Flushing of systems with the open reactor has commenced at Unit 1 of Rooppur NPP in Bangladesh. The this stage, the chemically demineralized water is supplied to the reactor vessel through the connection pipeline and primary circuit pipelines, the water removes impurities left after the installation of the equipment and the pipelines. Flushing of active and passive safety systems with the open reactor is one of the most important pre-commissioning operations. This is the first of a whole series of inspections of equipment and process systems, it is the result of the long-term work of assemblers, adjusters and other specialists involved in the construction of the first nuclear power plant in Bangladesh. The flushing to make sure that everything has been installed correctly and is ready for operation. Besides, it checks the operability of pump units of the process safety systems and normal operation systems.