**4.1 Evaluation of existing correlations for fluid mixing phenomena**

Innovative water reactor for flexible fuel cycle (FLWR) is one of new generation light water reactor and has been developed at Japan Atomic Energy Agency (Uchikawa, 2007). In order to achieve a conversion ratio higher than unity, a hexagonal tight-lattice rod bundle with about 1mm of gap width was adopted. Due to narrower rod gaps and the channels being surrounded by rods, bubble/slug-to-bubble/slug and bubble/slug-to-wall interactions may occur more frequently within the FLWRs core than in those of current BWRs, and these may affect the two-fluid mixing characteristics by way of the deformation, separation and coalescence of bubbles/slugs caused by the interactions.
