The main component of the channel is the coolant-carrying tube constructed from separate end and center segments. The center segment (9) is an 88 mm outside diameter (4 mm thick wall) tube. made from a zirconium-niobium alloy (Zr + 2.5 % Nb). The top (3) and bottom (11) segments are made from a stainless steel tube. The choice of zirconium-niobium for the center part was made because of the relatively low thermal neutron absorption cross-section (0.2 - 0.3 E-29 m2) of the material and its adequate mechanical and anti-corrosive properties at high temperatures (up to 350 xC). The center and end pieces are joined by special intermediate couplings, made from a steel-zirconium alloy.
The fuel channel tubes are set into the circular passages which consist of the aligned central openings of the graphite blocks and the stainless steel guide tubes of the top and bottom core plate structures. The channel tubes are welded to the top (2) and bottom (10) metal-structure plates to maintain the core region hermetically sealed. The tube is welded to a support ledge (4) at the top, and at the bottom to the guide tube of the metal structure (11 ). A bellows (12)is used to compensate for the differences in thermal expansion between the reactor metal core plates and the rue channel. In case of failure of the bellows, additional sealing is provided by a pressure seal (13). The design life of the channel tube is about 20-25 years. If necessary, the channel tube can be replaced by removing the top and bottom welds.
The fuel coolant tubes also provide cooling for the energy deposited in the graphite moderator of the core region. In order to improve heat transfer from the graphite stack, the central segment of the fuel channel is surrounded by the 20 mm high split graphite rings (8). These rings are arranged next to one another in such a manner that one is in contact with the channel, and the other with the graphite stack block. The minimum clearance between the fuel channel and the graphite ring is 1.3 mm, and between the ring and the graphite stack - 1.5 mm. These clearances prevent compression of the fuel channel tube due to radiation and/or thermal expansion of the graphite stack.
The fuel assembly is suspended in the center of the channel by means of a bracket (5). The bracket is provided with a seal plug (7), which hermetically seals the fuel channel tube after the fuel assembly is installed. Since all work related to sealing, unsealing and fuel changing is accomplished by remote control via the refueling machine, the seal plug must have an appropriately simple design. Fuel channels may also contain supplementary absorbers. They may also be devoid of structural elements and just filled with cooling water.
