Annular fuel element-based lead-cooled fast reactor core

Abstract

An annular fuel element-based lead-cooled fast reactor core, comprising a fuel area (100), a reflection area (200), and a shielding area (300), wherein the reflection area (200) surrounds the fuel area (100), and the shielding area (300) surrounds the reflection area (200); the fuel area (100) comprises an inner fuel area (101) and an outer fuel area (102) which are arranged concentrically, the fuel area (100) is provided with a plurality of assembly positions, gaps between the assembly positions form a cooling channel, and the cooling channel is filled with a cooling liquid; fuel assemblies (110) are placed at some assembly positions, each fuel assembly (110) comprises a plurality of fuel elements (112), each fuel element (112) comprises a fuel pellet (1122) and a fuel pellet cladding (1121) wrapping the fuel pellet (1122), and the cross section of each fuel element (112) is annular so that heat on the inner and outer sides of the fuel pellet (1122) can be taken away by the cooling liquid; adjustment assemblies (120) are placed at some assembly positions, and the adjustment assemblies (120) are capable of moving up and down at the assembly positions; safety assemblies (130) are placed at some assembly positions, and the safety assemblies (130) are capable of moving up and down at the assembly positions.

Description

A lead-based fast reactor core based on annular fuel elements Technical Field

[0001] The present application relates to the technical field of nuclear reactors, and in particular to a lead-based fast reactor core based on annular fuel elements. Background Art

[0002] The development of nuclear energy technology, as one of the main topics of today's era, is an important means to solve the world's problems such as resource shortages and environmental pollution. It is inseparable from the design and optimization of nuclear reactors.

[0003] At present, the nuclear energy system has developed to the fourth generation. Among them, the lead-based fast reactor not only has low operating conditions requirements and is not prone to producing explosive gases during operation, but also has a harder energy spectrum and can effectively transmute fissile nuclides into fissile nuclides to achieve fuel proliferation. It is considered to be the most promising type of reactor.

[0004] International research on lead-based fast reactors has made some progress, including China's CLEAR reactor, Russia's BREST-OD-300 reactor, and the European Union's MYRRHA and ALFRED reactors. Most of these reactors are still in the development phase, while others are mostly in the conceptual design stage. Existing lead-based fast reactors mostly use rod-shaped or plate-shaped fuel elements. However, rod-shaped fuel, due to the distance between the center of the fuel pellet and the cladding, results in high maximum fuel temperatures, poor heat dissipation, and low safety. The unique shape of plate-shaped fuel elements leads to a wide variety of fuel plate sizes within the core, increasing manufacturing complexity. Therefore, further research is needed to explore and optimize fuel element structures to improve the economic efficiency and safety of lead-based fast reactors.

[0005] Annular fuel elements not only reduce the maximum fuel temperature at the same power level, providing a higher safety margin, but also increase the minimum critical heat flux ratio and fuel utilization. While maintaining the same safety margin, they can also boost the core's output power. However, research on annular fuel elements has largely remained at the level of their individual characteristics and concepts, rarely integrating the annular fuel with the core. Existing core designs based on annular fuel elements have primarily focused on water-cooled reactors or small reactors. Therefore, annular fuel elements hold great potential for optimizing various aspects of lead-based fast reactor performance.

[0006] Summary of the Invention

[0007] The present application aims to address, at least to a certain extent, one of the aforementioned technical problems in the prior art. To this end, embodiments of the present application provide a lead-based fast reactor core based on annular fuel elements, integrating the annular fuel elements with the lead-based fast reactor core, and exhibiting the characteristics of long life, excellent safety, and high fuel utilization.

[0008] According to the lead-based fast reactor core based on annular fuel elements according to the embodiment of the present application, it includes a fuel area, a reflection area and a shielding area, wherein the reflection area surrounds the fuel area, and the shielding area surrounds the reflection area; wherein the fuel area includes an inner fuel area and an outer fuel area which are separated inside and outside, and the fuel area is provided with a plurality of component positions, and the gaps between the component positions are cooling channels, and the cooling channels are filled with coolant, and the coolant is liquid metal lead bismuth; fuel assemblies are placed at some of the component positions, and the fuel assembly includes a plurality of fuel elements, and the fuel elements include fuel pellets and the fuel pellets are placed The fuel core cladding is wrapped, and the cross-section of the fuel element is annular, so that heat can be taken away from both the inner and outer sides of the fuel core block by the coolant; adjustment components are placed at some of the component positions, which are used to adjust the reactivity of the lead-based fast reactor core based on the annular fuel element, so that the lead-based fast reactor core based on the annular fuel element is critical during steady-state operation, and the adjustment components can be moved up and down at the component positions; safety components are placed at some of the component positions, which are used to implement shutdown operations, and the safety components can be moved up and down at the component positions, and the space vacated after a single safety component moves is filled with coolant.

