Transient two-phase blowdown experiments were carried out to simulate the effects of a Main Steam Line Break accident in a nuclear steam generator. A purpose-designed facility was built for this experimental investigation, which uses R-134a as the working fluid. The experiments are unique in that a sectional model of a nuclear steam generator tube bundle is included in order to study its effects on the two-phase transient. Valuable insights were revealed as to the nature of the dynamic flow phenomena courtesy of a meticulously developed instrumentation and high-speed video acquisition system. A significantly non-uniform two-phase void distribution was observed in the upstream depressurizing liquid region. Two-dimensional effects were largely negligible further downstream. Transient temperature measurements showed significant thermal non-equilibrium between the liquid and vapor phases. The liquid mostly remained in a superheated state whereas the vapor temperatures rapidly dropped towards saturated thermodynamic conditions. In the early stages of the transients, inertia significantly influences vapor growth. The flow through the tube bundle was choked for the majority of the transient. The precision thermo-fluid measurements and detailed high-speed flow images provide a unique experimental database for two-phase transients in a relatively large diameter pipe (15.4cm), with specific application potential in numerical code validation and verification. The analysis and insights given here will offer guidance for further code development and assist in numerical benchmarking and validation efforts for complex transient two-phase flow phenomena. (C) 2018 Elsevier Ltd. All rights reserved.