Patent Number: 047160071
Section: claims

1. A spectral shift pressurized-water reactor comprising: a pressure vessel enclosing a reactor core which includes fissile material fuel, said pressure vessel having an inlet and an outlet for circulating water coolant moderator in heat transfer relationship with said core, said core comprising a plurality of square-shaped adjacent fuel assemblies vertically disposed therein for generating heat by nuclear fission, and said fuel assemblies having a fuel enrichment which provides a measure of excess reactivity at the beginning of core life which is later drawn upon to lengthen core life;  a plurality of spaced vertical guide tubes disposed in each of said fuel assemblies and adapted to have rod members vertically moved therein and therefrom during reactor operation, and a portion of said guide tubes in each said fuel assembly disposed in a cross-like configuration along the two diagonals which connect the corners of said square-shaped fuel assemblies;  three separate types of rods adapted to be moved into and out of said guide tubes, a first type of said rods comprising neutron-absorbing control rods which are movable into and out of said core so that movement of said control rods into said core will substantially decrease reactivity and withdrawal of said control rods from said core will substantially increase reactivity, a second type of said rods comprising neutron-spectral-shift displacer rods which have a substantially lower absorptivity for neutrons than said control rods, each said neutron-spectral-shift displacer rod comprising a hollow thin-walled Zircaloy member containing a filling of solid or annular zirconium- or aluminum-containing material for providing internal support and mass for said thin-walled tubular member, each said displacer rod having overall neutron-absorbing and -moderating characteristics essentially not exceeding those of hollow tubular Zircaloy members with or without a filling of zirconium oxide pellets or aluminum oxide pellets, the third type of said rods comprising thick walled gray rods each of which have an absorptivity for neutrons intermediate that of each of said control rods and each of said neutron-spectral-shift displacer rods, and said rods all having substantially the same cross-sectional dimension;  said gray rods and said control rods operable to be moved into and out of said core in a portion of said guide tubes which are positioned in said cross-like configuration, and said neutron-spectral-shift displacer rods operable to be moved into and out of substantially all of the remainder of said guide tubes, approximately half of said fuel assemblies operable to have only said neutron-spectral-shift displacer rods moved therein and therefrom, those of said fuel assemblies into which only said neutron-spectral-shift displacer rods are to be moved being alternated in position in said core with those of said fuel assemblies into which said control rods and said gray rods are to be moved, the total number of said neutron-spectral-shift displacer rods very substantially exceeding the total number of said control rods and said gray rods, and the total number of said control rods substantially exceeding the total number of said gray rods;  spider members and associated shafts and drive members therefor positioned above said core, a separate spider member provided for substantially all of each of said fuel assemblies, each of said spider members being separately controllable and having only one type of said rods connected thereto in the form of a rod cluster, said control rods and said gray rods being connected to said spider members in the form of control-rod and gray-rod cross-like clusters to move into said guide tubes which are similarly disposed, and said neutron-spectral-shift displacer rods connected to said spider members as composite clusters which interfit into substantially all said guide tubes in a single fuel assembly in addition to those proximate guide tubes of adjacently positioned fuel assemblies so that those spiders which have said displacer rod clusters connected thereto serve one fuel assembly in addition to proximate portions of those fuel assemblies which are positioned adjacent thereto; and  each said neutron-spectral-shift displacer rod cluster having a total reactivity worth when fully inserted into said core, each said gray rod cluster having a total reactivity worth when fully inserted into said core, and the total reactivity worth of each said neutron-spectral-shift displacer rod cluster substantially exceeding the total reactivity worth of each said gray rod cross-like cluster so that during reactor operation predetermined reactivity worths can be obtained by movement of said displacer rod clusters and said gray rod clusters.  