Patent Number: 051241137
Section: claims

1. A pressurized water nuclear reactor comprising: a vessel;  a core in said vessel consisting of a plurality of fuel assemblies arranged side by side and vertically, immersed in pressurized light water forming the moderator and cooling fluid of the reactor, each of said fuel assemblies having a bundle of fuel rods each having a central portion containing enriched fuel material and upper and lower end portions containing fertile material, whereby two layers of neutron absorbing fertile material are formed at the lower part of the core and the upper part of the core, throughout the cross-section of the core and further having a plurality of guide tubes parallel to said fuel rods and distributed in said fuel assemblies;  a plurality of control rod clusters containing neutron absorbing material and movable vertically along some only of said guide tubes in certain ones of said plurality of fuel assemblies for control of the power of the reactor;  a filling of metallic material reflecting high energy neutrons arranged at the circumference of the core and over the entire height thereof;  a plurality of neutron energy spectrum variation rod clusters associated with certain ones only of said fuel assemblies distributed throughout the core, each containing fertile material and vertically movable between a position of full insertion into some only of said guide tubes in the associated one of said fuel assemblies and a position out of said guide tubes; and  a plurality of actuating devices each associated with one of said spectrum variation rod clusters for movement thereof between said two positions to vary the ratio of water to the volume of fissile material in the core and to shift the neutron energy spectrum, said spectrum variation rod clusters being distributed regularly throughout the cross-section of the core,  wherein some of the fuel assemblies are continuously devoid of some control rod clusters and said spectrum variation rod clusters and plugs are located in all those guide tubes which do not receive one of said control rod clusters and spectrum variation rod clusters.  a vessel;  a core in said vessel consisting of a plurality of fuel assemblies arranged side by side and vertically, immersed in pressurized light water forming the moderator and cooling fluid of the reactor, each of said fuel assemblies having a bundle of fuel rods each having a central portion containing enriched fuel material and upper and lower end portions containing fertile material, whereby two layers of neutron absorbing fertile material are formed at the lower part of the core and the upper part of the core, throughout the cross-section of the core and further having a plurality of guide tubes parallel to said fuel rods and distributed in said fuel assemblies.  a plurality of clusters consisting of a plurality of control rod clusters containing neutron absorbing material and movable vertically along some only of said guide tubes in first ones of said plurality of fuel assemblies to control the power of the reactor, said first ones of said fuel assemblies being distributed regularly throughout the core, and of a plurality of clusters of neutron energy spectrum variation rods containing fertile material, all rods of a same cluster being vertically movable between a position of full insertion into all those guide tubes of a same one of said first fuel assemblies which do not receive said control rod clusters and a position out of said guide tubes;  whereby all said fuel assemblies other than said first fuel assemblies in the core are continuously devoid of said clusters;  and a plurality of actuating devices each associated with one of said spectrum variation rod clusters for movement thereof between said two positions to vary the ratio of water to the volume of fissile material in the core and to shift the neutron energy spectrum.  a vessel;  a core in said vessel consisting of a plurality of fuel assemblies arranged side by side and vertically, immersed in pressurized light water forming the moderator and cooling fluid of the reactor, each of said fuel assemblies having a bundle of fuel rods each containing enriched fuel material and plurality of guide tubes;  a plurality of clusters of control rods containing neutron absorbing material and movable vertically along some only of said guide tubes in first ones of said plurality of fuel assemblies for control of the power of the reactor, said first fuel assemblies being distributed regularly throughout the core;  a plurality of clusters of neutron energy spectrum variation rods containing fertile material, all rods of a same cluster being vertically movable between a position of full insertion into those guide tubes of a same one of said first fuel assemblies which do not receive said control rod clusters and a position out of said guide tubes;  all of said fuel assemblies other than said first fuel assemblies in the core being devoid of said clusters of control rods and clusters of spectrum variation rods and having plugs closing the guide tubes thereof;  and a plurality of actuating devices each associated with one of said spectrum variation rod clusters for movement thereof between said two positions to vary the ratio of water to the volume of fissile material in the core and to shift the neutron energy spectrum.  all of said fuel assemblies are identical, and  substantially all of those fuel assemblies which are arranged to receive said spectrum variation rod clusters are also arranged to receive said control rod clusters while substantially all of those fuel assemblies which are continuously devoid of one of said spectrum variation rod clusters and control rod clusters are also continuously devoid of the other of said spectrum variation rod clusters and control rod clusters during operation of the reactor. 2. A pressurized water nuclear reactor comprising: 3. A nuclear reactor as claimed in claim 2, wherein each of said upper and lower end positions comprises pellets of U235 depleted uranium dioxide arranged in tubular sheaths of said fuel rods. 4. A nuclear reactor as claimed in claim 1, wherein said filling is of stainless steel. 5. A pressurized water nuclear reactor as claimed in claim 1, wherein each of said fuel rods has a sheath and a stack of pellets in said sheath, the pellets at the lower part and at the upper part of said sheath being of U235 depleted uranium dioxide while the central portion of said sheath contains U235 enriched uranium dioxide. 6. A pressurized water nuclear reactor as claimed in claim 2, wherein substantially one fuel assembly out of two in the core is associated with one spectrum variation rod cluster and one control rod cluster and the arrangement in the cross-section of the core of the fuel assemblies associated with said rod clusters and the fuel assemblies without said rod clusters is according to a checker board pattern. 7. A pressurized water nuclear reactor as claimed in claim 2, wherein all of said fuel assemblies are identical. 8. A pressurized water nuclear reactor comprising: 9. A pressurized water nuclear reactor as claimed in claim 8, wherein one fuel assembly out of two in the core is associated with one spectrum variation rod cluster and one control rod cluster and the arrangement in the cross-section of the core of the fuel assemblies associated with said clusters of control rods and said clusters of spectrum variation rods and the fuel assemblies without said clusters of control rods and said clusters of spectrum variation rods is according to a checker board pattern. 10. A pressurized water nuclear reactor as claimed in claim 8, wherein all of said fuel assemblies are identical. 11. A pressurized water nuclear reactor according to claim 1, wherein: