Simulation of a main steam line break accident using a coupled “system thermal-hydraulics, three-dimensional reactor kinetics, and hot channel analysis” code

For a realistic analysis of thermal-hydraulics (T-H) transients in light water reactors, KAERI has developed the best-estimate T-H system code, MARS. The code has been improved from the consolidated version of the RELAP5/MOD3 and COBRA-TF codes. Then, the MARS code was coupled with a three-dimensional (3-D) reactor kinetics code, MASTER. This coupled calculation feature, in conjunction with the existing hot channel analysis capabilities of the MARS and MASTER codes, allows for more realistic simulations of nuclear system transients. In this paper, a main steam line break (MSLB) accident is simulated using the coupled “system T-H, 3-D reactor kinetics, and hot channel analysis” features of the MARS code. Two coupled calculations are performed for a demonstration. First, a coupled calculation of the “system T-H and 3-D reactor kinetics” with a refined core T-H nodalization is carried out to obtain the global core power and the local departure from nucleate boiling (DNB) ratio (DNBR) behaviors. Then, for a more accurate DNBR prediction, another coupled calculation with subchannel meshes for the hot channels is performed. The results of the coupled calculations are very reasonable and consistent so that they can be used to remove the excessive conservatism in the conventional safety analysis.