A Study of the Oxidation Behaviour of Pile Grade A (PGA) Nuclear Graphite Using Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM) and X-Ray Tomography (XRT)

Abstract

Pile grade A (PGA) graphite was used as a material for moderating and reflecting neutrons in the UK’s first generation Magnox nuclear power reactors. As all but one of these reactors are now shut down there is a need to understand the residual state of the material prior to decommissioning of the cores, in particular the location and concentration of key radio-contaminants such as 14C. The oxidation behaviour of unirradiated PGA graphite was studied, in the temperature range 600–1050°C, in air and nitrogen using thermogravimetric analysis, scanning electron microscopy and X-ray tomography to investigate the possibility of using thermal degradation techniques to examine 14C distribution within irradiated material. The thermal decomposition of PGA graphite was observed to follow the three oxidation regimes historically identified by previous workers with limited, uniform oxidation at temperatures below 600°C and substantial, external oxidation at higher temperatures. This work demonstrates that the different oxidation regimes of PGA graphite could be developed into a methodology to characterise the distribution and concentration of 14C in irradiated graphite by thermal treatment.

Porosity analysis of superfine-grain graphite IG-110 and ultrafine-grain graphite T220

“…The gas-escape pores of pristine and oxidized nuclear graphite have been characterized by using mercury porosimetry [6,9], X-ray tomography [10][11][12][13][14], focus ion beam serial milling [15,16] and optical microscopy [17][18][19][20]. There is a difference in pore size distributions obtained from different techniques.…”

Examination of Surface Deposits on Oldbury Reactor Core Graphite to Determine the Concentration and Distribution of 14C

“…The work presented here uses a similar method that actively oxidises the graphite in a controlled manner to selectively remove the 14 C from the different fractions of irradiated graphite, similar to work performed by the National Nuclear Laboratory (NNL) [ 24 ]. For this a thorough understanding of the oxidation behaviour of the graphite is required, and this has previously been studied on virgin PGA graphite [ 25 ]. This study highlighted that PGA graphite exhibits the three regimes of thermal oxidation that have been observed on various other types of nuclear graphite [ 26 – 28 ].…”

Thermal oxidation of nuclear graphite: A large scale waste treatment option

“…Graphite oxidation is known to consist of several kinetic regimes, sometimes also referred to as ‘oxidation modes’, beginning at temperatures > 400°C. There is a wealth of literature in this area [ 35 – 37 ] and therefore only a short explanation will be recapped for scientific completeness. The modes are classified as follows:…”

“…Each mode occurs within a specific temperature range, however there is some dispute in the literature as to what these temperature ranges are [ 37 ] mainly due to experimental differences involved in much of the research and the complexity of the graphite oxidation itself. An interpretation of the oxidation mechanisms occurring in the various modes is shown in Fig 4 .…”