In the severe accident (SA) event of core meltdown, the ‘In-vessel retention (IVR)’ mitigation has been certified as an effective strategy for accident management in most advanced nuclear power plants. The traditional concept of IVR was based on the idea that the reactor pressure vessel (RPV) was fully submerged into the cooling water. However, the Fukushima accident showed that the cooling water was insufficient due to the malfunction of water supply system, so the RPV structural behavior had not been appropriately assessed. Therefore, the paper tries to address the structure-related issue on determining whether RPV safety can be maintained or not with the effect of various water levels created by the SA condition. In achieving this goal, the structural behaviors were numerically investigated in terms of several field parameters, such as temperature, deformation, stress (strain) and damage. Due to the melting pool on the inside and water cooling on the outside, the high temperature gradient was formed across the wall thickness, so RPV failure was found to be the consequence of creep, plasticity and thermal expansion. According to the requirement on RPV safety during the prescribed time, it must be ensured that the water cooling takes effect in preventing the structural failure under SA condition. Through vigorous investigation, it is found that the RPV safety is secured within the 100 creep h. Furthermore, the structural failure site, time and mode are predicted with consideration of the effect of various water levels. Most importantly, the failure is found to take place at the site aside around the water level.