[en] In case the electromagnetic pump in the primary sodium purification system of Monju trips up, the sodium flow in the system stops. If we restart the system without stopping the main primary sodium circuit system, the mixing piping structure which connects the branch flow from the purification system to the main flow is subjected to thermal transient load because the cold stagnant sodium is pushed out and the hot sodium from main flow follows. The aim of this study is to assess the fatigue damage of the mixing piping structure in the primary sodium purification system of Monju in case its electromagnetic pump trips up and then it is restarted with keeping plant operation. As the first step we conducted temperature analysis of the mixing piping structure in the primary sodium purification system by FEM by assuming conservative initial temperature condition. Then we conducted stress and strain analysis by FEM based on the temperature analysis results. Finally, we evaluated allowable numbers of trips by this event by calculating the fatigue damage based on the stress and strain analysis results. We considered the difference of flow rate at the electromagnetic pump restart, restarts with or without preheating, etc. as the analysis parameters. From the analysis results, we concluded that [1] the fatigue damages were mainly dependent on the temperature difference between the cold sodium at the initial stage and the hot sodium after restart, [2] the fatigue damages were considerably mitigated by preheating and [3] even the severest case allows more than 400 cycles of this events. (author)