W. Russell Alexander

Natural analogue studies: present status and performance assessment implications

Abstract Studies of natural geological and archaeological systems as analogues to long-term processes, which are predicted to occur within a radioactive waste repository environment, have become increasingly popular over the last 10 years or so, to the extent that such studies form an integral part of many national programmes for radioactive waste disposal. There is now a common consensus that the natural analogue approach is a very useful scientific methodology to: (a) identify and understand processes and mechanisms analogous to those which could occur in the vicinity of a repository over realistic timescales, (b) derive input data which have been successfully used to test some of the laboratory-based models which form the basis of long-term repository performance assessment, and (c) to produce data which can be input directly to performance assessment models. Increasingly, analogues are playing an important role in public awareness, enabling the layman to understand better the concept of radioactive disposal and demonstrating the reliability of the disposal system over long periods of geological time. The complexity of geological systems means that it is very often difficult and sometimes impossible to quantify precisely the physico-chemical boundary conditions necessary to model a particular geochemical process or mechanism. Consequently, the availability of quantitative analogue data is limited when repository performance assessments are considered. However, this in no way detracts from their value in building confidence by demonstrating that important processes do exist and by showing qualitatively that they behave in a way predicted by models based on laboratory-derived data. The transfer of natural analogue data from the complexity of field studies to simplistic models which, by necessity, are used in performance assessments, is an area of activity which is presently being addressed. Field analogue studies are now being planned to interface with laboratory experiments and, ultimately, with in situ field experiments. This is a promising avenue of research which should provide a more quantitative use of natural analogue data in testing and further developing models used in (or supporting) repository performance assessments.

Assessment of radionuclide retardation: uses and abuses of natural analogue studies

Abstract Various techniques which have been reported for the in situ determination of radionuclide sorption or retardation as part of natural analogue studies have been critically assessed. In particular cases, the tacit assumptions used to derive retardation data from field observations can be shown to be questionable or, indeed, totally incorrect. Some problems identified are due to ambiguous or inconsistent use of terminology, but a fundamental error which commonly arises is the failure to distinguish between sorption and precipitation — processes which are treated quite differently in transport models. Natural analogue studies can be used to test radionuclide migration models and their associated databases, but considerable efforts are required to adequately characterise the geochemical process occurring. Without such extensive studies, the general applicability of data produced is limited and claims to derive parameters usable in repository performance assessment should be treated with considerable caution.

Chapter 5 Modelling radionuclide transport in the geological environment: a case study from the field of radioactive waste disposal

Publisher Summary This chapter presents a case study from the field of radioactive waste disposal. The chapter provides a short introduction to modeling the distribution of radionuclides in the geological environment (or geosphere). The basic principles of radionuclide geosphere transport modeling are examined, with the emphasis on identifying the processes and structures of relevance to contaminant transport. Approaches for including these relevant features in transport models used to assess the long-term performance of radioactive waste repositories are then discussed, followed by an explanation of how such models should be tested to ensure that all relevant mechanisms have been included and are represented in an appropriate manner. In addition, practical applications also require rigorous testing of the model or databases used and this becomes increasingly difficult as the systems represented become more complex. However, in the context of increasing confidence in the applicability of transport models, regardless of their use, this remains the highest priority and must be implemented more widely in the immediate future.

Development of geological disposal concepts

Publisher Summary This chapter discusses the development of geological disposal concepts. It provides an introduction to the historical evolution of disposal of radioactive wastes and a consideration of the range of options available. The chapter provides a short overview on the alternatives to geological disposal (section 3.4), which are classed as politically blocked options (such as sub-seabed disposal), technically impractical options [such as partitioning and transmutation (P&T)], and “non-options” (such as long-term storage). It reviews the alternatives to conventional mined repositories that are the current focus of most national programs. Although, such options are receiving increasing attention as more national program move towards repository implementation and begin to realize the potential shortcomings in many of the conventional designs. For deep geological disposal, a wide range of concepts has been studied, but effort is usually focused on emplacement in specially constructed underground tunnels or caverns. A prerequisite is geological stability, but many potential host rocks have been identified, including crystalline basement, salt, basalt, tuff, and a range of argillaceous sediments.

