Development of a transfer model for design of sodium purification systems for fast breeder reactors

Les pièges froids sont des systèmes de purification du fluide caloporteur sodium indispensables au bon fonctionnement des réacteurs à neutrons rapides. Ils permettent de contrôler la teneur en impuretés du sodium, notamment celles de l’oxygène et de l’hydrogène. Le piégeage de ces impuretés est basé sur leur cristallisation sous forme d’oxyde et d’hydrure de sodium, sur garnissage et sur parois froides. Appréhender le remplissage de ces systèmes de purification permettra d’orienter les choix technologiques en termes de conception et de conduite. L’objectif est de développer un outil d’aide à la conception et à la simulation des pièges froids. Le modèle de cristallisation intègre le couplage des différents phénomènes mis en jeu lors de la purification du sodium, à savoir l’hydrodynamique, transfert thermique et transfert de matière.Operating a Sodium Fast Reactor (SFR) in reliable and safe conditions requires to master the quality of the sodium fluid coolant, regarding oxygen and hydrogen impurities contents. A cold trap is a purification unit in SFR, designed for maintaining oxygen and hydrogen contents within acceptable limits. The purification of these impurities is based on crystallization of sodium hydride on cold walls and sodium oxide or hydride on wire mesh packing. Indeed, as oxygen and hydrogen solubilities are nearly nil at temperatures close to the sodium fusion point, i.e. 97.8°C, on line sodium purification can be performed by crystallization of sodium oxide and hydride from liquid sodium flows. However, the management of cold trap performances is necessary to prevent from unforeseen maintenance operations, which could induce shut-down of the reactor. It is thus essential to understand how a cold trap fills up with impurities crystallization in order to optimize the design of this system and to overcome any problems during nominal operation. The objective is to develop a design and simulation tool for cold traps able to predict the location and the amount of the impurities deposited. Crystallization model involve phenomena coupling in a porous medium with hydrodynamics, heat and mass transfer, distinguishing nucleation and growth phases for each impurity. It enables to understand how thermo hydraulic conditions and growing impurities interact on each other. This analysis will adapt operating and management conditions in order to optimize purification requirements