Abstract:
A method for repositioning garter springs around the pressure tubes of Candu-type nuclear reactors. A primary coil having a high permeability core is inserted into the pressure tube. The coil is energized by an alternating current of a frequency and amplitude sufficient to produce a primary alternating electromagnetic field that penetrates the wall and induces an eddy current in the garter spring. A secondary alternating electromagnetic field is also produced and is reactive with the primary field to produce a travelling electromagnetic field of sufficient intensity and of such direction that the resultant of the electromagnetic forces acting on the garter spring causes the spring to move along the tube.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to an electromagnetic method for controllably moving a conductive body from a first position to a second position and more particularly to a method for remotely positioning weakly conductive objects located behind a relatively conductive wall. An application of the invention is in remotely repositioning spacers called &#34;garter springs&#34; used for separating coaxial pressure tubes and calandria tubes in Candu nuclear reactors. 
     BACKGROUND OF THE INVENTION 
     In some heavy water-moderated nuclear reactors, for example Candu reactors, fuel for the nuclear reaction is carried in an array of pressure tubes disposed within a calandria vessel forming the reactor core. Heavy water is circulated through the pressure tubes and is heated by the nuclear reaction; the heated heavy water can then be used to drive turbines for generating electricity. Each pressure tube is surrounded by and coaxial with a calandria tube and the nuclear reaction is moderated by heavy water disposed within the calandria vessel externally of the calandria tubes. The temperature of this heavy water is significantly lower than the temperature of the heavy water in the pressure tubes when the reactor is in operation. Each pressure tube is spaced from the associated calandria tube by means of spacers called garter springs which are fitted around the pressure tubes at intervals along its length. However, it has been found that, over a period of time, the garter springs tend to migrate along the pressure tubes under the influence of vibrations generated in normal operation of the reactor and that the garter springs may eventually move to positions at which they no longer act as effective spacers, permitting the pressure tubes to sag. 
     It is therefore desirable to be able to reposition the garter springs to ensure proper spacing between the pressure tubes and the calandria tubes. The difficulty is that the garter springs are not directly accessible since the space between the two tubes is closed in the assembled reactor. Disassembly of the pressure tubes and calandria tubes is not practicable because the tubes are radioactive and such disassembly would be very expensive. 
     Accordingly, a specific object of the invention is to provide a non-intrusive, non-destructive method for repositioning garter springs or other conductive bodies which are shielded by a relatively conductive wall (e.g. the wall of the pressure tube). 
     SUMMARY OF THE INVENTION 
     The invention provides a method for controllably moving a conductive body from a first position to a second position, the body being shielded by a relatively conductive wall, the method comprising the steps of: generating at a side of the wall remote from the body a primary alternating electromagnetic field that is capable of penetrating the wall and inducing an eddy current in the conductive body with the body in said first position; and, producing a secondary alternating electromagnetic field that is reactive with the primary field to produce a travelling electromagnetic field of sufficient intensity and of such direction that the resultant of the electromagnetic forces acting on the conductive body causes the body to move from said first position to said second position. 
     The present invention permits the use of continuous rather than pulsating energizing current for producing the primary electromagnetic field. By creating in effect a travelling electromagnetic wave, the level of energizing current that is required is much less than would be required if the conductive body were subjected only to high energy electromagnetic impulses, thus reducing the risk of destruction or deterioration of any components subjected to the electromagnetic field. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to permit a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made to the accompanying drawings which show preferred embodiments of the present invention by way of example, and in which: 
     FIG. 1 is a perspective view of a conventional nuclear reactor fuel channel assembly in association with a schematic representation of an apparatus of the form for carrying out the method provided by the invention; 
     FIG. 2 is a longitudinal sectional view through the apparatus shown in FIG. 1; and, 
     FIG. 3 is a view similar to FIG. 2, showing an alternative form of apparatus for carrying out the method provided by the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, FIG. 1 shows the principal components of a fuel channel assembly 1 of the type used in a Candu nuclear reactor. As is wellknown, a reactor of this type includes an array of similar fuel channel assemblies supported within a calandria vessel (not shown). FIG. 1 shows the fuel channel assembly from one end of the reactor core. An end shield generally denoted by reference numeral 3 separates the reactor core area or calandria chamber (generally indicated at 5) from an external fueling area 7. A similar end shield is provided at the opposite end of the reactor core. As shown in the drawings, end shield 3 includes a fueling machine-side wall or &#34;tubesheet&#34; 9, lead shielding balls 11 and a calandria-side tubesheet 13. A lattice tube 15 extends between the two tubesheets and provides a passageway for the fuel channel assembly throughthe end shield 3. Typically, end shield 3 is of a width of approximately 31/2 feet. 
