Nuclear fuel pellets

Abstract

The invention belongs to nuclear technology, namely the structure of fuel element fuel pellets for light water reactors, such as water-water type power reactors, such as water-water power reactors, which can use fuel elements of reactors with long fuel cycles and the maneuvering state of reactor power. Develop cylindrical pellets with external chamfers and central holes along the longitudinal axis, connecting the side cylindrical surface with the end face, the external chamfer and the end face area plane make an angle equal to 20-30°, the ratio of the chamfer width to the pellet diameter is 0.02 to 0.15, and at the same time, the size of the single defect of the appearance of the pellet is not greater than 1.3% of the area of ​​the outer surface of the pellet in terms of end face defects, 1.0% of the area of ​​the outer surface of the pellet in terms of side cylindrical surface defects, and 0.7% of the area of ​​the outer surface of the pellet adjacent to the chamfer outer edge, and the average particle size of uranium dioxide is 6 to 35 μm. The pellets can contain aluminum from 0.002 to 0.020% by mass and silicon from 0.001 to 0.010% by mass as oxides uniformly distributed throughout the volume of the pellets. The pellets may additionally contain gadolinium oxide from 0.3 to 10.0% by mass, which is uniformly distributed throughout the pellet as a solid solution with uranium dioxide or erbium oxide from 0.3 to 2.5% by mass, which is uniformly distributed throughout the pellet volume as a solid solution with uranium dioxide. The invention object of the application is the development of a nuclear fuel pellet design that should ensure safe operation of the fuel elements in normal operating mode, interruptions of normal operation and maneuvering mode of reactor operation as well as increased fuel combustion. The technical result lies in reducing the tensile hoop stress in the fuel element cladding by reducing the size of unidirectional defects in the pellet appearance and reducing the yield of gaseous fission products due to the addition of additives that can increase the particle size.

Description

Technical Field

[0001] The invention belongs to nuclear technology, namely the fuel element fuel pellet structure of a light water reactor, such as a water-water type power reactor, the fuel element of a reactor that can use a long fuel cycle and the maneuvering state of the reactor power.

[0002] The nuclear fuel requirements for nuclear reactors are being constantly limited. In particular, the fuel should ensure safe operation of the reactor during long fuel cycles, maneuvering of the reactor power and maintaining performance during high burn-up operations. Background Art

[0003] The known pellets (RU 2 376 665, IPC G21C 3 / 62, issued in 2009) contain oxides of aluminium and silicon, equal to uranium, uniformly distributed throughout the volume of the pellet, with an aluminium content of 0.005 to 0.03 mass%, a silicon content of -0.003 to 0.02 mass%, a mass ratio of aluminium to silicon of 1.5 to 4, and a particle size of uranium dioxide of 20 to 45 μm.

[0004] A disadvantage of the known pellets is the relatively high particle size, which reduces the strength of the fuel and increases the breakability of the pellets.

[0005] Known fuel pellets (RU 2 193 242, IPC G21C 3 / 62, C01G 43 / 025, published in 2002) are made of uranium dioxide with added aluminum additives of 0.03 to 0.10 mass% and silicon additives of 0.01 to 0.05 mass%, with aluminum and silicon distributed as oxides on the uranium grain boundaries. In a specific embodiment, the pellets may also contain: iron oxide additives, part of which are distributed along the grain boundaries and part of which are distributed in the uranium dioxide as a solid solution; and niobium oxide additives, which are distributed in the uranium dioxide as a solid solution. In a specific embodiment, the uranium dioxide pellets may have a grain size of 10 to 20 μm or 12 to 30 μm. In a specific embodiment, the pellets will contain 0.3 to 10.0 mass% of gadolinium oxide additives or 0.3 to 0.8 mass% of erbium oxide additives.

[0006] Disadvantages of known pellets are high alloying element content, higher than currently required, increasing the total boron equivalent of the fuel and small grain size, reducing the ability to retain fission gas products and preventing high burn-up from being achieved.

[0007] A nuclear uranium erbium ceramic fuel pellet (RU 2 339 094, IPC G21C 3 / 62, issued in 2007) is known, wherein erbium has 0.36 to 0.64 mass %, and aluminum oxide powder (Al 2 O 3 ) and a powder of one of the following series of oxides: TiO 2 , Nb 2 O 5、SiO 2 , CaO, MgO, or only one mineral aluminosilicate: kaolin, metakaolin, galazine, montmorillonite, vermiculite.

