cases were evaluated using a single stylised biosphere situation to convert releases into dose. As in most SAs, it was acknowledged that there will be some modification of the near-surface environment over the very long timescales involved due to a range of factors including climate change and the sensitivity of calculated doses to different stylised climate states was investigated in a number of stand-alone biosphere calculations, each based on the same releases from the geosphere.

6.3.3. Consequence analysis

Having identified the possibilities for system evolution to be considered in the SA, these must be analysed to evaluate their consequences for safety. Although some (usually less likely) possibilities may be discussed partly qualitatively, or by the use of simple approximations, the main effort in consequence analysis is generally on the quantitative modelling of assessment cases.

In most SAs conducted internationally, radionuclide releases from the repository near-field, geosphere transport and biosphere transport, accumulation and dilution are modelled using separate computer codes, coupled together in a ‘‘model chain’’. The near-field code provides a source term for the geosphere code, which in turn provides input (radionuclide release rates as a function of time) for the biosphere code. Figure 6.7 shows the model chain COMP23 ! FARF31 ! BIO42 used in the SR 97 SA (SKB, 1999).

Modelling inevitably involves a certain number of conservative assumptions and simplifications because of the complexity of the systems considered, the impossibility of complete characterisation (particularly in the case of the geosphere), the limited understanding that is available for some processes and the wish to avoid treating some poorly defined uncertainties explicitly. Some processes are well-understood and can be modelled using fairly simple relationships based on fundamental physical and chemical principles, such as Darcy’s Law for groundwater flow, Fick’s Laws for diffusion and the Bateman Equations for radioactive decay and ingrowth. These are incorporated, in some form, into most SA model chains. Other processes are more complex to model and, in some cases, less well-understood, examples being advection in flowing groundwater in highly heterogeneous geological media, the range of radionuclide retardation processes that are grouped together as ‘‘sorption’’ and the transport of radionuclides in association with colloids (Box 6.2). The approach used in many SAs for these processes is to incorporate them in a relatively simple form in the model chain codes and to develop separate, more detailed and realistic models to derive input parameters (e.g., the use of geo/hydro analyses, as illustrated in Fig. 6.7) to provide input parameters, with conservative margins applied to the parameter values to deal with uncertainties.

Some poorly understood processes are less amenable to modelling. There is, for example, considerable uncertainty in the radionuclide transport resistance provided by fractured waste forms (such as blocks of vitrified HLW, which may fracture during cooling after fabrication) and by breached canisters, and the way in which this evolves over time. Most SA models conservatively omit this transport resistance altogether. In many cases, the lack of the necessary model or code to treat a particular phenomenon (like fracturing of the waste form) in detail reflects the fact that uncertainties in the phenomenon are large and are unlikely to be reduced significantly by further R&D

Fig. 6.7. The model chain COMP23 ! FARF31 ! BIO42 used in the SR 97 SA and supporting models and data that provide input parameters (from Fig. 3–11 of SKB, 1999).

(Chapter 8). Even if more refined models and computer codes were available, a pessimistic case in which, say, the transport resistance was small or negligible might still have to be considered. This reduces the motivation to develop such models or codes in the first place, something which is not easy to explain to the general public (Chapter 9).

Over-simplified models and conservative assumptions have to be used with particular care if an aim of a SA is to support site selection or design optimisation. There may, at a given stage of a programme, be more information (and less uncertainty) about some site or design options compared to others, simply because these have been the focus of more intensive site characterisation and design studies. A conservative approach will tend to be most conservative for the least understood options and there is obviously a danger that this may unduly bias a decision against these options.

Safety assessors are often wary about using the word ‘‘prediction’’ to describe evaluations of the performance and levels of safety provided by a repository. This is because there is a danger that the word may be misinterpreted as meaning precise

Box 6.2. Transport processes in the geosphere

Advection is the process by which dissolved (or colloidal) species (e.g., radionuclides) are transported by the bulk motion of flowing groundwater. Pressure gradients driving groundwater flow may arise, for example, from variations in the hydraulic head (e.g., in a mountainous site), glacial rebound (e.g., Scandinavian and Canadian Shields) and variations in density associated with salinity and temperature contrasts (e.g., at a coastal or island site). In unsaturated systems, flow occurs under gravity following any period of rainfall. Groundwater flow rates may vary considerably even within a single rock formation due to the heterogeneity in fracture and pore space structures and to friction on flow path walls. The resulting spreading of transported solutes (or colloids) is known as mechanical dispersion.

In contrast, diffusion is the process by which radionuclides will migrate driven by gradients in chemical potential. In advective systems, diffusion causes solute dispersion which, when combined with mechanical dispersion, is called hydrodynamic dispersion. The rate of diffusion is determined by the magnitude of the concentration gradient and the diffusion coefficient of each particular solute. The diffusion coefficient is itself a function of the properties of the rock, such as the tortuosity of pore spaces, the properties of the groundwater and, in particular, its temperature, and the properties of the diffusing species, such as their charge and size.

Figure: The retardation mechanisms that may affect radionuclides in the geosphere (after McKinley and Hadermann, 1984).

(Continued )