The advanced accumulator in the advanced pressurized-water reactor is a system used to inject coolant into the reactor in the event of an emergency such as a loss-of-coolant accident, and can automatically change the injection flow rate from high to low using fluidics technology without moving parts. Its performance was demonstrated in a 1/2-scale injection test at the operating pressure. This paper describes the numerical analysis performed to investigate the generation mechanism of fluid resistance that reduces the flow rate in small flow injection. We show that the characteristics of the accumulator in small flow injection do not depend significantly on the scale of the test apparatus.