Abstract
Void fraction distribution and its estimation methods in tight-lattice rod bundles for R&D of Innovative Water Reactor for Flexible Fuel Cycle (FLWR) core have been investigated based on many kinds of void fraction data and summarized in this paper. Void fraction data were measured by neutron radiography technique, quick-shut-valve technique, and electro void fraction meter which were developed by JAEA using 7-, 14-, 19- and 37-rod bundle test sections under from atmospheric pressure to 7.2 MPa. The rod geometry in the test sections simulates the FLWR fuel assembly, that is, gap between rods of 1.0 or 1.3 mm, rod diameter of 12-14 mm with triangular tight-lattice arrangement. Mass velocity ranged from 121 to 2000 kg/m^2s, and mainly controlled to 400 - 600 kg/m^2s as nominal condition. And, also spacer effect test was evaluated in this project. In this paper, (1) boiling flow characteristics on void fraction in the tight-lattice rod bundle, (2) comparison of advanced numerical analysis codes with 3D void fraction data and (3) TRAC-BF1 code and drift-flux model for 1D void fraction prediction were discussed. Followings were made clear from this study; (a) lower void fraction was observed at peripheral than at center of rod bundle and higher void fraction was observed at rod gap part of lower core and at center of subchannel of upper core, (b) numerical analysis codes calculated the similar void fraction distribution to the data, and (c) TRAC-BF1 code and drift-flux model tended to overestimate the void fraction at lower quality region, on the other hand at higher quality, those methods tended to same characteristics to the data. It was confirmed throughout the void fraction study that several special features were existed in the tight-lattice rod bundle but a lot of advanced numerical analysis codes can be applied to the tight-lattice core design.