[0008] The known pellets have the disadvantage of high alloying element content, 0.10% by mass for each element, which is higher than the existing requirement and increases the total boron equivalent of the fuel. In one embodiment, the pellet grains reach 50 μm, which reduces the strength characteristics of the fuel and increases the crushability of the pellets.

[0009] Nuclear fuel pellets made of uranium dioxide or uranium dioxide (RU 2 268 507, IPC G21C 3 / 62, 21 / 00, issued in 2005), with the addition of 2.0-8.0% by mass of gadolinium oxide and the addition of aluminum oxide powder (Al 2 O 3 ) and a powder of one of the following series of oxides TiO 2 , Nb 2 O 5 、SiO 2 、CaO、MgO、BeO、Na 2 O.P 2 O 5 , the oxide content should ensure that the content of aluminum and uranium-related oxide elements in the pellets is in the range of 20-500ppm, with the ratio: 60-85% A1, the others are the specified series of oxide elements.

[0010] The disadvantage of known pellets is that the alloying element content is high, which is greater than the current requirement, increasing the total boron equivalent of the fuel. In one embodiment, the grain size reaches 100 μm, which reduces the fuel strength and increases the crushability of the pellets.

[0011] The closest analogue is the known pellet (RU 1 669 308, ICP G21C3 / 28, 3 / 16, published in 1994), in which the quality is improved by increasing the end strength and reducing the thermal deformation of the pellet and its thermomechanical effect with the shell, and the conical flat protrusion is adopted. 凸出部 =(0.005-0.009)H, where H is the height of the core block, which is equal to 0.25-2.0 of the core block diameter, and the height of the conical hemisphere or flat groove is h 凹槽 =(1.1-2.5)(h 外倒角 +h в ), where h 外倒角 -Outer chamfer height.

[0012] Disadvantages of the known pellets are the reduced uranium content due to the conical projections and recesses and the increased hoop stresses inside the fuel element casing at the pellet contact point in the fuel rod due to the increased overall height of the chamfer and the pellet projections. Summary of the invention

[0013] The invention of the present application aims to ensure the safe operation of fuel elements and fuel pellets under normal operating conditions, the normal operation of the reactor and the interruption of the maneuvering state, and the increase of the fuel combustion depth.

[0014] Technical achievements include reducing the circumferential tensile stress in the fuel element cladding by reducing the size of individual defects in the pellet appearance, and reducing the yield of fission gases due to the introduction of additives that increase the particle size of fuel elements.

[0015] The technical result can be achieved by the fuel pellets having a cylindrical body and an outer chamfer and a central hole along the longitudinal axis, while the outer chamfer connecting the side cylindrical surface and the end platform forms an angle a with the plane of the end surface equal to 20-30°, and the ratio of the chamfer width to the pellet diameter is l 倒角 / D=0.02-0.15, and a single defect in the pellet appearance is no greater than 1.3% of the pellet outer surface area at the end face, no greater than 1.0% of the pellet outer surface area at the side cylindrical surface, and no greater than 0.7% of the pellet outer surface area adjacent to the chamfered outer side, and the average particle size of uranium dioxide is 6 to 35 μm.

[0016] In a specific embodiment, the fuel pellet contains 0.002 to 0.020 mass % aluminum and 0.001 to 0.010 mass % silicon, uniformly distributed as oxides throughout the pellet volume.

[0017] In another embodiment, the fuel pellet contains 0.3 to 10.0 mass % of gadolinium, which forms a solid solution with the uranium dioxide and is uniformly distributed throughout the pellet volume.

[0018] In another embodiment, the fuel pellet contains 0.3 to 2.5 mass % erbium, uniformly distributed throughout the pellet volume as a solid solution with uranium dioxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] exist Figure 1 Schematic diagram showing fuel pellets.

[0020] exist Figure 2 Schematic diagram showing typical defect types in the appearance of fuel pellets.

[0021] exist Figure 3 Displays the maximum circumferential stress inside the fuel element cladding based on the defect (notch) area where the fuel pellet is close to the outer edge of the chamfer. DETAILED DESCRIPTION

[0022] In normal operation, normal operation interruption and maneuvering mode of reactor operation, the tensile hoop stress in the fuel element cladding is reduced by reducing the single defects in the appearance of the pellets, thereby ensuring the safe operation of the fuel elements of the fuel pellets. By optimizing the geometric parameters of the pellets, especially the external chamfers and the content and particle size of the aluminum and silicon additives in the nuclear fuel pellets, the residual stress is reduced during the transportation and loading manufacturing process to increase the pressing and pellet strength, reduce the number of single defects, and reduce the breakage of the pellets. Due to the additives that increase the grain size of the nuclear fuel pellets, the yield of gaseous fission products is reduced, which causes an increase in the depth of fuel combustion.

[0023] Fuel pellets ( Figure 1 ) A cylindrical body 1 with a central hole 2 and an outer chamfer 3 along the axis, and the outer chamfer connecting the side cylinder surface and the end surface is at an angle of 20-30° to the end surface plane, and the ratio of the chamfer width to the core block diameter is l 倒角 / D = 0.02-0.15. The safe operation of fuel for advanced water-hydrodynamic reactors is ensured by increasing the strength, reducing the number of notches and reducing the number of internal notches in fuel elements during loading and transportation. The effect of reducing residual stresses due to the external chamfer can be achieved, thereby increasing the strength of pressed and sintered pellets and reducing pellet damage during the manufacturing process. A mathematical pressing model for reducing residual stresses in fuel pellets has been developed [Mathematical Modeling of the Pressing Process of Uranium Dioxide Powder in the Manufacture of Nuclear Fuel Pellets OA Bakhteev, AV Resikov, EN Mikheev, AN Zhiganov, AD Istomin, AA Matolikin, MD Noskov, AA Cheglokov / / Journal of the National Research Nuclear University-Moscow Engineering Physics Institute International Academic Publishing Company (Moscow), 2014, Vol. 3, No. 6, p. 618], and multivariate computational model experiments were performed using experimental data of model pellet pressing. Reducing the components of the residual stress tensor, which has been optimized for geometric parameters under modern production conditions, can increase the strength of pressed and sintered pellets, thereby reducing the number and size of defects formed during the production of fuel pellets. Reduce the breakability of nuclear fuel pellets Reasonable geometric parameters - the outer chamfer is 20-30° with the end plane, and the ratio of the chamfer width to the pellet diameter is l 倒角 / D=0.02-0.15.

[0024] In nuclear science experiments, it is known that due to pellet pressing, fuel element loading or operation, fuel element failure occurs due to shell unsealing caused by fuel pellet defects [Krupkin A., Kuznetsov V., Nesterov B, Novikov V. Simulation of ramp tests on WWER-1000 fuel rod without a central hole in ANSYS software / / In Proc.: 11th International Conference on WWER Fuel Performance, Modeling and Experimental Support, Varna, Bulgaria, 2015. - pp416-421]. Through numerical modeling, the study of stress-strain state during power mutation and pellet notch has proved that the shell can corrode and crack under stress. The amplitude of the stress increase on the inner surface of the fuel element cladding has been tested under various geometric parameters of fuel pellet defects. According to the following curve ( Figure 3 ), the stress standard SC1 of the fuel element cladding in the short-term mode (350MPa) limits the allowable size of a single defect that contacts the chamfer of the nuclear fuel pellet, which is equal to 0.7% of the total pellet surface area. The maximum single defect area requirement on the surface of the nuclear fuel pellet is stipulated based on the above data, and the strength of the fuel element cladding has been confirmed in the excessive operation mode of the reactor. Based on probability statistics, the maximum area of ​​a single defect on the end face and side cylindrical surface of two adjacent pellets or two single defects close to the outer side of the chamfer of the end face and side face of a pellet is calculated as the equivalent area of ​​the two combined defects.

[0025] Since the allowable size of single surface appearance defects is reduced, safety can be improved during operation in maneuvering mode, normal operation mode and normal operation failure (2). Compared with the technical requirements of existing water-hydrodynamic reactor core blocks, the end surface single defect 1 is reduced from 1.7% to 1.3% of the core block outer surface area. The area of ​​the single defect 2 on the side cylindrical surface is reduced from 2.0% to 1.0% of the core block outer surface area. The area of ​​the single defect 3 adjacent to the outside of the chamfer is reduced from 1.7% to 0.7% of the core block outer surface area.

