Andrej Horvat, Ivo Kljenak

A possible severe accident scenario is a general meltdown and relocation of the reactor core during which molten core material accumulates in the lower plenum of the reactor vessel. External cooling of the lower plenum by flooding of the concrete cavity with subcooled water is one of the severe accident management strategies currently being considered for this type of accident. At present, uncertainty exists with respect to heat transfer coefficients for natural convection between the pool of molten core material and the wall of the lower plenum. 

In the present work, numerical simulations of turbulent natural convection in a geometry similar to the lower plenum cavity are conducted. A two-dimensional numerical code based on finite volume method, using large-eddy simulation, was developed. The modified Smagorinsky model is proven to be a robust and reliable numerical tool for solving turbulent natural convection cases in a fluid with internal heat generation. The spatial discretisation of momentum equations followed the direction of the Harlow and Welsh (1965) scheme. However, the temperature field in the energy equation required a particular shock treatment. The projection method was selected for time integration and separate Runge-Kutta and Crank-Nicolson treatments for convection and diffusion terms were used, respectively. Thus, false numerical diffusion and dispersion were kept as low as possible.

Numerical simulations were performed at Rayleigh numbers up to and Prandtl number , which corresponds to molten core material conditions in the lower plenum during the postulated severe accident scenario. The local heat transfer coefficient at the externally cooled wall was calculated. The heat transfer coefficient is the lowest at the bottom and increases with height. The maximum value of the heat transfer coefficient occurs slightly below the pool surface. Simulations also revealed that variation of boundary conditions on the pool surface cause only a slight difference in the average heat transfer coefficient. For the purpose of code validation, simulation results are compared with experimental measurements performed by Henry et al. (1993) and Asfia et al. (1996). Satisfactory agreement is obtained and differences are presented.

 REFERENCES: 

Harlow, F. H., Welsh, J. E., 1965, "Numerical Calculation of Time-Dependent Viscous Incompressible Flow of Fluid with Free Surface", The Physics of Fluids, Vol. 8, No. 12, pp. 2182-2189.

 Henry, R. E., Fauske, H. K., 1993, "External Cooling of a Reactor Vessel under Severe Accident Conditions", Nuclear Engineering and Design, Vol. 139, pp. 31-43.

 Asfia, F. J., Frantz, B., Dhir, V. K., 1996, "Experimental Investigation of Natural Convection Heat Transfer in Volumetrically Heated Spherical Segments", Journal of Heat Transfer, Vol. 118, pp. 31-37.