RU2417462C2 - Nuclear reactor fuel element - Google Patents

Abstract

FIELD: power engineering. ^ SUBSTANCE: nuclear reactor fuel element comprises a cylindrical metal shell, which contains nuclear fuel in the form of a column of uranium dioxide pellets, a fixing element, a lower and upper plugs, along with the coolant flow in the reactor core. Besides, the fixing element is arranged in the form of a twisted spring, in which the support turns at one side of the spring rests against the fuel column of pellets, and at the other side of the spring - against the internal end of the upper plug, which comprises a support turn alignment element. The spring has a cylindrical working part, turns of which are arranged with identical diametre, and a shaped part, turns of which are arranged with alternating diametre. The support turn of the spring shaped part of the fixing element at the side of the upper plug has external diametre, which is less than the diametre of the zone with modified structure, formed at the internal end of the upper plug when the shell is sealed, the length of the spring shaped part in previously compressed condition is more than the zone with modified structure formed along the length of the wall as a result of sealing, and the alignment element at the end of the upper plug is made in the form of a conical ledge with the base diametre that is less than the diametre of the zone with the modified structure by value of the diametre of the wire, from which the spring is made. The turns of the fixing element shaped part are arranged in pressed condition. The alignment element of the upper plug may be arranged as a conical groove at the internal end of the plug, has the base diametre of 0.75-0.9 of the fuel element shell internal diametre and depth of 0.1-0.2 of the length of plug and shell welded joint zone. ^ EFFECT: increased reliability of fuel element operation with reduction of costs for its production. ^ 2 cl, 10 dwg

Description

The invention relates to nuclear energy, mainly to the design of internal inactive devices of the rod fuel element (fuel element) of a nuclear reactor.

Known rod fuel rod, consisting of a cylindrical metal shell, inside which is placed nuclear fuel in the form of enriched uranium dioxide, end parts - plugs required for sealing the shell, a fixing element that provides the placement of nuclear fuel in the required position, free volume (gas collector) that reduces pressure gases to the shell arising from the accumulation of gaseous fission products (see F.G. Reshetnikov, Yu.K. Bibilashvili, I.S. Golovin, etc. Development, production and operation fuel elements of power reactors. - M .: Energoatomizdat, 1995, p. 45-46). The known design of a rod fuel rod is one of the first basic options for fuel elements of industrial thermal neutron reactors, designed primarily for the use of sleeve fixing elements, has a sophisticated design of sealing units and is not intended for use in sealing the most advanced contact-butt welding method.

Also known are fuel elements in which the locking elements are made in the form of coil springs, one end of which abuts against the fuel column, and the opposite end is fixed from moving at a certain distance from the plug due to the friction forces between the shell and the spring created by the spacer force of the group of spring-fixing coils, the outer diameter of which in a free state is greater than the inner diameter of the shell (see US patent No. 3310474, 1967).

The disadvantage of this design is the creation in the radial direction of local additional tensile stresses in the cross section of the shell with which the locking coils contact. During the technological operation of placing the fixing element inside the shell, additional devices are required that provide preliminary stretching of the locking coils to reduce their diameter to a size sufficient to pass through the inner section of the shell.

A fuel rod is also known, in which the fixing element is made in the form of a twisted stainless steel spring, the support turns of which are supported by the fuel column and the inner end of the top (in the direction of the coolant from the bottom up) plugs (see US patent No. 3022240, 1962). The disadvantage of this design is the direct contact of stainless steel with a plug made of zirconium alloy. When welding a plug with a shell due to heat conduction, the contact point of the support turn and the inner end of the plug are heated, which can provoke the contact melting process with the formation of a relatively low-melting eutectic, as well as intermetallic compounds (see State diagrams of double metal systems: Reference: 3 t. : T.1 / Under the general editorship of N.P. Lyakishev, - M.: Mechanical Engineering, 1996, pp. 566-590). The formation of a eutectic at the contact point between the support turn and the end face of the plug is an undesirable phenomenon that imposes certain restrictions on the choice of welding methods for sealing, since the parameters of the thermal state of the shell and the plug will have a great influence. However, in the process of sealing (welding), another phenomenon of a catastrophic nature may take place. As a result of the absence of centering elements on the end face of the plug during axial compression of the spring, an axial displacement of the support coil can occur up to the contact of its outer diameter with the inner surface of the shell. With subsequent heating in the sealing process, contact melting is also possible, but not at the end of a relatively massive plug, but on a thin-walled shell with the prospect of rapid depressurization of the fuel rod during operation.

