An important constraint on the management option chosen is the economics involved – even if this is rarely emphasised. Based on existing technology, this completely excludes some variants – e.g., space disposal or partitioning and transmutation of long-lived radionuclides (even if they are not excluded by other legal or safety issues). For other options, the economic constraints may be very sensitive to boundary conditions. Thus, despite all the caveats with respect to operational safety, a national programme such as Slovenia’s, which shares one NPP with Croatia and has a very small waste inventory, may well conclude that the cost/risk analysis favours a deep borehole disposal concept over a conventional repository, even if a larger programme, with a large number of NPPs such as France, came to exactly the opposite conclusion.

Deep borehole variants are an example of cases where it is difficult to build a robust concept based on existing technology – but it could be argued that the additional requirements for deep characterisation and operational robustness are within the limits of technological development which can be reasonably expected to occur over the next few decades. Such a situation can be contrasted to space disposal or complete transmutation, which do not appear feasible based on reasonably expected developments of any existing technology5.

Overall, however, it must be accepted that technology has developed in unpredictable ways over the half century in which radwaste management has been an issue and major new developments cannot be precluded. To make these ‘‘superscience’’ options practical, however, such progress would effectively have to involve completely new science (anti-gravity, cold fusion, isotope chromatography, teleportation) which may or may not ever develop. At present, even major advances would appear to be capable of dealing only with a small amount of the least technically problematic waste. Nevertheless, the huge value of the nuclear industry could make even very expensive options feasible. It would, however, also have to be considered if, from the viewpoint of the principle of sustainability, such use of valuable resources of materials, energy and manpower were justified, given the number of more pressing problems facing humanity at present.

Finally, socio-political acceptance must be considered. Even if they can be argued to be less favourable from the viewpoints of safety, practicality or cost, concepts which receive popular acceptance may be the only ones which are feasible in a democracy. This may lead to increased consideration of long-term monitoring, reversibility and institutional control. Under such conditions, the challenge will be to develop an acceptable option which combines the robustness of deep geological disposal with meeting the desires and concerns of the key stakeholders (see also comments in Umeki et al., 2004).

10.3.6. Materials technology

As noted above, the radwaste industry is very conservative, preferring to stick with tried and tested designs but, as more repositories come nearer to fruition, pragmatic thinking is leading to interesting new developments in materials planned for use in either construction, waste immobilisation or both. Here, a couple of examples are briefly

5 For example, in the final report of France’s National Scientific Assessment Committee (CNE, 2006), it was noted that P&T operations represent ‘‘. . . a long process that only makes sense if nuclear energy use continues for at least a century.’’ Adding that, in any case, transmutation of some radionuclides (notably 129I) ‘‘. . . appears particularly difficult’’.

presented along with recommendations for long-term testing of these (and other) new materials.

10.3.6.1.Novel construction/immobilisation materials: the example of low pH cement

The problems associated with the use of standard OPC cement in repositories have been covered in Chapters 3, 5 and 8. Briefly, the fact that the cement leachate pH (up to 13.3) is so different from the bentonite pH (8–11) and the host rock pH (near-neutral) means that it is clearly out of (chemical) equilibrium with the other repository components. This will induce reactions which are generally assumed to degrade the barrier behaviour of the nearand far-field components. Consequently, alternative materials (and repository designs) have been examined and, currently, one area of focus is on low-pH cement.

Although traditionally called low-pH cements in the literature (see also Chapter 8), these materials should more correctly be known as low-alkali cements (or, due to their leachate pH values of 10.5–11, at the very least lower pH cements). Although much of the cement grout used by the Romans over two millennia ago was effectively low-alkali cement (see, e.g., the discussions in McKinley and Alexander, 1992; Miller et al., 2000), little interest was shown in the development of modern low-alkali cements until about 20 years ago when AECL began further developing existing cements for use as grouts. In fact, the use of low alkali cement grouts was initially contemplated due to better handling and fracture penetration properties (Mukherjee, 1982) and lower heat generation (e.g., Gray and Shenton, 1998) and, while these properties remain of interest, much work is currently focused on the greater chemical compatibility with bentonite (e.g., JAEA, 2007) and the repository host rocks and preliminary results (e.g., Seidler and Faucher, 2004) from the ongoing EU-funded ESDRED (Engineering Studies and Demonstration of Repository Designs) programme are promising (see also www.esdred.info for further details).

However, one area where some doubt remains as to the relevance of low-alkali cement is that of long-term durability (e.g., Philipose et al., 1991) and this is discussed below.

10.3.6.2.Long-term testing of novel (and existing) materials

It is now widely accepted (e.g., Kickmaier et al., 2005) that conventional laboratory test data combined with empirical or mechanistic extrapolative modelling is not enough on its own. A rigorous safety case needs also to include more qualitative, demonstrative arguments to be accepted by all key stakeholders. Of particular importance here are long-term demonstration tests under relevant, in situ conditions. Such experiments need to consider timescales of decades and can include studies of the behaviour of individual materials used in various EBS designs (such as the low-alkali cements noted above) and also materials in relevant combinations. Such a database is particularly appropriate for building safety cases for repository licensing at the construction, operation and closure stages. Inclusion of qualitative, demonstrative arguments can enhance SA models that are, traditionally, based on conventional laboratory test data combined with empirical/ mechanistic extrapolative modelling. This can increase their chances of acceptance by all key stakeholders.

Generally, for most national radwaste programmes, the timescale until first licensing is several decades away, hence initiation of such long-term studies is now rather urgent if a long-term database is desired. Some materials testing areas which could usefully be examined (but note this is not an exhaustive list) include: