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Innovative in-pile instrumentation is crucial for advanced experimental programs in research reactors. In this field, we developed a specific acoustic sensor to improve the knowledge of fission gas release in Pressurized Water Reactor (PWR) fuel rods when irradiated in Material Testing Reactors (MTR). In order to perform experimental programs related to the study of the fission gas release kinetics, the CEA (French Nuclear Energy Commission) acquired the ability to equip a pre-irradiated PWR fuel rod with three sensors, allowing the simultaneous on-line measurements of the following parameters: 1) fuel temperature with a centreline thermocouple type C 2) internal pressure with a specific counter-pressure sensor, 3) fraction of fission gas released in the fuel rod with an innovative acoustic sensor. The third detector, which has been developed and is patent pending by CEA, SCK·CEN (Belgian Nuclear Research Center) and IES (French research laboratory of Montpellier II University and French National Research Center), is the subject of this paper. This original acoustic sensor has been designed to measure the molar mass and pressure of the gas contained in the fuel rod plenum. For in-pile instrumentation, the fraction of fission gas, such as Krypton and Xenon, in Helium, can be deduced on-line from this measurement. The principle of this non destructive and on-line acoustical sensor is the following: a piezoelectric transducer generates acoustic waves in a cavity connected to the fuel rod plenum. The acoustic waves are propagated and reflected in this cavity and then detected by the transducer. The data processing of the signal gives the velocity of the acoustic waves and their amplitude, which can be related respectively to the molar mass and to the pressure of the gas. The piezoelectric material of this sensor has been qualified in nuclear conditions (gamma and neutron radiations). The complete sensor has also been specifically designed to be implemented in MT- - R conditions. For this purpose some technical points have been studied in details: 1) fixing of the piezoelectric sample in a reliable way with a suitable signal transmission, 2) size of the gas cavity to avoid any perturbation of the acoustic waves, 3) miniaturization of the sensor because of narrow in-pile experimental devices, 4) appropriate cables to transmit high frequency signal under nuclear conditions.