Services > Integrated R&D: the example of laboratory studies, underground rock laboratories and natural analogues

 

Due to the complex nature of  the disposal of radioactive, it is necessary to develop integrated R&D programmes which cover laboratory experiments, studies in underground rock laboratories and natural analogues. This is because each of these approaches have positive and negative aspects: the advantage of natural analogues over  short-term laboratory experiments is that they enable study of  repository-like systems which have evolved over the geological timescales of relevance to a radwaste repository safety assessment (rather than the days to months usual in laboratory tests). However, by their very nature, natural analogues often have ill-defined boundary conditions which may be better assessed under the well constrained (if less relevant) conditions of a laboratory. Well designed, realistic experiments in an underground rock laboratory can bridge the gap between the laboratory and natural analogues by offering repository relevant natural conditions with some of the constraints of the laboratory (and intermediate timescales).

In short, combining information from the three sources (long-term and realistic, if poorly defined, natural analogues, medium-term, better constrained, in situ field experiments and short-term, less realistic but well defined laboratory studies) can provide greater confidence in the extrapolation of laboratory derived data to repository relevant timescales and conditions.

 

Thorough Testing of Performance Assessment Models:
The Necessary Integration of In Situ Experiments, Natural Analogue Studies and Laboratory Work

Extended abstract in Sci. Basis Nucl. Waste Manag. XXI, 1013-1014.

W.RUSSELL ALEXANDER, ANDREAS GAUTSCHI* & PIET ZUIDEMA*
GGWW (Rock-Water Interaction Group), Institutes of Mineralogy-Petrology and Geology, University of Berne, Baltzerstrasse 1, 3012 Berne, Switzerland.
*Nagra (National Co-operative for the Disposal of Radioactive Waste), Hardstrasse 73, 5403 Wettingen, Switzerland.

ABSTRACT

Repositories have to isolate radioactive waste (radwaste) and some toxic wastes from the environment for hundreds to hundreds of thousands of years. For many scientists and engineers, and especially the general public, such time spans are beyond comprehension and, consequently, they have grave doubts as to the safety of any such waste repositories. That repository performance over these long time scales can only be assessed by the use of complex mathematical models (normally called performance assessment, or PA, models) only adds to the mistrust of many. How then can people outside the radwaste community be convinced that it is possible to assess the performance (and thus ensure the safety) of a repository over the long timescales of interest? One way is to address the robustness of the PA models, by clearly indicating the form and extent of model testing carried out within the repository PA. Not only can this show that the individual component parts of the complex structure which constitutes most PA models have been checked, but also that the 'mathematical black boxes' have some basis in reality.

Part of the problem undoubtedly lies in the unusual nature of radwaste disposal: in most major engineering projects, such as bridge construction or aerospace engineering, the designs are tested against a range of laboratory experiments backed up by expert judgement based on experience with the same or similar systems. Here repository design deviates from standard engineering practice in that no high-level waste (and only a few low- and intermediate-level waste) repositories yet exist and, even when they do, testing their compliance to design limits will be somewhat difficult due to the time scales involved. In addition, the irrational fear of most things radioactive means that most people require some greater form of 'proof' that a repository is safe than they are willing to accept for a bridge loaded with rush-hour traffic or an aircraft crowded with holidaymakers. This being the case, significant additional effort must be expended within the radwaste industry to make it completely clear that the PA models can adequately predict the long-term behaviour of a repository.

In all fairness to the 'doubting Thomases', most PA models are somewhat less than transparent and, in several cases, information on the testing of the models is unobtainable. Arguably, the actual testing of some PA models also has been less than thorough. Traditionally, PA modellers have placed too much weight on laboratory data for the construction and testing of PA models and, with only a few exceptions (for example[1]), have ignored data from natural analogues and in situ experiments (see comments in [2,3]). The over-dependence on laboratory data is understandable in that the information is produced under well understood, fully controlled conditions and thus the modellers feel they can place a high degree of confidence in the results obtained. Unfortunately, a repository is not like a laboratory and it is necessary to address processes which are influenced by natural heterogeneities, which include large degrees of uncertainty and which operate over very long timescales. In this case, it is necessary to supplement laboratory data with information from in situ field experimentsand  natural analogues.

The advantage of natural analogues over  short-term laboratory experiments is that they enable study of  repository-like systems which have evolved over the geological timescales of relevance to a radwaste repository safety assessment (rather than the days to months usual in laboratory tests). However, by their very nature, natural analogues often have ill-defined boundary conditions which may be better assessed under the well constrained (if less relevant) conditions of a laboratory. Well designed, realistic in situ field experiments can bridge the gap between the laboratory and natural analogues by offering repository relevant natural conditions with some of the constraints of the laboratory (and intermediate timescales). In short, combining information from the three sources (long-term and realistic, if poorly defined, natural analogues, medium-term, better constrained, in situ field experiments and short-term, less realistic but well defined laboratory studies) can provide greater confidence in the extrapolation of laboratory derived data to repository relevant timescales and conditions.

This paper will concentrate on presenting a model testing scheme which both promotes transparency (for the sake of technical peer reviewers and the general public alike) and which aims at a thorough test of PA models. In addition, several recently published PAs will be critically examined and the form, extent and transparency of testing will be discussed with a view to improving confidence in the robustness of the models and thus the perceived safety of specific radwaste repository designs.

 

REFERENCES

1. W.R.Alexander and I.G.McKinley (1999) The chemical basis of the Swiss high-level radioactive waste disposal concept (near-field). in Chemical Containment of Wastes in the Geosphere (eds R.Metcalfe and C.A.Rochelle), Geol.Soc.Spec.Publ. No. 157, 47-69
2. J.A.T.Smellie, F.Karlsson and W.R.Alexander (1997) Natural analogue studies: present status and performance assessment implications. J.Contam. Hydrol. 26, pp3-17.
3. W.R.Alexander, I.G.McKinley, U.Frick and K.Ota (1997) The Grimsel field tracer migration experiment - what have we achieved after a decade of intensive work? OECD Proceedings: Workshop on  Field Tracer Experiments (Role in the Prediction of Radionuclide Migration), Cologne, Germany, 28-30 August, 1996, NEA/OECD, Paris, France.