For more than 30 years, the Institute for Radiological Protection and Nuclear Safety (IRSN) carries out research and development concerning accident scenarios in nuclear facilities and particularly fuel under accidental conditions and reactor core meltdown accident. The Research Programs implement experimental facilities able to reproduce nuclear fuel melting up to 2800 C. A specific instrumentation was developed to measure high temperatures in severe physicochemical environments (oxidising conditions for instance). In addition to C-type thermocouples, the IRSN systematically used Ultrasonic Thermometers in PHEBUS FP program tests. Initially in partnership with other research institutes, the IRSN alone maintains the knowledge in this field due to international research programs reduction. First of all, the IRSN recovered the mastery of the measurement line in particular the analogical signal processing as well as numerical. Then the sensor design was modified to turn from prototype stage to a model easier to implement and to maintain. Lastly, the effort is currently carried out on uncertainties reduction and reliable measurement. Calibrations procedures are re-examined and measurements campaigns engaged in analytical facilities of the IRSN (high temperatures furnaces). Within this framework the use of eutectic cells is considered.