The lumped heat transfer methodology is simple, and the solution is very fast, so the lumped parameter approach has been widely used in thermal-hydraulic analysis for fuel pin heat transfer in nuclear reactors. In the conventional lumped thermal analysis of fuel pin structure, each component (such as a pellet, gap, cladding, etc.) is characterized by a concentrated bulk temperature (or averaged temperature), and a bulk thermal resistance. In contrast to this conventional lumped thermal resistance model, in this paper another kind of lumped thermal resistance heat transfer model for fuel pin structure has been developed. In this model, each fuel pin component is still represented by a concentrated lumped mean temperature node, while the location of the mean temperature position of each component is no longer set on the geometrical midpoint center; rather, it is assigned exactly onto the analytical temperature profile. Two thermal resistance elements are assigned for each component in this new model; between each component surface and its associated lumped mean temperature node a thermal resistance is assigned. Heat conduction in the radial direction between the mean temperature nodes of different components is purposely defined to take place at the in-between surface nodes. With this new arrangement, the location of the mean temperature positions for each component can be determined analytically, and all the thermal resistances are redefined, accordingly. The advantage of the presented method is that the temperature profile in the whole pin at any radial position can be reconstructed after a quite easy lumped heat transfer calculation. This advanced methodology can be used in nuclear reactor simulation studies where the fastness of the solution is of concern. It is of great advantage, e.g., for the early prediction of the formation of an internal molten fuel cavity within a fuel pin using this temperature profile, before the lumped pellet average temperature reaches the fuel melting point. This lumped thermal resistance model can be readily used for the sodium-cooled fast reactor design, especially for the optimized design of the pin structure. It can be also extended to the restructured fuel pin through the way that each restructured zone is treated as an individual component, e.g., for taking into account temperature-dependent thermophysical properties.