Known fuel rod, in which the locking element is made with focus on the inner end of the plug. At the end of the plug there is a protruding centering element consisting of a conical and several cylindrical parts. The conical section acts as a catcher of the support coil of the spring during the assembly of the fuel rod; the coil is centered along the cylindrical part. The cylindrical part is made stepwise from steps of different diameters. A step larger than the diameter of the centering element, diameter serves to support the coil of the spring, a step with a diameter smaller than the diameter of the centering element creates a thermal resistance jumper to limit the heat flow coming from the weld to the end face of the plug, which prevents the development of contact heating and melting. The length of the inner part of the plug, located inside the shell after sealing, is also a kind of thermal resistance, in addition, due to it, the support coil is moved into the interior of the shell from the weld (see Japan Patent JP 2004069366A, Mcl. 7 G21C 3/16 of 04.03. 2004). The disadvantage of this stub is its relatively large size, which leads to an increase in the irretrievable loss of zirconium. In addition, as a result of a complicated design, the plug has a more complex and, accordingly, expensive manufacturing technology.

A fuel rod is known in which the lower plug, to which the fuel column is drawn, is made in the form of a cup, a deformable element is installed between the bottom of which and the fuel column (see RF patent No. 2188467, Ml. 7 G21C 3/18, published 2002.08.27). The disadvantage of this design is the presence of a deep inner cavity in the plug, which is located under the sealing weld. A flash butt welding cannot be checked for continuity by ultrasound, since the used serial control method involves a continuous plug in the controlled area (see Development, production and operation of fuel elements of power reactors. In 2 book Kn.2 / F. G.Reshetnikov, Yu.K. Bibilashvili, I.S. Golovin et al. / Under the editorship of F.G. Reshetnikov. - M.: Energoatomizdat, 1995, p. 275-276). If the plug is welded after the equipment (which is typical for the top plug, to which this solution can be applied) with the spring already placed, the use of other types of non-destructive testing methods will also be difficult.

To eliminate contact melting between the supporting coil of the spring and the inner end of the plug are placed additional elements of material that does not form a eutectic or intermetallic compounds with the material of the plug (see US patent No. 3963566, Ml. 2 G21C 3/16 from 06/15/1976). The disadvantage of this design is the presence of additional details in the design of the fuel rod and, accordingly, the complexity of its production technology.

The closest in technical essence and the achieved effect to the claimed solution - the prototype is a fuel element according to US patent No. 3378458, published 04/16/1968, the fuel element contains a shell, plugs, sealing ends of the shell, a fuel column, compensating volume and a spring located between the fuel a pillar and an adjacent plug, the support turn of which is bent 90 ° and is in contact with the plug in the center of the end (the coldest zone when sealing by welding). In addition, the support coil has a surface coating with a material at which the temperature of the formation of the eutectic with the plug material is significantly higher than the temperature in the zone of its contact with the plug. This patent also indicates the possibility of using a shaped end with a variable diameter of coils in the spring design.

The main drawback of this design is that during technological operations, the support turn is bent and the contact point is shifted to a hotter zone. To combat this drawback, an additional coating is used, which complicates the process.

