Analysis of critical pipe break sizes leading to reactor pressure vessel liquid level collapse and core uncovery with APROS
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Keywords:
Loss-of-coolant accident
Pressurized water reactor
Corium
During a severe accident in a nuclear reactor, the molten core—or corium—may be relocated into the reactor vessel’s lower plenum in case of core support plate failure. The severe accident management strategy for In-Vessel Retention—or IVR—consists in stabilizing the corium within the reactor pressure vessel by external cooling of the vessel’s lower head. If now, the vessel fails due to excessive thermal loading on its walls, the Ex-Vessel Retention—or EVR—strategy is adopted. In this case, the core melt stabilization can be achieved by effective corium spreading, either in the reactor vessel cavity or in a dedicated “core-catcher”, and cooling by water. The success of both strategies highly depends on the corium behavior at high temperatures, conditioning vessel’s integrity for IVR, and promotion for the spreading of the EVR. This involves a variety of fundamental mechanisms closely related to heat and mass transfer regimes prevailing at the system scale, which requires further analytical and experimental insight to determine the primary mechanisms and feed the modeling tools, allowing the numerical simulations of severe accident scenarios. Within the framework of corium characterization at high temperatures, the present study aims at filling the lack of such fundamental data as density, surface tension, liquidus and solidus temperatures, and viscosity. In order to accurately measure these properties at high temperatures, the VITI facility is designed with various configurations. Concerning IVR, the influence of density and surface tension is particularly highlighted through VITI-SD and VITI-MBP configurations, and practical applications of experimental results are finally discussed, in link with the focusing effect issue at the thin upper metallic layer of the corium pool. Concerning EVR, the properties of interest are solidus/liquidus temperature and dynamic viscosity, and typical experimental results obtained through VITI-VPA and VITI-GFL configurations are discussed in view of characterizing corium spreading.
Corium
Plenum space
Light-water reactor
Pressurized water reactor
Liquidus
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Corium
Plenum space
Pressurized water reactor
Boiling water reactor
Natural circulation
Light-water reactor
Decay heat
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Corium
Liquid metal
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Corium
Plenum space
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In the event of a core meltdown accident, one of the accident progression paths is fuel relocation to the lower reactor plenum. In the Heavy Water New Production Reactor (NPR-HWR) design, the reactor cavity is flooded with water. In such a design, decay heat removal to the water in the reactor cavity and thence to the containment may be adequate to keep the reactor vessel temperature below failure limits. If this is the case, the accident progression can be arrested by retaining a coolable corium configuration in the lower reactor plenum. The strategy of reactor cavity flooding to prevent reactor vessel failure from molten corium relocation to the reactor vessel lower head has been discussed in this document.
Corium
Plenum space
Natural circulation
Decay heat
Light-water reactor
Pressurized water reactor
Shut down
Containment (computer programming)
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Corium
Liquid metal
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External reactor vessel cooling (ERVC) is considered as one of the most promising severe accident mitigation strategies for an in-vessel corium retention(IVR). When a molten corium is relocated in the lower head of reactor vessel , the external reactor vessel cooling will be activated automatically as the coolant is supplied into the reactor cavity which is a annual gap between the external reactor vessel and the insulation designed for the external reactor vessel cooling to remove the heat of external vessel through the two-phase natural circulation flow. When the severe accident happened, the heat generated by the molten corium will accumulate in the lower head of reactor, in order to improve the margin of in-vessel reactor retention(IVR), the CHF of lower head of reactor must be improved. CHF on different angles will be different basing on various water levels. Flooding water level have a significant impact on the natural circulating ability and CHF of RPV outer wall. A large scale facility ,REPEC-II, has been built to investigate the flow characteristic and CHF under ERVC conditions. This paper will focus on the experimental effect of flooding water level on the natural circulation characteristics and critical heat flux.
Corium
Natural circulation
Decay heat
Pressurized water reactor
Light-water reactor
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Corium
Plenum space
Pressurized water reactor
Loss-of-coolant accident
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External reactor vessel cooling (ERVC) for in-vessel retention (IVR) has been considered one of the most useful strategies to mitigate severe accidents. However, reliability of this common idea is weakened because many studies were focused on critical heat flux whereas there were diverse uncertainties in structural behaviors as well as thermal–hydraulic phenomena. In the present study, several key factors related to molten corium behaviors and thermal characteristics were examined under multi-layered corium formation conditions. Thereafter, systematic finite element analyses and subsequent damage evaluation with varying parameters were performed on a representative reactor pressure vessel (RPV) to figure out the possibility of high temperature induced failures. From the sensitivity analyses, it was proven that the reactor cavity should be flooded up to the top of the metal layer at least for successful accomplishment of the IVR-ERVC strategy. The thermal flux due to corium formation and the relocation time were also identified as crucial parameters. Moreover, three-layered corium formation conditions led to higher maximum von Mises stress values and consequently shorter creep rupture times as well as higher damage factors of the RPV than those obtained from two-layered conditions.
Corium
Thermal hydraulics
Cabin pressurization
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