A method for loading burnable poison to reduce tritium emissions from nuclear power plants

Abstract

A method for loading burnable poisons to reduce tritium emissions in nuclear power plants, belonging to the field of nuclear power. The method includes the following steps: providing a first type of fuel assembly used in the reactor, providing tritium-reducing burnable poison rods, and loading Gd-containing burnable poisons in the tritium-reducing burnable poison rods; replacing at least part of the integral burnable poison fuel rods in the first type of fuel assembly with tritium-reducing burnable poison rods to obtain a second type of fuel assembly, iteratively calculating and obtaining the structure of the second type of fuel assembly that meets the safety design requirements, and using the second type of fuel assembly for fuel replacement. This method uses tritium-reducing burnable poison rods with a longer burnup consumption time, thereby reducing the demand for soluble boron in the primary loop and achieving the effect of reducing tritium emissions.

Description

Technical Field

[0001] The present invention belongs to the field of nuclear power, and particularly relates to a method for loading burnable poison to reduce tritium emissions from nuclear power plants. Background Art

[0002] With the increasing safety performance and environmental protection requirements of nuclear power plants, the emissions of radioactive substances during the operation of nuclear power plants have received more and more attention. Before the wastewater from nuclear power plants is discharged, it will be treated to remove most of the radionuclides. However, due to the very similar physical and chemical properties of tritium, the radioactive isotope of hydrogen, to hydrogen, it is difficult to completely separate. Currently, tritium has become the main source of radioactivity in the wastewater of various types of pressurized water reactor nuclear power plants worldwide. Under normal operating conditions, the tritium emissions of modern commercial pressurized water reactor nuclear power plants comply with the tritium emission standards of the International Atomic Energy Agency. However, further reducing tritium emissions is still helpful to improve the environmental friendliness of nuclear power plants. The main source of tritium is the primary loop of the nuclear power plant. Among them, the reaction of soluble boron in the primary loop coolant to produce tritium is an important source of tritium in the primary loop. The main role of soluble boron is to compensate for the excess reactivity of the fuel assemblies during the operation of the nuclear power plant. Therefore, providing a technical solution to optimize the excess reactivity of the fuel assemblies and reduce the concentration of soluble boron in the primary loop through the loading of burnable poison has positive significance for reducing tritium emissions from nuclear power plants. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for loading burnable poison to reduce tritium emissions from nuclear power plants, and to reduce the concentration of soluble boron in the primary loop by optimizing the design of burnable poison.

[0004] According to an embodiment of the present invention, there is provided a method for loading burnable poison to reduce tritium emissions from a nuclear power plant. The reactor core of the nuclear power plant uses a first type of fuel assembly. The first type of fuel assembly includes ordinary fuel rods and integral burnable poison rods. The reactor core includes 157 groups of fuel assemblies. The method includes the following steps:

[0005] Step a): Provide a reactor core model and provide the first type of fuel assembly model based on the first type of fuel assembly.

[0006] Step b): Establish a second type of fuel assembly model, and the second type of fuel assembly model is obtained by the following method:

[0007] Replace some of the integral burnable poison fuel rods in the first type of fuel assembly model with the ordinary fuel rods, and

[0008] Replace at least some of the remaining integral burnable poison fuel rods with tritium-reducing burnable poison rods, or add the tritium-reducing burnable poison rods in the guide tubes of the fuel assembly; wherein, the tritium-reducing burnable poison rods are loaded with Gd-containing burnable poison.

[0009] Based on the reactor core model, simulate and calculate whether the reactor core meets the safety design requirements of the reactor after replacing the first type of fuel assemblies that have undergone burnup in the reactor core with 64 sets of the second type of fuel assemblies during refueling;

[0010] Step c): When the calculation result of step b) does not meet the safety design requirements, adjust the number of the tritium-reducing burnable poison rods in the second type of fuel assembly model and the arrangement positions of the fuel assemblies in the reactor core model, and repeat the calculation process in step b) until the calculation result meets the safety design requirements;

[0011] Step d): According to the second type of fuel assembly model obtained in step c), provide 64 sets of the second type of fuel assemblies and use the second type of fuel assemblies for fuel replacement.

