The Significance of Carbon 14 in Graphite Reactor Components at End of Generation

Graphite is a crystalline form of the element carbon with its atoms arranged in a hexagonal structure. It is widely used in the nuclear industry and in research facilities due to its extreme purity and its ability to withstand extremely high temperatures and high irradiation doses (fast neutrons). The widely use of graphite has led to increasing amounts of irradiated graphite pending disposal.

It is estimated that there are at least 250,000 tonnes of irradiated graphite worldwide that will require eventual disposal. This graphite arises from a number of sources, but principally comprises moderator and structural materials for experimental reactors, production reactors, commercial power reactors and fuel assemblies.

The radionuclides in irradiated graphite presenting the most significant long-term hazard are Carbon 14 (C-14) and Chlorine 36 (Cl-36) with half-lives of 5730 and 301,000 years respectively. For a better understanding of the way in which C-14 is produced, its distribution within irradiated graphite and realistic quantification of activity can potentially lead to improved characterization to validate its status within current or future waste classifications, segregation to reduce Intermediate Level Waste volumes, or treatment to reduce activity enabling re-classification as Low Level Waste.

This paper reviewed all these issues and then focused on the significance of C-14. Some findings from a National Nuclear Laboratory study of C-14 levels in carbonaceous deposits and the underlying Magnox reactor graphite were presented to illustrate the need for thorough characterization of the waste material. These results were discussed in the context of aqueous leaching of C-14 from irradiated graphite and potential treatment options to minimize aqueous release.

The results of the review showed that while the general characteristics of irradiated graphite and the behavior of C-14 have been studied extensively, any assessment of graphite waste management options for material from a specific plant or facility will require an understanding of that material in relation to its manufacture, irradiation history and chemical environment during irradiation. The paper concluded with some broader observations on the significance of C-14 in nuclear reactor graphite components and how these issues should be considered when preparing the lifetime management of new nuclear plant.

Article by Martin Metcalfe and Athanasia Tzelepi, from UK.

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Image by Chaminda Perera, from Flickr-cc.