3D field data of flow structures in dead-ended, coolant loop reactor branch lines for CFD validation of thermal fatigue onset modeling

Abstract

Thermal fatigue phenomena in normally stagnant branch lines connected to the primary loop of pressurized water reactors has been identified as a primary cause for at least ten pipe cracking events in the past decade. Current prediction and inspection of at-risk piping is based on semi-empirical models, derived on the basis of low-resolution experimental data. This data does not include sufficient information on contributing factors that lead to thermal boundary development and fluctuation in dead-ended branch lines. Without comprehensive, high-resolution data of the phenomena involved, CFD models attempting to account for the re-laminarization or quasi-laminar flow that persists in the branch line cannot be validated.Validated CFD models for dead-ended branch line flow conditions will greatly expand the predictive capabilities the method currently offers. To build the database necessary for related CFD validation campaigns, 3D high-resolution measurements were made in the branch line of an experimental facility built at the University of Michigan. The facility is a scaled coolant flow loop with a reduced main line diameter. Nuclear power plant coolant loop flow rates near 10 m/s were achieved for study. High-resolution, 3D velocity field data in the dead-ended branch line was obtained using Tomographic Particle Image Velocimetry (T-PIV) techniques. With this data as a basis for validation, CFD models such as Low-Re will be able to accurately predict flow phenomena in dead-ended branch line simulations where the branch line diameter, opening geometry, or piping component (elbow) locations are altered.