Nuclear fuel assemblies

Purpose: To effectively cool a fuel even at the time of loss-of-primary coolant accident. Constitution: More than 35% of total fuel rods contained in a fuel assembly are projected at their upper ends above an upper tie plate. Since the diffusion of the air blown up into a space is suppressed due to the presence of the projection in this case and the air diffused to the upper portion of a through hole is thus reduced, water heat pressure becomes large proportionally thereto so that the quantity of the dropped water increases. Accordingly, even if not all the fuel rods are projected at their upper ends but partial fuel rods are merely projected, it can exert the effect of diffusing the air in the space. In this manner, since the quantity of sprayed flow rate into the fuel assembly is increased, the temperature of the fuel cladding tube at the time of failure can be reduced. (Yoshihara, H.)