Abstract: Pipe cracking events reported internationally have shown that, in stagnation branch pipes connected to key systems of nuclear power plants, high-temperature fluid from the main pipe can intrude into the low-temperature stagnant fluid during steady operation through turbulent penetration, forming a dynamically migrating hot-cold interface and inducing thermal cycling that threatens the structural integrity of the pipe. Addressing the need for rapid assessment of this type of risk and the current lack of high-temperature test data on stagnant structures, this paper builds a high-temperature test facility for down-horizontal (DH) stagnation branch pipes, conducts turbulent penetration and thermal cycling tests, and compares the Japan Society of Mechanical Engineers (JSME) guideline with the Materials Reliability Program (MRP) guideline. The test results show a clear hot-cold interface in the elbow and horizontal pipe sections, together with wall temperature fluctuations corresponding to typical thermal cycling. Relative to the 60 °C case, the 180 °C case (stagnation flange at 20 °C) raises the Reynolds number by a factor of 2.8 but the density difference between the main and branch pipes by a factor of 7.3; the buoyancy suppression of turbulent penetration then outweighs the enhancement of turbulence intensity, so the penetration length decreases rather than increases with rising main pipe temperature. The comparison shows that the JSME guideline applies only to DH configurations but covers a wide flow-velocity range and yields conservative predictions for small pipe diameters, whereas the MRP guideline covers more configurations and more extensive quantitative data but does not sufficiently account for the temperature dependence of kinematic viscosity. Using the same test data, it is further found that the penetration lengths predicted by both guidelines cover the measured maximum with a margin of about twice the branch pipe inner diameter, yet under identical cases the two guidelines may lead to opposite risk-assessment conclusions. In engineering practice, the envelope of the two predictions should be adopted as a combined criterion according to the operating case and pipe dimensions, so as to improve the reliability of the assessment.