The main circulation circuit (MCC) of the RBMK-1500 reactor of the INPP is a system consisting of fuel channels, steam/water and water piping, separator drums, MC pumps, suction, pressure and group distribution headers, and associated piping. The MCC consists of two autonomous loops, each having two separator drums, horizontal steel vessels interconnected at the steam-filled and water-filled ends. Within each drum there is a feedwater header. From each header feedwater is conveyed to downcomer pipes.
The separated water mixed with the feedwater that is already treated, heated and deaerated, along downcomer pipes is conveyed to the suction header, and from there along suction header to four MC pumps, one of which acts as stand-by. The MCPs are of a vertical, centrifugal, single-stage configuration. The nominal capacity of the pump is 8500 m³/hr, speed is 1000 rpm, and electric motor power is 5500 kW. From the MCPs water flows through pressure header pipes to the pressure header. The suction piping of each MCP contains a stop valve, and the pressure piping is equipped with a check, throttling and gate valves. The suction and pressure headers of each loop are connected with six bypass pipes each equipped with a gate and check valve.
The manual valves of the bypass pipes of the INPP are closed. The pressure header is connected with piping to twenty group distribution headers. At the upstream end of each distribution header there is a flow limiter, a check valve and a mixer for the water from the reactor emergency core cooling system. Each distribution header is connected to 40-43 water pipes leading to fuel channels. One loop of the circuit removes the heat of half of the reactor. The coolant is distributed along the fuel channels depending on the heat emission in each channel.