Void Redistribution Measurement Downstream of Part-length Rods in Rod Bundle with High-Energy X-Ray CT

Abstract

The fuel rod bundle of boiling water reactors (BWRs) is implemented with part-length rods (PLRs) to improve the pressure drop and optimize the power profile. The complex rod bundle structure can affect the void fraction distribution. Prediction of void fraction distribution is essential for achieving high burnup of fuel rods and optimizing the thermal margins of the rod bundle. This paper focuses on detailed void behavior downstream of PLRs under BWR operating pressure conditions. The test section is a 5×5 rod bundle with five PLRs arranged in the center, side, and corner of the rod bundle. The heated lengths of the full-length rod and PLR are 3708 mm and 1854 mm, respectively, and the power profile is uniform in both axial and radial directions. A boiling flow experiment is conducted at a pressure of 7.2 MPa with inlet mass flux, inlet water temperature, and rod bundle power as test parameters. The cross-sectional void fraction distributions are measured with a minimum axial interval of 25 mm using a linear accelerated-driven high-energy X-ray CT system with a working distance of 4 m in the axial direction and a pixel resolution of 80 microns in CT reconstructed images. The experimental results show the void redistribution behavior downstream of the PLRs, i.e., the void fraction reaches a localized maximum just downstream of the PLR end and then decreases due to the sudden expansion of the flow channel. Further downstream, it recovers to the comparable void fraction of the surrounding subchannels.