Water-to-water nuclear reactor fuel elements

Abstract

This invention pertains to fuel elements for nuclear engineering and the VVER-1200 type water-hydrodynamic reactor, and relates to improvements in the structure of VVER-1000 type fuel elements. Essentially, it modifies the structure of the bottom plug, increases the length of the fuel elements, and the length and mass of the fuel column, thereby increasing the reactor's thermal power to 3300 MW. The reactor core, composed of fuel assemblies, is manufactured taking into account the increased thermal power of the reactor and the increased height of the fuel elements within the fuel assemblies, and can be used within a reactor vessel with a geometry similar to that of a typical VVER-1000 type reactor vessel. As a result, fuel burnup is increased, thereby increasing the reactor's thermal power to 3300 MW, while maintaining the size of the VVER-1000 type reactor vessel and previous safety levels, while simultaneously increasing the overall power generation of the reactor.

Description

Technical Field

[0001] This invention pertains to nuclear engineering and relates to improving the structure of fuel elements that are part of an advanced fuel assembly, which are used to assemble the core of a water-cooled high-power pressure vessel nuclear reactor (i.e., the VVER1200 type). Background Technology

[0002] The future of nuclear energy development is largely determined by addressing the issues of increasing power generation and maintaining the previous safety levels of nuclear power plants.

[0003] There are several solutions to the problem of improving the economic efficiency of VVER-type reactors in existing nuclear power plants. However, currently, this problem is often solved with minimal changes to the core structural components. This approach allows for more efficient use of existing resources without major adjustments to the technical processes involved in manufacturing the structural components.

[0004] Currently, VVER-type nuclear reactors use fuel rod-type fuel elements. A fuel rod-type fuel element consists of a fuel column composed of individual cylindrical fuel pellets placed within a shell, which is the structural load-bearing element (see A.G. Samoilov, Fuel Elements of Nuclear Reactors, M., Energoatomizdat, 1985, pp. 99-107). To increase heat exchange area and reduce thermal stress caused by temperature differences, the diameter of fuel rod-type fuel elements is kept as small as possible, varying from 7.35 × 10⁻³ m to 15 × 10⁻³ m in actual pressurized water reactor designs (see D.N. Nushakov, Technical Access and Fuel Elements of Nuclear Reactors, M., Energoizdat, 1981, pp. 32-36). The structure of the fuel rod-type fuel elements, fuel assemblies, and the reactor core itself in VVER-type reactors must ensure the mechanical stability and strength of the fuel elements, including under high-temperature emergency conditions.

[0005] A known nuclear reactor comprises a sealed shell and fuel elements consisting of cylindrical fuel pellets stacked along the length of the shell on a fuel column and held in a predetermined position by a locator in the form of an open bushing. The shell of the fuel element is made of zirconium alloy (RU 2244347, G 21 С З / 00, 10 / 24 / 2002).

[0006] A known disadvantage of fuel elements is that they have a locator in the form of an open bushing, which, unlike a spring locator, cannot ensure the compaction of the fuel column in cases where it may shift during transport and technical operations, whether the fuel is fresh or irradiated.

[0007] Another drawback is that the outer diameter of the fuel element housing varies from 7.00·10-3m to 8.79·10-3, which requires a major adjustment to the manufacturing technology of all components of the fuel element structure and leads to greater technical complexity compared to existing manufacturing technologies.

[0008] The closest representation of the technological essence and achievements is the fuel element of the VVER-1000 water-to-water reactor core (Shmelev V.D., Dragunov Yu.G., Denisov V.P., Vasilchenko INVVER nuclear power plant core – M., Akademkniga Publishing Center, 2004, p. 106). This element consists of a top plug, a shell, a bottom plug, and a fuel column composed of uranium dioxide pellets and a fixative. The shell and plug are made of E110 alloy. To prevent the shell from being crushed during operation, helium is added to the internal volume of the fuel element at a pressure of (2.00 ± 0.25) MPa. The fuel element is sealed by welding. To reduce the pressure of gaseous fission products released under the shell during operation, a compensation volume is installed in the upper part of the fuel element. To prevent the fuel column from being affected by transport technology loads, it is secured with a positioner. The top plug is coupled to a handle for installing the fuel element during the extraction process—the fuel assembly process. The bottom plug is installed in the lower grille and secured with a cotter pin.

