TRACE5 Simulation of Condensation-Induced Water Hammer Phenomena

The purpose of this work is to test the capability of TRACE5 code in the simulation of thermal-hydraulic transients concerning Condensation-Induced Water Hammer (CIWH) phenomena in a horizontal branch pipe connected to the vessel downcomer. The CIWH is produced by the condensation of the steam by subcooled water counterflow in the horizontal pipe, which causes two-phase flow interfacial instability, and is capable of initiating a severe water hammer, possibly leading to significant plant damage. The work is developed in the frame of OECD/NEA ROSA Project Test 2, performed in the Large Scale Test Facility (LSTF) of the Japan Atomic Energy Agency (JAEA) [1]. The purpose is to provide an analytical model for the LSTF installation, in order to evaluate the critical inlet water flow rates and system pressures of CIWH in a long horizontal pipe without using interfacial friction factor or heat transfer coefficients and using the default TRACE criterion of transition from stratified to a slug flow. The analytical model is designed with the thermal-hydraulic code TRACE5 via 1D-components, reproducing the actual branch where the CIWH is produced. A TEE component is connected to a FILL component, which simulates the water injection, and to a BREAK component set to the boundary conditions that simulate the downcomer. Our model uses one-dimensional flow equations and default correlations of interfacial shear stress and heat and mass transfer available by TRACE. Several comparisons are performed, varying pressure system and water injection mass flow rates. Simulated pressure pulses are characterized, studying parameters such as geometry nodalization, time-step effect, Courant number, numerical diffusion, etc. Results show that 1D model slightly underestimates the maximum pressure pulse intensity in all cases considered.