Nuclear fuel assembly and operation process of a reactor including such assemblies

The nuclear fuel assembly includes a bundle of fuel pencils certain of which contain a burnable neutron poison, maintained at the junctions of a regular network by a skeleton having end pieces connected by guide tubes occupying certain of the nodes of the network. All the pencils containing poison are disposed to act on the reactivity of the adjacent assemblies in the reactor core. The invention aims at reducing the overall reactivity disequilibrium of the reactor core during the replacement of a fraction of the fuel by improving the power distribution especially near new assemblies and those having undergone a cycle. The choice of this type of assembly allows a higher burnup of the assembly fissile matter if the same duration of elementary cycle is kept as before. Nevertheless, this type of assembly allows to increase the duration of an elementary cycle