Résumé
Sodium Fast Reactors (SFR) are studied in the international generation IV project to preserve uranium resourcesand transmute minor actinides. The french Phenix experimentalreactor has already shown promising results. The reactor poweris commonly calculated simultaneously by heat balance on waterto vapour heat exchanger and by neutron flux measurements.Concentration of some gamma radionuclides in the primarycoolant is also directly correlated with power. Therefore, we thinkthat an on-line gamma measurement system could be settledas another help for generation IV SFR reactor exploitation.The 2°F is a particularly convenient tagging agent to designa fast time response system. This work shows that a gammaspectrometry system measuring the 2°F activity could be set asa power monitoring system for SFR reactor. This system wiD betested at the Phenix reactor with the high count rate and highresolution Adonis spectrometer.