Biosphere model report: Project near surface disposal of category A waste at Dessel

This report is one of a set of technical supporting documents in the safety case, submitted to the FANC in support of an application for a construction and operating licence for a near surface repository for category A waste at Dessel. The present “Biosphere model report” is part of a series on parameters and processes (or equations) used in biosphere studies for the near surface disposal facility. Although the biosphere is the most uncertain component of the calculation chain “near field  geosphere  biosphere”, especially on a long-term perspective (hundreds to thousands to tens of thousands of years), biosphere assessment is important in that it provides the basis to make the calculations needed to judge whether safety targets expressed in terms of dose or radiological risk are met. The concept of reference biospheres, whereby the assessment biosphere is stylised and based on general (human) habits and (biosphere) conditions, has been developed consequently. The reference biosphere approach for the category A disposal programme is elaborated in chapter 2. The identification of biosphere receptors (or geosphere/biosphere interfaces) as representative for the category A near surface repository assessment builds on the Dessel repository’s environment features and is documented in chapter 3. The present reports builds on this information and describes the assessment biosphere for each biosphere receptor and its associated representative persons. For each of three main biosphere receptors (private well, river and wetlands), the assessment biosphere is described in terms of its associated conceptual and mathematical models, taking future climate change into account. As such, this work is an update of studies on biosphere modelling approaches carried out by SCK•CEN during previous phases of ONDRAF/NIRAS disposal programmes. In comparison to previous assessments, some modifications have been made in the description of radionuclide behaviour in different biosphere components, and additional exposure pathways are now described. The parameters associated with the model equations are only mentioned here. Information on their physical meaning and selected value (it is considered to take forward a deterministic approach in the category A biosphere assessment) are elaborated in NIROND-TR 2008-26E V3 and NIROND-TR 2008-27E V3 for radionuclide specific parameters, and in NIROND-TR 2008-28E C3 for radionuclide independent parameters. This report also gives an overview of the qualification and verification efforts for the biosphere models. Particular attention is hereby also given to the issue of traceability – it is important to list modelling hypotheses and assumptions, the basis for the assumptions, and to describe how they are implemented.