The case of the channel of a nuclear reactor

 

(57) Abstract:

Usage: in nuclear engineering for housings of channels of nuclear reactors. The inventive housing channel of a nuclear reactor contains a tube of zirconium alloy and transitional elements mounted on the ends of the tube and is made in the form of sleeves of zirconium alloy and stainless steel. Tube of zirconium alloy is connected to the bushings of the zirconium alloy by fusion welding, and the sleeve of the zirconium alloy is connected with the sleeves of stainless steel in the form of lap diffusion welded joint. The stainless steel sleeve is installed covering sleeve of zirconium alloy. The mating surfaces of the bushings on the entire length of overlap have alternating between the protrusions and depressions that are included in each other. Lap diffusion welded connection is made with solid diffusion layer to a thickness of 5 μm. 6 C.p. f-crystals, 5 Il.

The invention relates to nuclear technology, and more particularly, to housings channels of nuclear reactors.

Known case of a channel of a nuclear reactor (C. E. Coleman and L. A. Simpson, Evaluation of a leaking crack in an irradiated candu pressure tube. Rep. AECL-9733, June 1988, fig.1), which is made in the form of labour of the circuit, made of stainless steel.

A disadvantage of the known case of the channel is cracking and the occurrence of leakage during long-term operation due to hydrogen embrittlement of the ends of the tubes of zirconium alloy, which valcovany elements of the circuit, made of stainless steel with a thermal expansion coefficient significantly greater than the coefficient of thermal expansion of zirconium alloy. Because of the high difference of coefficients of thermal expansion of metals, joined by the method of rolling, heating a body channel of a nuclear reactor covering part of the stainless steel elements to increase in diameter more than covered the ends of the tubes of zirconium alloy. To ensure a tight seal is necessary in such a rolled connection to provide a high level of residual stresses. In the area of expanding tubes of zirconium alloy having a high level of residual stresses, gradually increases the concentration of hydrogen, which is formed as a result of corrosion and radiation processes in the channel of a nuclear reactor. The hydrogen zirconium forms a brittle hydrides, which lead to okhrupchivaniem (A. W. L. Segel, Explosive bonding of dissimilar metal tubes. Rep AECL -2209, May 1964, 63 p), which contains a tube of zirconium alloy and transitional elements mounted on the ends of the pipe and is made in the form of bushings stainless steel ferritic grade with the average value between the coefficients of thermal expansion of zirconium alloy and stainless austenitic steels, which are made of the elements of the circuit. Transition elements are connected with a tube of zirconium alloy by cold expanding, and with elements of the circuit by fusion welding.

A disadvantage of the known body channel of a nuclear reactor is cracking and the occurrence of leakage during long-term operation due to hydrogen embrittlement of the ends of the tubes of zirconium alloy, which valcovany in the sleeve of stainless steel. Since the coefficient of thermal expansion of the ferritic stainless steel grade exceeds approximately twice thermal expansion coefficient of zirconium alloy, to ensure the tightness of the rolled connection it is necessary to create a high level of residual stresses by distributing the ends of the tubes of zirconium alloy in the stainless steel collet. In the quiet seal rolled connection. The higher the level of residual stresses in the zone of the expanding pipe, the better ensures a tight seal. However, during long-term operation of the housing channel in the conditions of a nuclear reactor in places expanding tubes of zirconium alloy begins to accumulate hydrogen, which is formed as a result of corrosion and radiation processes in the channel of a nuclear reactor. Hydrogen at high concentrations forms with zirconium brittle hydrides, which lead to embrittlement and cracking of the pipe walls in places expanding and as a consequence of the tightness of the housing channel.

The closest to the invention by the combination of essential features is the body channel of a nuclear reactor (Dollezhal N. A. Emelianov, I. J. Duct nuclear power reactor. M Atomizdat, 1980, S. 54-56), containing a tube of zirconium alloy, transition elements mounted on the ends of the pipe and is made in the form of sleeves of zirconium alloy is connected with a tube of zirconium alloy by fusion butt welds (electron beam welding), and sleeves of stainless steel is overlapped by diffusion welding. Sleeve of stainless steel are welded the circulation.

