The aim of this research is to characterize the human behavior, concerning the level adjustment of steam generator (SG) when the operator switches over the flow-lines in PWR nuclear power plants. From the results of the experiments, we observed that the behavior changed as the operation progressed. The process of operation was classified into two categories ; called as "in-progress of operation", and "operation-end". Among "in-progress of operation", there existed two strategies. When the

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... t could take the first strategy, the difference between the level of SG and the reference could be kept low. However, under the second strategy he could only control the flow through the bypass line so that the level of SG became unstable. In "operation-end", it was found difficult to stabilize the level of SG under each experimental condition. Then, we suggested two improvements ; (1) inform that the flow through the bypass line approaches 0, (2) after (1) was performed, decrease input gain of each controller automatically. At the verification experiment with the improvements, it was obtained that the level of SG could be stabilized even in "operation-end", and the validity was confirmed.