Dynamic analysis on reactivity oscillation by the movement of the piston in integrated space nuclear reactor

Autonomous Circulation Miniature Integrated nuclear Reactor (ACMIR) adopts a compact integrated design, putting the helium-cooled reactor core and the Stirling cycle into the same pressure vessel, however, the periodic reactivity oscillation introduced by the movement of the piston of Stirling engine has a significant influence on the power stability. If it is not suppressed, the reactor core power will continue to rise, which will affect safe operation. In this paper, the SARAX code system was used to calculate the core parameters in both steady state and transient. The results show that the core power cannot rely on its own reactivity feedback to maintain longterm stability. In order to analyze the factors that affect the core power, this paper discusses three factors that may affect the power oscillation, including the magnitude of reactivity oscillation, magnitude of reactivity feedback and initial core reactivity. Reducing the reactivity oscillation can slow down the rate of power increase, but cannot fundamentally keep the core power stable. Enhancing the reactivity feedback or keeping the core subcritical while the piston passing equilibrium position is effective to make the core stably operated.