Algorithms for thermal and mechanical contact in nuclear fuel performance analysis

The transfer of heat and force from UO₂ pellets to the cladding is an essential element in typical nuclear fuel performance modeling. Traditionally, this has been accomplished in a one-dimensional fashion, with a slice of fuel interacting with a slice of cladding. In this manner, the location at which the transfer occurs is set a priori. While straightforward, this limits the applicability and accuracy of the model. We propose finite element algorithms for the transfer of heat and force where the location for the transfer is not predetermined. This enables analysis of individual fuel pellets with large sliding between the fuel and the cladding. The simplest of these approaches is a node on face constraint. Heat and force are transferred from a node on the fuel to the cladding face opposite. Another option is a transfer based on quadrature point locations, which is applied here to the transfer of heat. The final algorithm outlined here is the so-called mortar method, with applicability to heat and force transfer. The mortar method promises to be a highly accurate approach which may be used for a transfer of other quantities and in other contexts, such as heat from cladding to a CFD mesh of the coolant. This paper reviews these approaches, discusses their strengths and weaknesses, and presents results from each on simplified nuclear fuel performance models.