A modelling approach to assess the environmental/radiological impact of C-14 release from radioactive waste repositories

Highlights: • A new integrated model was developed to assess the impact of C-14 release. • A biogeochemical near field code interfaces with a geo-biosphere far field model. • Microbial activity controlled by pH affects substantially the effective dose. • A faster mid chain scission rate of cellulose leads to a higher effective dose. • A higher effective dose is associated with a smaller release area in gas pathway. - Abstract: Assessments of the environmental impact of C-14 disposal often assume that C-14 is converted into gases that are able to migrate to the surface, where they pose a radiological risk. However, uncertainties, associated with the long-term release of C-14 from graphite and the evolution in the post-closure environment of a geological disposal facility (GDF), exist. In this paper, an integrated modelling framework has been developed to investigate these uncertainties. The modelling framework consists of a biogeochemical near field model which interfaces with a geosphere/biosphere model and it is verified by comparing the results to those obtained from other models. A sensitivity analysis discloses that a faster mid chain scission rate of stopped cellulose about four orders of magnitude assesses a twice higher effective dose. In another scenario, which is related to the control of microbial activity by pH and the availability of carbon dioxide to microbes, the effective dose is two orders of magnitude higher compared with a reference scenario. This modelling work illustrates also the importance of far field parameters, such as the rock permeability and the release area of gas pathway, to the assessment of effective dose.