AP-1000 passive in-vessel retention design for a severe accident

The AP-1000 is a Westinghouse two-loop 1100 MWe advanced pressurized water reactor (PWR) that uses passive safety features to enhance plant safety and to provide significant and measurable improvements in plant simplification, reliability, investment protection and plant costs. One of the passive safety features of the AP-1000 is In-Vessel Retention. In-Vessel Retention passively provides sufficient external cooling of the reactor vessel to retain a molten core inside the vessel in the unlikely event of a severe accident. This concept was proven by a series of tests and analyses, and offers numerous advantages over other severe accident core management designs. The testing that proved the In-Vessel Retention Concept for AP-1000 also identified a series of features and functions for the reactor vessel insulation, making it different from the reactor vessel insulation in any other nuclear power plant. This paper presents the key functional and interface requirements for the AP-1000 reactor vessel insulation and the design configuration that meets those requirements. Requirements are identified for both normal and severe accident conditions. The requirements for normal conditions are similar to those for any other reactor vessel insulation. The severe accident requirements include features that passively allow containment flood water in the reactor cavity to freely flow between the insulation and reactor vessel and other features that passively allow the resulting steam to freely vent. The AP-1000 reactor vessel insulation must also provide the appropriate annulus to optimally cool the reactor vessel and be structurally adequate to withstand the loads that occur during the severe accident and during certain pipe breaks in containment. (authors)