Patent Number: 058928065
Section: summary

FIELD OF THE INVENTION This invention relates to a spacer for maintaining an inner tube in spaced relation within an outer tube and in particular to a spacer for maintaining a distance between a pressure tube and a calandria tube in a nuclear reactor. The invention is particularly concerned with a spacer which is fixed to the outer surface of the pressure tube. BACKGROUND OF THE INVENTION In a nuclear reactor, the pressure tubes which contain the fuel bundles are each positioned within a calandria tube. It is necessary to have an annular space maintained between the pressure tube and the calandria tube to allow for the circulation of gases which thermally insulate the hot pressure tube from the relatively colder calandria tube and the heavy water moderator which flows in the space outside the calandria tube. Conventionally, a garter spring spacer has been used to maintain the space between the pressure tube and the calandria tube. A garter spring spacer is an endless helical spring disposed around the pressure tube. Its convolutions contact the walls of both the pressure tube and the calandria tube. The spring is unattached to either tube. A garter spring spacer was disclosed in U.S. Pat. No. 3,106,520 issued to Wolfe et al. Oct. 8, 1963. While the garter spring is effective to support the pressure tube within the calandria tube, there are a number of drawbacks inherent in its use. Because garter spring spacers are not attached to either the pressure tube or the calandria tube, they must be installed on the pressure tube after the pressure tube has been placed inside the calandria tube. As a result, installation of the garter spring spacers is a difficult procedure which requires tedious operations to be carried out at the reactor face. The problem is exacerbated over the operating time of the fuel channel as increased sag develops in the calandria tubes. The difficulty installing the spacers is of particular significance to the fuel channel replacement procedures because each time the fuel channels are replaced, the spacers must be re-installed. Consequently, the procedure is slow and expensive. An improved fuel channel replacement procedure is desirable not only to reduce the time and expense of the operation but also to reduce the radiation dose level to which those who replace the fuel channels may be exposed. Another problem with garter spring spacers is that they have a tendency to move axially along the pressure tube during operation of the reactor. This movement is due to vibration and thermal cycling. When axial movement occurs, the spacers must be repositioned. Repositioning the spacers is difficult and costly and may also result in radiation exposure to those who conduct the procedure. Furthermore, the geometry of the garter spring spacer has limited the materials which can be used in its manufacture. Garter spring spacers have conventionally contained Inconel. It would be desirable to use only zirconium alloy to reduce fuel burn-up and increase neutron efficiency, however the brittleness of that material renders it difficult to form into a spring. Further, there would be considerable concern that a garter spring formed only of zirconium alloy would break during the life cycle of the reactor. Garter spring spacers are also known to cause wear on the inner wall of the calandria tube which may reduce the burst strength of the tube. Thus there is a need for a pressure tube spacer which is effective to maintain a space between the pressure tube and the calandria tube and which overcomes some of the difficulties inherent in the use of prior art spacers such as the garter spring spacer. SUMMARY OF THE INVENTION In accordance with the present invention there is provided a spacer for maintaining an inner tube in spaced relation within an outer tube, said spacer comprising a split ring adapted to be disposed about the outer surface of said inner tube, said ring having a central annular body portion with a raised bearing surface thereon adapted to contact the inner surface of said outer tube and prevent contact between said outer surface and said inner surface, an annular land projecting from each side of said central body portion, and a transverse split across said central annular body portion and said lands, each of said lands adapted to receive a collar thereon effective to close said split and create an interference fit between said ring and said inner tube and thereby constrain axial movement of said spacer on said inner tube. In accordance with another aspect of the present invention there is provided a spacer for maintaining a pressure tube in spaced relation with a calandria tube of a nuclear reactor, said spacer comprising a split ring adapted to be disposed about the outer surface of said pressure tube, said ring having a central annular body portion with a raised bearing surface thereon adapted to contact the inner surface of said calandria tube and prevent contact between said outer surface and said inner surface, an annular land projecting from each side of said central body portion, and a transverse split across said central annular body portion and said lands, each said land adapted to receive a collar thereon effective to close said split and create an interference fit between said ring and said pressure tube and thereby constrain axial movement of said spacer on said pressure tube. In accordance with other aspects of the invention, the lands can each have a groove or ridge to retain said collar thereon. In accordance with other aspects of the invention, a concavity can be provided in the central annular body portion forming an annular void space between said raised bearing surface and said pressure tube. In accordance with other aspects of the invention, the said bearing surface can have a coating to reduce heat transfer or friction between said ring and said outer tube. The present invention provides a spacer which is fixed to the outer wall of the pressure tube to prevent axial movement of the spacer on the pressure tube. By limiting this axial movement, the invention overcomes many of the problems of the prior art spacers. The spacer can be installed on the pressure tube before the pressure tube is installed in the calandria thus improving the installation and fuel channel replacement procedures. Further, repositioning of the spacer under normal circumstances no longer necessary.