Irradiation behavior and post-irradiation examinations of an acoustic sensor using a piezoelectric transducer

The development of advanced instrumentation for in-pile experiments in Material Testing Reactor constitutes a main goal for the improvement of the nuclear fuel behavior knowledge. In the framework of high burn-up fuel experiments under transient operating conditions, an innovative sensor based on acoustic method was developed by CEA and IES (Southern Electronic Institute).This sensor is used to determine the on-line composition of the gases located in fuel rodlet free volume and thus, allows calculating the molar fractions of fission gases and helium. The main principle of the composition determination by acoustic method consists in measuring the time of flight of an acoustic signal emitted and reflected in a specific cavity. A piezoelectric transducer, driven by a pulse generator, generates the acoustic wave in the cavity. The piezoelectric transducer is a PZT ceramic disk, mainly consisting of lead, zirconium and titanium. This acoustic method was tested with success during a first experiment called REMORA 3, and the results were used to differentiate helium and fission gas release kinetics under transient operating conditions. However, during the irradiation test, acoustic signal degradation was observed, mainly due to irradiation effect but also due to the increasing of the gas temperature. Despite this acoustic signal degradation, the time of flight measurements were carried out with good accuracy throughout the test, thanks to the development of a more efficient signal processing. After experiment, neutronic calculations were performed in order to determine neutron fluence at the level of the piezoelectric transducer. In order to have a better understanding of the acoustic sensor behavior under irradiation, Post Irradiation Examination program was done on piezoelectric transducer and on acoustic coupling material too. These examinations were also realized on a non-irradiated acoustic sensor built in the same conditions and with the same materials and the same- design as the one which has been irradiated. The comparison between both results highlights microstructural evolutions due to irradiation effect. Firstly, metallographic examinations of polished low-angle cross-section of the sensor were performed. The use of low-angle cross-section sample gives geometrical characteristics (macroscopic swelling measurements for instance) with a very good accuracy. This preparation induces a significant magnification of observed region of interest, especially for the acoustic coupling material whose thickness is very small. Optical microscopy on irradiated and non-irradiated samples provides microstructural information about a possible evolution during irradiation test. Secondly, X-ray diffraction measurements on both samples give information about crystalline phase behavior under irradiation. In addition with the important feedback acquired during the in-pile test, these examinations allow us to improve the acoustic sensor design for the next experiments.