Evaluation of in-vessel corium retention margin for small modular reactor ACP100

In-vessel corium retention (IVR) through external reactor vessel cooling is adopted in China small modular reactor (SMR) ACP100 as an important severe accident strategy. In the present study, thermal load analyses and IVR margin evaluation for China SMR ACP100 are performed. The thermal load analyses for steady molten pool are carried out based on a two layer final bounding state (FIBS). For FIBS analysis, the maximum heat flux 0.36 MW/m2 is obtained based on conservative input, which is remarkably lower than the SBLB pool boiling CHF. Sensitivity analyses indicate that the maximum heat flux could reach 0.58 MW/m2 if an extreme decay heat is assumed. The FIBS studies show that enough thermal margins could be obtained for ACP100 IVR. Then, transient thermal load analyses are carried out based on the simplified SCDAP/RELAP5 system model and detailed COUPLE model. For transient analysis, the peak value of transient thermal load can be up to 0.37 MW/m2, which is also remarkably lower than the corresponding CHF. However, the maximum q/CHF ratio happens at the bottom of the vessel. The studies of transient thermal load based on SCDAP/RELAP5 also show that the transient heat transfer characteristics during molten pool formation could be divided into two different stages, namely the dramatic heat transfer stage at the beginning and the quasi steady state heat transfer after the crust formation.