Nuclear fuel assemblies.

摘要

A nuclear fuel assembly has fuel rods (5) and a lower tie plate (3) supporting their lower ends. Coolant apertures (9, 21-24) extending through the upper face of the tie plate and a channel box (17) surrounds the fuel rods to confine coolant. To restrict leakage of coolant between the channel box and the lower tie plate, the coolant apertures in the tie plate comprise a plurality of first apertures (21) distributed in a first area spaced from the periphery of the upper face, and a plurality of second apertures (22,23,24) smaller in cross-section than said first apertures (21) and distributed in a second area surrounded by the first area. The sizes and spacings of said first and second apertures are such that the open area ratio of the first area is substantially larger than the open area ratio of said second area. The invention also provides venturis in the leakage path between the channel box and the tie plate, to restrict deformation of the tie plate.