     The fuel channel assemblies 1 carry fuel, normally in the form of bundles of fuel rods (not shown), from the fueling area 7 to the reactor core 5. The assemblies also support the fuel rods within the core during normal operation of the reactor, all as is well known in the art. 
     Protruding from the fuelling area 7 side of the end shield 3 is a liner tube 17 surrounded by an end fitting body 19. At their outer ends, tube 17and body 19 are enclosed by a channel closure 21. The end fitting body 19 and an outboard end fitting sleeve 23 are connected near the machine-side tubesheet 9 by a roll joint 25. The end fitting body 19 is connected by a roll joint 27 to a pressure tube 29 for containing fuel rods (not shown), within the lattice tube 15 near the calandria-side tubesheet 13. Surrounding the pressure tube 29, beginning at a roll joint 31, is a calandria tube 33. The pressure and calandria tubes 29, 33 extend from tubesheet 13 into the calandria chamber forming the reactor core. Within the reactor core, the nuclear reaction is moderated by heavy water that circulates externally of the calandria tubes 33 while heavy water is also circulated separately through the pressure tubes for heating. This heated heavy water is then used to drive turbines for generating electricity, in a nuclear generating plant. 
     Due to the temperature differential between the relatively hot heavy water within the pressure tubes 29 and the cooler heavy water externally of the calandria tubes 33 it is necessary to maintain the pressure tubes and calandria at a spacing from one another, as shown in FIG. 1. Typically, this spacing may be nominally greater than about 1/2 inch. In a commercialreactor, the distance between end shield 3 and the corresponding end shieldat the other end of the reactor core may be significant, for example, of the order of 26 feet. Over this distance, the calandria tube and pressure tube are unsupported. It has been found that, during operation of the reactor, the pressure tubes tend to sag within the calandria tubes. If a pressure tube touches the surrounding calandria tube, the temperature differential between the two tubes may cause the pressure tube to deteriorate and eventually to crack. To guard against this, several gartersprings indicated at 35 in FIG. 1 are fitted around the pressure tube 29 toact as spacers. The garter springs are spaced along the length of the pressure tubes 29. 
     Each garter spring 35 comprises a long, narrow relatively tightly wound coil 34 with a wire 36 running through the center of the coil 34. The wire36 is closed to form a loop. The ends of the coil 34 are contiguous, but not joined. The loop is approximately 41/2 inches in diameter. The width of the coil 34 is approximately 1/4 inch. 
     The garter springs 35 are not fixed in position but held only by friction and it has been found that the springs tend to move along the pressure tubes in service, due to the effect of vibrations that occur in operation of the reactor. Over a period of time, the springs may in fact migrate to positions in which they no longer effectively protect against contact between the pressure tube and the calandria tube. FIG. 1 illustrates the relatively complex structure of the fuel channel assembly and the practical impossibility of gaining direct access to the garter springs after the reactor has been put in service. 
     In FIG. 1, reference character &#34;A&#34; denotes generally an electromagnetic apparatus that has been devised for repositioning the garter springs. The apparatus includes an alternating current device 37 (hereinafter called anAC device) that is designed to be inserted into the pressure tube and used to remotely manipulate a mispositioned garter spring, as described below. The AC device itself is shown in longitudinal section in FIG. 2, which will now be described. 
     AC device 37 is of generally cylindrical shape overall and in fact is dimensioned to approximate the shape of a fuel rod bundle normally contained within the pressure tube. In this particular embodiment the AC device is of three-phase construction and it includes three contiguous coils or windings 39, 41, 43, respectively, made from standard magnet wire. The windings are wound about a high-permeability core 45. The core 45 is divided into a solid core section 47 and individual laminated core sections 49. The core 45 is approximately 10 inches in length and approximately 3 inches in diameter at the outside of the laminated core sections 49. As a very high permeability core will create a stronger fieldand in turn a stronger force, vanadium permendur was used for the core 45. The windings 39, 41, 43 are wound about the core 45 at the positions of the sections 49. The windings 39, 41, 43 are formed of 200 turns of 14 AWGsquare magnet wire in five layers. The direction of each winding 39, 41, 43is the same. The windings 39, 41, 43 are connected to a three-phase currentgenerator 50, shown in FIG. 1, by wires 39a, 39b, 41a, 41b, 43a, 43b, respectively, in a delta configuration. Of course, it is possible to use other configurations which will produce a sufficient level of three-phase current. The wires 39a, 39b, 41a, 41b, 43a, 43b run through three channels51 in the solid core 47. 
