A remotely controlled method for the non-destructive inspection of the primary pressure containment means of a nuclear reactor, on the site of use, for the detection of degradation resulting from sustained exposure to radiation. This method utilizes dual probes operating on a send-and-receive basis, wherein an ultrasonic beam is sent into a thick vessel wall describing a Vlike path in the same. The transducers are fixed with respect to each other, and are transported about a programmed course in fixed relation to the vessel, so as to allow the beam to inspect 100 percent of the steel in such wall. If a defect was present in the steel, it would interfere with the path of the ultrasonic beam and a corresponding loss of energy would be detected and recorded. A comparison of the record of signals resulting from this inspection, with an established pattern of signals having its inception prior to service would indicate the initiation and propagation of a defect so that, if necessary, remedial procedures could be undertaken.