Effect of Fuel Temperature Profile on Eigenvalue Calculations Tom Greifenkamp 1 , Kevin Clarno 2 , Jess Gehin 2 1 University of Cincinnati Department of Mechanical, Industrial, and Nuclear Engineering P.O. Box 210072, Cincinnati, OH 45221-0072 greifete@email.uc.edu 2 Oak Ridge National Laboratory P.O. Box 2008, MS 6170, Oak Ridge, TN 37831-6170 INTRODUCTION Use of an average fuel temperature is a current practice when modeling fuel for eigenvalue (k-inf) calculations. This is an approximation, as it is known from Heat-transfer methods that a fuel pin having linear power q′, will have a temperature that varies radially and has a maximum temperature at the center line [1]. This paper describes an investigation into the effects on k-inf and isotopic concentrations of modeling a fuel pin using a single average temperature versus a radially varying fuel temperature profile. The axial variation is not discussed in this paper. A single fuel pin was modeled having 1, 3, 5, 8, or 10 regions of equal volumes (areas). Fig. 1 shows a model of a 10-ring fuel pin surrounded by a gap and then cladding. Fig. 1. Fuel pin divided into 10 rings A temperature profile for the pin was calculated using a simple, one-dimensional heat-transfer approximation [1] [2], f m k r q T T 4 ' ' ' 2 − = , and the average temperatures for each of the regions were analytically determined. Separate cases were analyzed for two different values of linear power: 1) q′=15 kW/m to represent a low power fuel pin with an average fuel temperature of 1065.8 K and 2) q′=45 kW/m to represent a high power fuel pin with an average fuel temperature of 2057.5 K. Temperature profiles for the 2 cases are shown in Fig. 2. 0 500 1000 1500 2000 2500 3000 0.00 1.00 2.00 3.00 4.00 5.00 Radial Distance from Center (mm) Temperature (K) High Power Temp Profile Low Power Temp Profile High Power Average Temp Low Power Average Temp Fuel Cladding Gap 1065.8 K 2057.5 K Cladding Gap Fig. 2. Radial Temperature profile of high power and low power fuel pins with associated average temperatures. DESCRIPTION OF THE ACTUAL WORK The eigenvalue calculations were performed using SCALE 5.1 [3] for the high and low linear power fuel pin cases. Fuel Rings 1-10 Using the analytic temperature, an eigenvalue calculation was performed for each of the 1-, 3-, 5-, 8-, and 10-ring models, where the 10-ring model will be the reference case and considered the best approximation. Moderator An analysis was then performed using average temperatures for each of the 1-, 3-, 5-, 8-, and 10-ring models. Note that the 1-ring models for both the analytic and average temperature profiles are identical. The purpose of the multi-ring, average-temperature eigenvalue calculation was to investigate what impact the refined spatial meshing, without regard to temperature, would have between the 1-ring model and the 10-ring, analytic