Oxygen diffusion analysis of lead-bismuth-cooled natural-circulation reactor

Lead-Bismuth Eutectic (LBE) coolant has been selected as one of the possible options for fast breeder reactors (FBR). To control a corrosion of structural materials in LBE is a key issue for LBE-cooled reactor feasibility, so that oxygen concentration is necessary to be controlled in the appropriate range. A concentration diffusion analysis code (COCOA: COncentration COntrol Analysis code) was developed to evaluate diffusivity and controllability of oxygen in the LBE-cooled natural-circulation reactor design. The controllability was investigated through the sensitivity surveys on two ordinary models of oxygen control (ON-OFF and PID methods) and detection regions of oxygen concentration, which is referred in the control system. It is concluded that the oxygen concentration should be detected at the center of the secondary vortex near the inner shell for both control method to decrease the peak concentration below the upper constraint. The PID control method can decrease the peak concentration 400 s faster than the ON-OFF control method, when concentration is detected at the center of the vortex. It is also concluded that the diffusion and the equilibrium coefficients in LBE have negligible effects on the oxygen controllability through the sensitivity survey. It is prospected that oxygen concentration control is possible even in the natural-circulation reactor. (author)