Patent Number: 047626720
Section: claims

1. A fast breeder reactor comprising: a core having a core region encompassed by a blanket region; at least one hollow guide tube disposed inside said core and permitting the passage of a coolant therethrough; control rod means including a hollow cover member and a hollow elongated sleeve connected in series with said cover member at a lower part thereof, said control rod means adapted to be inserted into said guide tube for vertical movement therein; control rod driving means for vertically moving said control rod means via said cover member; said control rod means including a plurality of absorber rods each encased in said cover member, a neutron absorber encased in each of said absorber rods, said hollow sleeve filled with a gas and hermetically sealed with respect to said coolant, the interior of said hollow elongated sleeve having a vertical length of 10% to 100% of the vertical length of said core region and having a total volume greater than the total volume of neutron absorber encased in said plurality of absorber rods, the interior of said hollow elongated sleeve being void of coolant and solid material, said hollow elongated sleeve enabling an increase of core resistivity of said fast breeder reactor when positioned within said core region. 2. A fast breeder reactor according to claim 1, wherein the interior of said absorber rod communicates with the interior of said hollow elongated sleeve through an opening. 3. A fast breeder reactor comprising: a core having a core region encompassed by a blanket region, at least one hollow guide tube disposed inside said core and permitting the passage of a coolant therethrough, a coolant passing through said core; control rod means arranged for insertion into said core and vertically movable with respect to said core; control rod driving means for driving said control rod means vertically with respect to said core; said control rod means including absorber rod means disposed in a neutron absorption region of said control rod means and an elongated hollow sleeve in series with said neutron absorption region below said neutron absorption region, neutron absorber encased in said absorber rod means, said elongated hollow sleeve being filled with a gas and hermetically sealed with respect to said coolant, the interior of said elongated hollow sleeve having a vertical length extending from 10% to 100% of the vertical length of said core region, having a total volume greater than the total volume of all of the neutron absorber of said absorber rod means disposed in said neutron absorption region, the interior of said elongated hollow sleeve being void of coolant and solid material, said elongated hollow sleeve enabling an increase of core reactivity of said fast breeder reactor when positioned within said core region. 4. A fast breeder reactor according to claim 3, wherein the interior of said absorber rod means communicates with the interior of said elongated hollow sleeve through an opening.