The Influence of the Wall Stored Energy on the Natural-Circulation Mode in the Core Make-Up Tank Test Facility

Abstract

Scaled down thermal-hydraulic test facility is widely used in the nuclear reactor safety analysis. The wall stored energy is one of the common problem in a scaled down test facility, which will cause distortions in the simulation of the transients or local phenomena. For the natural-circulation mode in the Core Makeup Tank (CMT), the cold wall will work as heat sink and absorb heat from the heated water, and then has an influence in energy balance on the simulation of the prototype phenomena with scaled down test facility. In order to study the influence of the wall stored energy on the natural-circulation mode in the CMT test facility, this paper established the CMT test facility model with RELAP5/MOD 3.4, and studied the influence caused by the wall stored energy distortion. The simulation results show that the natural-circulation process in the CMT test facility can be divided into two stages: 1. Before the thermal stratification reaches the outlet, the natural-circulation flow rate decreases gradually. 2. Once the thermal stratification reaches the outlet, the natural-circulation flow rate has an apparent turning point and decreases faster. That is because the heated water reaches the outlet and fills the discharge line quickly, which reduces the density difference between the hot leg and cold leg, thus causing the natural-circulation flow rate decrease faster. Besides, before the turning point, the wall stored energy basically has no influence on the natural-circulation process, and the influence can be neglected in the design of test facility. However, after the turning point, the wall stored energy distortion results in a slower descent speed of natural-circulation flow rate, which is not conservative. Therefore, we can conclude that the scaled down CMT test facility can only simulate the prototype properly and conduct the natural-circulation simulation experiment before the thermal stratification reaches the exit, while afterwards, the simulation of the test facility won’t be accurate and conservative.