Patent Number: 052157075
Section: summary

TECHNICAL FIELD This invention relates to in-core instrument thimble tubes for nuclear reactors and more particularly to an instrument thimble tube shroud which surrounds the tube to reduce flow induced vibrations. BACKGROUND Typically, a nuclear reactor has a vessel containing a core made up of a plurality of fuel assemblies. The lower regions of the fuel assemblies include narrow annuli within a fuel assembly flow distribution plate and a nozzle block. In-core instrumentation for monitoring reactor operations is supplied through the vessel and distribution plate into the core and reside in one or more thimble guide tubes which provide a guide path for the insertion of the instrumentation into the core. The instruments may be movable within the thimble guide tubes to monitor operation at various axial locations in the fuel assemblies. Typically, a small amount of the reactor coolant water circulates through the instrument thimble tube as well as about the fuel rod assembly. It has been found that the coolant water flow through the narrow annuli between a flow distribution plate and the core lower support plate and around the thimble guide tubes, at the entry point of the instrumentation into the thimble guide tube, causes vibration and interaction between the thimble tube and adjacent structures which cause fretting wear and damage to the instrumentation, which may result in premature failure of the instrumentation. SUMMARY OF INVENTION It is an object of the present invention to provide means to protect the in-core instrumentation from vibration damage. It is another object to prevent damage to the in-core instrumentation yet allow fluid flow through the instrument thimble tube for proper instrument cooling. It is a further object of the present invention to provide means to prevent instrument vibration damage through incorporation of an instrument thimble tube shroud having anti-rotation means. These and other objects of the present invention are achieved by providing a thimble tube shroud comprising a cup disposed between a reactor flow distribution plate and a support plate, the cup having passages for diverting part of the fluid flow away from the instrument thimble and part of the fluid flow controllably into the thimble, one or more arms extending from the cup, each arm having means to engage the adjacent fuel assembly structure, the arms preventing rotation of the cup due to fluid flow.