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Description of Fast Reactors
Published in G. Vaidyanathan, Dynamic Simulation of Sodium Cooled Fast Reactors, 2023
The entire primary sodium circuit is contained in a large-diameter vessel (Ø 12,900 mm), called the main vessel, and consists of core, primary pumps, intermediate heat exchanger, and primary pipe connecting the pumps and the grid plate (Figure 2.10). The vessel has no penetrations and is welded at the top to the roof slab. The main vessel is cooled by cold sodium to maintain it at lower temperatures and enhance its structural integrity. The core subassemblies are supported on the grid plate, which in turn is supported on the core support structure. A core catcher provided below the core support structure is designed to take care of a meltdown of seven subassemblies and prevents the core debris from encountering the main vessel. The main vessel is surrounded by the safety vessel, closely following the shape of the main vessel, with a nominal gap of 300 mm to permit robotic and ultrasonic inspection of the welds in the vessels. The safety vessel also helps keep the sodium level above the inlet windows of the intermediate heat exchanger, ensuring continued cooling of the core in case of a leak of the main vessel. The interspace between main and safety vessels is filled with inert nitrogen. An inner vessel separates the hot and cold pools of sodium. The main vessel is closed at its top by a top shield, which includes a roof slab, large and small rotatable plugs, and a control plug.
France, nuclear power, and safety policy
Published in David Toke, Geoffrey Chun-Fung Chen, Antony Froggatt, Richard Connolly, Nuclear Power in Stagnation, 2021
David Toke, Geoffrey Chun-Fung Chen, Antony Froggatt, Richard Connolly
The design of the EPR is certainly marketed to be in line with such ideas of beyond design accidents. The EPR includes various extra safety devices on top of those included in previous (so-called Generation II) reactors. The new measures include what is called an ‘advanced passive design’, double containments around the reactor (that is in addition to enhanced aircraft protection), multiple redundancy in safety injection systems, and a ‘core catcher’. There are four safety injection systems for both the primary and secondary systems, whereas strictly speaking only one is needed provided it works properly. The primary system involves the ability to inject material into the core to take it offline, and the secondary system is in effect a water pump to intervene in the case of a loss of coolant situation. The core catcher is a device that is aimed to prevent a core meltdown by having a shield between the reactor and the ground.
Heavy Water Reactors
Published in Kenneth D. Kok, Nuclear Engineering Handbook, 2016
Alistair I. Miller, John Luxat, Edward G. Price, Paul J. Fehrenbach
The shield tank contains a large volume of water surrounding the calandria. In the case of beyond-design-basis accidents (BDBAs), for example, severe core damage accidents such as a LOCA plus LOECC plus loss of moderator heat removal plus failure of make-up to the moderator, the shield tank can provide water to the outside of the calandria shell, ensuring that it remains cool and therefore intact, thereby confining the damaged core material within the calandria. Recent HWR designs have added make-up to the shield tank and steam relief to ensure that this remains effective. Heat can be transferred from the debris through the thin-walled calandria shell to the shield tank without the debris melting through. This inherent core catcher provides debris retention and cooling functions. Because a severe core damage sequence can be stopped in the calandria, the challenge to containment is much reduced.
CFD Simulation of Melt Pool Coolability in a Simulated Core Catcher Model
Published in Nuclear Science and Engineering, 2023
Samyak S. Munot, Arun K. Nayak
To mitigate and manage the consequences of a severe accident involving core meltdown, especially after Fukushima, advanced nuclear reactors require a core melt retention strategy within the design as a key element in the severe accident management strategy.1 The principal objective of the core melt retention strategy during an accident condition is to contain, cool, and stabilize molten corium for a prolonged period. High melt temperature and decay heat present a difficult challenge to containing melt relocation in the desired region for stabilizing and cooling.2–4 Thus, to confront the challenge, a core catcher device has been introduced in modern nuclear reactor designs. The core catcher is a melt retention device that is placed in the containment or reactor to retain molten corium, quench molten corium debris, and sustain coolability for a longer period.5,6
Numerical Modeling and Experimental Validation of Melt Pool Coolability Under Bottom Flooding with Decay Heat Simulation
Published in Nuclear Technology, 2018
Nitendra Singh, Arun K. Nayak, Parimal P. Kulkarni
In the ex-vessel condition, severe accidents are generally managed by a core catcher. The core catcher is an arrangement placed outside the reactor vessel and is designed to retain, quench, and stabilize the corium in a cold state for a long time. Coolability of a molten pool is a complex phenomenon that includes multiple components and multiphase coupled heat and mass transfer. This phenomenon has not yet been very well understood. Thus, there have been quite a few attempts in the literature to model it. Models for melt coolability are necessary to extrapolate the results from experiments to actual reactor scenarios. For this, a mathematical model has been developed and validated with experimental measurements. The major findings are presented in this paper.
A molten metal jet impingement on a flat spreading surface
Published in Journal of Nuclear Science and Technology, 2020
Nassim Sahboun, Shuichiro Miwa, Kazuhiro Sawa, Yasunori Yamamoto, Yuta Watanabe, Tomomasa Ito
One of the major challenges in nuclear thermal-hydraulics for the severe accident’s evaluation and management is the ability to predict core meltdown behaviors [1–3]. Following the fuel meltdown and the displacement of the molten core, the corium goes through interactions with the remaining coolant within the reactor pressure vessel (RPV) and sedimented to the bottom of the vessel. Depending on the conditions of RPV’s vessel wall, the corium may be discharged to the lower level of the reactor building. Therefore, to assure the integrity of the reactor building, it will be important to retain the spreading molten core in a confined space such as core-catcher. Large-scale experiments to investigate the spreading of the molten core over a flat area for the design enhancement of the core-catcher were conducted internationally since the Three-mile Island (TMI) accident [1]. Most of those experiments were focused on molten core spreading behavior and investigation of the molten core concrete interaction (MCCI) mechanisms. In addition to the core catcher design, following the lessons learned from the Fukushima-Daiichi accident, advancement in the severe accident code has become a crucial issue in thermal-hydraulic fields, especially to improve predictive capabilities of MCCI and molten metal spreading behaviors [1]. Ogura et al. [4,5] took into account the spreading of downward jet on both dry and wet surfaces. The experimental facility, depicted in Figures 1 and 2, was utilized for those works. The authors investigated the effective dimensionless numbers of molten metal spreading and deposition. Matsumoto et al. [6], utilized an exact same test facility to develop a scaling criteria for the molten Copper. A summary of previous experiments that investigated the corium deposition and spreading behaviors through a falling jet is tabulated in Table 1. Note that well-known severe accident experimental programs such as SPREAD [7], CORINE [8,9], KATS [10,11], ECOKATS [12,13], S3E [14], and VULCANO [15,16] are excluded from the list since these programs investigated spreading of the corium (or simulant materials) on the substrate through bottom/side injection.