Numerical investigation of the depressurization performance in blowdown pipe using method of characteristics

Abstract

The coolant leaks quickly after the break of the high-pressure pipeline system. The rapid pressure decrease and fluctuation will further affect the integrity of the pressure boundary and the effective heat removal of the reactor core in the early phase of blowdown. Generally, numerical simulation plays a vital role in the safety analysis of nuclear power plants (NPPs) under severe accidents like loss of coolant accident (LOCA) conditions. In this paper, one-dimensional two-phase hydrodynamic equations and essential closure models are established to describe the internal pipe flow firstly. Then, method of characteristics (MOC) realized by a self-programming code is introduced, which is different from the numerical scheme of system analysis code RELAP5 in the simplification and discretization. Finally, combined with the Edwards blowdown experiments, the two numerical solution methods are compared on modeling, sensitivity analysis, and calculation results under various working conditions. The influence of back pressure variation on the solution of thermal–hydraulic parameters has been considered during the blowdown process. The solution result of RELAP5 is highly related to the set discharge coefficients, which are usually given according to experience. However, the solution result of MOC can be enveloped by that of RELAP5 with different discharge coefficients. In addition, the results solved by MOC are closer and more conservative to the experimental values during the early phase of blowdown with low void fraction fluid. It lays a foundation for the further calculation and safety analysis of the complex loop system under LCOA conditions.