[en] A feedback control system of primary pump is proposed for a PIUS-type reactor based on the temperature distribution in the lower density lock. This control system maintains the fluid temperature at the axial center of the lower density lock at the average of the fluid temperatures below and above the lower density lock in order to prevent the poison water from penetrating into the core during normal operation. The effectiveness of this control system was examined in a series of startup tests under different conditions. It has been confirmed from the test results that the control system is quite effective to start up the reactor from a very small fluid temperature difference between the primary loop side and the borated water pool side and even from an isothermal condition by using a preheater installed at the exit of the primary pump. (author)