A criticality safety control method for spent fuel storage pool

Abstract

The present invention relates to a criticality safety control method for a spent fuel storage pool. The criticality safety control method for a spent fuel storage pool provided by the present invention arranges spent fuel assemblies with a certain burnup depth at the edge of the grid in Zone I of the spent fuel storage pool, and coordinates the rational arrangement of neutron absorption plates. This avoids the need for blocking measures at the edge of the grid in Zone I of the spent fuel storage pool due to failure to meet criticality safety limit requirements under conditions where one side of the assembly falls, thereby saving storage space, ensuring the criticality safety of the spent fuel storage pool under normal conditions and various credible accident conditions, and improving the utilization rate and economic efficiency of the spent fuel storage pool.

Description

Technical Field

[0001] The invention belongs to the technical field of criticality safety design of spent fuel storage pools and relates to a criticality safety control method for spent fuel storage pools. Background Art

[0002] In nuclear power plants, spent fuel storage pools are crucial facilities for fuel handling and storage. Their primary function is to safely store fuel assemblies, including unirradiated new fuel assemblies, irradiated replacement assemblies, spent fuel assemblies no longer in service, and potentially damaged assemblies. Spent fuel discharged from the core is highly radioactive and emits significant decay heat, necessitating wet storage, which places high demands on storage conditions. To improve the economic efficiency of wet storage, advanced spent fuel storage systems employ denser storage methods, reducing the spacing between storage cavities and increasing storage capacity. This, in turn, places higher demands on the criticality safety control of the storage system.

[0003] Spent fuel storage pools at nuclear power plants are generally divided into two areas. Area I grids are used to store fresh fuel assemblies before loading, fuel assemblies unloaded from the core during unplanned outages, and spent fuel assemblies that have not met burnup limits and cannot be stored in Area II. Fresh fuel is assumed during criticality safety design. Area II storage grids are designed using a burnup credit system and are used to store spent fuel assemblies that have reached specified burnup limits.

[0004] The criticality safety design of the spent fuel storage pool grid requires consideration of the accidental fall of a new fuel assembly onto one side of the grid. For the grid in Area I of the spent fuel storage pool, if the edges are loaded with new fuel assemblies with a high initial enrichment, the criticality safety analysis results for this condition will exceed the criticality safety control limits and fail to meet criticality safety requirements. In engineering practice, the edges of the grid in Area I of the spent fuel storage pool are typically sealed, reducing the storage capacity of the spent fuel storage pool. Therefore, effectively utilizing the storage pool space while ensuring criticality safety becomes a critical issue. Summary of the Invention

[0005] In response to the shortcomings of the prior art, the present invention aims to provide a spent fuel storage pool criticality safety control method that avoids the need for blocking measures at the edge of the grid in Zone I of the spent fuel storage pool due to failure to meet criticality safety limit requirements under the condition of a component falling on one side. This saves storage space and improves the utilization rate and economic efficiency of the spent fuel storage pool while ensuring the criticality safety of the spent fuel storage pool under normal operation and various credible accident conditions.

[0006] To achieve this object, the present invention provides a spent fuel storage pool criticality safety control method, the method comprising the following steps:

[0007] Arrange the spent fuel assemblies with a certain burnup depth at the edge of the grid in Area I of the spent fuel storage pool;

[0008] Neutron absorption plates are installed between the fuel storage cavities and outside the storage cavities to reduce the reactivity of the storage system and improve the utilization rate of the spent fuel storage pool while ensuring criticality safety.

[0009] Furthermore, corresponding burnup limits are set for different initial spent fuel enrichments;

[0010] The spent fuel assembly with a certain burnup depth is a spent fuel assembly that meets the initial enrichment-burnup limit.

[0011] Furthermore, a method for determining whether a spent fuel assembly is a spent fuel assembly that meets the initial enrichment-burnup limit is as follows: if the burnup depth of the spent fuel assembly is not less than the burnup limit corresponding to the initial enrichment of the spent fuel assembly, then the spent fuel assembly is a spent fuel assembly that meets the initial enrichment-burnup limit.

[0012] Furthermore, the burnup limit corresponding to each initial spent fuel enrichment is smaller than the burnup limit of the spent fuel assemblies placed in area II of the spent fuel storage pool at the same initial enrichment.