A disadvantage of the known body channel of a nuclear reactor is a limited resource of his work in the conditions of a nuclear reactor in hot water and steam at temperatures up to 350oC because of the low indicators of long-term corrosion resistance. Under long-term corrosion of the hull strength of the channel is defined as its durability in conditions of prolonged exposure to the corrosive environment, workers and thermal stresses. This is because the lap-diffusion welded joint bushings zirconium and stainless steel characterized by low corrosion and strength properties. During long-term operation of lap diffusion welded joint is destroyed by corrosion under stress.

In the operation of the housing channel in the hot water and steam at temperatures from 150 to 350oC lap diffusion welded joint corrode with the release of free (atomic) hydrogen that accumulates in zirconium alloy in places with a high level of stress. Due to the cramped conditions of an active nuclear reactor, the wall thickness of the sleeve of zirconium alloy in the diffusion zone of the weld is reduced compared to the time, in the field of boundary points of overlap at the beginning of lap diffusion welded joint from the side of zirconium alloy ring hub voltage where the accumulated free hydrogen, which is formed with a zirconium alloy brittle hydrides of zirconium. In addition, the constant vibration of the housing channel from movement of the fluid creates in places of education annular hub voltage peaks, which in combination with the axial tensile stresses from the high-pressure coolant (70-100 MPa) accelerate the process of concentration of hydrogen in these places and the formation of brittle hydrides of zirconium.

For lap-diffusion welded joints bushings made of dissimilar metals with different coefficients of thermal expansion is characterized by the formation of stress peaks in the boundary points of overlap, the magnitude of which is directly proportional to the stiffness (thickness) of the walls of the joined sleeves and on the difference of coefficients of thermal expansion. The greater the wall thickness of the sleeves, the higher their rigidity, and therefore, the higher the level of stress in the boundary points of overlap.

Maintaining the desired axial ravnopravnosti housing channel is in the body channel, that is unacceptable for the cramped conditions of the active zone of the nuclear reactor. When the heating water temperature increase is directly proportional to the temperature peaks (hubs) stresses in the boundary points of overlap. The increase of stress peaks to values exceeding the value of the strength of lap diffusion welded joints, leads to the breakdown of the weld at the boundary points. In places strain stress concentration increases, which leads to gradual destruction of lap diffusion welded joints.

The objective of the invention is to create channel of a nuclear reactor, which is characterized by a long service life in hot water and steam at temperatures up to 350oC and a small radial dimensions, which allows to use it in cramped conditions the active zone of the nuclear reactor.

The technical result obtained by carrying out the invention, is to improve long-term corrosion of the hull strength of the channel in terms of hot water and steam at temperatures up to 350oC by increasing the corrosion resistance and strength of the diffusion weld. Lap diffusion welded joint the spine in hot water and steam up to 350oC (set experimentally). In addition, in combination with mechanical gearing, made in the form of projections and depressions, solid diffusion layer thickness to 5 μm provides high strength tubular lap welded joint. In the prior art it is known that solid diffusion layer thickness of 3-5 mm provides end-diffusion welded joint of titanium (metal one group with zirconium and stainless steel sufficiently high strength. However, tubular lap diffusion welded joint of the above metals with solid diffusion layer in the cooling process spontaneously decay due to the high residual thermal stresses in the diffusion layer through a twofold difference in the coefficients of thermal expansion between titanium and stainless steel (Cossacks N. In. Diffusion welding materials, M., "engineering", 1976, S. 186 to 190).

The prior art end-diffusion welded zirconium compounds with stainless steel with high strength due to the introduction between the welded surfaces of a thin intermediate layers: niobium, copper, Nickel, however, these connections because trehkantnyj compounds (K. Bhanumurthy, J. Krishnan, G. B. Kale, S. Banerjee, Journal of Nuclear Materals, V. 217 (1994) 67-74. "Transition joints between Zircaloy-2 and Stainless steel by diffusion bondig"). In the stated case a large part of the residual thermal stress is perceived by the mechanical engagement of the bushings, so solid diffusion layer is practically unloaded from them, despite a threefold difference of coefficients of thermal expansion of zirconium and stainless steel.

High corrosion resistance solid diffusion layer is not thicker than 5 μm in combination with high strength properties of lap diffusion welded joint by means of downloading it from thermal stresses mechanical gearing provides the body long-term corrosion strength under hot water and steam at temperatures up to 350oC.