[0012] This method utilizes the neutron absorption performance of the Gd-containing burnable poison in the tritium-reducing burnable poison rods, enabling the tritium-reducing poison rods to compensate for the core's excess reactivity for a longer time during reactor operation, thereby reducing the demand for soluble boron in the primary coolant, achieving the effect of reducing the soluble boron concentration and thus reducing the tritium emissions generated by soluble boron in the primary loop.

[0013] Further, in some embodiments, in step a), the tritium-reducing burnable poison rods containing the Gd-containing burnable poison adopt a non-full-length structure axially in the second type of fuel assembly and are arranged asymmetrically relative to the axial midpoint of the second type of fuel assembly.

[0014] Further, in some embodiments, the reactor core includes 157 sets of fuel assemblies. In each fuel replacement, 64 sets of the second type of fuel assemblies are used to replace the fuel assemblies that have undergone burnup in the reactor core.

[0015] Further, in some embodiments, in step a), 64 sets of the first type of fuel assemblies are provided, and all the integral burnable poison fuel rods therein are replaced with ordinary fuel rods or tritium-reducing burnable poison rods.

[0016] Further, in some embodiments, in step c), among the 64 sets of the second type of fuel assemblies used for fuel replacement, the tritium-reducing burnable poison rods contain Gd-containing burnable poison; among them, 8 sets of the second type of fuel assemblies are provided with 4 tritium-reducing burnable poison rods, 8 sets of the second type of fuel assemblies are provided with 8 tritium-reducing burnable poison rods, 20 sets of the second type of fuel assemblies are provided with 12 tritium-reducing burnable poison rods, and 28 sets of the second type of fuel assemblies are provided with 16 tritium-reducing burnable poison rods.

[0017] Further, in some embodiments, after fuel replacement using the second type of fuel assembly in step c), the average soluble boron concentration of the primary coolant is reduced by 19% compared to using the first type of fuel assembly, and the average service life of the second type of fuel assembly is not shorter than 95% of the average service life of the first type of fuel assembly. Description of the Drawings

[0018] Figure 1 It is the critical boron concentration curve of the primary loop in a comparative example;

[0019] Figure 2 It is a schematic diagram of the arrangement of the first type of fuel assembly in an embodiment;

[0020] Figure 3 It is a schematic diagram of the arrangement of the second type of fuel assembly in an embodiment;

[0021] Figure 4 It is a comparison curve of the critical boron concentration of the primary loop between an embodiment and a comparative example;

[0022] Figure 5 It is a comparison curve of the nuclear heat flux hot channel factor between an embodiment and a comparative example;

[0023] Figure 6 It is a comparison curve of the nuclear enthalpy rise hot channel factor between an embodiment and a comparative example.

[0024] The purpose of the above-mentioned drawings is to illustrate the present invention in detail so that those skilled in the art can understand the technical concept of the present invention, rather than limiting the present invention. For the sake of simplicity of expression, the above-mentioned drawings only schematically show the structures related to the technical features of the present invention, and do not draw the complete structure and all details in strict accordance with the actual ratio. Detailed Embodiments

[0025] The following further elaborates on the present invention through specific embodiments in conjunction with the drawings.

[0026] The mention of "embodiment" in this article means that the specific features, structures or characteristics described in connection with the embodiment may be included in at least one embodiment of this article. The phrase appearing at various positions in the specification does not necessarily refer to the same embodiment, nor is it limited to mutually exclusive independent or alternative embodiments. Those skilled in the art should be able to understand that the embodiments in this article can be combined with other embodiments without structural conflicts.

