The graphite stack of the RBMK-1500 reactors serves several functions. The primary one is neutron moderation and reflection, but it also provides structural integrity and in the event of a temporary cooling malfunction, a relatively large heat capacity.
The graphite blocks are assembled within the inner cavity of the reactor on a supporting metal structure. The stack can be visualized as a vertical cylinder, made up of 2488 graphite columns, constructed from various types of graphite blocks. The blocks are rectangular parallelepipeds, with a base of 0.25xO.25 m, and heights of O.2, 0.3, 0.5 and 0.6 m of which the 0.6 m blocks are most common. The short blocks are used only in the top and bottom end reflectors, as required to provide a staggered fit to neighboring columns. The total mass of graphite is about 1700 tons. The material must meet stringent purity requirements and has a density of 1650 kg/m3.
The outer edge of the graphite stack is covered by a metal liner. The four rows of columns at the outer edge make up the radial reflector, and a 0.5 m thick layer at the top and bottom make up the end reflectors. The blocks possess a 0.114 m diameter bore opening through the vertical axis. This provides a total of 2052 channels which are used for placing fuel clusters, reactivity regulating control rods and several types of instruments into the core. In the remaining 436 columns located within the radial reflector the central holes are filled by graphite rods, increasing the density and neutron reflecting effectiveness of this part of the graphite stack. The graphite columns rest on a steel support plate (5) which, in turn, is supported by a steel bushing (4). The bushing is welded to the top plate of the bottom biological shield. At the top of the stack. the columns are fastened and centered with respect to the guide pipes (9) welded into the top biological shield, by means of shield plates (7) and junction sleeves (8).
Radial creep of the graphite stack is restrained by 156 hollow reinforcing bars (10). These bars are positioned in the peripheral columns of the radial reflector. At the bottom, the reinforcing bars are welded to the support plate, while at the top they fit loosely into the guide tubes welded to the bottom plate of the top biological shield. This connection at the top allows freedom for thermal expansion. Since the reinforcing bars are hollow, they also serve as reflector cooling channels. Cooling water to these channels is supplied from above. The reinforcing bars are made from stainless steel tubes, with outside diameters of 0.110 m and wall thickness of 5 mm.
The corners of the rectangular cross-sections of the graphite columns in the stack are hollow and incorporate 17 vertical 45mm diameter instrumentation channels used for measuring the temperatures of the graphite stack itself as well as the support and the shielding plates. Thirteen of these channels are positioned within the boundaries of the core, while four are in the radial reflector. Within each channel the temperature is measured at 5 vertical positions.
The graphite stack, including its hermetically sealed cavity. is called the sealed reactor space. This space is filled with a circulating helium-nitrogen mixture at a pressure of 0.49 kPa. During normal operation, the gas is supplied by means of 0.3 m tubes, and removed through the fuel channel integrity monitoring system. Four drainage tubes (inside diameter of 0.147 m) are provided in order to guard against accidental releases.
When the reactor is in operation, all the components listed above are subjected to conditions of high temperature and intense neutron/gamma radiation. For example, the temperature of the support structures in the top part of the bottom biological shield reaches 350 C. The temperature of the bottom support plates reaches 440 C, while the maximum calculated graphite temperature is 750 C.