[en] In order to operate in reliable and safe conditions a Sodium Fast Reactor, it is necessary to master the quality of the coolant. The chemical control of sodium is performed versus the different chemical compounds : oxygen (corrosion control), hydrogen (detection of the sodium-water reaction), and to a less degree carbon (carburization, decarburization phenomena). Moreover, other detrimental effects could be avoided: plugging of narrow sections, loss of heat transfer efficiency in heat exchangers, contamination and dosimetry,...Oxygen and moisture are introduced mainly during handling operations; hydrogen is due to aqueous corrosion of the Steam Generator Unit and thermal decomposition of hydrazine, used to control the oxygen content in the water. The purification of oxygen and hydrogen is performed satisfactorily thanks to cold traps and mainly because of the fact that the solubilities are nearly nil for temperatures close to the fusion point, i.e. 97.8^oC. This paper deals with the developments of the purification systems for the primary and intermediate circuits of a SFR. Basis studies were carried out to understand the basic mechanisms of crystallization of sodium oxide and sodium hydride and to establish their respective nucleation and growth kinetics. Then, new concepts were developed for Phenix, Superphenix and EFR in order to provide high efficiency, large filling rate and reliability in operation. The methodology of development, based on calculations, tests on mock-up, will be addressed. Thanks to these developments, sodium purification was never considered as a key issue during steady state operation and purification campaigns necessary to deal with the main pollutions ie sodium-water reaction or air ingress. The sodium purification always allowed to fulfil the operational requirements. (author)