COVER STORY | IRRADIATED GRAPHITE
The complexities of irradiated graphite
Since the world’s first reactor, graphite has been used as a moderator in multiple nuclear plant designs so that today the world has accumulated a quarter of a million tonnes of irradiated graphite. It’s a major waste management challenge
By Judith Perera
GRAPHITE HAS BEEN USED AS a moderator and reflector of neutrons in more than 100 nuclear power reactors and research and plutonium producing reactors. In some reactor designs, it is also used as a fuel sleeving material leading to the generation of large amounts of material that is less irradiated but still requires management. Many of the older reactors are now shut down, with more approaching the end of their lives. Approximately 250,000 tonnes of irradiated graphite have now accumulated worldwide. Increasing amounts of irradiated graphite are being placed in temporary storage facilities pending disposal. Because of the complexity of how graphite reacts under
irradiation (see box) it is evident that no two reactor designs will give rise to irradiated graphite with the exact same set of properties. Different operational histories will affect the radioisotope content as well as the activity of those radioisotopes. Large variations may even arise in the graphite from reactors of a similar type as a result of impurity variations during manufacturing at different facilities.
“Irradiated graphite is a specific type of radioactive
waste. Because of the different levels of radiation based on different radiological characteristics – but also differences in chemical, physical characteristics,” Tetiana Kilochytska, Decommissioning Specialist at the International Atomic Energy Agency’s (IAEA’s) Department of Nuclear Energy told NEI. “And all these characteristics affect the disposal route. It depends on many features that are country specific, including the waste management infrastructure available to deal with this type of waste,” she added.
Managing irradiated graphite Many different approaches for managing irradiated graphite have been, and are being, researched around the world. Although it is generally planned to remove graphite from the majority of reactors as intact blocks, the effects of irradiation, combined with the presence in some cases of metallic components like pins or wires within the structures, means alternative dismantling strategies are also being considered. For example, dismantling of a graphite stack
Above: Graphite bricks are still used in the cores of the Advanced Gas-Cooled Reactors (AGRs) at seven of the UK’s power stations Photo credit: Jacobs
16 | February 2024 |
www.neimagazine.com
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