Validation Of RELAP5 Code To Simulate Flow In Low Pressure Natural Circulation System

The passive safety technology is extensively used in the third-generation NPP, and natural circulation has gained more and more attention as one important mechanism to realize system passive safety. However, natural circulation do well in simplifying system equipment and improving system security, but it also has problems like complex operating conditions,low heat capacity and easy to appear two-phase flow instability. Thus, for a new natural circulation system, it's necessary to carry out sufficient experiment study and numerical simulation on it. Among those methods, RELAP5 as one of the best estimate code in nuclear thermal hydraulic, it has got satisfied record in the calculation of NPP accident and forced circulation system, but its ability to calculate transient behaviors happened in low pressure natural circulation system still has controversy. In view of this, two RELAP5 models were made and calculation were conducted on two sets of low pressure natural circulation system,and the capability of RELAP5 to simulate flow in low pressure circulation system were tested by using experiment results.Firstly, through sensitivity study on calculation models, we found that the choice of heat transfer conditions in heating section will make significant influence on final results. Since experiment system in this paper is heated by mixture gas,and also involve condensation process outside the heating section, assessment will be made on the condensation model in RELAP 5 using experiment data. The results show that, in pure steam condensation condition,the calculated outer tube heat transfer coefficient in the use of Nusselt-Shah model has good agreement with experimental data with a deviation of 4%-11.7%. While in the two steam-air condensation conditions, the condensation heat transfer coefficient predicted by Colbum-Hougen model is lower than that of experiment data by 74%-96% and 67%-94%,respectively, and the deviation increases with an increase in the air mass fraction. The results given by Dehbi empirical correlation have deviation of 5%-25% and 13%-30%, separately.The single-phase flow and two-phase flow in system 1 and 2 were investigated. With the comparison of experimental data and calculated results, the results show that RELAP5 code could give accurate simulation results in single-phase flow condition. With the inlet temperature constant, the deviation of outlet temperature is less than 3% against on experiment results, and derivation of mass flow rate could be ± 16% and ± 5%. For the case of two-phase flow condition, the phenomena of subcooled boiling instability, boiling coupled flashing instability and pure flashing instability could be well predicted. The oscillation frequency from calculation shows good agreement with that of experimental results and the mean flow rate deviation is in reasonable range. But the geyser instability mass flow rate was lower than that of experimental values for the underestimate of outer-tube condensation heat transfer coefficient. In addition, the calculated results show that the RELAP5 code is able to predict instability boundary of system 1 correctly.Based the precious validation, the operation characteristic of system were analyzed using RELAP5,including the influence of air injection process (single-phase injection, instable two-phase flow injection and stable two-phase flow injection), change of system pressure and heat power. The calculation results show that air injection during single-phase flow region could increase the mass flow rate effectively, reduce the instable two-phase flow region and help to improve the effectiveness and stability of system operation. The changing of system pressure will affect the evolution process of system,and the flow instability area decrease with the increase of system pressure. Through the study on the relationship of heating power and mass flow rate,system mass flow rate increase with the heating power under a same inlet temperature and mass flow rate decrease with the increase of inlet subcooled temperature.There is an approximate 1/3 power relation between heat power and mass flow rate.