The ACC Configuration Sensitivity Study for AP1000 Large Break LOCA

Similar to the traditional nuclear power plant (NPP), the Accumulator (ACC) of AP1000 is one of the most important facility against Large-Break LOCA (LBLOCA). Following a LBLOCA, the Reactor Coolant System (RCS) pressure will be decreased rapidly. And the Core Makeup Tank (CMT) and Passive Residual Heat Removal (PRHR) will be actuated following “S” signal. However, the transient is so rapid that the CMT and PRHR could not be actuated timely, because the ACC will inject water into the reactor vessel downcomer through Direct Vessel Injection (DVI) line and it will stop the CMT injection immediately when RCS has depressurized to the ACC pressure. Therefore, the ACC configuration is very important to LBLOCA mitigation for AP1000. And the Peak Cladding Temperature (PCT) highly relies on ACC configuration. Several sets of different configuration of both ACC, including initial pressure and initial water volume, are discussed. Different initial conditions (e.g. ACC initial pressure) are considered in the sensitivity study on ACC depressurization phase by phase. WCOBRA/TRAC code was used to perform the LBLOCA sensitivity study. The results of each sensitivity case are presented and analyzed. And the suggestion of how to make the optimal ACC configuration is provided in this paper.