Review of models for Carbon 14 Dose Assessment
Interest in improved dose assessment methods for long-term releases of C-14 to the biosphere gave rise to a model review within the BIOPROTA programme [BIOPROTA, 2005]. This qualitative review of models concluded that the major difficulty surrounding the conceptual model for C-14 concerns identifying the conditions under which mixing and isotopic equilibrium may reasonably be assumed. Such system understanding needs to take account of the temporal and spatial scale of the assessment, including the nature of the release from the geosphere. A key issue is the decision as to whether kinetic models are useful and/or necessary, compared with an equilibrium approach. It was also argued that more data are needed for gaseous exchange from surface waters and soils to the atmosphere and how this affects the specific activity in the water and soil compartments, as well as uptake into plants.
The overall objective of the current project is to improve confidence in dose assessments for long-term releases into the biosphere of C-14 disposed in radioactive waste repositories, focussing upon the soil and plant components of the biosphere. This project compares quantitative estimates of soil, plant and canopy atmosphere C-14 concentrations arising from a scenario with unit C-14 activity in groundwater that is used to irrigate the crops.
The comparison of results for the different models, including uncertainty estimates, will be used to indentify:
- the implications of adopting different modelling approaches,
- the relevant processes, and
- the site specific information necessary for applying different models.
The work programme is continuing through 2010-11.