GB2119056A

GB2119056A - Double wall steam generator tubing

Info

Publication number: GB2119056A
Application number: GB08234833A
Authority: GB
Inventors: Thomas Roy Padden; Craig Fielding Uber
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1982-04-08
Filing date: 1982-12-07
Publication date: 1983-11-09
Also published as: DE3243423A1; FR2524967A1; JPS58178102A

Abstract

Double-walled steam generator tubing for the steam generators of a liquid metal cooled fast breeder reactor prevents sliding between the surfaces due to a mechanical interlock. Forces resulting from differential thermal expansion between the outer tube and the inner tube are insufficient in magnitude to cause shearing of base metal. The interlock is formed by jointly drawing the tubing, with the inside wall of the outer tube being already formed with grooves. The drawing causes the outer wall of the inner tube to form corrugations locking with the grooves. <IMAGE>

Description

SPECIFICATION Advanced design for double wall steam generator tubing This invention relates to an advanced design for double-walled tubes for a steam generator used in a liquid metal cooled breeder reactor.

One of the concepts being developed for steam generators for a liquid metal cooled breeder reactor uses double-walled tubes for the sodium/water heat transfer boundary. The inner tube may contain water while the shell side of the steam generator contains liquid sodium. The heat transfer is from the liquid sodium to the water on the inside of the tubes. An inner space between the two tubes contain a gas. If the outer tube wall is breached by sodium or the inner tube wall is breached by water, either leak can be detected before there is a sodium water reaction.

The inner space is filled with flowing helium at pressures intermediate between that of the water or steam in the bore of the double wall tube and that of the sodium on the shell side. The inner face between the two tubes contains fourfull-length longitudinal grooves which are connected to an isolated gas plenum. This plenum is formed using double tubeto-tube sheet joints. The plenum is in turn connected to water leak detection instrumentation in order to monitor for breach of the inner wall. The sodium region of the steam generator is connected to helium detection instrumentation to monitor for a breach of the outer tube wall.

The double-wall tube design is very conservative with respect to sodium/water reactions but it has the potential for wear at the inner face of the tubes. The tube walls are in contact at some locations even if there is a nominal gap between them. In the steam generator, the tubes experience both longitudinal and radial temperature gradients. One of the net effects is longitudinal sliding at the interface between the tubes. These conditions lead to the potential for cyclical wear at the interface as the tube temperatures are changed at start-up, during power ramps, and other thermal transients. Because of the geometry of the double wall tube, any wear metal formed at the interface is trapped in the gap and this can further increase the loading stress at the local wear sites. The fact that such wear can occur has been demonstrated by test results.One potential solution to the problem of sliding and wear between the inner and outer tube is to pre-stress the tubes during manufacture. The purpose of pre-stressing is to inhibit sliding motion at the interface. Improved heat transfer may also be expected due to pre- stressing. At the present time, some question remains as to whether pre-stressing is adequate to eliminate the problem of interface sliding. It is not known to what extent the pre-stressed tubes will become stress-relieved during service life.

Consequently, it is the principal object of the present invention to provide a new tube design completely eliminating the interface wear problem without sacrificing heat transfer properties or leak detection and a process of making the same.

With this object in view, the present invention resides in a steam generator for a liquid metal cooled fast breeder reactor of the double-walled tube variety characterized in that the interface between the surfaces of the double-walled tubes has a mechanical interlock which interlock is adapted to prevent sliding of the inner wall tube with respect to the outer wall tube.

The invention also resides in a process for mechanically interlocking the outer surface of an inner wall tube located within an outer tube to the inner surface of said outer tube characterized in that a non-smooth surface is formed onto the outer surface of the inner tube; said inner tube is inserted into said outer tube, said outer tube having a smooth inner surface, and said inner and said outer tubes are duplexed so as to cause the outer surface of said inner tube to deform the inner surface of said outer tube providing for a mechanical interlock of both tubes.

The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings, in which: Figure 1 is a sectioned schematic of a prior art double-walled tube for a steam generator; Figure 2 is a schematic section taken as indicated from Figure 1; and Figure 3 is a schematic section of the invented concept.

Figure 1 illustrates the basic configuration of double-walled tube for the steam generators.

