Boiling Heat Transfer Behavior in Core Catcher of Advanced Reactors

Abstract

Engineers and Scientists are working all over the world to make the nuclear reactors more and more safe by optimizing the strategies of defense in depth and using multiple redundant systems. With the vast experience and develo** safety features, the nuclear reactors were accepted as the viable energy source for the energy production. However, this sound and peaceful nuclear energy met with hurdles that made people reconsider the usage of nuclear energy as a safe option, in the year 1979, a shock in 1986, and a major setback in 2011 when three major tragic accidents occurred. These events were as follows:

• Three mile island, USA, 1979

• Tchernobyl, Russia, 1986

• Fukushima Daiichi, Japan, 2011

These accidents are categorized as severe accidents, which are Beyond Design Basis Accidents (BDBA). Severe accidents can be defined as the scenarios where it was extremely challenging to maintain the reactor core in safe condition. These incidents can be initiated by human error or a natural calamity and turn into a severe accident due to unavailability of safety and redundant systems. The severe accidents which involve melting of reactor core are difficult to manage as molten reactor core, or “Corium,” consists of molten fuel and metallic structural material. Corium is highly radioactive molten substance with temperature of the order of 3000 K. Upon the severe accident involving the reactor core melt down, corium relocates to the lower plenum of the reactor pressure vessel, and upon further failure of reactor pressure vessel, corium relocates in lower containment cavity. The lower cavity of the containment consists of basemat of concrete material, which interacts with molten corium and leads to ablation of concrete. This provides a pathway for highly radioactive molten corium to enter into the environment through the ground and may even cause environmental hazard by groundwater contamination.

Due the high temperature and radioactivity of molten corium, it is very challenging to arrest and cool it for a prolonged period in a desired location. The concern for coolability of corium grows when water comes into the picture. In dry conditions, where the water is absent in lower plenum or reactor pit, there exists a potential threat of reactor pressure vessel rupture and molten core concrete interaction at basemat. In case of wet conditions, when water may be present in the vessel or in the reactor cavity, there exists a potential threat of steam explosion which may damage containment walls. This is due to interaction of molten corium with water. Cooling of this corium is of utmost importance to arrest the accident propagation. The coolant or water in case of light water reactors has to come in contact with corium directly (top flooding and bottom flooding) or indirectly. Thus, using different cooling strategies and location of arrest of molten corium, various melt retention strategies are developed. The location of interaction defines the name as in-vessel, where the interaction takes place within the reactor vessel, and ex-vessel, where the interaction occurs within the containment but outside the reactor vessel. Thus, on the basis of different strategies of cooling the molten corium, core catchers are designed for advanced nuclear reactors. Core catcher is an arrangement where corium can be contained and cooled for a long duration in a stabilized state. Considering the type of reactor and severe accident management strategy adopted, different core catchers have been designed.

The following chapter elaborates briefly about the different core catcher techniques along with different cooling methodologies used to cool the molten corium. It describes the heat transfer characteristics of the molten pool upon stratification in different layers and associated empirical natural circulation correlations.