The nuclear fuel used is slightly enriched uranium (2%) in the form of uranium dioxide. This is a chemically-stable and heat-resistant ceramic material. It is prepared in powdered form, pressed into small, 11 5 mm diameter and 15 mm long pellets and sintered in the presence of a binder. The pellet shape is adapted to an intensive, high-temperature operating mode. For example, the pellets have hemispherical indentations, in order to reduce the fuel column's thermal expansion and thermo-mechanical interaction with the cladding. The 2 mm diameter hole through the axis of the pellet reduces the temperature at the center of the pellet, and helps release the gases formed during operation. Fuel assembly parameters are presented at the end of this page.
The pellets are placed into a tube with an outside diameter of 13.6 mm, a wall thickness of 0.825 mm and an active length of 3.4 m. Tube material is an alloy of zirconium with one percent niobium. This alloy has good anti-corrosive properties and a low neutron absorption coefficient. The initial clearance between the UO2 pellets and the wall of the tube varies from about 0.22 to 0.38 mm.
The tubes are pressurized with helium at 0.5 MPa and sealed. In the radial direction, the fuel clad is augmented by retaining rings which help to withstand the pressure of the fuel channel and improve the heat transfer from the pellet to the zirconium tube. In the axial direction, the fuel pellets are held in place by a spring.
At this stage the RBMK reactor fuel differs little from the analogous fuel elements manufactured for the widely prevalent BWR-type reactors. For example, a typical BWR fuel tube in the United States is also manufactured from a zirconium-niobium alloy, has a similar wall thickness and an outside diameter ranging from 12 to 13.5 mm. The uranium enrichment is also similar; namely, 2 % in the case of RBMK-1500, 2.2 to 3 % in the case of the BWR. Greater differences arise only when the fuel elements are' mounted into a structurally integral fuel assembly (or fuel cluster). The shape of the assembly is determined by the geometric characteristics of the core fuel channel. In the case of a BWR this results in a square-shaped (usually 8 x 8) fuel cluster which fits into the square core spaces between the control rod blades. For an RBMK reactor, the fuel assembly must fit into a circular channel having an inside diameter of 80mm and an active core height of 7 m. In order to achieve the required height, two fuel elements must be joined end-to-end. The radial spacial restriction determines the arrangement and the number of the fuel rods which can fit into a fuel assembly.
The assembly contains 18 fuel elements arranged within two concentric rings in a central carrier rod which is a 15 mm diameter tube with a 1.25 mm wall thickness and is made of a zirconium (2.5 % niobium) alloy. The complete fuel assembly is made up of two segments which are joined by means of a sleeve (7) at the central plane. Thus, along the axis of the core there is a region in which fission does not take place. This generates a flattening of the fast neutron flux and a dip of the thermal neutron flux at this location and influences the neutron kinetic characteristics of the core.
The lower segment of the fuel assembly is provided with an end grid and ten spacing grids. The central tube and the end spacer are also made from the zirconium ( 2.5 % niobium ) alloy. The remaining spacers are made from stainless steel and are rigidly fixed (welded) to the central tube and are positioned 360 mm apart. The top segment has 10 spacing grids placed 360 mm apart, and in addition, at every 120 mm this segment is provided with specifically designed spacers which act as turbulence enhancers to improve the heat transfer characteristics. The fuel tubes are mounted so that axial expansion of the upper or lower segments takes place in the direction towards the center of the core. For ease of manipulation, the fuel assembly is provided with appropriate fittings at both ends.
The fuel assemblies described in this section are utilized in two different ways: regular fuel assemblies and other fuel assemblies which contain a neutron flux detector. In an instrumented fuel assembly the detector is contained within a tube which replaces the main carrier rod. This tube has an outside diameter of 15 mm and a wall thickness of 2.75 mm.
Fuel assembly parameters
Fuel pellet
Fuel Uranium dioxide
Fuel enrichment in U-235, % 2
Edge pellet enrichment, % 0.4
Fuel pellet density, kg/m3 10400
Fuel pellet diameter, mm 11.5
Fuel pellet length. mm 15
Pellet central orifice diameter, mm 2
Maximum pellet temperature, C 2100
Fuel element
Fuel element cladding material Zr-1%Nb
Outside diameter of fuel element. mm 13.6
Length of fuel element. m 3.64
Active length of fuel element, m 3.4
Cladding wall thickness. mm 0.825
Clearance between fuel and cladding, mm 0.22-0.38
Mass of fuel pallets within fuel element, kg 3.5
Helium pressure in the cladding, MPa 0.5
Maximum permissible temperature of fuel element, C 600
Average linear thermal flux, W/cm 218
Maximum linear thermal flux, Wlcm 485
Fuel assembly
Number of segments per fuel assembly 2
Number of fuel elements per segment 18
Total length of fuel assembly. m 10.015
Active length of fuel assembly, m 6.862
Fuel assembly diameter (in the core), mm 79
Mass of fuel assembly without bracket, kg 185
Total mass of fuel assembly with the bracket, kg 280
Total steel mass of fuel assembly, kg 2.34
Total mass of zirconium alloy within assembly, kg 40
Mass of uranium within fuel pellets, kg 111.2
Mass of uranium within edge fuel pellets. kg 1.016
Maximum permissible power of fuel channel, MW 4.25
Authorized fuel assembly capacity, MWday/assembly 2500
Authorized lifetime of fuel assembly, year 6