This technical result is achieved by the fact that in the famous case of the channel of a nuclear reactor containing a tube of zirconium alloy and transitional elements mounted on the ends of the tube and is made in the form of sleeves of zirconium alloy and stainless steel, with the tube of zirconium alloy is connected to the bushings of the zirconium alloy by fusion welding, and the sleeve of zirconium alloy juice stainless steel set covering sleeve of zirconium alloy; the mating surfaces of the bushings on the entire length of overlap have alternating between the protrusions and depressions that are included in each other, and lap diffusion welded connection is made with solid diffusion layer to a thickness of 5 μm.

In addition, to avoid tears weld along the edges of overlap, the sleeve in the area of overlap is made with a variable wall thickness so that the thickness of the wall of one of the bushings decreases and the other increases. This allows in combination with the high corrosion resistance of the diffusion layer to slow down the corrosion rate, which is inevitable after prolonged use of the adapter in the conditions of hot water and steam, to values, ensuring integrity of a weld, for example, a nuclear reactor, to complete development of the operational channel capacity of a nuclear reactor. The implementation of the sleeves in the region of overlap with variable wall thickness will reduce the radial dimensions of the channel.

In addition, the minimum wall thickness of each sleeve at the edge of the overlap is no more than 25% of the thickness of its walls, free from overlap, and the mating surfaces of the bushings on the entire length of overlap of the imp is to overlap, and a truncated cone, smoothly connecting steps between them.

In addition, to increase the axial strength maximum wall thickness of each sleeve at the edge of the overlap is not less than the thickness of the wall, free from overlap, as well as to increase the axial strength and increase the tightness of (density) lap diffusion welded joint protrusions and depressions made ring.

In Fig. 1 shows a body channel of a nuclear reactor (longitudinal section); Fig. 2 speed lap diffusion welded joint sleeves of zirconium alloy and stainless steel; Fig. 3 step-tapered lap diffusion welded joint sleeves of zirconium alloy and stainless steel; Fig. 4 some of the steps of lap diffusion welded joints (longitudinal section on an enlarged scale; Fig. 5 - weld with solid diffusion layer (enlarged scale).

The case of the channel of a nuclear reactor comprises a tube 1 of zirconium alloy and transitional elements 2, 3, are installed at both ends of the pipe 1, which is made in the form of sleeves of zirconium alloy 2 and stainless steel 3. Pipe 1 of the zirconium alloy of CO2 from zirconium alloy joined by the weld seam 5 lapped by diffusion welding with sleeves 3 stainless steel with formation lap diffusion welded joint. Sleeve 3 made of stainless steel connected by fusion butt welds with elements of the circuit, made of stainless steel (not shown). Bushings 2, 3 in the area of overlap is made with a variable wall thickness, so that the wall thickness of one of the bushings decreases and the other increases. Maximum wall thickness of the sleeve at the edges of overlap is not less than the thickness of the wall, free from overlap, and the minimum wall thickness of the sleeve at the edges of overlap is no more than 25% of the thickness of its walls, free from overlap. The mating surfaces of the sleeves can be performed stepwise, in the form of a cylindrical steps 6. Step 6 are of equal length, and the height of the steps is equal to approximately 10-20% of the wall thickness of the end of the sleeve free of overlap. The mating surfaces of the sleeves can be made in the form of at least two cylindrical steps 6, placed one on each side of the overlap, and a truncated cone 7, smoothly connecting steps between them. The mating surfaces of the sleeves 2, 3 throughout the length of overlap are evenly alternating between the projections 8 and socket 9, tightly with no gap incoming into each other. The weld bead 5 is made in the form n the LASS="ptx2">

The tabs 8 and socket 9 can be made in the form of threads or annular.

Low level thermal stresses is provided in the area of boundary points of overlap from the sleeve of zirconium alloy by reducing the wall thickness of the sleeve of stainless steel, and in the field of boundary points of overlap from the sleeve of stainless steel by reducing the wall thickness of the sleeve of zirconium alloy, i.e., by reducing the rigidity of the wall of one of connected sleeves. At the same time the conditions for the conservation of the axial ravnopravnosti housing channel along the edges of the diffusion welded joints, because of the increased wall thickness of the sleeve of zirconium alloy side tubes of zirconium alloy and increasing the thickness of the sleeve stainless steel from stainless steel pipe. In addition, since the total thickness of the overlap is less than twice the thickness of one of the joined sleeves, the radial dimensions of the channel is reduced compared to the known case.

