Abstract: The old split-pin assemblies of the lower guide tube of a nuclear reactor are replaced by new split-pin assemblies in a robotic work station under a pool of water. The work station includes a plurality of robotic tools which are remotely actuable from a robotic command center outside of the pool to position the guide tube precisely for a replacement operation, to fragment the old split-pin assemblies and dispose of the fragments under water, and to install a new split-pin assembly. The lower-guide-tube positioning means has a hydraulic cylinder, also remotely actuable externally to the pool, for rotating the guide tube so that it may be oriented properly to be processed by different tools in succession and also so that both split-pin assemblies in each guide tube may be replaced by the same set of tools.