[0009] In an optional or preferred embodiment, the components are arranged layer by layer from the center of the fuel inner zone to the outer circle of the fuel outer zone and are arranged in a honeycomb shape. The adjustment components are dispersed at the junction of the fuel inner zone and the fuel outer zone and in the middle of the fuel inner zone, and the safety components are evenly dispersed in the area of ​​the fuel zone.

[0010] In an optional or preferred embodiment, the fuel assembly is in the shape of a hexagonal prism, and the fuel assembly includes a fuel shell, the fuel elements are arranged in the fuel shell, and a first flow channel for the coolant to flow is provided between each of the fuel elements in the fuel shell.

[0011] In an optional or preferred embodiment, in the fuel element, a first air cavity is provided between the fuel pellet and the fuel core cladding, and a first air cavity section, a first reflection section and a first shielding section are sequentially provided at the upper and lower ends of the fuel pellet.

[0012] In an optional or preferred embodiment, the fuel pellets utilize MOX fuel, with the inner fuel zone containing 93% UO2 and 7% PuO2, and the outer fuel zone containing 88.3% UO2 and 11.7% PuO2. This MOX fuel zoning arrangement ensures that the inner and outer fuel zones have the same U235 enrichment, but differ in their UO2 to PuO2 ratios: 7% and 11.7%, respectively, relative to the MOX fuel. This not only fully utilizes the fissile nuclide U238 for fuel proliferation but also facilitates power leveling within the reactor. This technical solution can reduce core refueling costs and extend core life.

[0013] In an optional or preferred embodiment, the adjustment component is cylindrical, and the adjustment component includes a first guide tube and a plurality of adjustment elements disposed in the first guide tube, and a second flow channel for the coolant to flow is provided between each of the adjustment elements in the adjustment component.

[0014] In an optional or preferred embodiment, the adjusting element includes an adjusting rod and an adjusting rod shell that wraps the adjusting rod, a second air cavity is provided between the adjusting rod and the adjusting rod shell, the adjusting rod is made of boron carbide material, and the upper end of the adjusting rod is sequentially provided with a second reflecting section, a second air cavity section and a second shielding section.

[0015] In an optional or preferred embodiment, the safety component is cylindrical, and includes a second guide tube and a plurality of safety elements disposed in the second guide tube. A third flow channel for the coolant to flow is provided between each of the safety elements in the safety component.

[0016] In an optional or preferred embodiment, the safety element includes a safety rod and a safety rod shell that wraps the safety rod, a third air cavity is provided between the safety rod and the safety rod shell, the safety rod is made of boron carbide material, and weight-bearing materials are provided at both ends of the safety rod.

[0017] In an optional or preferred embodiment, the inner fuel area is provided with 217 component positions, wherein the inner fuel area has 10 component positions for placing the regulating component or the safety component, and the outer fuel area is provided with 180 component positions, wherein the outer fuel area has 28 component positions for placing the regulating component or the safety component.

[0018] Based on the above technical solution, the embodiments of the present application have at least the following beneficial effects: To ensure the safety of the reactor core and reduce the possibility of a core rod jam, the present application utilizes two control systems in the fuel zone: a regulating assembly and a safety assembly. The regulating assembly is used to adjust the core reactivity to maintain criticality during steady-state operation, while the safety assembly is used to respond to emergency shutdowns and refueling. Both control systems independently regulate the reactor to ensure safety both during operation and in the event of a failure. Furthermore, the fuel elements utilize an annular design. Due to the unique double-sided cooling structure, heat can be removed from both the inside and outside of the fuel elements via coolant. The coolant is liquid lead-bismuth, which has a higher heat transfer coefficient than water, significantly improving the core's heat transfer capacity and safety. The present application combines annular fuel elements with a lead-based fast reactor core, resulting in a long lifespan, excellent safety, and high fuel utilization. This design not only provides power for power reactors but can also be used as a research reactor for analysis and research on fast breeder performance, providing a reference for the optimization of subsequent fast breeder reactor structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present application is further described below with reference to the accompanying drawings and embodiments;

