abbie.jones@manchester.ac.uk
0161 3062602
F Floor
Pariser Building
Sackville St.
M60 1QD

WP2

Nuclear Graphite in KNOO

Scope

Nuclear graphite is manufactured from coke a bi-product of the oil or coal industry. The coke is mixed with a pitch and then extruded, or moulded, into large blocks which are baked at about 800oC, re-impregnated, at least once, and then graphitised at about 1800 oC.

The final product is very pure polycrystalline artificial graphite with about 20% porosity. The properties, both unirradiated and irradiated, of the final product are mainly defined by the original coke. As coke supplies vary with time, the properties and irradiation behaviour of the final product changes. At present irradiated graphite properties are based on empirical data obtained on samples irradiated in a material test reactor (MTR). The cost of such a programme may take about 6 years and amount to 8m. It is therefore desirable that the relationship between irradiation induced changes to graphite structure and change in property are fully understood, thus reducing the scope and size required of MTR programmes. In addition, such an understanding may lead to the development of new types of graphite that will have longer lifetimes. This would reduce the amount of irradiated graphite waste in future VHTR designs.

Objectives

The crystalline structures of unirradiated nuclear graphites and their thermal strain behaviour have been evaluated using HRTEM with in situ TEM thermal experiments. The objective is to obtain thermal strain measurements by image correlation, with which to validate models for structure / property relationships. Sample preparation methods have been compared using both traditional mechanical polishing and state of the art Focused Ion Beam FEG-SEM. The mechanical properties of the virgin graphites are being correlated with the microstructure through a series of compressive loading experiments using both in situ X-ray diffraction and Raman spectroscopy.
In addition to analysis of the thermal strain experiments, the examination of neutron irradiated graphites by HRTEM will be undertaken following visits to Petten (Netherlands) and Nexia Solutions. Ion beam irradiated graphites from Cardiff inconjuction with KNOO WP 4, will be compared with neutron irradiated samples. Three-dimensional studies of the structure / property relationships will be initiated using TEM tomography.