[0026] Improved burn-up is achieved primarily by using fuel with increased particle size. Existing requirements provide for an average particle size of 10-25 μm for "standard" fuel pellets made of uranium dioxide without trace additives; not less than 25 μm for pellets containing trace additives of aluminum and silicon; and 6 to 25 μm for pellets with gadolinium oxide. The reasonable particle size for pellets applied for using additives is 6 to 35 μm. Due to the addition of aluminum and silicon additives that can increase the particle size to 25-35μm, the yield of gas fission products is reduced [AV Medvedev, JKBibilashvili, OM Milovanov, SM Bogatyr Influencing and optimizing fuel pellet parameters for achievement of extended burnup / / IAEA-TECDOC-1036 In Proc.: Technical Committee meeting, Tokyo, Japan, 1996.-pp257-266.], gas fission products are retained in the grain pores, thereby reducing the release of gas fission products into the pores connected to the grain boundaries and slowing down the formation of edge structures. When the particle size increases to more than 35μm, first, it will cause the fuel to have a reduced ductility and a reduced creep rate, which is inversely proportional to the square of the grain size, and finally increase the interaction between the fuel and the shell. Second, the increase in particle size above 35 μm leads to an increase in brittleness due to a change in the crack propagation mechanism from intergranular to transgranular, causing fuel element breakage during loading and transportation [Fracture toughness of uranium dioxide water-hydrodynamic reactor and PWR fuel pellets of different particle sizes. VV Novikov, RB Sivov, EN Mikheev, AV Fedotov / / Atomic Energy News, 2015, Vol. 118, No. 2, pp. 94-96] and changes in porosity properties.

[0027] Since the specific embodiment of the nuclear fuel pellet contains aluminum-0.002 to 0.020 mass % and silicon-0.001 to 0.010 mass %, the oxides are evenly distributed in the entire pellet volume, and the particle size of uranium dioxide is 25 to 35 μm, which improves the operating safety under high burnout conditions. The above particle size range effectively reduces the gas fission products on the one hand, and limits the breakability on the other hand, and reduces the amount of additives added to activate particle size growth during the pellet manufacturing process. At the same time, in other nuclear fuel pellet manufacturing embodiments, the pellet contains 0.3 to 10.0 mass % of gadolinium or 0.3 to 2.5 mass % of erbium, which is evenly distributed in the entire pellet volume as a solid solution with uranium dioxide, and the particle size is 6 to 25 μm.

[0028] Industrial Applicability

[0029] The nuclear fuel pellets are manufactured in a known manner using common tools on standard equipment. The fuel should have a specific phase composition, average particle size, and other standard parameters.

[0030] The nuclear fuel pellet manufacturing technology is as follows. Multi-component uranium dioxide (UO 2 ), uranium oxide (U 3 O 8 ), foaming agent, SiO 2 and Al 2 O 3 or A1(OH) 3 A mixture of trace additives of silicon and aluminum. Later, the mixture is granulated, and then the plasticizer is mixed through the intermediate alloy in the mixture and sieved. Then the pellets are pressed, soft-packed in molybdenum boats, and sintered at high temperature. Then they are ground on a centerless grinder and automatically stacked on a tray.

[0031] The preparation of the homogeneous powder mixture stage in the production of pellets with gadolinium or erbium additives can additionally include mixing of the constituent master alloys and subsequent processing in a mixing device and / or a vibrating mill.

[0032] Development of nuclear fuel pellet design to ensure safe operation of fuel elements in normal operation, interruption of normal operation and maneuvering mode of reactor operation by adding additives that increase grain size and reduce the yield of gaseous fission products, by reducing the size of single defects in pellet appearance, reducing circumferential stress in fuel element cladding and increasing fuel burning depth.

Claims

1. A nuclear fuel pellet made of uranium dioxide for a water-water power reactor type, which is a cylindrical pellet with an outer chamfer and a central hole along the longitudinal axis, characterized in that: The outer chamfer connecting the side cylindrical surface to the end face forms an angle а with the end face plane, which is equal to 20-30°, and the ratio of the outer chamfer width to the core block diameter is l 倒角 / D=0.02-0.15, The size of a single defect on the end surface is not greater than 1.3% of the outer surface area of ​​the core block, the size of a single defect on the side cylindrical surface is not greater than 1.0% of the outer surface area of ​​the core block, and the size of a single defect adjacent to the outer edge of the outer chamfer is not greater than 0.7% of the outer surface area of ​​the core block. wherein the pellet contains 0.002 to 0.020 mass % aluminum and 0.001 to 0.010 mass % silicon, with the oxides being uniformly distributed throughout the pellet volume, and the particle size of the uranium dioxide is 25-35 μm, or The pellet contains 0.3 to 10.0 mass % of gadolinium oxide or 0.3 to 2.5 mass % of erbium oxide in a solid solution with uranium dioxide uniformly distributed throughout the pellet volume, and the average particle size of the uranium dioxide is 6 to 35 μm.

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