[0020] FIG1 is a radial cross-sectional view of an embodiment of the present application;

[0021] FIG2 is a radial cross-sectional view of a fuel assembly according to an embodiment of the present application;

[0022] FIG3 is a radial cross-sectional view of a fuel element in an embodiment of the present application;

[0023] FIG4 is an axial cross-sectional view of a fuel element in an embodiment of the present application;

[0024] FIG5 is a radial cross-sectional view of the adjustment assembly in an embodiment of the present application;

[0025] FIG6 is a radial cross-sectional view of an adjusting element in an embodiment of the present application;

[0026] FIG7 is an axial cross-sectional view of an adjusting element in an embodiment of the present application;

[0027] FIG8 is a radial cross-sectional view of a safety component in an embodiment of the present application;

[0028] FIG9 is a radial cross-sectional view of a safety element according to an embodiment of the present application;

[0029] FIG10 is an axial cross-sectional view of the safety element in an embodiment of the present application. DETAILED DESCRIPTION

[0030] This section will describe the specific embodiments of the present application in detail. The preferred embodiments of the present application are shown in the accompanying drawings. The purpose of the accompanying drawings is to supplement the description of the text part of the specification with graphics, so that people can intuitively and vividly understand each technical feature and overall technical solution of the present application, but it cannot be understood as a limitation on the scope of protection of the present application.

[0031] In the description of this application, it should be understood that descriptions involving orientations, such as up, down, front, back, left, right, etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, they cannot be understood as limitations on this application.

[0032] In the description of this application, "several" means one or more, "many" means more than two, "greater than," "less than," and "exceed" are understood to exclude the number itself, while "above," "below," and "within" are understood to include the number itself. The terms "first" and "second" are used solely to distinguish technical features and are not to be construed as indicating or implying relative importance, or as implicitly specifying the number or order of the technical features indicated.

[0033] In the description of this application, unless otherwise clearly defined, terms such as setting, installing, and connecting should be understood in a broad sense, and technicians in the relevant technical field can reasonably determine the specific meanings of the above terms in this application based on the specific content of the technical solution.

[0034] Referring to Figures 1 to 10 , a lead-based fast reactor core based on annular fuel elements is shown, comprising a fuel region 100, a reflective region 200, and a shield region 300. The reflective region 200 surrounds the fuel region 100, and the shield region 300 surrounds the reflective region 200. The overall structure of the lead-based fast reactor core based on annular fuel elements is a short hexagonal prism, with a circumscribed diameter of 626 cm and a height of 380 cm. The height of the fuel region 100 is 120 cm. The thicknesses of the reflective region 200 and the shield region 300 surrounding the fuel region 100 are 400 mm to 460 mm, respectively. In this embodiment, the core is a lead-based fast reactor core based on annular fuel elements.

[0035] As shown in Figure 1, in order to make the core power distribution as flat as possible, the fuel area 100 includes an inner fuel area 101 and an outer fuel area 102, which are separated inside and outside. Combined with Figure 2, the fuel area 100 is provided with multiple component positions. The gaps between the component positions are cooling channels. The cooling channels are filled with coolant, and the coolant is liquid metal lead bismuth. Liquid metal lead bismuth has a higher heat transfer coefficient than water, which can significantly improve the heat transfer capacity and safety of the core.

[0036] Some assembly positions are provided with fuel assemblies 110, some assembly positions are provided with regulating assemblies 120, and some assembly positions are provided with safety assemblies 130. In this embodiment, the inner fuel area 101 is provided with 217 assembly positions, and the outer fuel area 102 is provided with 180 assembly positions.

[0037] The inner fuel area 101 has 10 locations for regulating assemblies 120 or safety assemblies 130; the outer fuel area 102 has 28 locations for regulating assemblies 120 or safety assemblies 130, and the remaining locations are for fuel assemblies 110. The regulating assemblies 120 are used to adjust criticality, while the safety assemblies 130 can handle emergency shutdowns and refueling situations to ensure core safety.

