Abstract
Due to large uncertainty, the effectiveness of in-vessel retention (IVR) with external reactor vessel cooling (ERVC) for high-power reactors cannot be fully demonstrated during the transient process. To optimize the current IVR-ERVC strategy, the concept of IVR-ERVC with in-vessel injection (IVI) is studied. Two feasible IVI designs are proposed: 1) using the passive IVR water tank to implement simultaneous water injection in-vessel and ex-vessel and 2) using the passive severe accident dedicated in-vessel injection tanks (SADITs) to implement water injection in-vessel. The research on the feasibility and effectiveness of IVI is performed, and the corresponding negative effects are analyzed. The calculation results by using MIDAC show that the two proposed IVI concepts can greatly delay the accident progression in the core and reduce the decay heat peak of the molten pool in the lower head, thereby improving the effectiveness of the current IVR-ERVC strategy. And the negative effects are within acceptable ranges.
Highlights
After the Fukushima nuclear accident, nuclear regulatory authorities of various countries put forward more stringent requirements for nuclear safety
The key principle of the IVR-ERVC strategy lies in that the external cooling water is quickly injected into the reactor pit through active or passive means after severe accidents, forming a natural circulation in the annular flow channel
It can be found that the creep damage fraction and the jet ablation fraction are far less than 1.0, and the hoop stress is much smaller than the yield strength
Summary
After the Fukushima nuclear accident, nuclear regulatory authorities of various countries put forward more stringent requirements for nuclear safety. Considering not challenging the current severe accident mitigation strategies and not significantly increasing the pressure boundary of the primary circuit, the following two IVI strategies are proposed: 1) Using the passive IVR water tank to implement simultaneous water injection in-vessel and ex-vessel. It can be found that the implementation moment of IVI is earlier, and the core damage fraction at 5 and 10 h after LB-LOCA is lower, which indicates that the water injection in-vessel in the early core degradation phase can reduce the melt mass relocated into the lower head and decrease the decay heat of the molten pool. In the process of water injection in-vessel, a large amount of steam will be generated after the contact between the coolant and fuel rods at high temperature, which may cause the pressure in the primary circuit to increase significantly. The risk of fission products being released into the environment can be eliminated
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