Abstract: When the molten core debris relocates to the lower plenum of the Reactor Pressurized Vessel（RPV） during a severe accident,penetrations may fail due to the thermal attack from the molten core debris,then the RPV loses its integrity,and molten core debris releases into the reactor cavity,resulting in the failure of In-Vessel Retention（IVR）.Based on the two kinds of penetration failure modes,i.e.penetration ejection and debris entering the penetration channel,the ablation of weld and friction force due to thermal expansion are calculated by VTA code,and flowing distance of debris along the penetration channel are analyzed with MBF model respectively.The results demonstrate that 300MW PWR plant RPV lower head will not lose its integrity due to penetration failure,and the molten core debris will not relocate to the reactor cavity through the failed penetrations if the External Reactor Vessel Cooling（ERVC） is performed.