The position of the reactor core and its main components in a RBMK-1500 plant is shown below in the schematic cross-section through the main reactor building.
The core of the reactor is housed in a 25 m deep, 21 x 21 m cross-section concrete vault. The core volume is dominated by a large cylindrical graphite stack (1).The graphite stack is constructed of closely packed graphite blocks stacked into columns and provided with an axial opening. Most of the openings contain fuel channels. A number of them also serve other purposes (e.g. instrumentation, regulations) and they are referred to as "special channels".
The graphite stack is located in a hermetically sealed cavity consisting of cylindrical walls and top and bottom metal plates. The entire reactor cavity is filled with a helium (about 40 % by mass) and nitrogen mixture which prevents graphite oxidation and improves heat transfer from the graphite to the fuel channels. In order to prevent loss of helium, the space surrounding the cylindrical graphite stack is filled with nitrogen at a pressure of about 0.2-1.2 kPa greater than that of the helium-nitrogen mixture. In the radial direction as well as above and below the reactor it is surrounded by the primary biological shield structures.
The coolant channels penetrating the reactor core are divided into two essentially independent cooling loops: one containing 830, the other 831 vertical channels with fuel assemblies. Each loop is provided with four main circulation pumps, one of which is kept in reserve during normal operation.
Water from the main pump pressure header (9) is distributed by means of regulating valves to the feeder pipes (3), these then lead to the individual fuel channels. The water rises past the fuel assemblies. attains its saturation temperature, partly vaporizes (average steam quality is 23 %) and, in the form of a steam-water mixture, flows through the header pipes (13) to the separator drums (16). Here the two-phase mixture is separated, and the steam continues to the turbines. The condensate passes through deaerators. returns to the separator drums where it mixes with the unvaporized water. From there the condensate flows via standpipes to the pump suction header (11). and then to the main circulation pumps (7), which return it to the fuel channels.
As noted in previously, one of the important characteristics of RBMK reactors is their online refueling capability. Refueling at full reactor power is accomplished by means of the fueling machine (15). Under normal operation and nominal reactor power, it is feasible to change up to two fuel assemblies per day (24-hours). The maximum capacity of this machine is 5 fuel assemblies per day.
These systems provide integral information regarding the operation of the entire reactor and information regarding specified reactor core segments and individual fuel channels. Because of the very large size of the core and the resulting weak neutronic coupling of distant core segments, reactor operation requires a detailed spatial resolution of the main operational parameters.