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

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 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’ constitute an acceptabe representation of the repository system. 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 other engineered systems. 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. Traditionally, PA modellers have placed much weight on laboratory data for the construction and testing of PA models and, with only a few exceptions, have not integrated in their PA reports data from natural analogues and in situ experiments. 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, the full complexity of a repository cannot be re-created in 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 experiments and 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.

Development of Comprehensive Techniques for Coastal Site Characterisation: Integrated Palaeohydrogeological Approach for Development of Site Evolution Models

Radioactive waste repository designs consist of multiple safety barriers which include the waste form, the canister, the engineered barriers and the geosphere. It is widely considered that the three most important safety features provided by the geosphere are mechanical stability, favourable geochemical conditions and low groundwater flux. To guarantee that a repository site will provide such conditions for timescales of relevance to the safety assessment, any repository site characterisation has to not only define whether these features will function appropriately today, but also to assess if they will remain adequate up to several thousand to hundreds of thousand years into the future, depending on the repository type. The case study described here is focussed on the palaeohydrogeology of the coastal area around Horonobe in northern Hokkaido, Japan. Data from JAEA’s ongoing underground research laboratory project is being synthesised in a Site Descriptive Model (SDM) with new information from the collaborating research institutes to develop a Site Evolution Model (SEM), with the focus very much on changes in the Sea of Japan seaboard over the last few million years. This new conceptual model will then be used to assess the palaeohydrological evolution of the deep geosphere of coastal sites of Japan.Copyright

Development of a Quality Management System (QMS) for Borehole Investigations: Part 2—Evaluation of Applicability of QMS Methodology for the Hydrochemical Dataset

An appropriate QMS, which is among the first tools required for repository site characterisation, will save on effort by reducing errors and the requirement to resample and reanalyse–but this can only be guaranteed by continuously assessing if the system is truly fit-for-purpose and amending it as necessary based on the practical experience of the end-users on-site. A QA audit of hydrochemical datasets for boreholes HDB-1–11 from Horonobe URL project by JAEA has been carried out by the application of a formal QA analysis which is based on the methodology previously employed for groundwaters during the recent site characterisation programme in Sweden. This methodology has been successfully applied to the groundwaters of the fractured crystalline rocks of the Fennoscandian Shield and has now been adapted and applied to some of the ground- and porewaters of the Horonobe URL area. This paper will present this system in the context of the Japanese national programme and elucidate improvements made during hands-on application of the borehole investigation QMS. Further improvements foreseen for the future will also be discussed with a view of removing inter-operator variability as much as is possible. Only then can confidence be placed in URL project or repository site hydrochemical datasets.Copyright

The Study of Radionuclide Retardation in Fractured Rock by Means of In Situ Resin Impregnation

