core respectively The first enrichment and the second enrichment are arranged in a checkerboard pattern; the fuel assemblies using the third enrichment are separated from the outer boundary of the small pressurized water reactor core by one fuel assembly using the second enrichment; the number of fuel assemblies using the first enrichment is 5, the number of fuel assemblies using the second enrichment is 40, and the number of fuel assemblies using the third enrichment is 12; the first enrichment is 0.92%, the second enrichment is 1.90%, and the third enrichment is 3.00%; Step b): After the initial cycle, five or six transition cycles are performed; in each transition cycle, 17 burned-up fuel assemblies are discharged and 17 replacement fuel assemblies are loaded; each transition cycle lasts 21-24 months; the 17 discharged burned-up fuel assemblies include the fuel assembly at the center of the small pressurized water reactor core and the 16 most burned-up fuel assemblies among the remaining fuel assemblies; during refueling, the core loading maintains 1 / 4 rotational symmetry; the replacement fuel assemblies include the fuel assembly with the highest reserve reactivity remaining in the spent fuel pool and 16 new fuel assemblies with an enrichment of 3.40%-3.50%; Step c): After all transition cycles are completed, a balancing cycle is entered; in each balancing cycle, 17 burned-up fuel assemblies are unloaded and 17 replacement fuel assemblies are loaded; each balancing cycle lasts 24 months; the 17 burned-up fuel assemblies unloaded include the fuel assembly at the center of the small pressurized water reactor core and the 16 most burned-up fuel assemblies among the remaining fuel assemblies; during refueling, the core loading maintains 1 / 4 rotational symmetry; the replacement fuel assemblies include the fuel assembly with the highest reserve reactivity remaining in the spent fuel pool and 16 new fuel assemblies with an enrichment of 3.50%-3.60%; In the balancing cycle, the average batch discharge fuel consumption is 35,000-37,000 MWd / tU, and the maximum rod fuel consumption is 42,000-45,000 MWd / tU.

2. A method for managing fuel in a small pressurized water reactor core, wherein the small pressurized water reactor core comprises 57 groups of 17×17 fuel assemblies, characterized in that: The fuel assembly uses an integral burnable poison, and the small pressurized water reactor core fuel management method includes the following steps: Step a): In the first cycle of the small pressurized water reactor core, a first batch of fuel assemblies with three different enrichments is loaded, and the length of the first cycle is 24 months; the first batch of fuel assemblies includes a first enrichment, a second enrichment, and a third enrichment, wherein the first enrichment < the second enrichment < the third enrichment; the fuel assembly arranged in the center of the small pressurized water reactor core adopts the first enrichment; the fuel assemblies at the outermost edge of the small pressurized water reactor core adopt the third enrichment; 3×3 fuel assemblies in the middle of the small pressurized water reactor core adopt the first enrichment and the second enrichment respectively and are arranged in a checkerboard pattern; the number of fuel assemblies using the first enrichment is 5, the number of fuel assemblies using the second enrichment is 32, and the number of fuel assemblies using the third enrichment is 20; the first enrichment is 1.40%, the second enrichment is 1.90%, and the third enrichment is 2.70%; Step b): After the initial cycle, five or six transition cycles are performed; in each transition cycle, 17 burned-up fuel assemblies are discharged and 17 replacement fuel assemblies are loaded; each transition cycle lasts 21-24 months; the 17 discharged burned-up fuel assemblies include the fuel assembly at the center of the small pressurized water reactor core and the 16 most burned-up fuel assemblies among the remaining fuel assemblies; during refueling, the core loading maintains 1 / 4 rotational symmetry; the replacement fuel assemblies include a new fuel assembly with an enrichment of 2.40%-2.60% and 16 new fuel assemblies with an enrichment of 3.55%-3.65% disposed at the center of the small pressurized water reactor core; Step c): After all the transition cycles are completed, a balancing cycle is entered; in each balancing cycle, 17 burned-up fuel assemblies are unloaded and 17 replacement fuel assemblies are loaded; the length of each balancing cycle is 24 months; the 17 burned-up fuel assemblies unloaded include the fuel assembly at the center of the small pressurized water reactor core and the 16 fuel assemblies with the highest degree of burn-up among the remaining fuel assemblies; when refueling, the core loading maintains 1 / 4 rotational symmetry; the replacement fuel assemblies include a new fuel assembly with an enrichment of 2.40%-2.60% and 16 new fuel assemblies with an enrichment of 3.40-3.60% arranged at the center of the small pressurized water reactor core; the replacement fuel assemblies include a new fuel assembly with an enrichment of 2.40%-2.60% and 16 new fuel assemblies with an enrichment of 3.40-3.60% arranged at the center of the small pressurized water reactor core; In the balancing cycle, the average batch discharge fuel consumption is 35,000-37,000 MWd / tU, and the maximum rod fuel consumption is 42,000-45,000 MWd / tU.

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