Detailed evaluation on reactor vessel upper head voiding formation during pwr npp rapid natural circulation cooldown

摘要

Since system analysis code such as RETRAN-02 for reactor vessel upper head (RVUH) voiding formation was not accurate enough, successfully developed was a code named RVUHVF that is used to analyze the RVUH voiding formation during rapid natural circulation cool down. In this paper included are the establishment of physical models, the selection of numerical methods, the debugging and verification of the code and the application of it in a modern U.S. Combustion Engineering Power Plant. The fundamental conservation equations about the various control volumes and the heat conductors are established in the physical models. The approach of backward differential named Gear that is generally used to stiff problem is chosen for the solution of the ordinary differential equations that derived from the physical models. Considering the RETRAN-02 system analyses, we have developed detailed evaluation and sensitivity research in this special paper for 43 different calculation conditions of the RVUH as an independent isolated plenum during rapid natural circulation cool down transient of the plant. The results calculated by the RVUHVF show that the RVUH region will be the hottest point of the system and the coolant in this region will be saturated. In the paper, it was concluded that during rapid natural circulation cool down, at any calculation condition, the minimum water level in RVUH is above the upper guide structure plate and the temperature of the liquid in the reactor vessel upper plenum is still lower than the saturation temperature at the corresponding pressure. So, the RVUH voiding formation during rapid natural circulation cool down of the plant is not great enough to impair the reactor coolant circulation or core cool ability. In this paper also made was some sensitivity research.