DILUTION AND MIXING CHARACTERISTICS OF BORIC ACID IN REACTOR PRESSURE VESSEL WITH DIRECT VESSEL INJECTION

Abstract

Direct vessel injection (DVI) device connected to the reactor pressure vessel has been applied in most passive pressurized water reactors. During the emergency cooling injection process, concentrated boric acid solution is injected to the reactor through the DVI tube, and it mixed non-uniformly with the cooling water from the cold tube in descent section of reactor pressure vessel (RPV). The non-uniform mixing will affect the reactivity distribution of the core and bring core return to critical state. Therefore, it is of great significance to know accurately the boric acid concentration distribution during direct injection in pressure vessel and to study the influence of the mixing process during injection in coolant to improve the safety of reactor operation. According to the structural characteristics of ACP1000 pressure vessel, the 1/6 scaled numerical model was established. The transient flow mixing process of direct injection solution and coolant in the annular descent section and lower chamber of RPV was obtained by numerical simulation method. Variation tendency of boric acid diffusion process changes with the Reynolds number of safety injection fluid and cold pipe fluid. Results show that flow rate ratio of injection fluid and cold pipe fluid mainly affect the mixing degree, and then dominated the concentration distribution of boric acid on the same section. Gravity causes the injected fluid to form an envelope in the vertical direction and gradually dilute and diffuse in the circumferential direction.