Complementary shutdown device for an undermoderated nuclear reactor

Claim 1

In an undermoderated nuclear reactor comprising a core immersed in a moderating fluid, (a) a plurality of fissile fuel assemblies arranged side by side in upright position in said core and having a casing and a bundle of parallel elongated tubular rods grouped in said casing and containing fissile material, (b) a plurality of fertile fuel assemblies arranged side by side in upright position around said fissile fuel assemblies in said core and having a casing and a bundle of parallel elongated tubular rods grouped in said casing and containing fertile material, (c) wherein a first group of said fissile fuel assemblies includes a plurality of first guide tubes each substituted for one of said rods of fissile material and arranged to slidably receive control rods movable along said guide tubes for adjusting the reactivity of said core, and a second group of said fissile fuel assemblies including a plurality of second guide tubes having a larger cross-section than said first guide tubes, each substituted for at least three of said rods of fissile material and arranged for each receiving a shutdown rod of non-fission strongly neutron-absorbing material having a cross-section approximating that of said guide tube, each said second guide tube being adjacently surrounded by at least one layer of fertile rods, and (d) means for retaining said shutdown rods above and out of said core and allowing said shutdown rods to fall into said second guide tubes into a position where they exhibit maximum neutron absorption in said core.