Natural convection heat transfer in fluid-saturated porous media has in recent years gained considerable attention especially in high-temperature reactors. It is proposed in this study that light water reactors (LWRs) can be made safer by redesigning the fuel in the fuel assembly. The proposed design is aimed at increasing the safety level in LWRs by the use of fuel in the form of loose coated particles in a helium environment inside the nuclear fuel cladding tubes of the fuel elements. The coated particle fuel being a heat source forms a bed in the cladding tube closed at both ends, the heat from the particles is transferred to the gas in the tube, and the gas movement is due to natural convection. In this study, we investigate the heat transfer characteristics inside a cladding tube containing packed beds of spherical particles by simulating a porous region whose medium properties are defined; that is, the geometrical model representing the packed bed is specified as a porous region. The finite volume method was used in solving the three-dimensional Navier-Stokes equation while the heat transfer coefficient h and the dimensionless numbers such as Ra = f(Gr, Pr) and Nu are used in analyzing the results. Simulated results from this investigation were validated with experimental results. The discrepancy in the results may be due to uncertainties, experimental errors, numerical errors, and the consequence of the lump parameter effect in the porous region modeling approach. This approach may be considered a unique means of estimating heat transfer characteristics in porous media.