A coupled analysis of system thermal-hydraulics and three-dimensional reactor kinetics for a 12-finger control element assembly drop event in a PWR plant

A coupled system thermal-hydraulics (T-H) and three-dimensional reactor kinetics code, MARS/MASTER, was developed to attain more accurate predictions for nuclear system transients that involve strong interactions between neutronic and T-H phenomena. In this paper, a 12-finger control element assembly (CEA) drop event in a two-loop pressurized water reactor (PWR) plant under a full power condition was analyzed, where the 12-finger CEA that is nearest to the hot leg of Loop 2 is assumed to incidentally drop. This instantaneously results in an asymmetric radial power distribution and, in turn, asymmetric loop behavior, which may lead to a reactor trip due to a low departure from nucleate boiling (DNB) ratio at the intact side of the core or an excessive difference between the cold leg coolant temperatures. This event clearly requires a coupled calculation of system T-H and three-dimensional reactor kinetics to realistically investigate the thermal-hydraulic behavior of the reactor core. A simple theoretical modeling is also devised to evaluate the cold leg temperature difference under a quasi-steady state.