Resumen de Development of an ap1000 3d full containment model using an innovative approach
Samanta Estefanía Estévez Albuja
In order to make the reactor accident management independent from alternating current supply, Generation III+ reactors utilize passive mechanisms for their safety systems. A prominent type is the Westinghouse AP1000® reactor which Passive Containment Cooling System (PCS) uses the atmosphere as the ultimate heat sink for evacuating thermal energy from the containment. Additionally, the AP1000 design includes an In-containment Refueling Water Storage Tank (IRWST) that acts as a heat sink of the Passive Reactor Hear Removal Heat Exchanger (PRHR-HX) and as the water source for the long-term cooling phase. The performance of these systems is not only dependent on the thermo-hydraulic state of the Reactor Coolant System (RCS), but also the containment. Under certain circumstances the core cooling, by means of the PRHR, would be determined by the heat transfer between the PRHR HX and the IRWST, being the IRWST thermo-hydraulic state dependent on the containment thermo-hydraulic state. The accurate simulation of this phenomenology is complex and demands 3D models that integrate all the AP1000 containment safety systems and spaces, which has not been the case in the state of the art.
Therefore, the goal of this thesis is the development of a detailed AP1000 3D full containment model that encompass the PCS and IRWST-PRHR detailed systems for its subsequent use on design basis accidents and severe accidents.
In a first step, the IRWST-PRHR model development is described. The best modeling strategy is obtained after modeling a down-scaled experiment. With the obtained insights, a full-scaled IRWST model is generated. The proper simulation of thermal stratification, which is a safety-relevant phenomenon for the containment integrity, is proven and its influence on containment pressure is evaluated. This model is used to study the ADS steam injection during an SBLOCA accident. The importance of a proper nodalization is presented.
Then, the PCS is modeled in GOTHIC taking the conservative hypothesis that no water is available from the Passive Containment Cooling Water Storage Tank (PCCWST). Due to its design characteristics different assumptions and simplifications were applied in this model. The model performance is tested against a steady state case and it was found that the modeling of natural convection of such complex geometry is not trivial. The modeling parameters should be studied carefully.
In a third step, the AP1000 containment (SCV) is also modeled in 3D. A methodology for its construction in GOTHIC is described: a detailed CAD model is constructed, then it is simplified into simple geometry forms and finally it is implemented in the GOTHIC code. The several modeling assumptions for the correct compartmentalization are discussed and presented.
Finally, the full containment model, containing all the previous developed models, is used for simulating two application cases: LBLOCA and SBLOCA. For each accident, different AP1000 approaches are used. In the case of the LBLOCA, only the SVC and the PCS are used as the IRWST does not play a significant role. In the case of the SBLOCA, the IRWST detailed model is implemented as the stratification of the pool may be relevant in the transient evolution.
The results obtained from the application cases show that the AP1000 containment GOTHIC model could be used for simulating these kinds of accidents giving enough resolution to see the evolution of the different containment compartments. For that purpose it was needed to develop different isolated models of the IRWST, SCV or PCS in detail, in order to understand and simulate all the complex phenomena that may occur. The computational competitiveness of the model, together with its accuracy allowed to understand phenomena that have not be seen in any other AP1000 models.
