Patent Number: 058928065
Section: description

DETAILED DESCRIPTION OF THE INVENTION The fuel channel arrangement commonly used in a CANDU reactor is shown in FIG. 1. Fuel bundles 10 are arranged end-to-end in pressure tube 12 which in turn is encased by calandria tube 14. Gas, typically carbon dioxide, circulates within annular space 16 between pressure tube 12 and calandria tube 14 to thermally insulate pressure tube 12 from calandria tube 14 and the heavy water moderator which flows in the space 17 outside calandria tube 14. Heavy water coolant is contained within pressure tube 14. Pressure tube 12 and calandria tube 14 are fixed in coaxial relation at either end to the calandria end shield tube sheets and are unsupported therebetween. As the reactor ages, pressure tube 12 is subject to sag. The spacer of the present invention is applied to the outer surface of pressure tube 12 to prevent contact of the outer surface of pressure tube 12 with the inner surface of calandria tube 14. The spacer of the present invention is shown in FIG. 2 and FIG. 3. The spacer, generally designated by reference numeral 18, has an annular ring 20 of generally circular cross-section which has a central annular body portion 21 and a land 24, 26 projecting at either side of central body portion 21 in contact with pressure tube 12. Central body portion 21 has a raised bearing surface 22. Annular ring 20 is a split ring with split 28 through central body portion 21 and lands 24, 26. The inside diameter of lands 24, 26 is approximately equal to the outer diameter of pressure tube 12. Annular ring 20 can be advantageously formed with a concavity 27 under central body portion 21. When assembled to pressure tube 12, concavity 27 forms a void space which reduces heat transfer between pressure tube 12 and calandria tube 14. Annular collars 30, 32 are disposed on lands 24, 26 and are sized to be forced onto lands 24, 26 and thereby narrow split 28 to create an interference fit between spacer 18 and pressure tube 12. In this manner, collars 30, 32 are effective to constrain spacer 18 against axial movement on pressure tube 12. In practice, a plurality of spacers 18 are installed at regularly spaced intervals along each pressure tube 12. To account for diametrical variation of pressure tube 12, collars 30, 32 are selected from a sets of collars having small variances in diameters in order to achieve the required interference fit at the positions to be installed. Annular collars 30, 32 can be retained on lands 24, 26 by any suitable means. If desired, lands 24, 26 can be formed with shallow grooves into which collars 30, 32 can be forced, or optionally can have raised ridges over which collars 30, 32 can be forced. As shown in FIG. 3, lands 24, 26 have grooves 34, 36 formed thereon. While pressure tube 12 is shown in FIG. 3 as being concentrically spaced within calandria tube 14, it will be understood that as pressure tube 12 sags, spacer 18 becomes offset in relation to its position within calandria tube 14, and eventually, raised bearing surface 22 will abut inner surface of calandria tube 14 thereby preventing contact between pressure tube 12 and calandria tube 14 and maintaining tubes 12, 14 in spaced relation. The extent to which bearing surface 22 is raised above lands 24, 26 depends upon the relative diameters of pressure tube 12 and calandria tube 14. Sufficient space must be maintained between spacer 18 and calandria tube 14 to allow for the circulation of gases in annular space 16 taking into account the diametrical creep of pressure tube 12 which occurs over its operating time. As the pressure tube ages, it undergoes a slight increase in diameter, known to those skilled in the art as diametrical creep. Conventional garter springs are sized to loosely fit about the pressure tube in order to accommodate diametrical variations and creep of the pressure tube. This has the significant disadvantage that conventional garter springs can be displaced from their initial position over the life of the reactor with the result that the distance between adjacent garter springs can exceed that required to maintain the pressure tube out of contact with the calandria tube. In such circumstances, time consuming and costly procedures are required to reposition the garter springs. In the present invention split 28 accommodates diametrical variations of the pressure tube and may allow for the diametrical expansion of annular ring 20. Collars 30, 32 are preferably formed of the same material used to manufacture pressure tubes 12. In the alternative, collars 30, 32 may be formed of a different material than pressure tubes 12 provided the creep coefficient of the materials is substantially the same. This matching of creep coefficients is advantageous because collars 30, 32 will not come loose (as they would if they had a higher creep rate than the pressure tube) nor will they restrict the expansion of the pressure tube (as they would if they had a lower creep rate than the pressure tube). For pressure tubes made of zirconium alloy, collars 30, 32 are preferably formed of the same zirconium alloy. However other suitable materials can also be used. Annular ring 20 is preferably also manufactured from zirconium alloy. However, because of split 28, it is not necessary to ensure that the annular ring 20 be formed from the same material, or one having the same coefficient of diametrical creep as the pressure tube. One of the advantages of the present invention is that because spacer 18 maintains its position on pressure tube 12, spacer 18 can be applied to pressure tube 12 before pressure tube 12 is inserted in calandria tube 14 and spacer 18 is not displaced during insertion. As a result, the difficult and tedious procedure of installing spacers at the reactor face is avoided. The use of the spacers of the present invention consequently results in a significant reduction in installation time and a corresponding reduction in radiation exposure to those persons conducting the installation procedure. Similarly, the ability to apply spacer 18 to pressure tube 12 before pressure tube 12 is installed in the reactor improves the safety and efficiency of the fuel channel replacement procedures. During operation, the spacer of the present invention also maintains its location on the pressure tube and does not suffer the axial movement which characterize some of the prior art spacers. Accordingly, under normal circumstances, there is no need to reposition the spacers. The geometry of the spacer of the present invention provides a number of advantages. It allows for the spacer to be constructed of zirconium alloy. The use of zirconium alloy is beneficial because of the reduced fuel burn-up and increased neutron efficiency inherent in that materials as compared with Inconel used in the manufacture of conventional garter spring spacers. The geometry also provides bearing surface 22 which advantageously can have a zirconium oxide coating applied thereto. The coating can be applied by the plasma spray technique or any other suitable technique well known to those skilled in the art. The oxide coating has a number of beneficial functions. Firstly, it minimizes wear on the inner wall of calandria tube 14 where that tube contacts bearing surface 22. This minimal wear maintains the burst strength of the tube which is necessary for safe operation of the reactor and which is necessary to meet regulatory requirements. Secondly, it permits relative movement between bearing surface 22 and calandria 14 which is necessary due to the differential thermal expansion between pressure tube 12 and calandria tube 14 and to allow for the differential creep between these two components. Thirdly, it provides low heat transfer between pressure tube 12 and calandria tube 14. Other suitable coatings which provide the above functions can also be used. In addition, concavity 27 provides a thermal barrier between pressure tube 12 and calandria tube 14 and prevents cold spots on pressure tube 12 which may result in undesirable blister formation. Its presence also reduces the amount of material required to form annular ring 20. Although the present invention has beendescribed with particular reference to its use to maintain pressure tubes in spaced relation within calandria tubes of a nuclear reactor, the invention has more general application to maintain concentrically disposed tubes in spaced relation.