Hold-down spring unit for top nozzle of nuclear fuel assembly having improved hold-down performance and top nozzle for nuclear fuel assembly having the same

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;
• and
• a 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 including
• a 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, and
• a 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 including
• a 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, and
• a 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;
• and
• at least one intermediate spring provided between the first spring and the second spring, each of the at least one intermediate spring, including
• an intermediate ramp having an upper end and a lower end,
• an intermediate support part extending from the lower end of the intermediate ramp, and
• an intermediate neck part insert slot receiving the first neck part and formed through the upper end portion of the intermediate ramp, and
• a support end holding the second support part, the intermediate support part and the first support part all together.