[0013] Furthermore, the neutron absorption plate is made of a plate-shaped neutron absorption material used for criticality safety control, and the neutron absorption material includes one or a combination of boron carbide, boron-containing silicate material, boron-containing aluminum-based composite material, boron-containing organic material, metal cadmium with a coating layer, metal gadolinium, gadolinium oxide, boron-containing stainless steel, cadmium-containing stainless steel, and gadolinium-containing stainless steel.

[0014] Furthermore, neutron absorption plates of different materials or thicknesses are arranged in different partition grids, at different positions and in different directions of the spent fuel storage pool to ensure that the spent fuel storage pool meets criticality safety limit requirements under normal and credible accident conditions.

[0015] Furthermore, the neutron absorption plates are arranged in the same manner as the existing neutron absorption plates between the fuel storage cavities and outside the storage cavities.

[0016] The beneficial effect of the present invention is that, by adopting the criticality safety control method of the spent fuel storage pool provided by the present invention, by arranging spent fuel assemblies with a certain burnup depth at the edge of the grid in zone I of the spent fuel storage pool, and by rationally arranging neutron absorption plates, the need for blocking measures to be taken at the edge of the grid in zone I of the spent fuel storage pool due to failure to meet criticality safety limit requirements under the condition of component falling on one side is avoided, storage space is saved, criticality safety of the spent fuel storage pool under normal conditions and various credible accident conditions is ensured, and the utilization rate and economy of the spent fuel storage pool are improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of the criticality safety control method for a spent fuel storage pool according to an embodiment of the present invention.

[0018] Figure 2 2D schematic diagram of a storage grid when spent fuel assemblies with a certain burnup depth are arranged at the edge of the grid in zone I of the spent fuel storage pool according to an embodiment of the present invention.

[0019] Figure 3 2D schematic diagram of a storage rack with a new fuel dropped from its side according to an embodiment of the present invention.

[0020] Figure 4 3D schematic diagram of a storage rack with a new fuel dropped from its side according to an embodiment of the present invention. DETAILED DESCRIPTION

[0021] In order to enable those skilled in the art to better understand the technical solution of the present invention, the technical solution in the implementation mode of the present invention will be further clearly and completely described below in combination with the drawings in the implementation mode of the present invention. Obviously, the described implementation mode is only a part of the implementation mode of the present invention, not all implementation modes.

[0022] like Figures 1 to 3 As shown, this embodiment provides a spent fuel storage pool criticality safety control method, the method comprising the following steps:

[0023] Arrange the spent fuel assemblies with a certain burnup depth at the edge of the grid in Area I of the spent fuel storage pool;

[0024] Neutron absorption plates are installed between the fuel storage cavities and outside the storage cavities to reduce the reactivity of the storage system and improve the utilization rate of the spent fuel storage pool while ensuring criticality safety.

[0025] The spent fuel assembly having a certain burnup depth satisfies the initial enrichment-burnup limit. The method for determining whether a spent fuel assembly satisfies the initial enrichment-burnup limit is as follows: corresponding burnup limits are set for different initial spent fuel enrichments; if the burnup depth of the spent fuel assembly is not less than the burnup limit corresponding to the initial enrichment of the spent fuel assembly, the spent fuel assembly satisfies the initial enrichment-burnup limit.

[0026] Furthermore, the burnup limit corresponding to each spent fuel initial enrichment is smaller than the spent fuel assembly burnup limit at the same initial enrichment for the spent fuel assemblies placed in area II of the spent fuel storage pool.

[0027] The neutron absorption plate is made of a plate-shaped neutron absorption material used for criticality safety control, and the neutron absorption material includes one or a combination of boron carbide, boron-containing silicate material, boron-containing aluminum-based composite material, boron-containing organic material, metal cadmium with a coating layer, metal gadolinium, gadolinium oxide, boron-containing stainless steel, cadmium-containing stainless steel, and gadolinium-containing stainless steel.

[0028] Neutron absorption plates of different materials or thicknesses are arranged in different partition grids, positions and orientations of the spent fuel storage pool to ensure that the spent fuel storage pool meets criticality safety limit requirements under normal and credible accident conditions.

[0029] Furthermore, the neutron absorption plates are arranged in the same manner as the existing neutron absorption plates between the fuel storage cavities and outside the storage cavities.

