Modelling the mechanisms that govern the oxidation of graphite

Abstract

The Pebble Bed Modular Reactor (PBMR) design is one of the High Temperature Gas Cooled Reactors (HTGR) under the Generation IV initiative. These designs incorporate numerous inherent passive safety features. Graphite is an important material of construction for the reactor core and the fuel pebbles. Knowledge of the high temperature oxidative behaviour of the graphite materials utilized in such reactors is important for design and accident modelling purposes. Despite large amounts of research into the oxidation of graphite, a coherent framework for the comparison and assessment of the relative reactivity’s of graphite samples from different origins has not yet been established. This is mainly due to a lack of clarity regarding the relative contribution of different factors which influence the overall behaviour of a given sample. The objective of this work was to identify and isolate the key factors which influence the oxidation of graphite and understand their operation across the entire range of conversion. Based on this understanding a comprehensive model for the oxidation can be established. The framework of this model will allow the sensible comparison of samples from different origins, based on the relative contribution of the relevant mechanisms. The work focused purely on the kinetic factors which influence the oxidation and extreme care was taken to avoid mass transfer limitations where possible. Through a carefully established experimental methodology three key factors were found to influence the progression of oxidation in a given sample: <ul><li> First and foremost, the development of the active surface area of a given sample</li><li> Secondly, the presence of catalytic impurities</li><li> Thirdly, the influence of inhibiting impurities/li></ul> Based on these three effects, a finite element type, Monte Carlo simulation was developed. In this simulation virtually any geometry can be easily represented and the progression of the active surface area as the oxidation proceeds can be monitored. Furthermore, catalytic impurities could be easily incorporated into the simulation in a clear, consistent manner. This leads to an overall simulation which produces results that visually reflect the observed behaviour as well as accounting for the kinetic aspects of the experimentally determined conversion behaviour. This work provides a starting point for assessing samples from different origins to first determine differences in the three basic governing effects, followed by a relative assessment of their reactivity’s on a common basis. Future work should focus on refining the understanding of the mechanistic aspects of each of the individual governing effects, especially the influence of surface complexes and different reaction pathways. In addition, the work should be extended to cover a more comprehensive selection of graphite samples from different origins and a wider variety of impurity behaviours.