An approach to modelling the impact of 14C release from reactor graphite in a geological disposal facility

Charalampos Doulgeris,Simon Rout,Paul Humphreys

doi:10.1180/minmag.2015.079.6.24

Abstract

AbstractCarbon-14 (C-14) is a key radionuclide in the assessment of a geological disposal facility (GDF) for radioactive waste. In the UK a significant proportion of the national C-14 inventory is associated with reactor-core graphite generated by the decommissioning of the UK's Magnox and AGR reactors.There are a number of uncertainties associated with the fate and transport of C-14 in a post-closure disposal environment that need to be considered when calculating the radiological impacts of C-14-containing wastes. Some of these uncertainties are associated with the distribution of C-14-containing gaseous species such as 14CH4 and 14CO2 between the groundwater and gaseous release pathways. As part of the C14-BIG programme, a modelling framework has been developed to investigate these uncertainties. This framework consists of a biogeochemical near-field evolution model, incorporating a graphite carbon-14 release model, which interfaces with a geosphere/biosphere model. The model highlights the potential impact of the microbial reduction of 14CO2 to 14CH4, through the oxidation of H2, on C-14 transport. The modelling results could be used to inform the possible segregation of reactor graphite from other gasgenerating wastes.

Highlights

APPROXIMATELY 80,000 tonnes of irradiated graphite will arise from the decommissioning of Magnox and AGR reactors (NDA, 2011, 2014)
The analysis presented in those studies demonstrated that, in principle, it should be possible to dispose safely of irradiated graphite wastes in isolation, in a wide range of disposal systems, including near-surface, shallow and deep geological disposal, and in a wide range of host rocks
Scenarios B and C2 have broadly similar dose profiles, the increased doses over and above those seen in scenario A are due to the generation of bulk gases which in turn drive the release of C-14 bearing gases

Summary

Introduction

APPROXIMATELY 80,000 tonnes of irradiated graphite will arise from the decommissioning of Magnox and AGR reactors (NDA, 2011, 2014). Bracke and Muller (2008) discussed possible scenarios for the release pathways and the associated processes towards a less conservative approach in the release of C-14 from a low-level waste repository. One key process considered by the model is carbonation, where dissolved CO2 has the potential to be retained in the near field via carbonation reactions with cementitious materials within the GDF (NDA, 2010a).

Results

Conclusion