The bent support turn is ideally in contact only with the center of the inner end of the plug. Between the outer diameters of the coil of the spring and the inner diameter of the shell there is a gap for the shaped part - significant. In the absence of centering elements at the end face of the plug, it is possible to shift the support coil from the center of the plug within the gap with the formation of contact with the inner surface of the shell. When sealing fuel rods with one of the fusion welding methods, this phenomenon is not significant. The plug is pre-pressed in the cold state before welding, and its inner end, when pressed, moves the support coil away from the future heating zone. In flash butt welding, for example, according to patent RU 2082574, Mcl. 6 B23K 11/02, G21C 3/00, publ. 06/26/97, bull. No. 18 (see Fig. 1), the outer diameter of the plug (5) exceeds the inner diameter of the shell (1) by (0.2-1.4) δ, where δ is the wall thickness of the shell, so the plug is pressed into an integral, sealed weld compounds in a preheated state into a preheated portion of the shell. The welding current for heating the shell and the plug is turned on after providing preliminary tight contact between the end of the shell and the end of the plug. The movement (press-fitting and welding) of the plug into the shell will begin only when the zones of the welded parts are heated, sufficient for plastic deformation of the metal of the plug and the shell. Under such welding conditions and possible displacement of the support coil to contact with the inner surface of the shell, the contact point may be in a zone heated to a plastic state. For example, according to patent RU 2082574 (Ml. 6 В23К 11/02, G21C 3/00, publ. 06/26/97, bull. No. 18), such a zone (for creating a high-quality welded joint) is defined in (2-5) δ, although it can be larger, since the size of the type-setting part of the current lead (7, Fig. 1), which determines the entire heating zone, is made in the range (2.5-6) δ. With a nominal inner diameter of the cladding of a WWER1000 fuel rod - 7.72 mm, the radius of the support coil of the spring, taking into account the gap between the outer diameter of the coil of the spring and the inner diameter of the shell, can be 3.6 mm, with this radius, the contact point of the coil with the shell will be approximately the same distance from the inner end of the plug. With a fuel cladding thickness of up to 0.73 mm, the heating zone can be 3.65–4.38 mm from the plane of the cladding end face without regard to the material’s thermal conductivity. Pressing in (the amount of penetration) of the plug into the shell under the action of the welding force is also (2-5) δ. The metal of this zone of the shell is involved in the formation of the welded joint, that is, it is heated and plastically deformed. Therefore, it is possible to heat the outer surface of the support coil of the spring (3, figure 1). The contact of the outer surface of the coil with the inner surface of the shell, which was formed as a result of the lack of alignment, becomes more dense as a result of plastic deformation caused by heating of the layers of the shell metal located on the side of the inner surface. Conditions are created for the fixing metal of the retainer to the shell metal (the temperature of the formation of the Zr-Fe eutectic is 934 ° C). When the plug is moved, the setting point is destroyed and the inner surface of the shell (scuff with metal displacement) and the outer surface of the contacting coil are mechanically damaged.

The hot metal of the inner layers of the shell heated to a plastic state can simply be shifted by the support coil of the spring as a result of misalignment and without setting, which also reduces the life of the shell.

An object of the invention is to increase the reliability of the fuel element while reducing the cost of its production.

The solution to the problem is achieved by the fact that the fuel element of a nuclear reactor, consisting of a cylindrical metal shell, inside of which there is nuclear fuel in the form of a column of tablets made of enriched uranium dioxide, a fixing element that provides a given position of the column of tablets made in the form of a coil spring, consisting of a cylindrical working and fittings, lower and upper, in the direction of the coolant in the reactor core, plugs, sealing the shell from the open ends as a result of cont butt welding, the coils of the working part of the spring have the same diameter, and its support coil abuts against the fuel column, the coils of the shaped part of the spring have a variable diameter, and its support coil rests on the inner end of the upper plug having centering elements of the support coil, according to the invention, support the coil of the shaped part of the spring from the side of the upper plug has an outer diameter smaller than the diameter of the zone with a changed metal structure resulting from heating during sealing of the shell at the inner end plugs, the length of the shaped part of the spring in a precompressed state, when the inner end of the plug in contact with the support turn abuts against the end of the shell before starting sealing, more than the size of the zone with a changed structure formed along the length of the shell wall as a result of sealing, and the centering element at the end the upper plug is made in the form of a conical protrusion with a diameter of the base smaller than the diameter of the zone with a changed structure by the diameter of the wire from which the spring is wound.

In the particular case of implementing the invention, in order to eliminate non-linearity in the operating characteristic of the spring, the turns of the shaped part of the fixing element are made in a pressed state.

The use of a centering element in the top plug in the form of a conical recess on the inner end with a base equal to 0.75-0.9 of the inner diameter of the end face of the plug and a depth of 0.1-0.2 of the length of the continuity zone of the welded joint of the plug with the shell obtained by sealing by flash-butt welding, it is possible to center the support turn of the fittings with sufficient accuracy, to provide reliable non-destructive testing of the continuity zone of the welded joint and to reduce irretrievable losses of zirconium.

Taking into account the quantitative results of heating the cladding and plugs of a fuel rod during sealing by flash butt welding, implemented in the geometry of the springs and plugs, it is possible to exclude high-temperature contact of dissimilar materials in the heated state, prevent the formation of intermetallic phases, increase the reliability of the fuel rod and reduce the cost of manufacturing elements that provide these properties.

The proposed technical solution is illustrated by graphic materials.

Figure 1 presents a diagram of the sealing of a known fuel rod by a known method of contact-butt welding, illustrating the design flaws of a fuel rod.

Figure 2 presents the design of the claimed fuel rod.

Figure 3 shows the placement of the locking element with the free state of the turns in the fuel element equipped with fuel before moving to the sealing position by flash-butt welding.

Figure 4 presents a diagram of the initial sealing period of the claimed fuel rod (with pre-clamped turns of the locking element and the focus of the end face of the plug in the end face of the shell) in a known contact-butt welding method, illustrating the design advantages of the claimed fuel element.

