Development of a thermal–hydraulic safety analysis code RETAC for AP1000

In the present study, a thermal–hydraulic safety analysis code for AP1000 named RETAC (REactor Transient Analysis Code) has been developed using FORTRAN 90 language. A point reactor neutron kinetics model with six groups of delayed neutrons was adopted to describe the core thermal power transient. A distributed parameter model with two-phase drift flux model was used in the U-tube steam generator simulation. In the pressurizer simulation, the RETAC code was equipped with three-region non-equilibrium model and multi-region non-equilibrium model respectively. Similar to current large commercial codes such as RELAP5 and RETRAN series, the four-quadrant analogy curves were adopted for the solution of the transient behaviors of the main coolant pumps. In this paper, a new and reasonable model for the passive residual heat removal heat exchanger (PRHR HX) was proposed based on basic equations of mass, momentum and energy. Gear method and Adams predictor-corrector method were adopted alternately for a better solution to ill-condition differential equations corresponding to detail processes.The PRHR HX inadvertent operation accident and the ADS (automatic depressurization system) inadvertent operation accident were chosen in the transient accident analysis. Furthermore, the simulation results obtained by RETAC were compared with that by Westinghouse-developed LOFTRAN code. The comparison results showed a good agreement and thus proved the accuracy and reliability of the RETAC code. With the adoption of modular programming techniques, the RETAC code can be easily modified and applied to higher power passive safety reactors in the future.

► A thermal–hydraulic analysis code for AP1000 named RETAC was developed.
► The transient accident analysis was performed with the code developed.
► This article is useful for design and operation of AP1000 and other plants.