In nuclear boiling water reactor, an improved lower tie-plate and fuel channel interface for a boiling water reactor fuel bundle is disclosed. The fuel bundle has a lower tie-plate (L) for supporting fuel rods and permitting the introduction of fluid interior of the fuel bundle. An upper tie-plate maintains the lower tie-plate supported rods in side-by-side relation and has apertures for discharging a mixture of water and steam. A polygon sectioned channel (C), preferably square, surrounds the tie-plates and fuel rods for the confining of fluid flow between the tie-plates interior of the bundle. The interface of the channel as it surrounds the lower tie-plate is reconfigured to include means for inducing a rapid pressure drop from the interior juncture of the lower tie-plate and channel to and towards the exterior juncture of the lower tie-plate and channel. Because of this rapid pressure drop the bottom portion of the square sectioned channel is not subject to the pressure loading, and the bottom portion reinforces the upper portion. In one embodiment, a labyrinth seal configuration is made in the region of overlap between channel and the lower tie-plate, being configured either in the lower tie-plate or channel. In a second, preferred embodiment, a diffuser (54) is incorporated in the region of overlap between the channel (C) and lower tie-plate (L). The diffuser causes a pressure distribution in the overlap region which limits the deflection of the channel.
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