COVER STORY | IRRADIATED GRAPHITE
Irradiated graphite A crystalline allotrope of carbon, synthetic graphite has been produced on an industrial scale for more than a century. Blocks of graphite display good mechanical properties and chemical inertness, hence their use in some reactors. Radioactive graphite retrieved from nuclear installations has different physical and chemical
properties compared with other radioactive waste. In addition, after irradiation, it contains a significant amount of long-lived radioisotopes, such as carbon-14 (14 years, and chlorine-36 (36 isotopes, such as tritium (3
waste-management process. In graphite, 14
C is mostly generated by the interaction of nitrogen with reactor neutrons.
Nitrogen is present in graphite as an impurity and also in the reactor coolant or cover gas. Tritium results from the reaction of neutrons with impurities in graphite, as well as during
fission of the fuel. Similarly, neutron activation of chlorine impurities in graphite can produce 36
Cl. A number of significant changes take place in the graphite during irradiation in a reactor,
driven by different components of the radiation field to which it is exposed. The results may depend on temperature or other factors, such as the pressure of a coolant gas. The main irradiation-induced changes are caused by fast neutrons; ionising radiation; slow neutrons; and the operational effects and irradiation environment.
Fast neutrons
The purpose of a graphite moderator is to slow high energy neutrons, maximising capture by further fissionable isotopes. During this process, collisions between fast neutrons and carbon atoms displace the carbon atoms, affecting the graphite structure. Changes also occur in the porosity of the graphite. These fast-neutron effects change its physical and mechanical properties, leading to dimensional change, embrittlement and strength reduction, as well as changes in thermal properties. The extent of such changes depends on the total fluence and the flux, and also on temperature as some effects are mitigated at a higher irradiation temperature. The effects can be complex. For example, in power reactors such as AGRs and RBMKs initial shrinkage of the graphite components may reverse leading to expansion at different times and in different regions of the core. One effect, specific to low-temperature graphite irradiation, is the storage of potentially large amounts of energy – known as Wigner energy – within the damaged graphite structures. This can be released later if the graphite is heated to approximately 50°C above its former irradiation temperature. It was the unplanned release of this Wigner energy which led to the Windscale Pile 1 accident in the UK in 1957. However, in most cases it is controlled by regular annealing and remains well below any threshold for release.
Ionising radiation
Gamma and beta radiations are present in reactor graphite, primarily arising from fuel fission-products. These do not affect the graphite structure directly but interact with coolant gases to generate species from the gas phase which may then interact with graphite. The most important effect is oxidation of the graphite.
Slow neutrons
A number of impurities within the graphite have the potential for activation to become radioactive isotopes. Depending upon their half–lives, these radioisotopes may be important during the dismantling phase and the operational period of a repository (3 be very long lived becoming an issue for containment (14
H or cobalt-60) or C, 36 Cl Calcium-41). The physical location of these newly created isotopes within the graphite structure is also a factor.
Operational effects and irradiation environment The nature of the environment in which the graphite has been irradiated is also significant. For example, graphite in the RBMK design is usually irradiated in a helium/nitrogen mixture which is essentially static. This can give rise to the formation of a high concentration of 14 reaction with the 14
C from a C located on accessible graphite surfaces. Secondly, in dynamic gas circuits, extraneous material such as oxides from boiler tube
oxidation may be transported around the circuit and trapped in the graphite transport pore structure. It is then activated in the higher fluence of soft neutrons present in the graphite. An additional effect to be considered as part of the dismantling process is the presence of graphite dusts which may include graphite containing substantial quantities of Wigner energy. ■
C) with a half-life of 5,730
Cl) with a half-life of 300,000 years. There are also many short-lived H), with a half-life of 12.3 years. All these have implications for the
18 | February 2024 |
www.neimagazine.com
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