The current status of primary coolant technology for VVER reactor types is outlined. Importance of the water coolant technology in fuel and materials integrity and radiation field formation is emphasized. Different types of VVER water chemistry (standard, modified and hydrazine) are reviewed and compared. A brief description of the radiation situation at some VVER units is given. Experimental work carried out at the Nuclear Research Institute Rez on the basic understanding of hydrazine water chemistry phenomena is described.