University of Manchester

The pore structure evolution of nuclear graphite under heavy ion beam irradiation

Zhoutong He

Scientific Case

The development of Generation IV reactors requires the development of new nuclear graphite grades and as such their behaviour under fast neutron irradiation is critical to their application in a nuclear reactor. This topic is of particular international interest at present due to the Chinese development of a graphite moderated prototype High-Temperature Reactor due to start operation in 2018 and development of a demonstration graphite moderated Molten Salt Reactor at SINAP. There are also similar programmes in progress in the USA and some UK studies. Graphite life prediction behaviour and property changes have previously been obtained by using materials test reactors (MTR), however, this is limited to existing grade, and hugely expensive and time-consuming irradiation experiments which are often piggy-backed on to fuel irradiation programmes with limited suitability. It is believed that the irradiation behaviour is closely related to the microstructure and its evolution under irradiation, which was demonstrated by the previous study on fast neutron irradiated graphite. However, these studies are limited by the inhomogeneous nature of nuclear graphite, variation of the fast neutron irradiation environment and varying coolant compositions. For new graphite grades emerging, MTR irradiation is not fully feasible and alternative routes to predict behaviour evolution can be used using STFC facilities such as the Dalton Cumbrian Facility (DCF). Ion beam irradiation shares the same damage mechanism with fast neutron in graphite, and is frequently used as a surrogate for fast neutron irradiation without resulting in radioactive properties of the material. The depth profile induced by heavy ion irradiation provides an ideal scenario to compare the microstructure change induced by irradiation. At Manchester, we have undertaken initial experiments’ at DCF to irradiate nuclear graphite with a heavy ion beam and study the microstructure and its evolution under irradiation.
Nuclear graphite

Experiment Design

1. Identify the implanted Ni layer in graphite, which is about ~2µm in thickness and 10 µm beneath the surface. The Ni content in this layer is about 4 at. %. To identify this feature, a resolution about 500 nm is required.
2. Identify the pores which are sized range from 0.5~3µm.
3. Identify the microcracks in the graphite crystallite, which is lenticular-shaped. The width of the microcracks is range from 10~1000 nm, and the length of them is usually more than 5 µm. After ion beam irradiation the microcracks will be closed, and we want to see microcrack density reducing or microcrack disappearance with Nano-CT.
Scanners and Rigs
Xradia Ultra XCT
Not Required

Sample & Safety

The samples we are going to test are two Ni+ ion beam (~30 MeV) irradiated nuclear graphite foil with a thickness of 50 µm. The samples are cut from the graphite foils and they are wedge-shaped. The elemental composition in the samples is carbon, with an implanted Ni layer ~1 µm in thickness, which is about 10 µm beneath the surface. And the Ni content in this layer is about 4 at. %. In the samples there is both ion beam damaged part and virgin part, so we could compare them.
Low Hazard

Scan Records

Project Report