Radium is a naturally occurring radioactive element commonly found at low levels in natural systems such as lacustrine or marine sediments. Anthropogenic activities including former uranium mining activities can lead to the dissemination of radium isotopes having high radiological toxicities, which potentially threaten the safety of impacted nearby environments. Although radium mobility in oxidized environments is known to be largely governed by sorption/desorption onto Fe and Mn oxyhydroxides and coprecipitation with sulfate minerals (e.g. barite), little is known regarding its behavior under reducing conditions, which are the conditions typically encountered in organic-rich systems such as wetlands and sediments. The present study aims at understanding the behavior of long-lived radium isotopes (226Ra and 228Ra), during early diagenetis of lake sediments contaminated by former uranium mining activities. Solid and pore water concentrations of 226Ra and 228Ra were determined using ultra low background gamma spectrometry, which allowed us to improve detection limits and measurement accuracy. This study shows that the downcore distribution of radium isotopes is closely related to the reductive dissolution of iron and manganese oxyhydroxides below the sediment-water interface. The resulting diffusive fluxes of 226Ra and 228Ra (4.1 10-25 and 4.7 10-28 mol.cm-2.s-1) are however significantly lower than other radium-impacted environments, such as uranium mill tailings pond and phosphate industry-impacted sediments, and are similar to those reported for natural marine environments. Hence, in the reduced lacustrine sediments of Saint-Clement, the major fraction of radium trapped by the solid phase, while early diagenesis only induces a slight mobility of this radioelement.