     The device 37 is coated with a polyurethane resin 52 to make it substantially impermeable to heavy water. Of course, other materials suitable for use within a reactor that serve this function, may be adopted. A handle 53 (see also FIG. 1) extends from one end of device 37 for permitting remote manipulation thereof. 
     In operation, the relevant fuel channel assembly 1 is emptied of fuel. The channel closure 21 (FIG. 1) is opened and the AC device 37 is inserted through the liner tube 17 and into the pressure tube 29, and is moved to aposition within the pressure tube such that one end of the device is in close proximity to the garter spring 35. The position of the garter spring35 within the pressure tube 29 may be determined using devices embodying standard eddy current techniques. The windings are energized by the generator 50. The frequency should be sufficiently low to penetrate the wall of the pressure tube 29 and high enough to produce sufficient electromagnetic force on the garter spring 35 to cause it to move. In practice, main frequency (50-60 Hz) is satisfactory. 
     Generator 50 supplies an approximate average current of 100 amps in the windings 39, 41, 43 and an approximate average voltage of 400 volts acrossthe windings The phase shift between the current in the windings is approximately 120 degrees. 
     The winding which at any given moment is in close proximity to the garter spring 35 will act as a primary electromagnetic field generator. The windings beside the primary winding will combine to generate a secondary electromagnetic field. If it is assumed that winding 39 is closest to the garter spring, the current in all three windings will produce an electromagnetic field that travels toward winding 43. This induces a current in the wire 36 of the garter spring 35 (wire 36 is the only part of the garter spring that forms a closed loop about the pressure tube 29).To induce maximum current in the wire 36, the longitudinal axis of the device 37 should be generally parallel to the longitudinal axis of the pressure tube 29 (which it will be in practice). The field then acts to produce a force on the garter spring 35, moving it towards winding 43. As the force is continuous and may be controlled by the level of current in the windings 39, 41, 43, the garter spring 35 can be made to come to rest approximately at the end of device 37 nearest the winding 43. In a practical test, the force acting on the garter spring 35 was found to be in the range of approximately 30 to 40 grams. 
     If it is desired to move the spring 35 in the opposite direction the phase sequence between the windings 39, 41, 43 may be changed so that the field travels from winding 43 to winding 39. 
     If longer continuous motion is desired AC device 37 may be made of greater length. Alternatively, the device may be made to move along the pressure tube 29, with the garter spring 35 electromagnetically coupled to the device. 
     An alternative embodiment of the invention is shown in FIG. 3. In FIG. 3, the AC device is denoted by reference numeral 63 and has a primary coil 65wound about a high permeability core 67. The core 67 is composed of solid core material 69 and laminated core material 71. A copper ring 73 is placed about a portion of the coil 65. Coil 65 is connected to a single phase current generator similar to generator 50 shown in FIG. 1. Device 63has an exterior configuration similar to that of the device shown in FIGS. 1 and 2. Device 63 is similarly coated with a polyurethane resin indicatedat 75. 
     In operation, device 63 is placed inside the pressure tube 29 with the coil65 in close proximity to the garter spring that is to be moved. The coil 65is energized by an associated current generator (not shown), producing a primary electromagnetic field. The frequency of the generator used is 60 hz although, as mentioned for the previous embodiment, other relatively low frequencies may be used. The primary field induces an eddy current in the wire 36. The primary field also induces an eddy current in the copper ring 73. The current in the copper ring 73 produces a secondary electromagnetic field which is phase shifted approximately 90 degrees withrespect to the primary field. The primary and secondary fields produce a travelling electromagnetic field which reacts with the induced current in the wire 36 to exert a force on the garter spring 35. This moves the garter spring 35 from the end of the device remote from the copper ring 73to the end at which the copper ring is located. Simply stated, the copper ring acts as a shield providing a low magnetic field region where the garter spring tends to &#34; hide&#34;, occupying a minimum energy position. In this embodiment, the garter spring 35 can only be moved toward the copper ring 73. 
     While the present invention has been disclosed in connection with the specific embodiments described, it should be understood that there may be other embodiments which fall within the spirit and scope of the invention.For example, other electromagnetic devices may be used to produce a travelling electromagnetic field. It should also be noted that use of the method provided by the invention is not restricted to the repositioning ofgarter springs in a nuclear reactor. The invention may be used in other environments for moving a conductive body from a first position to a second position. The apparatus for carrying out the method of the invention may incorporate other features such as more complex winding arrangements to produce a rotational electromagnetic field in superposition with a linear travelling field so as to create a helical type of motion of the garter spring along the pressure tube.