[0030] If the neutron absorption plates are arranged in the same manner as the existing arrangement of neutron absorption plates between and outside the fuel storage cavities, it is necessary to analyze the burnup limits of spent fuel assemblies that can be arranged at the edge of the grid in Zone I of the spent fuel storage pool without changing the existing neutron absorption plate arrangement. A three-dimensional Monte Carlo program is used to simulate and model the actual grid configuration of the spent fuel storage pool in Zone I. Then, for each different initial enrichment, spent fuel assemblies with a certain burnup depth are arranged at the edge of the grid in Zone I of the spent fuel storage pool. Criticality safety analyses are then conducted under normal storage conditions and under conditions where one side of the assembly falls (i.e., the potentially most reactive accident condition) to confirm that all spent fuel assemblies with a certain burnup depth meet criticality safety requirements.

[0031] In this embodiment, the embodiment of the present invention is described in further detail by taking the actual design of a spent fuel storage pool of a nuclear power plant as an example.

[0032] The nuclear power plant uses new AFA-3G fuel assemblies with a maximum initial enrichment of 4.95%. The spent fuel storage pool is divided into two zones: Zone I and Zone II. Zone I consists of three 6x9 spent fuel storage grids with boron-aluminum composite neutron absorbers. Zone II utilizes a burnup credit design technique and consists of three 7x10 spent fuel storage grids with boron-aluminum composite neutron absorbers. These are used to store spent fuel assemblies whose burnup has reached the specified limit. The effective multiplication factor limit for Zone I of the spent fuel pool is 0.9358.

[0033] Comparative Example

[0034] Using existing criticality safety design methods, the outermost storage unit of the spent fuel storage pool's Area I grid facing the pool wall failed to meet criticality safety limits under a side-dropped component condition, necessitating containment measures. The containment ratio reached 28%. When Area I was fully loaded with fresh fuel with a 4.5% enrichment, the calculation results of the effective multiplication factor for varying distances between the side-dropped component and the grid are shown in Table 1. It can be seen that the criticality safety limit requirement was not met under the side-dropped component condition.

[0035] Table 1 Calculation results of effective multiplication factor when the distance between the new fuel assembly and the grid changes when the new fuel assembly is dropped from the side of the I area

[0036]

[0037] Example

[0038] A criticality safety control method for a spent fuel storage pool provided in this embodiment is firstly analyzed for the burnup limits of spent fuel assemblies that can be arranged at the edge of the grid in Zone I of the spent fuel storage pool without changing the arrangement of neutron absorption plates. A three-dimensional Monte Carlo program is used to simulate the actual grid configuration of the grid in Zone I of the spent fuel storage pool. The boundary conditions around the computational model are all set to mirror reflection. The physical meaning of this computational model represents an infinite arrangement of storage grids on a plane. The upper and lower top surfaces are set to sufficiently thick full-density water reflection layers. The water in the spent fuel storage pool is pure water, ignoring soluble boron in the water, and the maximum water density is set to 1.0 g / cm. 3 .

[0039] Then, for each type of spent fuel with different initial enrichment, spent fuel assemblies with different burnup depths were arranged at the edge of the grid in Area I of the spent fuel storage pool. A criticality safety analysis was then conducted under normal storage conditions, overhead drop conditions, and side assembly drop conditions (i.e., the most reactive and potentially accidental condition). When calculating the side assembly drop condition, only the boundary condition on one side of the dropped assembly was set to non-specular reflection, while the boundary conditions on the other three sides remained unchanged. The effective multiplication factor was also calculated for the dropped assembly at different distances from the grid.

[0040] Table 2 shows the effective incremental factors for the I-zone grid fully loaded with spent fuel assemblies or new fuel assemblies of a single initial enrichment under normal operating conditions, the condition where a group of spent fuel assemblies were dropped from above, and the condition where a group of new fuel assemblies were dropped from the side. The burnup depth of the spent fuel assembly is the burnup limit corresponding to the initial enrichment of the spent fuel assembly.

[0041] Table 2 Effective proliferation factors of storage racks fully loaded with a single species

[0042] Initial enrichment, % Burnup depth, MWd / tU <![CDATA[Normal operating condition k eff > <![CDATA[Falling from above k eff > <![CDATA[Side drop k eff > 4.45 38500 0.8872 0.9149 0.9098 4.95 44000 0.8876 0.9143 0.9104 4.5 New fuel 0.8926 0.9193 0.9455

[0043] Table 2 shows that when the spent fuel storage rack is fully loaded with new fuel assemblies, if a new fuel assembly falls from the side, the effective multiplication factor will exceed the critical safety limit (the effective multiplication factor limit for Zone I of the spent fuel pool is 0.9358).

