Ultrasonic testing (UT) is being used to detect leaking BWR and PWR fuel rods. The testing method makes use of differences in scattering by water and gas of ultrasonic (pressure) waves as they are reflected between the inner and outer surfaces of fuel cladding. The UT process makes use of two probes, which move laterally across a FA. The probes are arranged in a “pitch-catch” configuration in which one probe transmits a signal, which is picked up by the cladding, reflected between the inner and outer surfaces and ultimately received by a second probe. The scattering by water at the inner surface of a failed rod is greater than the scattering by gas at the inner surface of a sound rod. Such differences are used to identify the specific rod or rods that are leaking.