1. A METHOD OF DETECTING CHANNEL BLOCKAGE IN A LIQUID METAL COOLED NUCLEAR REACTOR OF THE KIND HAVING WITHIN A VESSEL A REACTOR CORE COMPOSED OF FUEL RODS IN CHANNELS THROUGH WHICH THE LIQUID METAL COOLANT FLOWS, COMPRISING THE STEPS OF SENSING SONIC VIBRATIONS OCCURRING IN ALL OF A NUMBER OF RODS RESPECTIVELY HAVING FREE ENDS IN THE CHANNELS TO ACT AS SOUND GUIDES, AND SELECTING FROM SAID VIBRATIONS THOSE THAT ARE CHARACTERISTIC OF THE FORMATION OF BUBBLES IN THE LIQUID METAL COOLANT YET FREE FROM INTERFERENCE OF COMPETITIVE STRENGTH BY SONIC WAVES OF OTHER ORIGIN, WHEREBY SUCH SELECTED VIBRATIONS INDICATE, FOR WHICHEVER CHANNEL IS ASSOCIATED WITH THE ROD AT WHICH THESE SELECTED VIBRATIONS ARE SENSED, THAT IN THAT CHANNEL A BOILING OF THE COOLANT IS TAKING PLACE AND HENCE THAT A BLOCKAGE HAS OCCURRED.