[0044] This embodiment performs a criticality safety analysis for the side-dropped assembly condition: the outermost assembly is replaced with a spent fuel assembly whose burnup depth is the burnup limit corresponding to the initial enrichment of the spent fuel assembly. Under the side-dropped condition of a group of spent fuel assemblies, the calculation results of the effective multiplication factor as a function of the distance between the side-dropped assembly and the grid are shown in Table 3. Table 3 shows that the effective multiplication factor under the side-dropped condition of a group of spent fuel assemblies does not exceed the criticality safety limit, indicating that the spent fuels listed in Table 3 all meet criticality safety requirements.

[0045] It can be seen from this that when the outermost ring components are replaced with spent fuel assemblies and the burnup depth of the spent fuel assemblies is not less than the burnup limit corresponding to the initial enrichment of the spent fuel assemblies, the effective proliferation factor under the condition of a group of spent fuel assemblies falling sideways does not exceed the criticality safety limit, and can meet the criticality safety requirements.

[0046] Table 3 Effective proliferation factors when the outermost assembly is replaced by spent fuel in the case of a group of spent fuel assemblies dropped sideways

[0047]

[0048] The above embodiments are merely illustrative of the present invention. Those skilled in the art may make various modifications and variations to the present invention without departing from the spirit and scope of the present invention. Therefore, the present invention is intended to include such modifications and variations as long as they fall within the scope of the claims and their equivalents.

Claims

1. A method for criticality safety control of a spent fuel storage pool, characterized in that: The method comprises the following steps: Spent fuel assemblies having a certain burnup depth are arranged at the edge of the grid in zone I of the spent fuel storage pool; the spent fuel assemblies having a certain burnup depth are spent fuel assemblies that meet the initial enrichment-burnup limit; and a method for determining whether a spent fuel assembly is a spent fuel assembly that meets the initial enrichment-burnup limit is as follows: if the burnup depth of the spent fuel assembly is not less than the burnup limit corresponding to the initial enrichment of the spent fuel assembly, then the spent fuel assembly meets the initial enrichment-burnup limit. Then, a criticality safety analysis is conducted under normal storage conditions and under conditions where one side of the assembly has fallen. Combined with the effective proliferation factor limit for Zone I of the spent fuel pool, it is confirmed that all spent fuel assemblies with a certain burnup depth meet the criticality safety requirements. Neutron absorption plates are installed between the fuel storage cavities and outside the storage cavities to reduce the reactivity of the storage system and improve the utilization rate of the spent fuel storage pool while ensuring criticality safety.

2. A spent fuel storage pool criticality safety control method according to claim 1, characterized in that: Corresponding burnup limits are set for different initial enrichments of spent fuel.

3. The criticality safety control method for a spent fuel storage pool according to claim 1, characterized in that: The burnup limit corresponding to each initial enrichment of spent fuel is less than the burnup limit of the spent fuel assemblies placed in Area II of the spent fuel storage pool at the same initial enrichment.

4. A spent fuel storage pool criticality safety control method according to any one of claims 1 to 3, characterized in that: The neutron absorption plate is made of a plate-shaped neutron absorption material used for criticality safety control, and the neutron absorption material includes one or a combination of boron carbide, boron-containing silicate material, boron-containing aluminum-based composite material, boron-containing organic material, metal cadmium with a coating layer, metal gadolinium, gadolinium oxide, boron-containing stainless steel, cadmium-containing stainless steel, and gadolinium-containing stainless steel.

5. A spent fuel storage pool criticality safety control method according to claim 4, characterized in that: Neutron absorption plates made of different materials or thicknesses are arranged in different partition grids, at different positions and in different directions of the spent fuel storage pool to ensure that the spent fuel storage pool meets criticality safety limit requirements under normal and credible accident conditions.

6. The method for criticality safety control of a spent fuel storage pool according to claim 1, characterized in that: The neutron absorption plates are arranged in the same manner as the original neutron absorption plates between the fuel storage cavities and outside the storage cavities.

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