An inner tube 2 within an outer tube 3 forms a 'double wall tube" having an interface 5. Water/ steam is passed through the interior 1 of the tube while liquid sodium surrounds the tube. Grooves 4 are filled with gas and in communication with a gas plenum (not shown). The arrows in Figure 1 illustrate sliding movement as it may occur during temperature changes. For a straight double-walled tube having uniform walls the mean shear surface during longitudinal differential thermal expansion is the right cylindrical surface at the interface 5 between the walls. In the current pre-stressed tube design the interface 5 is under compression to inhibit sliding and the resulting wear. The resistance to sliding results from the surface friction and the shear strength of the asperities 6 on the tube surface as shown in Figure 2.The contact area is probably about 5% of the total in a real pre-stressed tube having a nominal interface 5 gap of 0.0025 to 0.0063 mm. A very small change in dimensions and/or a loss of interface 5 compression resulting from stress relaxation or large thermal gradients in service would allow the tubes to slide.

Figure 3 illustrates the basic concept of this invention; namely, mechanical interlocking the tube wall surfaces at interface 5. With this design, sliding at interface 5 cannot occur without shearing a significant amount of the base metal near the interface surface. It is only necessary that the interlocking have a configuration which will inhibit shear motion in the longitudinal direction. It is not necessary to inhibit thermal gradients in the radial direction providing that changes in gap size are small compared to the magnitude of the inner penetration of the interface. The required interlock ing could be either on a macro or a micro scale and have a configuration as simple as a wavy interface as illustrated in Figure 3.

Producing such an interface configuration as shown in Figure 3 is not difficult. There is experimental evidence from the micro structural characterizations of various double-walled tubes that the desired interface configuration could be shaped during a process called duplexing. In duplexing, starting with a a relatively smooth inside surface on an outertube and a shaped outside surface on an inner tube, the two tubes are inserted one in the other and both are stretched longitudinally. The outer tube is drawn down onto the surface of the inner tube, the shaped surface of which permanently deforms the inner surface of the outer tube.It has been observed that, as a result of the drawing step in the duplexing sequence of current manufacturing processes, the surface of the inner tube projects as much as 0.063 to 0.076 mils into the space of the longitudinal tube grooves which are preformed in the surface of the outer tube. This indicates that even in the present process, sufficient metal would flow during duplexing to achieve the interlock interface if the inner tube surface were properly shaped.

The production of circumferential hills and valleys on the outer surface of the inner tube with the maintenance of an inner surface which is smooth, round and of the proper diameter can be accomplished by planetary swaging or simply by coarsegrinding the outer surface of the inner tube before duplexing.

As part of a program to develop pre-stressed double-walled tubes, two samples of double-walled tubes having a 20 mm. interface engagement were tested at loads up to 4300 kg. and were cyclically loaded at 4100 kg. or ongreater for at least 95 cycles at 400 C. One sample was tested in the as-prestressed condition and one after a stress relief heat treatment.

Both samples had incidental mechanical interlocking on a micro-scale. Neither sample showed any sliding and there was no wear. Similar samples not having an engaged interface slid at loads as low as 80 pounds and showed interface wear.

The addition of mechanical interlocking to the design of double-walled tubes may make it possible to delete pre-stressing.

This invention was conceived during performance of a contract with the United States Government designated DOE-AC02-77ET37201.

Claims

1. A steam generator for a liquid metal cooled fast breeder reactor of the double-walled tube variety characterized in that the interface between the surfaces of the double-walled tubes has a mechanical interlock which interlock is adapted to prevent sliding of the inner wall tube with respect to the outer wall tube.

2. A process for mechanically interlocking the outer surface of an inner wall tube located within an outer tube to the inner surface of said outer tube, characterized in that a non-smooth surface is formed onto the outer surface of the inner tube; said inner tube is inserted into said outer tube, said outer tube having a smooth inner surface, and said inner and said outer tubes are duplexed so as to cause the outer surface of said inner tube to deform the inner surface of said outer tube providing for a mechanical interlock of both tubes.

3. A process according to claim 2, characterized in that said non-smooth outer surface of said inner tube has a pattern produced by coarse grinding said outer surface.

4. A steam generator according to claim 2, characterized in that a wavy surface is swaged onto said inner tube for engagement of said inner tube with said outer tube.