[0027] In the description of this article, terms such as "first" and "second" are only used to distinguish different objects, and cannot be understood as indicating relative importance or limiting the quantity, specific order or primary-secondary relationship of the described technical features. In the description of this article, the meaning of "a plurality of" is at least two.

[0028] At present, tritium is an important source of radioactivity in the liquid waste discharged from pressurized water reactor nuclear power plants. The main production pathways of tritium include:

[0029] Tritium is produced by ternary fission in the fuel and penetrates through the fuel cladding into the primary coolant;

[0030] B10 in the core burnable poison reacts with neutrons to 10 produce tritium through the B(n,2α)T reaction and 10 B(n,α) 7 produce tritium through the Li(n,nα)T reaction, and then penetrates through the shell containing the burnable poison into the primary coolant;

[0031] In the secondary neutron source rod, the 9 B(n,α) 6 He(β - )Li(n,α)T reaction produces tritium, which penetrates through the secondary neutron source rod into the primary coolant;

[0032] In addition, the soluble boron in the primary coolant undergoes 10 the B(n,2α)T reaction and 6 the Li(n,α)T reaction and other reactions to produce tritium.

[0033] However, at present, the technical solutions for reducing tritium emissions mainly target the tritium emissions generated by the secondary neutron source rod. For example, Patent CN209374067U and CN114220562A respectively propose optimized secondary neutron source rod structures to reduce the production of tritium during the service of the secondary neutron source rod; the literature "Benefit Analysis of Canceling the Secondary Neutron Source Technology" (Automation Today, 2020.06) proposes a solution to cancel the secondary neutron source rod.

[0034] However, among the above tritium sources, the contribution of the secondary neutron source rod to tritium emissions only accounts for about 12%; the 10 B(n,2α)T reaction of the soluble boron in the primary coolant is the main pathway for tritium production, usually accounting for more than 60% of the total tritium emissions. Therefore, reducing the boron concentration in the primary coolant during the fuel cycle can effectively reduce the production of tritium.

[0035] In a pressurized water reactor, soluble boron in the primary loop is mainly used to compensate for the core's excess reactivity. At the beginning of the fuel cycle, due to the low degree of fuel burnup, it is necessary to control the core's excess reactivity to prevent the core power from being too high. The compensation for excess reactivity depends on soluble boron in the primary loop on the one hand and control rods and burnable poisons on the other hand. The use of burnable poisons is mainly used to control the excess reactivity in part of the core, reduce the initial soluble boron concentration, so that the moderator temperature coefficient is not positive, and is used to flatten the radial power distribution of the core. During the core burnup process, the neutron absorbers in the burnable poisons react with neutrons and are consumed, gradually releasing the core's excess reactivity. At present, some pressurized water reactor nuclear power plants use integral burnable poison fuel rods (IFBA). The loading amount of burnable poisons is determined during the core design. Usually, its loading amount is determined according to the limit condition that the moderator temperature coefficient is not positive. At present, some technical solutions disclose burnable poisons containing Gd. However, for commercial nuclear power plants using integral burnable poison fuel rods (IFBA), there is no technical solution to reduce tritium emissions by adjusting the burnable poisons. As Figure 1 shown, at the initial stage of the fuel cycle, due to the establishment of the equilibrium tritium poisoning level, the critical boron concentration decreases rapidly; subsequently, as the consumption rate of the burnable poison in the IFBA is greater than the fuel burnup, the critical boron concentration rises again and monotonically decreases with the increase of the fuel burnup depth after the burnable poison is exhausted. Patent CN118299081A discloses a technical solution to partially replace the compensation effect of soluble boron in the primary coolant on the excess reactivity by keeping some low-value control rods inserted in the core, so as to achieve the effect of reducing the soluble boron concentration. However, this technical solution will cause the reactor to lose the ability to track the load without boron adjustment, and its engineering application value is relatively limited.

