Additional grid for a nuclear reactor fuel assembly, and assembly comprising an application thereof

A fuel assembly comprising:

• (a) a support structure having two end fittings;
• (b) guide tubes interconnecting said end fittings;
• (c) structural grids distributed along said fuel assembly, each structural grid having at least two mutually crossed sets of parallel plates and an outer belt defining cells, some of said cells receiving said guide tubes and others of said cells receiving fuel rods, said plates being provided with means for limiting movment of said fuel rods relative to nodes in a regular array, and being fixed to at least some of said guide tubes, at least some of said structural grids being provided with fins for mixing streams of coolant;
• (d) at least one coolant mixing grid placed between two structural grids in a downstream portion of said fuel assembly, said coolant mixing grid being devoid of a belt and comprising at least two crossed sets of mutually parallel plates fixed to each other at cross-points of said plates, said plates being provided with coolant stirring fins and defining cells, some of said cells receiving said guide tubes and others of said cells receiving fuel rods;
• and
• (e) each of said plates being provided, on each of faces of each of said cells receiving fuel rods, with bosses that project radially inwardly into said cell by an amount which is sufficiently small to leave radial clearance for a respective rod passing through said cell, an amount of projection of said bosses being proportioned relative to a projection of said fins so that in case of bending vibration, the respective rod comes into contact with said bosses without reaching said fins, lengths of individual plates being such that a footprint of said grid is less than a polygon constituting a virtual envelope of said rods.