Drift-flux modeling of void fraction for boiling two-phase flow in a tight-lattice rod bundle

Abstract

The concept of a resource-renewal boiling water reactor (RBWR) that uses spent nuclear fuel as a nuclear fuel has been proposed to reduce the period required for the radiotoxicity of spent nuclear fuel decaying to the same level as natural uranium ore from about 100,000 years to about 300 years. The RBWR core is designed based on the tight-lattice core concept to reduce the ratio of a moderator to fuel. Since the void fraction is one of the most critical design parameters for the RBWR, this study developed the drift-flux correlation applicable to two-phase flow in the tight-lattice rod bundle. The prediction error of the newly developed drift-flux correlation is ±11.7 %. The validated rod bundle geometrical conditions are the number of rods from 4 to 37, the rod spacing from 1.0 to 2.0 mm, the rod diameter from 9.0 to 13.7 mm, the mixture volumetric flux from 0.11 to 211 m/s, and the pressure from 0.1 to 7.2 MPa. The comparison in the distribution parameter between the tight-lattice rod bundle and the conventional BWR rod bundle indicates that the distribution parameter for the tight-lattice rod bundle is larger than that for the conventional BWR rod bundle. The increased distribution parameter in the tight-lattice core may be due to the small rod spacing and the triangular array of the rods. Both of the small rod spacing and the triangular array of the rods tend to increase the distribution parameter.