High corrosion resistance of solid diffusion layer is not thicker than 5 μm in combination with a low concentration of thermal stress (peak stress) in the edges of the pouring rate of corrosion lap diffusion welded connection to values, providing up to full production operational resource body channel of a nuclear reactor.

To increase the specific strength of lap diffusion welded joint is provided by the increase in the area of overlap of diffusion welded joints by creating a jagged shape of the weld. In addition, the toothed shape of the weld provides axial ravnopravnosti housing channel in the zone of overlap of diffusion welded joints.

Reducing the radial dimensions of the channel of a nuclear reactor, namely, the thickness of the casing wall in places lap-welded diffusion of compounds of zirconium alloy with stainless steel is made possible by reducing thermal stresses in the boundary points of overlap while maintaining the desired operating conditions axial ravnopravnosti housing channel. This is due to the fact that one of the bushings in the field of boundary points of overlap is made with a minimum wall thickness that allows the other sleeve in this area to have a wall thickness greater than the thickness of the wall of its free end.

Getting in lap diffusion weld seal the other, maintains integrity (density) even when random low quality lap diffusion welded joint, which is possible when the failure modes of welding. The execution of the projections and depressions of the ring shape allows you to enhance the effect of the labyrinth seal.

The case of the channel of a nuclear reactor works as follows.

When the channel in a nuclear reactor housing channel experiences radial and longitudinal tensile loads arising from exposure to temperature and pressure of the coolant, and hydraulic vibratory loads resulting from the pulsation of the flow of coolant. Heated to the working temperature of the fluid in the passage inside the channel oxidizes the surface of the zirconium alloy. When one oxygen atom of the water involved in the formation of the oxide film, frees them from the water molecule two hydrogen atoms, which form with zirconium brittle hydrides. During long-term operation on the body surface of the zirconium alloy is formed oxide film with a thickness of several microns, and in proportion to the magnitude of this oxide film in zirconium alloy arise hydrides. Hydrides relative to equal the high stress concentration where the hydrides can be collected above the maximum permissible limits, which leads to embrittlement of zirconium alloy. In the case of the channel in the joints of zirconium alloy with stainless steel and especially on the edges of the lap diffusion welded joint may have a greater axial and radial thermal and working voltage due to a more than threefold difference in the coefficients of thermal expansion of zirconium alloy and stainless austenitic steels. However, the mechanical engagement of the paired surfaces of the sleeves, made in the form of alternating projections and depressions, unloads solid diffusion layer from workers and thermal stresses, resulting in higher long-term corrosion strength, and therefore, the operability and reliability of the buildings of the channels of a nuclear reactor. In addition, the implementation of lap diffusion welded joints in areas of overlap with the reduced thickness of the sleeve and the increased thickness of the related sleeve along the edges of the lap-joints helps to enhance the effect of the solids discharge diffusion layer from stress.

1. The case of the channel of a nuclear reactor containing the tube of conewago alloy and stainless steel, with the tube of zirconium alloy is connected to the bushings of the zirconium alloy by fusion welding, and the sleeve of the zirconium alloy is connected with the sleeves of stainless steel in the form of lap diffusion welded joint, and the stainless steel sleeve is installed covering sleeve of zirconium alloy, characterized in that the mating surfaces of the bushings on the entire length of overlap have alternating between the protrusions and depressions that are included in each other, and lap diffusion welded connection is made with solid diffusion layer to a thickness of 5 μm.

2. The case under item 1, characterized in that the sleeve in the area of overlap is made with a variable wall thickness so that the thickness of the wall of one of the bushings decreases and the other increases accordingly.

3. Building on p. 2, characterized in that the minimum wall thickness of each sleeve at the edge of the overlap is no more than 25% of the thickness of its walls, free from overlap.

4. The case under item 2 or 3, characterized in that the mating surfaces of the bushings on the entire length of overlap is performed stepwise.

5. The case under item 2 or 3, characterized in that the mating surfaces of the bushings for AC, the second side of the overlap, and a truncated cone, smoothly connecting steps between them.

6. Building on p. 2, characterized in that the maximum thickness of the wall of each sleeve at the edge of the overlap is not less than the thickness of the wall, free from overlap.

7. The case under item 1, characterized in that the protrusions and depressions are ring.

 

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