a pressure vessel enclosing a reactor core which includes fissile material fuel, said pressure vessel having an inlet and an outlet for circulating water coolant moderator in heat transfer relationship with said core, said core comprising a plurality of square-shaped adjacent fuel assemblies vertically disposed therein for generating heat by nuclear fission, and said fuel assemblies having a fuel enrichment which provides a measure of excess reactivity at the beginning of core life which is later drawn upon to lengthen core life;  a plurality of spaced vertical guide tubes disposed in each of said fuel assemblies and adapted to have rod members vertically moved therein and therefrom during reactor operation, and a portion of said guide tubes in each said fuel assembly disposed in a cross-like configuration along the two diagonals which connect the corners of said square-shaped fuel assemblies;  three separate types of rods adapted to be moved into and out of said guide tubes, a first type of said rods comprising neutron-absorbing control rods which are movable into and out of said core so that movement of said control rods into said core will substantially decrease reactivity and withdrawal of said control rods from said core will substantially increase reactivity, a second type of said rods comprising neutron-spectral-shift displacer rods which have a substantially lower absorptivity for neutrons than said control rods, each said neutron-spectral-shift displacer rod comprising a hollow thin-walled Zircaloy member containing a filling of solid or annular zirconium- or aluminum-containing material for providing internal support and mass for said thin-walled tubular member, each said displacer rod having overall neutron-absorbing and -moderating characteristics essentially not exceeding those of hollow tubular Zircaloy members with or without a filling of zirconium oxide pellets or aluminum oxide pellets, the third type of said rods comprising thick walled gray rods each of which have an absorptivity for neutrons intermediate that of each of said control rods and each of said neutron-spectral-shift displacer rods, said control rods and said neutron-sepctral-shift displacer rods and said gray rods when fully inserted into said core displacing an equivalent volume of water, and the volume of said water coolant moderator displaced by said neutron-spectral-shift displacer rods when fully inserted into said core very substantially exceeding the volume of said water coolant moderator which is displaced by said control rods and said gray rods if fully inserted into said core;  said gray rods and said control rods operable to be moved into and out of said core in a portion of said guide tubes which are positioned in said cross-like configuration, and said neutron-spectral-shift displacer rods operable to be moved into and of substantially all of the remainder of said guide tubes, approximately half of said fuel assemblies operable to have only said neutron-spectral-shift displacer rods moved therein and therefrom, those of said fuel assemblies into which only said neutron-spectral-shift displacer rods are to be moved being alternated in position in said core with those of said fuel assemblies into which said control rods and said gray rods are to be moved, and the volume of said water coolant moderator displaced by said control rods if fully inserted into said core substantially exceeding the volume of said water coolant moderator displaced by said gray rods when fully inserted into said core;  spider members and associated shafts and drive members therefor positioned above said core, a separate spider member provided for substantially all of each of said fuel assemblies, each of said spider members having only one type of said rods connected thereto in the form of a rod cluster, said control rods and said gray rods being connected to said spider members in the form of control-rod and gray-rod cross-like clusters to move into said guide tubes which are similarly disposed, and said neutron-spectral-shift displacer rods connected to said spider members as composite clusters which interfit into substantially all said guide tube in a single fuel assembly in addition to those proximate guide tubes of adjacently positioned fuel assemblies so that those spiders which have said neutron-spectral-shift displacer rod clusters connected thereto serve one fuel assembly in addition to proximate portions of those fuel assemblies which are positioned adjacent thereto; and  each said neutron-spectral-shift displacer rod cluster having a total reactivity worth when fully inserted into said core, each said gray rod cluster having a total reactivity worth when fully inserted into said core, and the total reactivity worth of each said neutron-spectral-shift displacer rod cluster substantially exceeding the total reactivity worth of each said gray rod cross-like cluster so that during reactor operation predetermined reactivity worths can be obtained by movement of said displacer rod clusters and said gray rod clusters. 2. The reactor as specified in claim 1, wherein the substantial majority of said fuel assemblies which have only displacer rods movable therein and therefrom are surrounded by four adjacent fuel assemblies, and those of said spiders and associated shafts and drives which serve said fuel assemblies which have only displacer rods movable therein also control the movement of displacer rods into the proximate guide tubes of said four adjacent fuel assemblies. 3. The reactor as specified in claim 1, wherein said reactor has chemical shim control capability. 4. The reactor as specified in claim 1, wherein the total reactivity worth of each said displacer rod cluster is approximately four times the total reactivity worth of each said gray rod cluster. 5. A spectral shift pressurized-water reactor comprising: 6. The method as specified in claim 5 wherein said gray rods comprise material having an overall neutron absorptivity approximating that of hollow heavy walled stainless steel tubes.