Abstract: The In-vessel retention (IVR) of molten core has been part of the effective management strategies of the severe accidents for nuclear power plant. To make IVR succeed, the thermal failure criterion must be met, so the local heat flux of reactor pressure vessel (RPV) wall should be lower than the critical heat flux (CHF) of the external coolant. Besides, the pressure boundary integrity of the RPV should also be ensured to avoid the structural failure. Accordingly, it’s urgently necessary to analyze the structural integrity of the RPV under CHF. Firstly, the thermal analysis of a RPV under CHF is carried out to obtain effective geometric model with some of the RPV wall molten and temperature distribution along the wall. Then the limit load and IVR 72 hour creep behavior are analyzed for this model subjected to the thermal load and internal pressure, and the ultimate load capacity of the RPV is subsequently determined. The results show that the whole wall along the thinnest thickness of the RPV yields and the RPV reaches the ultimate load capacity under internal pressure of 4.9 MPa at the beginning of the IVR. The maximum local creep strain of 4.1% is reached under 3.6 MPa within 72 hours, while the internal pressure of 3.7 MPa results in a significant increase of maximum creep strain up to 42.5%. Therefore, the ultimate load capacity of this RPV under CHF in 72 hours is determined to be 3.6 MPa.

Key words: creep, critical heat flux, in-vessel retention (IVR), reactor pressure vessel (RPV), ultimate load capacity