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.
Full access: http://t.cn/E5FZheV
Image by Chaminda Perera, from
Flickr-cc.
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