Abstract: An ultrasonic cleaning method for cleaning the film, scale and sludge from internal surfaces of tubes, tubesheets, tube support plates and channel heads of steam generators previously exposed to water or steam at temperatures of more than 200° C. introduces an ultrasonic transducer or transducer array into the steam generator and submerges the ultrasonic transducer and the surface to be cleaned in water or aqueous solution. Ultrasonic energy is introduced into the water at a power level of 20 watts/gallon or more and at frequencies of from 10 to 200 KHz. An array of transducers is employed to introduce 20 to 60 watts/gallon of water or more into the steam generator. In one practice, the transducer or transducer array is suspended in the steam generator. An array of transducers can be assembled (and later disassembled) in situ where individual transducers or small transducer subassemblies must introduced into the steam generator through small hand holes or other small diameter nozzles.