REMORA 3: The first instrumented fuel experiment with on-line gas composition measurement by acoustic sensor

With the aim to improve the knowledge of nuclear fuel behaviour, the development of advanced instrumentation used during in-pile experiments in Material Testing Reactor (MTR) is necessary. To obtain data on high Burn-Up MOX fuel performance under transient operating conditions, especially in order to differentiate between the kinetics of fission gas and helium releases and to acquire data on the degradation of the fuel conductivity, a highly instrumented in-pile experiment called REMORA 3 has been conducted by CEA and IES (Southern Electronic Institute - CNRS - Montpellier 2 University). A rodlet extracted from a fuel rod base irradiated for five cycles in a French EDF commercial PWR has been re-instrumented with a fuel centerline thermocouple, a pressure transducer and an advanced acoustic sensor. This latter, patented by CEA and IES, is¹ used in addition to pressure measurement to determine the composition of the gases located in the free volume and the molar fractions of fission gas and helium. This instrumented fuel rodlet has been re-irradiated in a specific rig, GRIFFONOS, located in the periphery of the OSIRIS experimental reactor core at CEA Saclay. First of all, an important design stage and test phases have been performed before the irradiation in order to optimize the response and the accuracy of the sensors: - To control the influence of the temperature on the acoustic sensor behaviour, a thermal mock-up has been built. - To determine the temperature of the gas located in the acoustic cavity as a function of the coolant temperature, and the average temperature of the gases located in the rodlet free volume as a function of the linear heat rate, thermal calculations have been achieved. The former temperature is necessary to calculate the molar fractions of the gases and the latter is used to calculate the total amount of released gas from the internal rod pressure measurements. - At the end of the instrumented rod manufacturing, specific inter- al free volume and pressure measurements have been carried out. Preliminary calculations of the REMORA 3 experiments have been performed from these measurements, with the aim to determine free volume evolution as a function of linear heat rate history. - A tracer gas has been added to the filling gas in order to optimize the accuracy of the helium balance at the time of the post irradiation examination. The two phases of the REMORA 3 irradiation have been achieved at the end of 2010 in the OSIRIS reactor. Slight acoustic signal degradation, observed during the test under high neutron and gamma flux, has led to an efficiency optimization of the signal processing. The instrumentation ran smoothly and allowed to reach all the experimental objectives. After non destructive examination performed in the Osiris reactor pool, typically gamma spectrometry and neutron radiography, the instrumented rod and the device have been disassembled. Then the instrumented rod has been transported to the LECA facility in Cadarache Centre for post irradiation examination. The internal pressure and volume of the rodlet as well as precise gas composition measurements will be known after puncturing step performed in a hot cell of this facility. That will allow us to qualify the in-pile measurements and to finalize the data which will be used for the validation of the fuel behaviour computer codes.