Fuel assembly spacer grids

The fuel assembly spacer grids are fabricated either of Inconel or of Zircaloy materials, depending on fuel assembly type and manufacturer. While the central grids are located in the region of maximum neutron flux, the upper and the lower... 

Figure 3.27. Specific activities of activation products in PWR fuel assembly spacer grids (according to Robertson, 1990)...

C-E Report, 1975, C-E Critical Heat Flux Correlation for C-E Fuel Assemblies with Standard Spacer Grids, Part I, Uniform Axial Power Distribution, CENPD-162, Combustion Engineering Co., Winsor, CT. (5)... 

As shown in Fig. 12, the active core is made up of 241 fuel assemblies, all of which are mechanically identical. As indicated by Fig. 13. each fuel assembly contains 236 Zircaloy-clad, UO2 fuel rods retained in a structure consisting of Zirculoy spacer grids welded at about 15-inch (38.1-centimeter) intervals to five Zirealoy control element assembly guide tubes which, in turn, are mechanically fastened at each end to stainless... 

It is necessary to implement advanced uranium- gadolinium (U02-Gd203) fuel assemblies with Zr alloy guide tubes (GT) and Zr alloy spacer grids (SG) for continuous improvements of WWER-1000 fuel utilization. It is expected to improve fuel assembly dimensional stability and to save uranium consumption by increasing uranium utilization. 

Ukrainian NPPs have been operating advanced fuel assemblies with Zr alloy-110 and Zr alloy-635 guide tubes (GT) and Zr alloy-110 spacer grids (SG) since 1995. According to the calculations the substitution of the steel by Zr alloy in materials of guide tubes and spacer grids increases the fuel utilization efficiency by 8.2%. Advanced fuel implementation allows to increase fuel burnup by 5-7%. 

The fuel rods are joined to the fuel assembly by the fuel assembly structure (see Fig. 1.7.), which connects the fuel assembly top and bottom end pieces. In an 18 X 18 array, 300 fuel rods are fixed in position by the spacer grids, with nine of them being distributed over the length of the bundle. In earlier designs the material of the spacer grids was Ni-coated Inconel X 713 for reasons of neutron economy and in order to minimize the amount of material forming long-lived radionuclides in the neutron field, Zircaloy-4 is now commonly used. Only the very small spacer... 

The reactor core contains 349 hexagonal fuel assemblies, each of them consisting of 129 fuel rods with a diameter of 9.1 mm and a length of 3.21 m the fuel rods are kept in position by 15 honeycomb-type spacer grids which are fixed on a central channel. Seventy-three of the fuel assemblies contain movable control assemblies with boron steel as an effective material in the V213 fuel assemblies, six of the fuel rods are replaced by fixed burnable poison rods. 

In the fuel subassembly, fuel pins are assembled with grid spacers and a top shield is installed to prevent activation of the EM pumps and the secondary sodium in the MX. Coolant inlet modules located beneath the fuel subassembly provide a lower shielding for the reactor internal structures including the core support plate and air in the RVACS. 

Core type and characteristic dimensions Homogeneous, with two zones of different enrichment. The active core region is 1150 mm in diameter and 1000 mm high with a central channel of 220 mm diameter. The core consists of approximately 14 000 fuel elements, combined by a core support grid and several spacer grids, and assembled into a fuel cartridge. 

The coupled neutronics/thermo-hydraulic/thermo-structural reactivity feedback design approach for the STAR-H2 reactor has achieved the proper ratio between that reactivity which is vested in the coolant temperature rise relative to inlet temperature vis-a-vis that reactivity which is vested in the fuel temperature rise above the coolant, and at the same time in having designed an overall coolant flow circuit pressure drop tailored to cause coolant flow rate to adjust properly to changes in pressure driving head caused by source/sink temperature difference. A non-conventional open-pitch ductless fuel assembly structural design coupled with a non-conventional core support approach (the assemblies tend to neutral buoyancy in the dense Pb coolant) has been proposed to simultaneously provide low pressure drop, structural reliability of grid spacers, and an appropriate value for coolant power/flow reactivity temperature coefficient. 

Only minor deformation of fuel assemblies has been observed at German PWRs as a result of radiation induced creep. This caused damages at some fuel assemblies (FA) spacer grids during refueling, but did not affect movement or fast insertion of control rods. New improved FAs have been constructed to minimize bending. For this purpose, very stiff blind guide thimbles have been implemented within the FAs. 

Wide (or open ) fuel rod lattices, application of spacer grids to fix fuel rod bundles, fuel assemblies without shrouds (ducts), a cluster-type control and protection system, developed for light water reactors [XXIII-6]. 

The use of wide fuel rod lattice and spacer grids allows the manufacture of a RBEC-M fuel assembly as a dismountable component, which could also facilitate implementation of the IAEA safeguards. In this case, there is an opportunity to secure all the fuel rods by measurements and to exclude the possibility of an undeclared replacement of intermediate fuel rod rows by pins with fertile materials. In the case of a non-dismountable fuel assembly design, the sensitivity of contemporary methods of measurement for nuclear materials allows measurements only of the external fuel rod rows and up to two internal fuel rod rows (if a guide tube for control in the fuel assembly center is available). 

The fuel rod free volume is filled with helium at 1 MPa pressure. A gas plenum of 500 mm height is located in the lower part of the fuel and fertile rods to mitigate the effect of fission gas release. Because of the high pin pitch-to-diameter ratio in the RBEC-M core, fuel and fertile rods are fixed with spacer grids (Fig. XXIII-8). Fuel assemblies of all three active core zones have no shrouds (ducts). A subassembly of the lateral blanket has a shroud and differs from the core fuel assemblies by a lower number of higher diameter pins. 

Mean mass of uranium per assembly, kg Spacer/support grids per sub-assembly Active fuel length, mm... 

Material property (MP) correlations for irradiated fuel, cladding, and other assembly components (e.g., grid spacers) have been provided in the Material Properties Handbook that was developed under this initiative. This document also includes material property correlations for the materials in the generic burnup cask (GBC)-32. 

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