A pressurized water nuclear reactor comprising: a pressure vessel; an upper removable head for sealably engaging an upper opening in the pressure vessel; a core having an axial dimension supported within the pressure vessel; a plurality of nuclear fuel assemblies supported within the core, at least some of the fuel assemblies having at least one instrumentation thimble extending axially therethrough; an upper internals assembly supported above the core and having axially extending instrumentation guide paths supported therethrough with each of the instrumentation thimbles that are configured to receive instrumentation through the upper internals assembly being aligned with one of the instrumentation guide paths, the upper internals assembly including an instrumentation grid assembly plate supported above the instrumentation guide paths and axially movable relative to a lower portion of the upper internals; and a plurality of in-core instrumentation thimble assemblies extending through a corresponding one of the instrumentation guide paths into an instrumentation thimble, the in-core instrumentation thimble assemblies having an upper portion connected to the instrumentation grid assembly plate and retractable with raising of the instrumentation grid assembly plate to move a portion of the in-core instrumentation thimble assemblies within the instrumentation thimbles, into the upper internals assembly, the in-core instrumentation thimble assembly including a lower section comprising a sensor region and an upper section through which signal cabling is routed, both the lower section and the upper section being enclosed within an outer sheath, the outer sheath having an upper portion connected to the instrumentation grid assembly plate with the signal cabling extending at least partially through the instrumentation guide path, characterised by the signal cabling passes through and around an outside of the outer sheath in a tight spiral that extends substantially from the top of the outer sheath to within a protective tube where the signal lead exits through a passage from the interior of the reactor vessel to an exterior thereof, with the number of revolutions sufficient to stretch along an extended length of the outer sheath as the outer sheath is extended with a raising of the instrumentation grid assembly plate to remove the in-core instrumentation thimble assembly from the corresponding instrument thimble, without disconnecting the signal cabling that was housed within the pressure vessel and upper removable head. a pressure vessel; an upper removable head for sealably engaging an upper opening in the pressure vessel; a core having an axial dimension supported within the pressure vessel; a plurality of nuclear fuel assemblies supported within the core, at least some of the fuel assemblies having at least one instrumentation thimble extending axially therethrough; an upper internals assembly supported above the core and having axially extending instrumentation guide paths supported therethrough with each of the instrumentation thimbles that are configured to receive instrumentation through the upper internals assembly being aligned with one of the instrumentation guide paths, the upper internals assembly including an instrumentation grid assembly plate supported above the instrumentation guide paths and axially movable relative to a lower portion of the upper internals; and a plurality of in-core instrumentation thimble assemblies extending through a corresponding one of the instrumentation guide paths into an instrumentation thimble, the in-core instrumentation thimble assemblies having an upper portion connected to the instrumentation grid assembly plate and retractable with raising of the instrumentation grid assembly plate to move a portion of the in-core instrumentation thimble assemblies within the instrumentation thimbles, into the upper internals assembly, the in-core instrumentation thimble assembly including a lower section comprising a sensor region and an upper section through which signal cabling is routed, both the lower section and the upper section being enclosed within an outer sheath, the outer sheath having an upper portion connected to the instrumentation grid assembly plate with the signal cabling extending at least partially through the instrumentation guide path, characterised by the signal cabling passes through and around an outside of the outer sheath in a tight spiral that extends substantially from the top of the outer sheath to within a protective tube where the signal lead exits through a passage from the interior of the reactor vessel to an exterior thereof, with the number of revolutions sufficient to stretch along an extended length of the outer sheath as the outer sheath is extended with a raising of the instrumentation grid assembly plate to remove the in-core instrumentation thimble assembly from the corresponding instrument thimble, without disconnecting the signal cabling that was housed within the pressure vessel and upper removable head.