[0036] To solve the above problems, an embodiment of the present invention provides a method for loading burnable poisons to reduce tritium emissions in a nuclear power plant. This method is applicable to nuclear power plants that use the first type of fuel assembly in the reactor core, that is, the fuel assembly including ordinary fuel rods and integral burnable poison rods, and includes the following steps:

[0037] Step a): First, provide a reactor core model, and based on the fuel assembly currently used in the reactor core (i.e., the first type of fuel assembly), provide a first type of fuel assembly model. The first type of fuel assembly model is established based on the first type of fuel assembly. The first type of fuel assembly includes ordinary fuel rods and IFBA rods. Usually, the burnable poison in the IFBA rod uses a ZrB2 coating coated on the surface of the fuel pellets. The first type of fuel assembly model reflects the relative position relationship and reaction performance of the ordinary fuel rods and IFBA rods in the first type of fuel assembly.

[0038] Step b): Establish a model of a second type of fuel assembly. The model of the second type of fuel assembly includes a tritium-reducing burnable poison rod, and the tritium-reducing burnable poison rod is loaded with Gd burnable poison.

[0039] Specifically, when using Gd burnable poison, the cladding structure of the tritium-reducing burnable poison rod is the same as that of the IFBA rod, and the arrangement form of the burnable poison is distributed in the fuel pellets in a dispersed form.

[0040] The specific structure of the model of the second type of fuel assembly can be obtained by the following scheme: Replace some of the IFBA rods in the model of the first type of fuel assembly with ordinary fuel rods, and replace at least some of the remaining IFBA rods with tritium-reducing burnable poison rods containing Gd burnable poison. Use the reactor core model to carry out calculations for the refueling scenario, and judge whether the reactor core meets the safety design requirements such as the power limit and power flattening of the reactor after replacing some of the first type of fuel assemblies that have undergone burnup in the reactor core with the second type of fuel assemblies. The burnup rate of Gd burnable poison is slower than that of the IFBA rod. According to the different operating states of the reactor, the number of fuel assemblies replaced during the refueling process is different.

[0041] Step c): When the calculation result of step b) does not meet the safety design requirements, adjust the number of tritium-reducing burnable poison rods in the second type of fuel rods and the arrangement of the fuel assemblies in the core, and repeat step b) until the calculation result of step b) meets the safety design requirements.

[0042] Step d): According to the structure of the model of the second type of fuel assembly obtained in step c), provide the corresponding second type of fuel assembly with the corresponding structure, and use these second type of fuel assemblies for fuel replacement.

[0043] In one embodiment, 157 groups of fuel assemblies are arranged in the reactor core, and the soluble boron concentration in the primary coolant during the fuel cycle is reduced by optimizing the loading of burnable poison.

[0044] In the comparative example where only IFBA rods are used in the core, the arrangement of the first type of fuel assemblies is as Figure 2 shown. In this embodiment, the tritium-reducing burnable poison rod uses Gd burnable poison doped with Gd oxide in uranium dioxide. The arrangement of the fuel assemblies that meets the safety design requirements obtained through calculation is as Figure 3 shown. To ensure that the moderator temperature coefficient is not positive and to flatten the radial power distribution, there are a total of 1184 Gd burnable poison fuel rods in the second type of fuel assembly, and integral burnable poison is not used.

[0045] Specifically, among the 64 newly loaded second-class fuel assemblies, 8 second-class fuel assemblies are provided with 4 tritium-reducing burnable poison rods, 8 second-class fuel assemblies are provided with 8 tritium-reducing burnable poison rods, 20 second-class fuel assemblies are provided with 12 tritium-reducing burnable poison rods, and 28 second-class fuel assemblies are provided with 16 tritium-reducing burnable poison rods.