[0038] As shown in Figure 1, the components are arranged layer by layer from the center of the inner fuel zone 101 to the outer perimeter of the outer fuel zone 102 in a honeycomb pattern. The regulating assemblies 120 are dispersed at the junction of the inner and outer fuel zones 101, 102, and in the center of the inner fuel zone 101. The safety assemblies 130 are evenly dispersed throughout the fuel zone 100. Specifically, the inner fuel zone 101 is equipped with 207 fuel assemblies 110, 4 regulating assemblies 120, and 6 safety assemblies 130; the outer fuel zone 102 is equipped with 152 fuel assemblies 110, 14 regulating assemblies 120, and 14 safety assemblies 130.

[0039] Referring to Figure 2 , the fuel assembly 110 is hexagonal in shape, with a regular hexagonal cross-section. The fuel assemblies are not in close contact; the gap between them is 4.6 mm, and the distance across their flats is 197 mm to 198 mm. The fuel assembly 110 includes multiple fuel elements 112. In this embodiment, each fuel assembly 110 contains 169 fuel elements 112.

[0040] As shown in Figures 3 and 4 , the fuel element 112 specifically comprises a fuel pellet 1122 and a core cladding 1121 encasing the fuel pellet 1122. The fuel element 112 has an annular cross-section, allowing heat to be removed from both the inner and outer sides of the fuel pellet 1122 by the coolant. The fuel assembly 110 includes a fuel casing 111, within which the fuel elements 112 are disposed. A first flow channel 113 for coolant flow is defined between the fuel elements 112 within the fuel casing 111. It is understood that the fuel element 112 is generally cylindrical, with both its inner and outer circumferential surfaces in contact with the coolant, enhancing heat dissipation.

[0041] Fuel pellets 1122 use MOX fuel, with the inner fuel zone 101 containing 93% UO2 and 7% PuO2, and the outer fuel zone 102 containing 88.3% UO2 and 11.7% PuO2. This MOX fuel zoning arrangement ensures that the inner and outer fuel zones have the same U235 enrichment, but different ratios of UO2 and PuO2. The PuO2 ratios in the inner and outer zones relative to the MOX fuel are 7% and 11.7%, respectively. This not only fully utilizes the fissile nuclide U238 for fuel proliferation but also facilitates power leveling within the reactor. Specifically, varying fuel enrichments contribute to a flatter core power distribution, further reducing core refueling costs and extending core life.

[0042] The regulating assembly 120 is used to adjust the reactivity of the lead-based fast reactor core based on annular fuel elements, ensuring that the core is critical during steady-state operation. The regulating assembly 120 can be moved up and down. Referring to Figures 5 to 7 , the regulating assembly 120 is cylindrical and includes a first guide tube 121 and a plurality of regulating elements 122 disposed within the first guide tube 121. In this embodiment, 31 regulating elements 122 are evenly distributed within each first guide tube 121. A second flow channel 123 for coolant circulation is defined between each regulating element 122 within the regulating assembly 120.

[0043] Safety assembly 130 is used to implement a reactor shutdown operation. It can be moved up and down within its assembly position, and the space vacated by the movement of a single safety assembly is filled with coolant. Referring to Figures 8 to 10 , safety assembly 130 is cylindrical and includes a second guide tube 131 and multiple safety elements 132 positioned within the second guide tube 131. Each second guide tube 131 has 21 safety elements 132 evenly distributed within it. A third flow channel 133 for coolant circulation is located between each safety element 132 within the safety assembly 130.

[0044] This embodiment utilizes a lead-based fast reactor core with annular fuel elements. To ensure core safety and reduce the possibility of rod jams, two control systems are employed: a regulating component and a safety component. The regulating component adjusts core reactivity to maintain criticality during steady-state operation, while the safety component responds to emergency shutdowns and refueling. Both control systems independently regulate the reactor to ensure safety during operation and in the event of a failure.

