Rod bundle channel is widely applied as reactor core coolant passage

in which the flow characteristic is of great significance for safe operation of the reactor

it is necessary to do some experimental investigations on it. The flow resistance characteristics experiments were performed under natural circulation condition. It was found that the friction factor is less than predicted value by Cheng & Todreas correlation and has no obvious rise when the flow transition happens

which is different from the results in regular channel. A correlation to calculate the friction factor in laminar regime was proposed and it also has the same form with the correlation in regular channel. In addition

there is still a large transition region for friction factor in rod bundle channel under natural circulation condition. Based on the experimental results with different heat flux

it was found that the friction factor decreased with the rise of heat flux and viscosity revision was considered. As for local resistance at the spacer grid

no obvious flow transition occurred and correlations for local resistance calculation were fitted in different range of Reynolds number.