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tube assembly. The guide channels held by individual hold-sown springs pass through the
spacer grid cells with a gap ~ 0.3 mm per diameter and are bolted to the perforated plate of
the tailpiece. The guide channels impact the axial force generated by the hold-down
springs on the perforated plate of the tailpiece, while bending moments of the guide chan-
nels are impacted on shaped angle pieces through spacer grids.
Fuel-graded fuel assemblies containing FRs with different concentrations of nuclear
elements are used to feed fuel to the UVR core. Two or three types of FR, which are dif-
ferent in uranium concentration, can be used in the same duel assembly. FRs with reduced
uranium concentration are usually positioned at the corners of the fuel assembly, where
they are «covered» with angle stiffeners, and in the center of the fuel assembly where the
guide channels pass.
Gadolinium integrated into fuel pellets is used in UVR cartridges as the burnable
absorber (such FRs are called FR with a burnable gadolinium absorber). An UVR car-
tridge can comprise from 12 to 24 FRs with the burnable gadolinium absorber (FR-G),
which are evenly distributed across the fuel assembly or arranged in the center close to the
guide channel tubes. Uranium concentration in such FRs is ~ 20 % lower than that in gad-
olinium-free FRs, since their thermal conductivity factor is lower. Natural gadolinium
density in FR-G is 0.3–0.6 g/cm
3
. In order to level the energy distribution along the fuel
assembly (and, first of all, in the «recent» core of the initial fuel), gadolinium distribution
along FR is standardized – the ends of such FR are free of gadolinium (such gadolinium-
free ends are called «cuts-off»).
In the process of the development of the permanent load-bearing frame structure in
the fuel assembly, the following measures were taken to prevent or minimize bending of
the frame and the entire structure of the fuel assembly:
•
angle pieces of the frame are made of radiation-resistant E635 alloy (to reduce
the difference in their height caused by their radiation-induced expansion);
•
spacer grid cells and guide cannels have radial gaps between them;
•
the rim of the spacer grid is designed so that it can compensate for deformity in
case of irregular expansion of the angle pieces and slanting of the spacer grid;
•
spacer grid cells are optimized in terms of the force with which FR is drawn
through a spacer grid cell in case of spacer grid slanting or irregular expansion of the angle
pieces.
Furthermore, the attachment of the guide channels in the fuel assembly to the head-
piece through a system of bushes and individual hold-down springs and the radial gap be-
tween the guide channels and spacer grid cells provide for the thermal and radiation-
induced expansion of the guide channels independently of each other.
Functionality of the major components of the fuel assembly is defined as follows:
•
stiffness of the frame formed by the angle pieces connected to the spacer grids
and tailpiece is fixed and does not change in the process of operation and, thus, the load-
bearing capacity and shape of the frame remain the same during the entire useful life of the
fuel assembly;
•
the guide channels play no role in the frame stiffness and act as elements ensur-
ing a reliable operation of control absorber rods;
•
the gap between a FR passing through a spacer grid cell and the spacer grid cell
wall is enough to reduce its impact on the frame.
