Proceedings of ICAPP ‘12 Chicago, USA, June 24-28, 2012 Paper 12022 Diffusion Velocity Correlation for Nuclear Graphite Gasification at High Temperature and Low Reynolds Numbers Mohamed S. El-Genk 1,2,3,4 and Jean-Michel P. Tournier 2,3 2 Institute for Space & Nuclear Power Studies, 3 Chemical & Nuclear Engineering Dept. and 4 Mechanical Engineering Dept. University of New Mexico, Albuquerque, NM, USA Abstract – The safety analysis of High-Temperature and Very High Temperature gas-cooled Reactors requires reliable estimates of nuclear graphite gasification as a function of temperature, among other parameters, in the unlikely event of an air ingress accident. Although the rates of prevailing chemical reactions increase exponentially with temperature, graphite gasification at high temperatures is limited by oxygen diffusion through the boundary layer. The effective diffusion velocity depends on the total flow rate and pressure of the bulk air-gas mixture. This paper developed a semi-empirical Sherwood number correlation for calculating the oxygen diffusion velocity. The correlation is based on a compiled database of the results of convective heat transfer experiments with wires and cylinders of different diameters in air, water and paraffin oil at 0.006 < Re < 1,604 and 0.068 < Sc < 35.2, and of mass transfer experiments at 4.8 < Re < 77 and 1,300 < Sc < 2,000. The developed correlation is within + 8% of the compiled database of 567 data points and consistent with reported gasification rate measurements at higher temperatures in experiments using different size specimens of nuclear graphite grades of NBG-18 and NB-25, IG-11, IG-110 and IG-430 in atmospheric air at 0.08 < Re < 30. Unlike the Graetz solution that gives a constant Sh of 3.66 at Re < 1.0, in the present correlation Sh decreases monotonically to much lower values with decreasing Re. 1 Corresponding Author: Regents’ Professor and Director, Phone: (505) 277 – 5442, and E-mail: mgenk@unm.edu I. INTRODUCTION Gasification of nuclear graphite in the unlikely event of massive air ingress is a primary focus of the safety analysis of High-Temperature and Very High Temperature gas-cooled Reactors (HTGRs and VHTRs) with prismatic cores. In such an accident, partial gasification of the massive graphite support columns in the lower plenum could compromise their strength and possibly result in a collapse of the reactor core. A weight loss of as little as 10% could reduce the mechanical strength of nuclear graphite by about 50%. 1,2 Furthermore, air ingress into the helium coolant channels and subsequent weight loss of nuclear graphite in the core could expose coated particles in the fuel columns and release fission products trapped within the graphite matrix. Models developed to calculate the total gasification rate of different grades of nuclear graphite use Arrhenius relations with apparent activation energies and pre- exponential rate coefficients, determined from empirical fits of experimental measurements of the total gasification rate. While easy to implement, this empirical approach, besides being limited to the experimental conditions and range of measurements used to determine the apparent activation energies and the pre-exponential coefficients, offers little insight into the kinetics of the chemical reactions taking place. In addition, the large variances in the reported values of the apparent activation energy and pre-exponential coefficient by different investigators, even for the same grade of nuclear graphite, result in a wide range of predictions. 2-6 A challenging, but practical and consistent approach is using a chemical-reaction kinetics model that is fundamentally, rather than empirically based, to estimate the gasification rates of nuclear graphite. Such an approach has been developed and successfully validated with the reported measurements of gasification rates and weight loss in experiments. Experiments used different size specimens of different nuclear graphite grades of NBG-18, NBG-25, IG-11, IG-110 and IG-430. 7,8 The developed gasification model employs the chemical kinetics rate and pre-exponential coefficients of four primary gasification reactions. It also uses specific activation energies and their Gaussian-like distributions for adsorption of oxygen and desorption of CO gas, and the surface area of free active sites. These quantities for