released from the repository (most probably hyperalkaline to alkaline resulting from the use of cement and concrete in many designs). The former might be facilitated, for example, by carbonate minerals, while the latter might result from the presence of volcanic glasses.

It may be worth emphasising that the processes influencing pH and redox are often strongly coupled (as suggested above) and are typically catalysed by microbial activity. The role of microbes may need to be specifically taken into account if attempts are made to quantify the effects of such processes on barrier performance, due to their additional impact on material degradation and the mobilisation of radionuclides.

Finally, total salinity can play a significant role in defining the effectiveness of the barrier system. Most information supporting the design and performance of the EBS and radionuclide retention in the geosphere is defined for conditions of low to medium salinity and hence making the safety case for very saline conditions (brines) can be complicated and may place special design requirements on the EBS. However, the presence of old, very dense brines can provide evidence of geological stability and, due to density stratification, contribute a mechanism for increasing performance of this barrier.

3.3.3.4. Overview

A general observation from the previous sections is that all key characteristics of the geological environment have pros and cons with regard to practicality of repository construction, protection of the EBS and geological barrier performance. This absence of a clearly preferable option explains why so many different geological settings have been considered for repositories. Selecting a particular geological setting involves balancing these factors in a way which is very dependent on boundary conditions – such as the type and quantity of waste involved, regulatory requirements, socio-political boundary conditions, budget constraints, etc. A particularly important factor is the timescale over which safety must be assured – which can be set by either the characteristics of the waste or specific regulations. Siting environments which may provide assured performance for millennia may be unsuitable if safety has to be demonstrated for hundreds of thousands or even millions of years. This is currently a very topical issue (see Box 3.1 and Chapter 7).

Nevertheless, great flexibility is provided by the wide range of options available for the engineered barriers, which allow these to be tailored to the characteristics of the site. Such a tailoring process was already implied in the discussion of the evolution of the SKB and Nagra designs in section 3.2.2 above and is considered in more detail in Chapter 5 and, for example, by NUMO (2004).

Further consideration of geological disposal options which do not involve conventional repositories is presented in section 3.5. Before this, however, alternatives to geological disposal are discussed.

3.4. Alternatives to geological disposal

3.4.1. Introduction

Although these options can generally be considered as ‘‘discarded’’, as they are not presently considered as practical substitutes for geological disposal in any national programme, it is worth reviewing them here as future societal, technological or economic