Patent Number: 
Section: claims

1. A hold-down spring unit coupled to an upper end of a top nozzle of a nuclear fuel assembly, the hold-down spring unit comprising:a first spring configured to provide a hold-down force upon the nuclear fuel assembly under start-up conditions or hot full power conditions of a nuclear reactor, and includinga first ramp having an upper end and a lower end,a first support part extending from the lower end of the first ramp,a first neck part bent downwards from the upper end of the first ramp and having a predetermined length, anda first neck part slot formed in a lower end of the first neck part and extending in a longitudinal direction along a central axis of the first neck part,a second spring configured to provide an additional hold-down force upon the nuclear fuel assembly under start-up conditions of the nuclear reactor, and includinga second ramp having an upper end and a lower end,a second support part extending from the lower end of the second ramp,a second neck part bent downwards from the upper end of the second ramp and having a length shorter than the predetermined length of the first neck part, anda neck part insert slot receiving the first neck part and formed in a through-hole shape at a bent portion where an upper end of the second ramp and the second neck part are connected to each other; andat least one intermediate spring provided between the first spring and the second spring, each of the at least one intermediate spring, includingan intermediate ramp having an upper end and a lower end,an intermediate support part extending from the lower end of the intermediate ramp, andan intermediate neck part insert slot receiving the first neck part and formed through the upper end portion of the intermediate ramp, anda support end holding the second support part, the intermediate support part and the first support part all together. 2. A top nozzle for a nuclear fuel assembly, the top nozzle comprising:a coupling plate coupled to a guide thimble of the nuclear fuel assembly;a perimeter wall protruding upwards from a perimeter of the coupling plate, with a spring clamp provided on an upper surface of the perimeter wall; anda hold-down spring unit mounted to the upper surface of the perimeter wall, the hold-down spring unit comprising:a first spring configured to provide a hold-down force upon the nuclear fuel assembly under start-up conditions or hot full power conditions of a nuclear reactor, and includinga first ramp having an upper end and a lower end,a first support part extending from the lower end of the first ramp,a first neck part bent downwards from the upper end of the first ramp and having a predetermined length, anda first neck part slot formed in a lower end of the first neck part and extending in a longitudinal direction along a central axis of the first neck part,a second spring configured to provide an additional hold-down force upon the nuclear fuel assembly under start-up conditions of the nuclear reactor, and includinga second ramp having an upper end and a lower end,a second support part extending from the lower end of the second ramp,a second neck part bent downwards from the upper end of the second ramp and having a length shorter than the predetermined length of the first neck part, anda neck part insert slot receiving the first neck part and formed in a through-hole shape at a bent portion where the upper end of the second ramp and the second neck part are connected to each other; andat least one intermediate spring provided between the first spring and the second spring, each of the at least one intermediate spring, includingan intermediate ramp having an upper end and a lower end,an intermediate support part extending from the lower end of the intermediate ramp, andan intermediate neck part insert slot receiving the first neck part and formed through the upper end portion of the intermediate ramp, anda support end holding the second support part, the intermediate support part and the first support part all together. 3. The top nozzle as set forth in claim 1, wherein the first neck part further includes first neck part hooks protruding outwards from opposite sides of a lower end of the first neck part. 4. The top nozzle as set forth in claim 1, wherein the first neck part further includes first neck part hooks protruding outwards from opposite sides of a lower end of the first neck part.