INFLUENCE OF ROD-PITCH ON VOID FRACTION DISTRIBUTION IN AN UNHEATED 5 × 5 ROD BUNDLE UNDER HIGH PRESSURE AND LOW FLOW RATE CONDITIONS

摘要

The void fraction distributions in two unheated 5 × 5 rod bundle test sections (10 mm OD (common), 13 mm or 12 mm pitch width) were experimentally investigated by a three-dimensional time-averaged X-ray CT system. This test simulated the thermal-hydraulics conditions of the boil-off process in a BWR (boiling water reactor) fuel bundle. The steam and water were supplied from the multi-purpose steam-water test facility known as HUSTLE (Hitachi Utility Steam Test Leading Facility). Total mass flux was lower than 500 kg/m~2/s (natural circulation mass flux rate of BWRs) and pressure ranged from 5.0 to 7.5 MPa(abs), including the operating pressure of BWRs. The following conclusions were obtained: (1) The volume-averaged void fractions evaluated by the X-ray CT system well agreed with those evaluated by a differential pressure transducer. (2) A comparison of the void fractions evaluated experimentally with those obtained by the Ishii correlation (M. Ishii, ANL-77-47, 1977) showed good agreement at high total mass fluxes. On the other hand, at low total mass fluxes (less than 150 kg/m~2/s), the void fractions evaluated by the Ishii correlation were larger than those evaluated experimentally. (3) The void fractions in a narrow-rod-pitch-type rod bundle became larger than those in a reference-rod-pitch-type rod bundle.