Self-leveling Behavior of Debris Bed

Debris bed heat transfer and self leveling experiment have been carried out to provide information needed for postaccident heat removal analysis of sodium-cooled reactor. The postaccident heat removal is the final step in the analysis of hypothetical core disruptive accidents (HCDAs) for fast reactor.1) In core disruptive accident of a sodium cooled fast breeder reactor, degraded core material can form debris beds on the core support structure and/or in the lower inlet plenum of the reactor vessel. Typically, the debris will form conically shaped mounds in the subcooled sodium plenum. Decay heat within the debris bed heats the subcooled sodium, transports and vaporizes it. Its heat convection and its vaporization then will lead ultimately to leveling the debris bed. This mechanism defines the term ‘debris bed self leveling’. This behavior is crucial in the relocation of the molten core and the heat removal capability of the debris bed.2)

After decades of research on heat transfer of debris bed, this self leveling behavior has not gained much attention. In theoretical and experimental investigation, much of the research generally assumes that the upper surface of the debris bed is level.2) Hesson et al. and Gabor, researchers who are pioneer in this endeavor, validated the existence of the self leveling behavior by respectively introducing a bubbling airflow through a particle bed and volume heating of a particle bed composed of UO2-salt water in separate experiment. They reached the same conclusion that self-leveling occurs easily, even under condition of low heat flux. Gabor further identified the potential contribution of boiling inside the particle bed.2)

Recently, Zhang et al have proposed a criteria model for self leveling onset in a debris bed. The model was based on force balance equation with experimental setup used in two methods; depressurization method and bottom heating method. The future study of this topic, that is going to be performed in PhD program, is adding one covering wider conditions of its experiment in particular for gas velocity in the bed. It will be performed using the gas injection method from the bottom of bed.

Same with previous study, to obtain the general characteristics of the self leveling behavior, the experiment will take parameters; particle size (0.5 – 6), particle shape (spherical and nonspherical), and particle density (alumina, zirconia, lead, and Stainless steel). While, the experimental setup will be more developed by adding a gas container which is the gas inside will be injected through the bottom of debris bed, called gas injection method. The aim of this method is to realize the gas velocity corresponding to the decay heat level. Furthermore, the experimental result of this study could be analyzed by a computational tool which is currently under development at JAEA.

For conducting experiment above, it is important to study the overall concepts of experimental methods, principal work of experimental apparatus and the characteristic of particle bed itself. Those will be done in the first year of doctoral program, while the other two year are going to be used for full scale research on experimental self leveling behavior of debris bed and related research which is needed to fulfill an appropriate outcome.

References:

1) J. D. Gabor et al., Studies and experiments on heat removal from fuel debris in sodium, Proc. Fast Reactor Safety Mtg., Beverly Hills, California, USA, Apr.24, 1974, CONF-740401-P2,823(1974).

2) B. Zhang, T. Harada, D. Hirahara, T. Matsumoto, K. Morita, K. Fukuda, H. Yamano, T. Suzuki, Y. Tobita, Self-leveling Onset Criteria in Debris Beds, Journal of nuclear science and technology, vol. 47, no. 4, p.384-395(2010).

3) S. Cheng et al. , Experimental investigation of bubbling in particle beds with high solid hold up, Exp. Therm. Fluid Sci.(2010), doi:10.1016/j.expthermusci.2010.11.003

*) research plan for kyushu university 2011 by antphy