A method and an apparatus for test an annular weld at a main coolant line connected to a reactor pressure vessel in a nuclear power plant, include a self-propelled submersible vehicle which is introduced into an opened, flooded reactor pressure vessel. The submersible vehicle has at least one test device to be placed against an inner circumferential surface of the main coolant line and moved in circumferential direction of the main coolant line. The submersible vehicle is advanced into the main coolant line, which is also flooded, and is fixed in the region of the weld by radially deployable spreading or extension arms. The test device is then placed against the weld and is moved therealong in the circumferential direction.
