Of the 2052 channels which penetrate the reactor block, 1661 of them are loaded with fuel, and the remainder, including additional channels in the radial reflector, are termed "special purpose channels" and contain control rods or various types of instrumentation. The reactivity control system channels, the channels carrying vertical traverse flux distribution instruments and the fissiling chamber channels are all identical.
The upper segment of the guide tube incorporates a compensator bellows (2) to accommodate the significant thermal expansion of the channels due to the large temperature difference between the top biological shield and the cold guide tubes. The lower section of the channel guide tube, unlike that in the fuel channel, also contains a thermal expansion compensator (6). The top and bottom sections are made of stainless steel, while the middle section is of a zirconium - niobium alloy.
The reactor control and protection system (CPS) channels are equipped with a metal cover which is employed for mounting the control rod drive mechanisms and for providing access to cooling water. The lower ends of the ionization chamber and flux detector channels are sealed by metal caps. The ionization chamber channel caps are made of stainless steel, and also serve to support the ionization chambers. The caps of the other channels are made of an aluminum alloy. At the bottom of this type of a channel is a throttling device, which provides some resistance to water flow, and helps to ensure reliable filling of the channel.
The CPS channels are cooled by an independent water circuit equiped with its own pumps and heat exchangers. The cooling water is supplied to the channels from above, and flows over the exterior and interior casings of the absorber rods. In this way, the water is heated from 40 C to a temperature of 70 C. During reactor operation, regardless of the position of the control rod, the inside of the channel is filled with water. When the absorber rod is withdrawn from the core. and if no special provisions are taken, its volume would be replaced by water. Because water is a moderately strong neutron absorber. most control rods have not only a boron carbide absorber, but also a graphite follower which displaces the water and improves the reactor's neutron balance.
The above-described channels are always filled with either a control rod, or a graphite control rod follower and cooled with water. Post-Chernobyl modifications often include the addition of fast-acting scram-type rods. To achieve higher insertion speeds, rod drops into a gas-filled channel without having to displace water which leads to a different rod design and requires some changes in the top and bottom fittings of the special purpose coolant channels. These channels are cooled by a flowing film of water and by internally supplied Nitrogen. Therefore, the top cover includes both access to the liquid and the gas coolants and a distributor which creates the falling liquid film. The bottom cap is provided with a liquid collector.
Cooling channels also provide cooling to the radial reflector of the graphite stack. These channels also cool the radial reflector fastening rods and reduce the heat flow toward the shell of the graphite stack and its compensator. The channel is made of stainless steel. The cooling water is supplied to this channel from above through a central tube. It flows down to the bottom biological shield, then rises again to the top in the annular space between the two tubes, and finally leaves the channel.
Radial reflector cooling channels. CPS channels, and fuel channels are inside the central part of the graphite columns, the openings of which are of 114 mm diameter. Total number of channels inside the graphite columns is 2052.
The corners of the rectangular cross-sections of the graphite columns in the stack are hollow and incorporate 17 vertical 45 mm diameter instrumentation channels used for measuring the temperature 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. One channel of the same type is placed in the core and is used for gas sampling. Consequently, there are 18 channels in the graphite stack of the reactor which are outside the graphite columns.
ln the radial biological shielding of the reactor there are a total of 24 channels into which radial ionization chambers are positioned. Reactor startup channels are made of a shell inside of which there is a suspension bracket and a convex lead shield (its mass is 122D kg) which protects startup ionization chambers from the gamma radiation. The normal reactor operation ionization chamber channels are quite analogous to the cannels mentioned above except that there are no lead shields. This is because at normal reactor operation the measured neutron flux significantly exceeds the gamma radiation.