Figure 5 presents a macro section of the surface of the inner end of the upper plug after sealing using flash butt welding with a dedicated zone of unchanged metal structure of the plug (D).

Figure 6 presents the x-ray of the upper part of the fuel element after sealing using contact-butt welding, illustrating the displacement of the support coil of the shaped part of the spring in the absence of centering elements on the inner end of the plug.

Figure 7 presents the x-ray of the upper part of the fuel element after sealing using contact-butt welding, illustrating the displacement of the support coil of the shaped part of the spring with preloaded turns in the absence of centering elements on the inner end of the plug.

On Fig presents a x-ray of the inventive fuel rod after sealing using flash butt welding.

Figure 9 presents the x-ray of the inventive fuel rod with tightened turns of the shaped part with a centering element on the inner end of the plug in the form of a conical recess.

Figure 10 shows a variant of the centering element at the inner end of the plug in the form of a conical recess.

The fuel element of the nuclear reactor (figure 2) consists of a shell 1, in which are placed the tablets 2 of the fuel column. The fuel rod is sealed by flash-butt welding with the shell of the plugs of the lower 4 and upper 5. Between the fuel column and the upper plug there is a fixing element 3 made in the form of a twisted spring with a cylindrical working and shaped parts. The reference turn of the working part rests on the end face of the upper tablet 2 of the fuel column. The reference turn of the shaped part rests on the inner end of the upper plug 5 and is fixed from axial displacement by the centering element (a). Zones (b) and (c) (in longitudinal section) correspond to zones with a metal structure changed as a result of welding in the plug and shell, respectively.

This device operates as follows.

The shell 1 with a welded bottom plug 4 is equipped with tablets 2 fuel columns. In the compensation volume of the shell, not filled with tablets, the fixing element 3, made in the form of a twisted spring, is installed in the free state. Part of the spring in the free state extends beyond the shell 1 (see figure 3). Departure value depends on the operating characteristics of the spring. In this state, a fuel rod is fed by a transport module of an automatic line, for example, such as declared according to the patent of the Russian Federation No. 2070740, to the position of welding of welding installations, for example, as declared according to USSR copyright certificates No. 941070 or No. 1490829, RF patent No. 2245769, or others, designed for sealing fuel rods by contact butt welding. The feed device of the welding units feeds the shell 1 with its open end, from which the locking element 3 protrudes, into the welding chamber, which isolates the end of the shell 1 from the environment for a period of sealing. Depending on the design of the welding installation, the plug 5 can be fed into the one-piece welding chamber by pushing the open end of the sheath 1 (the protruding part of the fixing element) through a special hole or a specialized device in the presence of a detachable camera. Since the beginning of the supply of the shell 1 and the plug 5 to the welding chamber, their relative position can neither be controlled nor adjusted by the welder-operator. In the absence of centering elements (a) at the end of the plug 5 at this moment, a mutual displacement of the support coil of the spring and the plug can occur, which will then be fixed when the plug stops against the end of the shell. The displacement of the support turn may occur as a result of loss of stability during compression of the part of the fixing element protruding significantly beyond the shell. In the presence of a centering element, the loss of stability does not matter much, since the displacement of the support coil does not occur, and the spring deflection is eliminated by further compression and placement of the protruding coils inside the shell.

The final stage of the supply of the shell 1 and the plug 5, which is the initial state for starting the sealing process, is shown in Fig.4. The cladding 1 of the fuel element is fixed from displacement by a special current lead 8 and an additional clamping device (not shown in Fig. 4). After a series of operations related to filling a fuel rod with helium, a welding force (P) is applied to the plug 5, and a tight contact is created between the end face of the plug and the end face of the cladding. After creating a contact, a welding current pulse is passed through the plug and shell. A heating zone (s) is created. Due to the use of 8 typesetting plates 7 in the current lead, the cladding heating zone has an anomalously long extent, which is the main characteristic sign of contact-butt welding of fuel rods. However, the turns of the shaped part of the fixing element do not come into contact with the heated sections of the shell. The reference turn of the shaped part with the inner end of the plug comes into thermal contact. Fig. 5 shows the macrostructure of the metal of the end of the plug that arose as a result of heating during sealing, the zone (D) with an unchanged structure is highlighted. Its diameter depends on the diameter of the fuel rod and welding modes - the amplitude and duration of the welding current. In accordance with the state diagram of the zirconium alloys from which the fuel rods are made, in the equilibrium state, the temperature of their recrystallization is 580-610 ° C (see Zaimovsky A.S., Nikulina A.V., Reshetnikov N.G. Zirconium alloys in nuclear energy - M .: Energoatomizdat, 1994, p.194-195). Given the high degrees of overheating and supercooling that occur at high welding speeds of heating and cooling (the passage of the welding current in one of the special cases is 20 ms, the time of a complete welding cycle is 49 s), the boundary of the zone with an unchanged structure can be moved 50-100 ° FROM. However, this is much lower than the temperature of formation of the Zr-Fe eutectic, which is 934 ° С. Therefore, the boundary between zones with a changed and unchanged structure at the end face of the plug can serve as a criterion for the absence of contact melting and the formation of intermetallic compounds during contact of the materials of the fixing element and the plug during heating during sealing by flash butt welding.

