Effective Models for Simulation of Thermal Stratification and Mixing Induced by Steam Injection into a Large Pool of Water

University dissertation from Stockholm : KTH Royal Institute of Technology

Abstract: Steam venting and condensation in a large pool of water creates both a source of heat and a source of momentum. Complex interplay between these two sources leads to either thermal stratification or mixing. If heat source dominates, development of thermal stratification in a Pressure Suppression Pool (PSP) of a Boiling Water Reactor (BWR) increases temperature of the free surface which reduces the steam condensation capacity of the pool and can lead to significant pressure increase in the containment. If mixing is dominant it is important to know how fast a stratified pool can be mixed to restore the steam condensation capacity and reduce containment pressure. Advanced modeling and simulation of direct contact condensation in large systems remains a challenge as evident in commercial and research codes mainly due to long transients and small time-steps to resolve direct contact condensation on the free surface of steam-water interface. In this work, effective models, namely, the Effective Heat Source (EHS) and Effective Momentum Source (EMS) models are proposed to model and simulate thermal stratification and mixing during a steam injection into a large pool of water. The EHS/EMS models are developed for steam injection through a vertical pipe submerged in a pool under two condensation regimes: complete condensation inside the pipe and chugging. These models are (i) computationally efficient, since small scale phenomena of steam injection and direct contact condensation are not resolved explicitly, and (ii) sufficiently accurate, since the integral effect of these phenomena on the large scale flow structure and temperature distribution in the pool is taken into account. These effective models are implemented in GOTHICĀ® software and validated against POOLEX and PPOOLEX tests. Excellent agreement in averaged pool temperature and water level in the pool between the experiment and simulation has been achieved. The development of thermal stratification and mixing of the pool are also well captured in the simulation. Lastly, a scaling approach is proposed to generalize available data on the effective amplitude and frequency of oscillations during chugging.