| 摘要 |
PURPOSE:To prevent an increase in pressure drop and to increase the water to fuel volume ratio in a fuel assembly by providing a diameter changing part of a streamline shape which is gradually decreased in diameter from the lower to the upper part between the upper part and the lower part of an ultra-large- diameter water rod. CONSTITUTION:This fuel assembly is smaller in the diameter of the upper part of the water rod W than the lower part and is provided with the diameter changing part of the streamline shape between the upper part and the lower part of the water rod. The length of wetted perimeter can be decreased by forming the upper part of the water rod to the diameter smaller than the diameter of the lower part in such a manner as compared to the case in which the diameter of the water rod is held uniform to the diameter of the lower part. The pressure drop is thus decreased. The smaller pressure drop in a high void region is more preferable in terms of the reactor core safety and, therefore, the reactor core safety is greatly improved by forming the upper part of the water rod to the diameter smaller than the diameter of the lower part. The water to fuel volume ratio in the fuel assembly is thus increased while the reactor core safety is improved. |
