Patent Number: 052157075
Section: claims

1. In a nuclear reactor having a core, a core support plate, a flow distribution plate, an instrument guide thimble extending into the core from the flow distribution plate, in-core instrumentation means for monitoring reactor operations extendable through the core support plate and flow distribution plate into the guide thimble, and, means for reducing flow induced vibration of the instrumentation means comprising: an instrument shroud disposed between the core support plate and flow distribution plate and having a cup with first and second openings for passing the instrument means therethrough, the first opening disposed adjacent an opening in the flow distribution plate and being of sufficient diameter to allow fluid flow into the instrument guide thimble, the second opening disposed adjacent to the core support plate and being of sufficient diameter to allow fluid flow into the cup, the cup having passages for diverting part of the fluid flow received from the second opening away from the instrument guide thimble, and, one or more shroud arms extending radially from the cup for engaging a substantially parallel core structure, each shroud arm having means for engaging the adjacent core structure at an end thereof, said engaging means having a locking pad disposed at the end of each shroud arm and spring means extending resiliently from each pad for biasing the shroud into position, the shroud arms preventing movement of the shroud due to fluid flow.  providing an instrument guide thimble shroud having a cup with first and second openings for passing the instrument means therethrough, the first opening disposed adjacent an opening in the flow distribution plate and being of sufficient diameter to allow fluid flow into the thimble tube, the second opening disposed adjacent to the core support plate and being of sufficient diameter to allow fluid flow into the cup, the cup having passages for diverting part of the fluid flow received from the second opening away from the instrument guide thimble, and, one or more shroud arms extending radially from the cup for engaging a substantially parallel core structure, each shroud arm having means for engaging the adjacent core structure, the engaging means having a locking pad disposed at the end of each shroud arm and spring means extending resiliently from each pad for biasing the shroud into position, the shroud arms preventing movement of the shroud due to fluid flow;  placing the instrument thimble tube shroud between the fuel assembly flow distribution plate and the reactor support plate; and,  placing an instrument guide thimble within the shroud. 2. The shroud of claim 1 wherein the cup has a cylindrical shape. 3. The shroud of claim 1 wherein the cup has an upper inwardly tapered surface with the passages located therein. 4. The shroud of claim 1 wherein four shroud arms extend from the cup. 5. The shroud of claim 1 wherein the spring means comprise a spring plate having one or more spring arms for biasing the shroud arms into position, recesses provided in the adjacent core structure for accepting the spring arms therein. 6. The shroud of claim 5 further comprising a retaining spring extending from the spring plate for locking the shroud arm in position. 7. The shroud of claim 5 wherein each spring arm has first and second sloped surfaces. 8. The shroud of claim 6 wherein the retaining spring has an angled surface and an angled locking tab extending therefrom. 9. The shroud of claim 1 wherein the shroud is composed of a material from the group consisting of stainless steel, zirconium, Zircaloy-2, Zircaloy-4, Inconel and nickel alloys. 10. The shroud of claim 1 wherein the shroud is composed of stainless steel. 11. A method for preventing damage to an instrument guide thimble in a nuclear reactor having a core, a core support plate, a flow distribution plate, an instrument guide thimble extending into the core from the flow distribution plate, in-core instrumentation means for monitoring reactor operations extendable through the core support plate and flow distribution plate into the guide thimble, and, means for reducing flow induced vibration of the instrumentation means, the method comprising: