Patent Number: 047708409
Section: claims

1. A method of operating a light water moderated and cooled nuclear reactor having a core consisting of a plurality of adjacent fuel assemblies and having internals arranged to slidably receive only spectral shift fertile rod clusters and neutron absorption control rod clusters at predetermined locations, for movement into and out of said core, comprising the steps of: (a) operating said reactor during at least one life cycle of the core with a core consisting of fuel assemblies of a first type, each fuel assembly of said first type having a plurality of fuel elements distributed at nodes of a regular polygonal array, some of those nodes of the array which are devoid of fuel elements being occupied by guide tubes so located as to slidably receive one of said spectral shift fertile rod clusters and said neutron absorption control rod clusters while all other nodes of the array which are devoid of fuel elements are continuously occupied during operation only by water, the number, size and spacing of said fuel elements being so selected that the core operates with thermal neutrons when said spectral shift fertile rod clusters are out of said core, operating said reactor with said fertile rod clusters in said core during an initial part of the life cycle of the core and with said fertile rod clusters withdrawn from said core after said cycle has partly elapsed, whereby a shift of the neutron energy spectrum down to the thermal range is achieved, wherein said step of operating said reactor includes operating said reactor in an arrangement wherein there is no provision for insertion of spectral shift rods or neutron absorption control rods into said nodes of the array which are continuously occupied only be water during reactor operation, and  (b) operating said reactor with a conversion rate which is higher than during said at least one life cycle during a subsequent life cycle after some at least of the fuel assemblies in said core are substituted with fuel assemblies of a second type each having the same plurality of fuel elements distributed at the same nodes of the same polygonal array as in the fuel assemblies of the first type, additional fuel elements located at those nodes of the array which were continuously occupied during operation only by water in said fuel assemblies of the first type, and guide tubes occupying the same nodes as the guides tubes in said fuel assemblies of the first type, operating said reactor with spectral shift during said subsequent life cycle, wherein said step of operating said reactor includes operating said reactor in an arrangement wherein there is no provision for insertion of spectral shift rods or neutron absorption control rods into said nodes of the array which are continuously occupied only by water during reactor operation.  (a) operating said reactor during at least one life cycle of the core with a core consisting of fuel assemblies of a first type each fuel assembly of said first type having a plurality of fuel elements distributed at nodes of a regular polygonal array, containing U235-enriched uranium, some of those nodes of the array which are devoid of fuel elements being occupied by guide tubes located to slidably receive one of said spectral shift fertile rod clusters and neutron absorption rod clusters while all other nodes of the array which are devoid of fuel elements are continuously occupied during operation only by water, the number, size and spacing of said fuel elements being so selected that said core operates with thermal neutrons when said spectral shift fertile rod clusters are out of said core, operating said reactor with said fertile rod clusters in said core during an initial part of the life cycle of the core and with said fertile rod clusters withdrawn from said core after said cycle has partly elapsed, whereby a shift of the neutron energy spectrum down to the thermal range is achieved, wherein said step of operating said reactor includes operating said reactor in an arrangement wherein there is no provision for insertion of spectral shift rods or neutron absorption control rods into said nodes of the array which are continuously occupied only by water during reactor operation, and  (b) operating said reactor with a conversion rate during a subsequent life cycle which is higher than during said at least one life cycle, with a core consisting of a combination of fuel assemblies of the first type and fuel assemblies of a second type, each of said assemblies of the second type having the same plurality of fuel elements distributed at the same nodes of the same polygonal array as in the fuel assemblies of the first type, additional fuel elements located at those nodes of the array which were continuously occupied during operation only by water in said fuel assemblies of the first type, and guide tubes occupying the same nodes as the guide tubes in said fuel assemblies of the first type, operating said reactor with spectral shift during said subsequent life cycle and all fuel elements in said assemblies of the second type containing fuel material consisting of plutonium-enriched natural or depleted uranium, wherein said step of operating said reactor includes operating said reactor in an arrangement wherein there is no provision for insertion of spectral shift rods or neutron absorption control rods into said nodes of the array which are continuously occupied only by water during reactor operation. 2. A method according to claim 1, coprising operating said reactor during said subsequent life cycle entirely in undermoderated condition. 3. A method according to claim 2, comprising operating said reactor during said one life cycle with a moderation ratio in the range of from 1.5 to 2 and operating said reactor during said subsequent life cycle with a moderation ratio in the range of 1.1 to 1.4 with fuel assemblies containing fertile material only at the periphery of the core. 4. A method according to claim 1, wherein each of said fuel assemblies is associated with a respective one of said rod clusters, the number of spectral shift fertile rod clusters being approximately twice the number of neutron absorption control rod clusters. 5. A method of operating a pressurized light water moderated and cooled nuclear reactor having a core consisting of a plurality of adjacent fuel assemblies and having internals arranged to only receive a plurality of spectral shift fertile rod clusters and a plurality of neutron absorption control rod clusters at predetermined locations, for movement into and out of said core, comprising: