Guide thimble tubes are vacant tubes for control rods or incore instrumentation. Control rods usually constitute cluster control rod assemblies (PWR) and are inserted into guide thimbles within a nuclear fuel assembly. The absorbing material (e.g. pellets of Boron Carbide) is protected by the cladding usually made of stainless steel. The absorber rods in the control clusters slide within these guide tubes. There are designs of fuel assemblies that contain also an instrumentation tube. An intrumentation tube is a vacant tube only for incore instrumentation such as the incore neutron flux monitoring system. This system, for example, utilize:
These movable flux detectors are usually placed into the instrumentation tube of a fuel assembly. They can monitor the entire length of selected fuel assemblies to provide an extremely accurate, three-dimensional map of the neutron flux distribution. Using these data, neutron flux reconstruction can be performed also in the rest of the reactor core.