Abstract: The additional mixing grid has no belt and comprises at least two sets of crossed plates that are fixed to each other at their cross-points, the plates being fitted with fins for stirring the coolant, and delimiting cells, some for receiving guide tubes and the others for receiving fuel rods. Each plate is provided, on each of the faces of those internal cells that receive the rods, with bosses that project sufficiently little to leave radial clearance for the rods passing through the cells and the projection of the bosses is proportioned relative to the projection of the fins so that in the event of bending vibration the rods come into contact with the bosses without reaching the fins, the lengths of the plates being such that the footprint of the additional grid is less than the footprint of the polygon constituting the virtual envelope of the rods.

Abstract: The lower connector comprises an adaptor plate of square shape, traversed by water passage orifices, and a filtration plate pierced with holes of small dimensions and abutting against the adaptor plate. The water passage orifices of the adaptor plate are arranged completely symmetrically in relation to the medians and to the diagonals of the adaptor plate. The set of orifices in each of the zones of the adaptor plate limited by a diagonal and a median comprises orifices have an oblong cross-section and, if appropriate, water passage orifices of a different shape. The filtration plate comprises sets of holes of small dimensions arranged in the zones of the plate which come into alignment with the oblong orifices of the adaptor plate.