[0009] The disadvantages are that the fuel elements and fuel columns in the VVER-1000 reactor are shorter than those in the VVER-1200 reactor, resulting in a smaller total fuel load into the core. This leads to a decrease in the power generation of the VVER-1000 water-to-water reactor compared to the VVER-1200. The use of E110 alloy as the hull material, with a higher hafnium content than E110 alloy, increases neutron absorption in the fuel hull, also contributing to the reduced power generation. The use of cotter pins to secure the bottom plug to the support grid makes the process of removing fuel elements from the fuel assembly more complex and time-consuming. Summary of the Invention

[0010] The purpose of this invention is to develop and create a novel fuel element for the VVER-1200 type water-to-water power reactor, which has increased power generation while maintaining the same level of safety, and simplifies the assembly of fresh (unirradiated) fuel assemblies using existing technology and equipment through optimization of the fuel element structure.

[0011] The technological achievements include increasing power generation and fuel consumption while maintaining the reliability and safe operation of water-to-water reactor fuel elements, and simplifying the process of assembling fresh fuel assemblies.

[0012] The technological achievement is realized as follows: the fuel element of the water-hydrodynamic reactor consists of a cylindrical shell, which is sealed with a top plug and a bottom plug. The plugs are concentrically welded to the shell containing inert gas inside the fuel element. The fuel element contains fuel columns concentrically arranged in the auxiliary shell. The fuel columns are composed of fuel pellets with a central hole. In this case, the fuel columns are axially pressed against the bottom plug by a spring positioner. The spring positioner consists of coils of a compensation group. The coils of the compensation group ensure the axial clamping force between the fuel columns and the coils of the positioning group. The spring positioner is fixed in a defined position by an interference fit on the inner surface of the cylindrical shell. The shell and the plug are connected by contact butt welding. The length of the weld joint is one to three shell wall thicknesses. The weld joint area does not protrude beyond the diameter of the original shell. The bottom plug is a chuck-type element with a stepped cross-section convex edge and a slot at its lower part. The slot is located on the longitudinal plane facing downwards.

[0013] The fuel element length - L0 is 4030 to 4036 mm.

[0014] The length of the fuel column, L1, is 3720 to 3740 mm.

[0015] The free volume length under the fuel element housing - L2 is 250 to 270 mm.

[0016] The length of the casing - L3 - is 3995 to 4005 mm.

[0017] The fuel column has a mass of 1600 to 1800g.

[0018] The cylindrical shell is made of high-purity zirconium alloy E110, which is composed of zirconium and alkaline impurities added in the following mass ratios, %: zirconium-based; niobium 0.8-1.5; iron 0.02-0.08; oxygen 0.05-0.1; carbon less than 0.01; silicon less than 0.02; hafnium less than 0.010.

[0019] The spring positioner is made of stainless steel.

[0020] The coil of the spring positioner that contacts the upper fuel pellet is pressed into contact and polished to form a contact plane between the coil and the fuel pellet.

[0021] Helium, as an inert gas under the shell, accounts for 90% to 99% of the final product by mass.

[0022] The specified sum of features is innovative, unknown in the prior art, and solves the proposed problem because:

[0023] (i) Increasing the length of fuel columns and fuel elements will increase the total fuel load in the reactor core. Calculations show that, with a relatively large fuel load, it provides similar performance and reliability indicators to the fuel elements of the VVER-1000 reactor, and has a higher service life and fuel burnup for similar parameters of the VVER-1000 fuel elements;

[0024] (ii) Due to the reduction in hafnium content, using high-purity alloy E110 as the shell material can reduce the absorption of neutrons by the shell, thereby increasing power generation;

[0025] (iii) Using a clamp-type bottom plug in the structure of the fuel element can assemble the fuel elements into bundles and secure them firmly without the need for additional tools and fixing elements (cotter pins, etc.);

[0026] (iv) The use of contact welding can improve reliability and simplify the process of fuel element assembly;

[0027] (v) When installing the positioner, the inner surface of the housing is cold-worked from the bottom of the top plug to the coil area of ​​the positioning group, thereby increasing the safety factor during the water pressure test according to the loss of stability criterion.