[0045] The first guide tube 121 enables the adjustment component 120 to be inserted and removed from the component position, and the second guide tube 131 enables the safety component 130 to be inserted and removed from the component position. In conjunction with Figures 1 and 5, when the core is in normal service, the safety components 120 are all pulled out and the gaps are filled with coolant lead and bismuth. When an emergency occurs or the reactor needs to be shut down for refueling, the safety component 120 can be quickly inserted into the core to implement a shutdown operation. Referring to Figures 1 and 8, and in conjunction with Figure 5, the principles of the adjustment component 130 and the safety component 120 are the same. Both can be moved up and down independently along the guide grooves of the component position to achieve core reactivity regulation.

[0046] As shown in Figure 1, the regulating assemblies 120 are positioned as close as possible to the junction of the inner fuel zone 101 and the outer fuel zone 102, as well as in the center of the inner fuel zone 101. This prevents excessive peak power factors in the center of the core and mitigates core power instability caused by changes in enrichment at the separation between the inner and outer fuel zones. Fuel assemblies 110 are connected by non-fuel segments at both ends, ensuring a compact layout.

[0047] 3 and 4 , in the fuel element 112 , a first air cavity 1123 is defined between the fuel pellet 1122 and the core cladding 1121 , and a first air cavity segment 1124 , a first reflection segment 1125 and a first shielding segment 1126 are sequentially provided at the upper and lower ends of the fuel pellet 1122 .

[0048] 6 and 7 , the adjusting element 122 includes an adjusting rod 1222 and an adjusting rod shell 1221 that wraps the adjusting rod 1222. A second air cavity 1223 is provided between the adjusting rod 1222 and the adjusting rod shell 1221. The adjusting rod 1222 is made of boron carbide material. The upper end of the adjusting rod 1222 is sequentially provided with a second reflecting segment 1224, a second air cavity segment 1225 and a second shielding segment 1226.

[0049] 9 and 10 , the safety element 132 includes a safety rod 1322 and a safety rod cladding 1321 that wraps the safety rod 1322. A third air cavity 1323 is defined between the safety rod 1322 and the safety rod cladding 1321. The safety rod 1322 is made of boron carbide. Weight-bearing materials 1324 are provided at both ends of the safety rod 1322. Tungsten is used to ensure that the safety component 130 is not affected by the buoyancy caused by the coolant when inserted into the component position of the core.

[0050] The regulating rod 1222 is made of boron carbide material, and the safety rod 1322 is made of boron carbide material. That is, the absorbers of the regulating component and the safety component are both made of boron carbide, among which the abundance of 10-B10 is 92%, thereby effectively reducing the neutron leakage rate and enabling the core to achieve the purpose of long-term continuous combustion.

[0051] In some embodiments, the core cladding 1121, the regulating rod cladding 1221, and the safety rod cladding 1321 are made of T91 stainless steel. Similarly, the cladding structure used for the reflector and shield assemblies is also made of T91 stainless steel. Furthermore, the first reflector segment 1125 and the second reflector segment 1224 are also made of T91 stainless steel. To mitigate the corrosion of the cladding by liquid lead-bismuth, an anti-corrosion coating is applied to the surfaces of the core cladding 1121, the regulating rod cladding 1221, and the safety rod cladding 1321. The first shield segment 1126 on the fuel pellet 1122 is made of boron carbide, as is the shielding material of the shielding area 300. The second shield segment 1226 on the regulating rod 1222 is made of lead-bismuth.

[0052] In addition, the shielding area 300 includes a plurality of shielding assemblies. The shielding assemblies have the same structure as the fuel assemblies, except that the material in the fuel assemblies is replaced with boron carbide.

[0053] The lead-based fast reactor core based on annular fuel elements in this application has a thermal power of 300MW and a refueling cycle of more than 30 full-power years. It is characterized by excellent safety performance, long life, and high economy. This design significantly saves refueling costs while improving fuel utilization. The design concept of the ultra-long-life lead-based fast neutron reactor based on annular fuel elements in this application can not only be used as a long-term heat and power supply for nuclear power plants and various industries, but can also be used as a research reactor to conduct relevant fast neutron breeder reactor performance analysis and research work, and provide reference value for the subsequent optimization of fast neutron breeder reactor structures.

[0054] The embodiments of the present application are described in detail above in conjunction with the accompanying drawings, but the present application is not limited to the above embodiments. Various changes can be made within the scope of knowledge possessed by ordinary technicians in the technical field without departing from the purpose of the present application.

