Abstract: A complementary shutdown device for an undermoderated nuclear reactor, comprising, inside certain arrays (3) of the core, guide tubes (7) of which the cross-section is at least equal to the sum of the cross-sections of three rods (6), and which are surrounded by at least one row of fertile rods (8), and a set of shutdown rods, the cross-section of which matches the cross-section of the guide tubes (7). The shutdown is obtained by allowing the shutdown rods, containing a neutron-absorbing material, to fall into the arrays (3) of the core, in the position of maximum insertion. The invention applies, in particular, to pressurized water nuclear reactors.