A Study Of The Flow Characteristics Of The Natural Circulation In The PCCS System

As the last barrier of nuclear power plants, the containment can withstand the pressure arising from serious accidents,prevent the leakage of radioactive substances. In a nuclear power plant of the latest generation, the passive containment cooling system (PCCS) is one of the key facilities for preventing the containment from being damaged in serious accidents.The PCCS takes advantage of natural circulation to remove the heat from the core. Therefore,it is very important to study the flow characteristics of the natural circulation in the PCCS,which has a greatly impact on the safe operation of the system. A simplified system was designed to roughly study the behaviors of the natural circulation in the PCCS.To illustrate the principles of PCCS, experimental research was conducted on the flow characteristics of the natural circulation in the loop during startup and steady-state at low pressure and low outlet gas quality. During startup, the loop can be divided into three regions—single-phase region, transition region and stable region, where the stable region can be further divided into transition-I region and transition-? region. In transition-I region, the flow oscillation is caused by geysering. In transition-? region, the flow instability shows different characteristics at different heating power: at low heating power, it is induced by both geysering and flashing; at high heating power by flashing and saturated boiling. In steady-state stage, five typical circulation modes were observed, which are periodical flashing with geysering, flashing with periodical boiling eruption, continuous flashing, boiling eruption and stable circulation of two-phase flow. It was shown that flashing and geysering or boiling eruption are the two key factors determining the flow behaviors, and that the circulation mode changes with the power is due to the different contributions of the two factors accordingly. A flow pattern maps was given,which can provide a reference to the modes in the system.A model for calculating the mean flow rate of the natural circulation in steady-state stage was developed by taking into consideration of driving head, flow resistance, subcooled boiling, flashing and heat loss. The analytical results agreed well with the experiments,proving that the concept of averaged stable flow is feasible. The trend of the circulation flow at high heating power and flow excursion were predicted based on the model. Additionally,the instability of the natural circulation as well as the influence of the loop structure was analyzed.