Claims

1. A lead-based fast reactor core based on annular fuel elements, comprising a fuel region, a reflector region, and a shielding region. The reflector region surrounds the fuel region, and the shielding region surrounds the reflector region. Among them, the fuel region includes an inner fuel region and an outer fuel region which are arranged inside and outside respectively. The fuel region is provided with a plurality of assembly positions, and the gaps between the assembly positions are cooling channels, which are filled with a coolant. The coolant is liquid lead-bismuth. Some of the assembly positions are filled with fuel assemblies. The fuel assembly includes a plurality of fuel elements. The fuel element includes a fuel pellet and a fuel cladding that wraps the fuel pellet. The cross-section of the fuel element is annular, so that both the inner and outer sides of the fuel pellet can transfer heat away through the coolant. Some of the assembly positions are filled with regulating assemblies, which are used to regulate the reactivity of the lead-based fast reactor core based on annular fuel elements, so that the lead-based fast reactor core based on annular fuel elements is critical during steady-state operation. The regulating assemblies can move up and down at the assembly positions. Some of the assembly positions are filled with safety assemblies, which are used to achieve shutdown operations. The safety assemblies can move up and down at the assembly positions, and the space vacated after a single safety assembly moves is filled with coolant.

2. The lead-based fast reactor core based on annular fuel elements according to claim 1, wherein, Each of the assembly positions is arranged layer by layer from the center of the inner fuel region to the outer circle of the outer fuel region, and is arranged in a honeycomb pattern. The regulating assemblies are dispersed at the junction of the inner fuel region and the outer fuel region and in the middle of the inner fuel region, and the safety assemblies are evenly dispersed in the fuel region.

3. The lead-based fast reactor core based on annular fuel elements according to claim 2, wherein, The fuel assembly is hexagonal prism-shaped. The fuel assembly includes a fuel housing, and the fuel elements are arranged in the fuel housing. There is a first flow channel for the coolant to flow between the fuel elements in the fuel housing.

4. The lead-based fast reactor core based on annular fuel elements according to claim 3, wherein, In the fuel element, there is a first gas cavity between the fuel pellet and the fuel cladding. Both the upper and lower ends of the fuel pellet are sequentially provided with a first gas cavity section, a first reflector section, and a first shielding section.

5. The lead-based fast reactor core based on annular fuel elements according to claim 3, wherein, The fuel pellet uses MOX fuel. Among them, 93% UO2 + 7% PuO2 is used in the inner fuel region, and 88.3% UO2 + 11.7% PuO2 is used in the outer fuel region.

6. The lead-based fast reactor core based on an annular fuel element according to claim 2, wherein, The regulating assembly is cylindrical. The regulating assembly includes a first guide tube and a plurality of regulating elements placed in the first guide tube. There is a second flow channel for the coolant to flow between the regulating elements in the regulating assembly.

7. The lead-based fast reactor core based on annular fuel elements according to claim 6, wherein, The regulating element includes a regulating rod and a regulating rod cladding that wraps the regulating rod. There is a second gas cavity between the regulating rod and the regulating rod cladding. The regulating rod is made of boron carbide material. The upper end of the regulating rod is sequentially provided with a second reflector section, a second gas cavity section, and a second shielding section.

8. The lead-based fast reactor core based on annular fuel elements according to claim 2, wherein, The safety assembly is cylindrical. The safety assembly includes a second guide tube and a plurality of safety elements placed in the second guide tube. There is a third flow channel for the coolant to flow between the safety elements in the safety assembly.

9. The lead-based fast reactor core based on annular fuel elements according to claim 8, wherein, The safety element includes a safety rod and a safety rod cladding that wraps the safety rod. There is a third gas cavity between the safety rod and the safety rod cladding. The safety rod is made of boron carbide material, and weight-bearing materials are respectively arranged at both ends of the safety rod.

10. The lead-based fast reactor core based on annular fuel elements according to any one of claims 2 to 9, wherein, There are 217 assembly positions provided in the inner fuel region, and 10 assembly positions in the inner fuel region are used to place the regulating assembly or the safety assembly. There are 180 assembly positions provided in the outer fuel region, and 28 assembly positions in the outer fuel region are used to place the regulating assembly or the safety assembly.

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