[0028] (vi) The installation method of the positioner is such that the end coil of the spring positioner is pressed to contact and polished, and then contacts the upper fuel pellet, forming a contact plane between the coil and the fuel pellet, thereby improving the reliability of the fuel element;

[0029] (vii) Helium is used as an inert gas under the casing, and its mass fraction in the final product is in the range of 90% to 99%, thereby improving the thermal conductivity and corrosion resistance of the inner surface of the fuel element casing.

[0030] (viii) The selected length of the welded joint is one to three times the thickness of the outer shell wall, and the welded joint area does not exceed the original shell diameter, thereby ensuring the reliability and safe operation of the fuel element. Detailed Implementation

[0031] Tables 1 and 2 show the confirmation of the technical results, which list a comparison of the calculated design parameters of the fuel elements for the VVER-1000 and VVER-1200 reactors.

[0032] Table 1. Calculated fuel consumption

[0033]

[0034] Table 2. Safety factors calculated under static operating conditions based on thermophysical, strength, and deformation standards.

[0035]

[0036]

[0037] The comparison of data in Table 1 shows that the average maximum design fuel burnup of both fuel elements and pellets in the VVER-1200 reactor is increased compared to the VVER-1000 reactor. Table 2 compares the safety factors of the design performance standards, showing that the design safety factor for the fuel elements of the VVER-1200 reactor is higher than the standard value and also higher than the same safety factor for the fuel elements of the VVER-1000 reactor.

[0038] Figure 1 A proposed longitudinal cross-section of a fuel element for a VVER-1200 reactor is shown.

[0039] Figure 2 A magnified image of the bottom plug is shown.

[0040] Figure 3 The cross-section of the bottom plug is shown.

[0041] Figure 4 The elongation of the fuel element is shown, calculated based on the burnup of the fuel element in the VVER-1200 reactor.

[0042] Figure 5 The design elongation of the fuel element is shown, depending on the fuel burnup of the VVER-1000 reactor fuel element.

[0043] Figure 6 The diagram shows the variation in fuel casing diameter calculated based on the burnup of fuel elements in a VVER-1200 reactor.

[0044] Figure 7 The diagram shows the variation in fuel casing diameter calculated based on the burnup of fuel elements in a VVER-1000 reactor.

[0045] Figure 8 The diagram shows the calculated circumferential stress on the inner surface of the overloaded fixed-cycle shell of a VVER-1200 reactor fuel element.

[0046] Figure 9 The diagram shows the calculated circumferential stress on the inner surface of the overloaded fixed-cycle shell of a VVER-1000 reactor fuel element.

[0047] Figure 10 The calculated gas pressure values ​​for the static cycle of fuel elements in the VVER-1200 reactor under both hot and cold conditions are shown.

[0048] Figure 11 The calculated gas pressure values ​​for the static cycle of fuel elements in the VVER-1000 reactor under both hot and cold conditions are displayed.

[0049] Fuel elements of a water-water nuclear reactor ( Figure 1 The fuel element consists of the following structural components: a fuel column composed of fuel pellets (4) with a central hole, a spring positioner (5), a cylindrical shell (3), a top plug (1), and a bottom plug (2). The fuel column is placed in the housing (3) of the fuel element, with the lower end of the fuel pellet (4) contacting the bottom plug (2) and the upper fuel pellet (4) contacting the spring positioner (5). The spring positioner is fixed in the cylindrical shell (3) by tension and provides compression and maintains the tightness of the fuel column. The top plug (1) and the bottom plug (2) are sealed and welded to the cylindrical shell (3), thereby providing a sealed cavity inside the fuel element. When the top plug (1) is welded to the cylindrical shell (3), an inert gas is supplied to the inside of the fuel element under pressure to ensure the corrosion resistance, strength, and thermal conductivity of the fuel element. The bottom plug (2) is characterized by having: a groove (7) with a length L4 from 9 to 13 mm; a cylindrical groove (6) with a length L5 of 9 to 13 mm; a clamp portion (9) with a length L6 of 15 to 16 mm; and a convex edge (8) with a tapered portion. Due to the elasticity of the grooved clamp portion, the bottom plug (2) of the fuel element is fixed in the support grid of the fuel element during fuel element assembly. When the fuel element is assembled in the fuel assembly, the groove is compressed and the outer diameter of the convex edge is reduced to the corresponding inner diameter of the grid of the fuel assembly. Then, the clamp portion of the plug is pushed into the grid of the fuel element until it stops. After that, the groove is released to its original state and the initial outer diameter of the convex edge is restored. The convex edge is close to the grid of the fuel element, thereby preventing axial movement of the bottom plug and the fuel element.

