Irradiate Graphite Waste: more than just 14C

Graphite has been used as a neutron moderator in the UK nuclear industry across several generations of nuclear reactor design, from early test reactors such as the Graphite Low Energy Experimental Pile (GLEEP) through to the fleet of Advanced Gas-cooled Reactors (AGRs) currently operating in the UK today. As a by-product of using graphite-moderated nuclear reactors the UK has a legacy of approximately 96,000 tonnes of irradiated graphite waste, corresponding to over a third of the worldwide irradiated graphite waste inventory, estimated at 260,000 tonnes.
Although a proportion of the material, particularly that from the core periphery, may be classified as Low Level Waste (LLW), practicalities of retrieval and segregation mean that it is expected that almost all of the UK legacy will be managed as Intermediate Level Waste (ILW). Current policy for the management of this in England and Wales includes a ‘safe storage period’ of the order of 100 years, during which the graphite core will remain in-situ, which will allow the activities of shorter-lived radioisotopes to diminish prior to disposal in a future Geological Disposal Facility (GDF). Scotland has opted for an alternative waste disposal destination, utilising near-surface long-term storage. There have, however, been calls in the UK for an accelerated programme of decommissioning for civil nuclear power stations[4], which would require unanticipated early dismantling of the graphite reactor cores and responsible management of the resultant radioactive material without the benefit of a long decay time.
The two prominent radioisotopes frequently highlighted as isotopes of concern in irradiated graphite waste are 14C and 36Cl, the former of which has been explored in a previous contribution to Nuclear Future. These long-lived radioisotopes are considered very mobile in the bio- and geosphere, and have half-lives of 5730 and 301000 years respectively meaning that their activities will not naturally diminish by radioactive decay over any practical timeframe. The activities of many other radioisotopes associated with irradiated graphite waste, including tritium, 60Co and 137Cs, will decay by orders of magnitude over a 100 year cool-down period. If, however, an accelerated decommissioning programme were to be pursued, the activities of even relatively short-lived radioisotopes may not have decayed significantly, thus requiring these to also be considered when developing a safe and effective waste management route.
As with all decommissioning activities there is always a complex list of factors and arguments which must be explored before deciding how best to approach the safe and effective management of radioactive waste. Many studies have been produced which examine the pros and cons of different management options for the UK’s irradiated graphite legacy, including treatment options to decontaminate the bulk material. The aim of this article, however, is to re-examine the origin of the radioactivity expected to be present in the material and consider the relative significance of the prominent radioisotopes to waste management strategy in the UK and worldwide.