Detailed analyses of a late-phase melt progression in the advanced power reactor (APR)1400 were completed to identify the melt and the thermal-hydraulic states of the in-vessel materials in the reactor vessel lower plenum at the time of reactor vessel failure to evaluate the candidate strategies for an in-vessel corium retention (IVR). Initiating events considered included high-pressure transients of a total loss of feed water (LOFW) and a station blackout (SBO) and low-pressure transients of a 0.0009-m2 small, 0.0093-m2 medium, and 0.0465-m2 large-break loss-of-coolant accident (LOCA) without safety injection. Best-estimate simulations for these low-probability events with conservative accident progression assumptions that lead to reactor vessel failure were performed by using the SCDAP/RELAP5/MOD3.3 computer code. The SCDAP/RELAP5/MOD3.3 results have shown that the pressurizer surge line failed before the reactor vessel failure, which results in a rapid decrease of the in-vessel pressure and a delay of the reactor vessel failure time of ~40 min in the high-pressure sequences of the total LOFW and the SBO transients. In all the sequences, ~80 to 90% of the core material was melted and relocated to the lower plenum of the reactor vessel at the time of reactor vessel failure. The maximum value of the volumetric heat source in the corium pool was estimated as 1.9 to 3.7 MW/m3. The corium temperature was ~2800 to 3400 K at the time of reactor vessel failure. The highest volumetric heat source sequence is predicted for the 0.0465-m2 large-break LOCA without safety injection in the APR1400, because this sequence leads to an early reactor vessel failure.