[0050] The presence of the bottom plug in the structure enables one of the set tasks to be accomplished, namely, simplifying the assembly process of the fresh fuel assembly.

[0051] Industrial Applicability

[0052] Taking into account Figure 4-11 The calculations shown indicate that, under normal operating conditions, compared to the fuel element design of the VVER-1000 reactor, the elongation and diameter variation of the fuel elements are reduced, circumferential stress is decreased, and the internal pressure of the VVEL-1200 fuel elements does not increase. Specifically, the power generation limit of the fuel elements is reduced, and the fuel element casing is one of the main obstacles to the spread of radioactive materials; in an emergency, it can lose its seal, primarily due to overheating. This decision was based on increased safety requirements for nuclear power plants and the successful operational experience of existing nuclear fuel designs over many years.

Claims

1. A water-to-water nuclear reactor fuel element consists of a cylindrical shell (3) sealed with a top plug (1) and a bottom plug (2). The plugs are concentrically welded to the cylindrical shell (3) containing inert gas inside the fuel element. The fuel element comprises fuel columns concentrically arranged in the cylindrical shell (3). The fuel columns are composed of fuel pellets (4) with a central hole. In this case, the fuel columns are axially pressed against the bottom plug (2) by a spring positioner (5). The spring positioner (5) consists of coils of a compensation group. The coils of the compensation group ensure the axial clamping force between the fuel column and the coils of the fixing group. The spring positioner is fixed in a defined position by an interference fit on the inner surface of the cylindrical shell (3). The fuel element is characterized by: The cylindrical shell (3) and the plugs (1, 2) are connected by contact butt welding. The length of the weld joint is one to three times the wall thickness of the cylindrical shell (3). The weld joint area does not protrude beyond the diameter of the original cylindrical shell (3). The bottom plug (2) is a clamp-type element (9) with a stepped cross-section convex edge (8) and a slot (7) at its lower part. The slot (7) is located in the plane of the longitudinal axis and extends to the lower end. The cylindrical shell (3) is made of a high-purity zirconium alloy E110, which is composed of zirconium with the following weight % main impurities: zirconium-based; niobium 0.8-1.5; iron 0.02-0.08; oxygen 0.05-0.1; carbon less than 0.01; silicon less than 0.02; and hafnium less than 0.

010.

2. The fuel element of the water-to-water dynamic nuclear reactor according to claim 1, characterized in that, The length of the fuel element, L0, is 4030–4036 mm.

3. The fuel element of the water-to-water dynamic nuclear reactor according to claim 1, characterized in that, The length of the fuel column, L1, is 3720–3740 mm.

4. The fuel element of the water-to-water nuclear reactor according to claim 1, characterized in that, The free volume length -L2 inside the cylindrical shell (3) of the fuel element is 250 to 270 mm.

5. The fuel element of the water-to-water dynamic nuclear reactor according to claim 1, characterized in that, The length L3 of the cylindrical outer shell (3) is 3995 to 4005 mm.

6. The fuel element of the water-to-water dynamic nuclear reactor according to claim 1, characterized in that, The mass of the fuel column is 1600–1800g.

7. The fuel element of the water-to-water dynamic nuclear reactor according to claim 1, characterized in that, The spring positioner (5) is made of stainless steel.

8. The fuel element of the water-to-water nuclear reactor according to claim 1, characterized in that, The coil of the spring positioner (5) that contacts the upper fuel pellet (4) is pressed into contact and polished, thus forming the contact plane between the coil and the fuel pellet (4).

9. The fuel element of the water-to-water dynamic nuclear reactor according to claim 1, characterized in that, Helium, as an inert gas under the cylindrical shell (3), has a mass fraction of 90-99% in the final product.

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