In the framework of studies on the control rods lifetime for Sodium Fast Reactor, three commercial steels were exposed to B4C powder in liquid sodium at 600°C for durations up to 3000 h. Analyses by optical and secondary electron microscopy, electron microprobe and glow discharge optical emission spectrometry revealed the formation of borides layers at the surface of the steels and slight carburization underneath. The growth of the boride layers followed parabolic kinetics. The nature of the formed boride layers was in good agreement with thermodynamic equilibrium predicted by Thermo-Calc software. The carburization depths were much lower than the ones obtained in pure carburizing sodium at 600°C. Finally, the carbon penetration depth did not grow with time revealing possible protective character of the boride layers against carbon penetration.
In order to preserve uranium natural resources and to decrease the quantity of nuclear waste, France is developing a Sodium-cooled Fast Reactor (SFR) prototype named ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration). Currently, research studies are under-going to evaluate the control rods lifetime. In the current concept, the control rods are made of B4C pellets encapsulated in stainless steel clads.1 The B4C pellets stacked in the stainless steel clads are used to absorb neutrons to control the fission rate in the core of the reactor. Feedbacks from former French SFRs revealed that lifetime of the control rolds was limited by the embrittlement of the cladding likely induced by its interaction with B4C.2
With the aim of better understanding this embrittlement, out-of-pile exposure of three different candidate steel grades in sodium containing nuclear grade B4C powder were carried out at 600°C, the maximum expected temperature in the reactor. The nature of the corrosion products was identified and their kinetics of formation were followed through several exposure times up to 3000 h. Particular attention was given to the carbon and boron concentration profiles through the steel surface as carburization and/or boriding were suspected to be responsible for the embrittlement of the control rods in in French SFRs.