[0046] Adopting this loading method of burnable poison, compared with the comparative example using the first-class fuel assembly as Figure 4 shown, in the embodiment, the critical boron concentration of the primary coolant is significantly reduced, the average soluble boron concentration of the circulating coolant is reduced by 19%, effectively reducing tritium emissions; at the same time, the curves of the nuclear heat flux hot channel factor and the nuclear enthalpy rise hot channel factor changing with the core burnup during the core operation of the embodiment and the comparative example are as Figure 5 shown, and the curve of the nuclear enthalpy rise hot channel factor changing with the core burnup is as Figure 6 shown. In the embodiment, the power peak factor of the core slightly increases in the initial stage of refueling but still meets the design limit requirements and has a large safety margin, and the change in the average lifetime of the fuel assembly is less than 5%.

[0047] The purpose of the above embodiments is to make a detailed description of the present invention in combination with the accompanying drawings so that those skilled in the art can understand the technical concept of the present invention. Within the scope disclosed by the present invention, optimizing or equivalently replacing the involved method steps, and combining the implementation manners in different embodiments without conflict in structure and principle all fall within the protection scope of the present invention.

Claims

1. A method for loading burnable poisons to reduce tritium emissions in a nuclear power plant, wherein the reactor core of the nuclear power plant uses a first type of fuel assembly, and the first type of fuel assembly includes ordinary fuel rods and integral burnable poison fuel rods, characterized in that, The reactor core includes 157 fuel assemblies and comprises the following steps: Step a): Provide a reactor core model and provide a first type of fuel assembly model based on the first type of fuel assemblies; Step b): Establish a second type of fuel assembly model, which is obtained by: Replacing some of the integral burnable poison fuel rods in the first type of fuel assembly model with the ordinary fuel rods, and Replacing at least some of the remaining integral burnable poison fuel rods with tritium-reducing burnable poison rods; wherein, the tritium-reducing burnable poison rods are loaded with Gd-containing burnable poison; Based on the reactor core model, simulate and calculate whether the reactor core after replacing the first type of fuel assemblies that have undergone burnup in the reactor core with 64 groups of the second type of fuel assemblies during refueling meets the safety design requirements of the reactor; Step c): When the calculation result of step b) does not meet the safety design requirements, adjust the number of the tritium-reducing burnable poison rods in the second type of fuel assembly model and the arrangement positions of the fuel assemblies in the reactor core model, and repeat the calculation process in step b) until the calculation result meets the safety design requirements; Step d): According to the second type of fuel assembly model obtained in step c), provide 64 groups of corresponding second type of fuel assemblies and use the second type of fuel assemblies for fuel replacement.

2. The combustible poison loading method for reducing tritium emissions from a nuclear power plant according to claim 1, wherein In step a), 64 groups of the first type of fuel assemblies are provided, and all the integral burnable poison fuel rods therein are replaced with the ordinary fuel rods or tritium-reducing burnable poison rods.

3. The combustible poison loading method for reducing tritium emissions from a nuclear power plant according to claim 1, characterized in that, In step c), among the 64 groups of the second type of fuel assemblies used for fuel replacement, the tritium-reducing burnable poison rods use Gd-containing burnable poison; wherein, 8 groups of the second type of fuel assemblies are provided with 4 tritium-reducing burnable poison rods, 8 groups of the second type of fuel assemblies are provided with 8 tritium-reducing burnable poison rods, 20 groups of the second type of fuel assemblies are provided with 12 tritium-reducing burnable poison rods, and 28 groups of the second type of fuel assemblies are provided with 16 tritium-reducing burnable poison rods.

4. The method for loading burnable poison to reduce tritium emission in a nuclear power plant according to claim 2, wherein In step c), all the burnable poisons in the second type of fuel assemblies used for fuel replacement are configured in the form of tritium-reducing burnable poison rods using Gd-containing burnable poison.

5. The combustible poison loading method for reducing tritium emissions from nuclear power plants according to claim 1, wherein After using the second type of fuel assemblies for fuel replacement in step c), the average soluble boron concentration of the primary coolant is reduced by 19% compared with using the first type of fuel assemblies, and the average service life of the second type of fuel assemblies is not shorter than 95% of the average service life of the first type of fuel assemblies.

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