Traditional in situ tracer tests estimate contaminant retardation by analysis of the degree and form of tracer breakthrough after transport through the rock. Unfortunately, this approach does not allow direct examination of in situ retardation mechanisms and, in the case of strongly retarded radionuclides, is highly impractical as tracer breakthrough may take months to decades. An alternative method to study retardation is therefore required in such a case and Nagra and PNC have recently employed one such variant to study radionuclide transport in fractured crystalline rock. Here, direct, detailed, examination of in situ radionuclide retardation following tracer injection is carried out by immobilising and recovering the intact fracture and associated rock matrix [1,2].The material can then be studied in the laboratory by standard surface analytical and radiochemical methods and the degree and form of radionuclide uptake can be readily assessed. As part of this work, Nagra and PNC have invested significant effort over the last four years in developing appropriate means of immobilising water-conducting fractures and undisturbed low porosity crystalline rock matrix in a manner which minimises physico-chemical disturbance[3]. After examining a range of options, it was decided to employ in situ resin impregnation as the immobilisation medium as this produced the best results with respect to minimising physico-chemical disturbance of the system while at the same time ensuring impregnation of very fine water saturated pore space. In addition, the polymerised resins improve the rigidity and strength of the rock such that the water saturated structures (pores, fractures or fault gouges) survive the subsequent overcoring and sub-sampling. Two experiments will be discussed: the first has been recently completed in Nagras underground laboratory in the central Swiss Alps (the Grimsel Test Site, or GTS) and the second is currently ongoing at PNCs Kamaishi In Situ Test Site (KTS) in north-east Japan. In the GTS, retardation of radionuclides is being studied in the Radionuclide Retardation Project (RRP) and two resins have been formulated for different aspects of the study. An epoxy resin has been injected into a complex water-conducting shear zone in a granodiorite following the injection of a cocktail of strongly retarding radionuclides (including 60 Co, 237 Np, 234. 235 U, 99 Tc, 152 Eu, 113 Sn and 75 Se [1,2]). To negate the hydrophobic nature of the epoxy resin, a trick has been imported from soil science where isopropanol is first injected to replace the water and only then is the epoxy resin injected. Laboratory tests showed that neither the isopropanol nor the resin should disturb the in situ radionuclide distribution, a result which has since been verified in the field. In parallel with this work, the low porosity ( in situ injection of an acrylic resin. The very low viscosity of the specially developed acrylic resin allows impregnation (and subsequent visualisation) of the connected microporosity of the matrix, so allowing detailed in situ examination of the depth of available matrix behind the shear zone. These methods have been further refined in the KTS and are currently being applied to several different types of water conducting features. The form and type of connected porosity in the associated granodioritic rock matrix is also being examined in detail [4]. As with the GTS work, the results of the in situ experiments will be compared with laboratory data on retardation and matrix diffusion to assess the transferability of the large volume of laboratory data to the field. The development of the various resins will be discussed along with the applicability of these specially developed resins to other rock types. Finally, the results of the recently concluded GTS tests and the ongoing KTS tests will be presented.

Development of Comprehensive Techniques for Coastal Site Characterisation: Part 1—Strategic Overview

Any assessment of long-term repository safety will require development of a set of analyses and arguments to demonstrate the persistence of the key safety functions of the geological environment up to several hundred thousand years into the future. However, likely future global climatic and sea-level fluctuations and uplift/subsidence would result in a dramatic change in the location of the current coastline with a subsequent significant change to hydraulic and hydrochemical conditions at coastal sites. It is thus of great importance in the Japanese disposal programme to establish comprehensive techniques for coastal site characterisation. To this end, a systematic framework, which is known as a ‘Geosynthesis Data Flow Diagram’, has been formulated, which outlines a basic roadmap of the geosynthesis methodology for characterising temporal and spatial changes of various properties and processes at coastal sites, with particular focus on the palaeohydrogeology. A basic strategy for stepwise surface-based investigations has also been proposed, which incorporates the geosynthesis methodology in an effective manner. This technique has been introduced in an ongoing collaborative programme for characterising the coastal geological environment around Horonobe in northern Hokkaido, Japan, and now tested and optimised based on accumulated technical knowledge and experience during the progress of the investigations.Copyright

Development of Comprehensive Techniques for Coastal Site Characterisation: Part 3—Conceptualisation of Long-Term Geosphere Evolution

A critical issue for building confidence in the long-term safety of geological disposal is to demonstrate the stability of the geosphere, taking into account its likely future evolution. An ongoing collaborative programme aims to establish comprehensive techniques for characterising the overall evolution of coastal sites through studying the palaeohydrogeological evolution in the coastal system around the Horonobe area, Hokkaido, northern Japan. Information on natural events and processes related to the palaeohydrogelogical evolution of the area have been integrated into the chronological tables and conceptual models that indicates the temporal and spatial sequences of the events and processes, such as climatic and sea-level changes, palaeogeography, and geomorphological and geological evolution in the area. The methodology for conceptualisation of the geosphere evolution will be applied to other analogous coastal areas on Japan’s western seaboard to produce comprehensive techniques to support understanding the geosphere evolution of potential coastal sites for deep geological repositories.Copyright