In Fig.6 and 7 presents the x-ray of the fuel rods, for sealing which were used plugs without centering elements. In the sample in Fig. 6 only for the fuel rod on the right, in the sample in Fig. 7 only for two fuel elements on the right, the location of the support turn can be considered acceptable, although its displacement from the X-ray plane is not excluded.

On Fig and 9 presents the x-ray of the fuel rods, for sealing which were used plugs with centering elements. 8, a centering element in the form of a conical protrusion is used. In Fig.9, the centering element is made in the form of a conical recess in the end face of the plug, the design of which is shown in Fig.10. In these samples, all fuel rods do not have a reference coil offset.

The choice of dimensions of the centering device in the form of a conical recess is associated with the features of the welded joint obtained by sealing the fuel rod by flash butt welding. When the diameter of the cone is more than 0.75-0.9, the inner diameter of the shell violates the stability of the “cold” planar contact of the pipe end and the end face of the plug, since the conical surface enters the plastic deformation zone when a welding force is applied. When the diameter is less than 0.75-0.9 of the inner diameter of the shell, the ability to center the support coil with large deviations is deteriorated.

The choice of the depth of the conical recess (0.1-0.2 of the length of the welded joint) is determined by the features of ultrasonic control (ultrasonic testing) of continuity. Since the beginning of the zone of the welded joint performed by flash butt welding along the shell practically coincides with the plane of the end face of the plug, any depression in the end face is determined by the ultrasonic testing device as lack of penetration with a corresponding decrease in the length of the zone of the welded joint. For reasons of reliability of fuel rod operation, the length of the welded joint zone should not be less than 2δ (see Design, production and operation of fuel elements of power reactors. In 2 books Kn.2 / F.G. Reshetnikov, Yu.K. Bibilashvili, I.S. Golovin et al. / Edited by F.G. Reshetnikov. - M.: Energoatomizdat, 1995, p. 219). By increasing technologically the length of the welded joint zone by 20-30%, which is easily achieved with modern welding equipment, it is possible to compensate for deviations in the readings of the ultrasonic testing device, taking into account the recess at the end face of the plug. The actual excess of the length of the weld zone is a margin of safety. An increase in the depth of more than 0.1-0.2 of the length of the welded joint will cause a significant adjustment of the welding conditions for technological compensation of the reduction in the zone of the length of the welded joint, which, in turn, can lead to overheating of the shell and loss of its corrosion resistance.

Claims (2)

1. The fuel element of a nuclear reactor, consisting of a cylindrical metal shell, inside which nuclear fuel is placed in the form of a column of uranium dioxide tablets, a fixing element, lower and upper, along the flow of the coolant in the reactor core, plugs, and the fixing element is made in the form coil spring, in which the support coils on one side of the spring abut against the fuel column of the tablets, and on the other side of the spring - in the inner end of the upper plug having a centering element of the support coil a, characterized in that the spring has a cylindrical working part, the turns of which are made with the same diameter, and a shaped part, the turns of which are made with a variable diameter, while the supporting coil of the shaped part of the spring from the side of the upper plug has an outer diameter smaller than the diameter of the zone with a changed structure formed on the inner end of the upper plug during sealing of the shell, the length of the shaped part of the spring in a pre-compressed state is greater than the zone with a changed structure formed along the length of the shell wall and as a result of sealing, and the centering element of the supporting coil at the end of the upper plug is made in the form of a conical protrusion with a base diameter less than the diameter of the zone with a changed structure by the diameter of the wire from which the spring is made, or in the form of a conical recess with a base diameter of 0.75- 0.9 of the inner diameter of the shell of the fuel element and a depth of 0.1-0.2 of the length of the weld zone of the plug with the shell. 2. The fuel element according to claim 1, characterized in that the turns of the shaped part of the spring are made in a preloaded state.

Images