A neutron flux mapping system for a nuclear reactor including: drivers each including a geared motor, a helical gear driven by the geared motor, and a storage reel adapted to supply, to the helical gear, a detector cable carrying a detector; and a double indexing path selector unit including upper and lower fixed plates, a fixed shaft, an outer path selector rotatably mounted to the upper fixed plate, and inner path selectors. The outer path selector includes an upper rotating plate rotatably mounted to the upper fixed plate while carrying a drive unit for rotating the outer path selector about the fixed shaft, and a control unit for controlling the drive unit, and a lower rotating plate connected to the upper rotating plate. Each inner path selector includes a hollow rotating shaft rotatably mounted between the upper and lower rotating plates, a path select tubing connected to the rotating shaft to rotate along with the rotating shaft, and a disc mounted to a lower end of the rotating shaft, and provided with a plurality of circumferentially-arranged paths.