ABSTRACT
Fuel rods in the BWR core are held in position by tie plates at the bottom and by grid plates at the top, and in between by spacers. Such obstructions in a channel that are designed in order to improve the mixing between the bubble layer and the liquid near the fuel wall cause an additional pressure drop and influence the subcooled boiling behavior. However, the influence of these obstructions has not been evident especially on the inception of subcooled boiling. In the present work, the effect of the turbulent mixing caused by the obstructions was analyzed experimentally, and the mechanism to retreat the initiation front of subcooled boiling downstream was clarified. Experiments were performed using a small boiling loop with a vertical annular channel test section that had a heated SUS tube simulating the fuel rod. Video image data of incipient boiling were analyzed by the image processing technique, and compared with numerical analyses carried out to simulate the incipient subcooled boiling with low void fraction and its upstream region of single phase flow. From the work, it was cleared that, in the case of short distance from the tie plate as the upstream obstruction, the initiation of the subcooled boiling was suppressed, and the position of boiling front moved downstream, that depends on thermohydrodynamic conditions and the obstruction arrangements.
