Experimental study on thermal cycling phenomena and load response in downward-horizontal (DH) stagnant branch pipes of nuclear power plants

Thermal cycling phenomena refer to periodic fluctuations of the cold-hot interface in pipes of nuclear power plants, which induce temperature fluctuations and cause fatigue damage, threatening long-term operational safety. The downward-horizontal (DH) stagnant branch pipe is commonly found in nuclear power plants and is particularly susceptible to this phenomenon. Several related incidents have been reported internationally, making it necessary to study its characteristics for developing engineering protection strategies. This study investigated thermal cycling phenomena in a DH stagnant branch pipe using a high-temperature, high-pressure thermal-hydraulic test facility. The geometry of the test section was designed through numerical simulation and iterative debugging to capture thermal cycling in a large stagnant branch pipe, and pipe wall temperatures and strains at key locations were measured synchronously. The synchronous measurement provides direct experimental evidence of the cyclic loading induced by thermal cycling, which in previous studies was mainly inferred from numerical simulations. Experiments covered steady and pulsating flow conditions, with temperatures ranging from 60 to 260 °C and main pipe flow velocities from 3.1 to 10 m/s. analysis of data from 14 test cases shows that, under typical conditions, thermal cycling exhibits a sawtooth-shaped fluctuation characterized by rapid rise, temporary stabilization, and slow decline, with superimposed multi-time-scale variations. Under thermal cycling, pipe wall stresses at the elbow and horizontal sections show highly synchronized periodic fluctuations with temperature, confirming that the phenomenon generates significant fatigue loading on the structure. The cold-hot mixing region can be divided into three distinct zones along the pipe, whose extents are influenced by the temperature and flow velocity of the main pipe. Excessively low or high temperature differences between the main and branch pipes both dampen the fluctuations, the former for insufficient mixing drive and the latter for buoyancy-dominated suppression. Within a sensitive intermediate temperature-difference range, however, the cold-hot interface becomes unstable due to turbulent penetration and heat dissipation, leading to complex flows and intensified fluctuations. A higher steady main pipe velocity enhances turbulent penetration, while a pulsating main flow causes the cold-hot interface to undergo repeated reconstruction-breakdown-reconstruction cycles, amplifying flow instability and significantly increasing temperature and load fluctuations. Dynamic flow variations should therefore be avoided during plant operation. The results provide experimental data and a mechanistic basis for the numerical validation, fatigue assessment, and engineering protection of DH stagnant branch pipes against thermal cycling