Graphite reactors have served the nuclear industry well. But they pose a challenge in generating isotopes that are itching to get out into the environment.
Graphite-moderated reactors have been at the heart at some of the nuclear industry’s finest and poorest moments. Graphite technology allowed the industry to get off the ground with the pioneering Chicago Pile-1 and X-10 Oak Ridge National Laboratory reactors.
Yet it was also at the heart of the world’s worst nuclear disaster; the Chernobyl Nuclear Power Plant was a graphite-moderated model.
More prosaically, graphite moderation formed the centrepiece of the UK’s early nuclear industry, as part of the Magnox and advanced gas-cooled reactor designs. So it is fair to say graphite has already left a lasting legacy.
In the UK, though, researchers are aware the graphite legacy is set to last a lot longer, owing to two of the isotopes found in the moderation material.
Carbon-14, which is formed from the activation of graphite-based carbon-13 or interstitial nitrogen-14, has a half-life of more than 5,700 years.
Meanwhile chlorine-36, which is created from residual traces of the chlorine formerly used to purify graphite during manufacturing, has a 301,000-year half-life.
Of course, handling long-lasting radioactive waste is not news in the nuclear industry; the longevity of these isotopes is nothing compared to the 703.8 million-year half-life of uranium-235, for instance.
And in the US, says Dr James Conca, director of the RJLee Group Center for Laboratory Sciences on the campus of Columbia Basin College, the nation’s Waste Isolation Pilot Plant has “disposed of graphite waste from the weapons programme with no trouble.”
However, in the UK graphite is problematic because there is rather a lot of it: more than 79,000m3, to be precise.
“For Magnox reactors, graphite is manufactured in the form of bricks, which surround the fuel and are used to moderate or slow down the nuclear reaction,” states Deborah Ward, corporate communications manager for the UK’s Nuclear Decommissioning Authority (NDA).
“The UK has a substantial graphite inventory. The waste arises from operational and reactor decommissioning activities.”
Plus the isotopes it contains are particularly adept at getting into the food chain.
Carbon-14, in particular, has such a strong affinity for living tissues that it has become a benchmark for when things die. The proportion of the isotope left in organic remains is the basis for radiocarbon dating.
Chlorine-36, meanwhile, is soluble and potentially hazardous because of the possibility that it might contaminate groundwater.
The good news is that graphite, which is classed as intermediate level waste (ILW), is a relatively stable and homogeneous material compared to some other types of ILW. It can survive unchanged in natural environments over geological timescales.
Geological disposal facility
The current default strategy for dealing with it is to bury it in a geological disposal facility (GDF), once one has been created. But it is estimated it would already take up a third of the space potentially available, so there is an understandable desire to look for alternative options.
In February 2011 the NDA identified three main ones. The first is to condition graphite waste so it can be disposed of in a low-level waste repository.
The second is to remove most of the contamination, potentially through gasification followed by sequestration or radionuclide immobilisation, or reuse the graphite where possible.
And the third is to create one or more separate disposal facilities for graphite, which the NDA says should be “including a near-surface disposal option and may include a pre-treatment step.”
In May 2012, the NDA reviewed is baseline plan for graphite disposal in a GDF and found that: “A disposal cost of £739m would be applicable for geological disposal of core graphite.”
The study did not look into detailed costs for other options, but concluded: “The impact of disposing of graphite in a GDF on the total cost of the GDF is relatively low, as are the potential environmental impacts associated with an increased GDF footprint.
“Geological disposal is therefore considered to offer a feasible and potentially cost-effective strategy for managing the long-term risk associated with radionuclide releases from core graphite wastes.”
James Penfold, principal consultant at Quintessa, a UK consultancy covering nuclear decommissioning, says that no matter what form of disposal is chosen, the isotopes “will ultimately get out.
“It is a question of trying to manage the containment as long as possible and ensure environments into which it is released minimise the pathway back to humans.”
Given that no other country has used graphite within nuclear reactors to the extent that the UK has, this remains a purely British challenge.
“It is one of the challenges of decommissioning our stations, it’s true,” says Saranne Postans, head of communications at Magnox.
For now it is not a pressing problem either; graphite processing is not expected to start until decommissioned reactors have completed their care and maintenance phase, typically 85 years. But with an estimated 99,000 tonnes to deal with, it is no small task, either.
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