This chapter reviews the gasification modes and fabrication methods of nuclear graphite used in Very High Temperature and High Temperature gas-Cooled Reactors (VHTRs and HTRs). It introduces chemical-reactions kinetics parameters and a model for the gasification of different grades of nuclear graphite. The chemical kinetics parameters are determined, using a multi-parameters optimization algorithm, from the reported experimental measurements of the total gasification rate and transient weight loss at different temperatures. The parameters for the nuclear graphite grades of IG-110 and NB-25, with super fine and medium size petroleum coke filler particles, are nearly the same, but different from those for NBG-18 and IG-430, with superfine and medium size coal tar pitch filler particles. Experimental measurements of the total gasification rate and transient weight loss for nuclear graphite grades of NBG-18, IG-430, NBG-25 and IG-110 successfully validate the chemical-reactions kinetics model for a wide range of temperatures and weight loss fractions up to 0.40. The model incorporates a recently developed correlation of Sherwood number for calculating the effective velocity of oxygen transport through the boundary layer in the diffusion-limited gasification mode at high temperatures. Results show that the initial specific surface area of free active sites is inversely proportional to the square root of the initial mass of graphite specimens in the experiments (≤ 50 g). However, the total gasification rate and the production rates of CO and CO2 gasses increase with increasing the initial mass of graphite. Also presented are transient gasification analysis results for NBG-18 graphite in a helium coolant channel of a typical VHTR prismatic fuel element. © 2013 Nova Science Publishers, Inc. All rights reserved.