Patent Number: 043303714
Section: claims

1. In a nuclear reactor of the type including a reactor vessel and a core assembly to be maintained in a fixed position within said vessel, a structural arrangement also located in said vessel for supporting said assembly in said fixed position, said arrangement comprising a plurality of components interconnected with one another by both welded joints and mechanical interlocks so as to define a unitary structure which supports said core assembly in said fixed position and which has sufficient structural integrity to support said assembly in a limited downwardly deflected position if all of said welded joints should fail, said plurality of components including a first group of horizontally extending, spaced apart and substantially parallel lower I-beams and a second group of horizontally extending, spaced apart and substantially parallel upper I-beams extending transverse to and across each of the lower I-beams in said first group of specific crossing point whereby to form an overall grillage of I-beams, each of said upper I-beams being interconnected to all of said lower I-beams at their crossing points by welded joints and also by mechanical interlocks such that said lower I-beams will support said upper I-beams in said limited downwardly deflected position in the event of total failure of the welded joints interconnecting said I-beams said components also including a box girder extending circumferentially around said grillage of I-beams, each of said upper and lower I-beams having opposite end sections, each of which is welded to and mechanically interlocked with said box girder at a predetermined point along its circumference. 2. A structural arrangement according to claim 1 wherein said upper I-beams are positioned against said lower I-beams such that the welded joints connecting the lower I-beams are maintained in a state of compression. 3. A structural arrangement according to claim 2 wherein each of said lower I-beams includes a vertically extending, upwardly opening slot at each of its crossing points for receiving and interlocking with an adjacent section of an upper I-beam and wherein each of said upper I-beams includes a vertically extending, downwardly extending slot at each of its crossing points for receiving and interlocking with an adjacent section of a lower I-beam, the slots at each crossing point cooperating with each other such that the top and bottom sides of said I-beams are co-planar. 4. A structural arrangement according to claim 1 wherein said box girder includes an inner circumferentially extending plate including spaced vertical through slots and an outer circumferentially extending plate spaced outwardly from and connected with said inner plate and wherein the end sections of said I-beams extend into said girder through a corresponding one of said vertical slots so as to mechanically interlock with said girder. 5. A structural arrangement according to claim 1 wherein said box girder includes a downwardly facing, horizontally extending circumferential shoulder having an outermost peripheral edge and wherein said components include a circumferential skirt supported by and within said reactor vessel, said skirt having a vertically upwardly facing top end welded to said circumferential shoulder for supporting said girder and grillage of I-beams even if the welded connection between the girder and skirt should fail. 6. In a nuclear reactor of the type including a reactor vessel and a core assembly to be maintained in a fixed position within said vessel as well as control rod position indicators associated with said core assembly, a structural arrangement also located in said vessel for supporting said assembly in said fixed position, said structural arrangement comprising a plurality of components interconnected to one another by welded joints so as to define a unitary structure capable of supporting said core assembly within said vessel, said components also being mechanically interlocked such that a total failure of said welded joints will result in limited downward deflection of said unitary structure when supporting said core assembly while, at the same time, retaining sufficient structural integrity to maintain said assembly in a downwardly deflected but supported position, said downward deflection being sufficiently large to be detected by said control rod position indicators. 7. A structural arrangement according to claim 4 wherein said outer plate includes inwardly extending and upwardly facing shoulder means in confronting relationship with each of said slots and wherein the end sections of said I-beams extending through associated ones of said slots are configured to engage and rest on confronting ones of said shoulders so as to further mechanically interlock with said box girder. 8. A structural arrangement according to claim 7 wherein said outer plate displays a stepped cross-sectional configuration along its entire circumference so as to define said inwardly and upwardly facing shoulder means. 9. In a nuclear reactor of the type including a reactor vessel and a core assembly to be maintained in a fixed position within said vessel, a structural arrangement also located in said vessel for supporting said assembly in said fixed position, said arrangement comprising a plurality of components interconnected with one another by both welded joints and mechanical interlocks so as to define a unitary structure which supports said core assembly in said fixed position and which has sufficient structural integrity to support said assembly in a limited downwardly deflected position if all of said welded joints should fail, said plurality of components including a first group of horizontally extending, spaced apart and substantially parallel lower I-beams and a second group of horizontally extending, spaced apart and substantially parallel upper I-beams extending transverse to and across each of the lower I-beams in said first group at specific crossing point whereby to form an overall grillage of I-beams, each of said upper I-beams being interconnected to all of said lower I-beams at their crossing points by welded joints and also by mechanical interlocks such that said lower I-beams will support said upper I-beams in said limited downwardly deflected position in the event of total failure of the welded joints interconnecting said I-beams, said components also including a box girder extending circumferentially around said grillage of I-beams, each of said upper and lower I-beams having opposite end sections, each of which is welded to and mechanically interlocked with said box girder at a predetermined point along its circumference, said box girder including an inner circumferentially extending plate including spaced vertical through slots and an outer circumferentially extending plate spaced outwardly from and connected with said inner plate, in the end sections of said I-beams extending into said girder through a corresponding one of said vertical slots so as to mechanically interlock with said girder, said outer plate including an inwardly extending and upwardly facing shoulder means in confronting relationship with each of said slots, the end sections of said I-beams extending through an associated one of said slots being configured to engage and rest on confronting ones of said shoulders so as to further mechanically interlock with said box girder; and said outer plate also including downwardly facing outwardly extending circumferential shoulder means, said components including a circumferential skirt supported by and within said reactor vessel, said skirt having a vertically upwardly facing top end welded to said outwardly extending shoulder means for supporting said girder and grillage of I-beams even if the welded connection between said skirt should fail. 10. A structural arrangement according to claim 9 wherein said reactor includes control rod indicators associated with said core assembly and wherein said downwardly deflected position is sufficiently large to be detected by said control rod indicators.