GEOTRAP is the OECD/NEA Project on Radionuclide Migration in Geologic, Heterogeneous Media carried out in the context of site evaluation and safety assessment of deep repository systems for long-lived radioactive waste.
Performance assessment of proposed waste disposal sites requires models of radionuclide transport through the geosphere. To be used in repository planning and development, these models must have the confidence of both national waste management programmes and the wider scientific community. The fourth GEOTRAP workshop, "Confidence in Models of Radionuclide Transport for Site-specific Performance Assessment" held in June 1999, addressed the issue of technical confidence building and provided an overview of current developments in this field. Proposed approaches to confidence building and approaches that have already proven successful were presented and discussed.
In addition to the material presented during the workshop, this publication includes a technical synthesis reflecting the discussions that took place as well as the conclusions and recommendations made, notably during the working group sessions.

PART A. SYNTHESIS OF THE WORKSHOP
1. Introduction
--.1 Background, Aims, and Scope of the 4th GEOTRAP Workshop
--1.2 Structure of the Present Synthesis
2. Findings of the Workshop
-2.1 Achievements and Observations
-2.2 Recommendations
3. The Evolution of Confidence in the Courseof Repository Planning and Development
4. General Aspects of Confidence in Performance Assessment
5. Confidence in Models of Radionuclide Transport for Site-Specific Performance Assessment
-5.1 The Role of the Geosphere within the Overall Disposal System
-5.2 Confidence in the Completeness of Phenomena Considered in Geosphere Performance Assessment and their Conceptualisation
-5.3 Enhancing Confidence through Site Characterisation and Research and Development
-5.4 Model Simplifications and its Implications for Confidence in Model Results
6. Concluding Remarks
-Annex 1. Structure and Programme of the Workshop
-Annex 2. Conclusions of Working Group A: Role of Independent Indicators to Support Transport Models
-Annex 3. Conclusions of Working Group B: Implications of Simplification in Geosphere - Transport Modeling in Performance Assessment
-Annex 4. Conclusions of Working Group C: Aspects and Sufficiency of Confidence in Supporting (Process) Modeling
-Annex 5. Conclusions of Working Group D: Strategies for Increasing Confidence in Transport Models through Data Acquisition
PART B. WORKSHOP PROCEEDINGS
Session I. Confidence in Transport Models in the Overall Context of Repository Development
-Confidence in the Long-Term Safety of Deep Geological Repositories - Approach Developed under theAuspices of the NEA by H. Rothemeyer
-Evolution of Confidence in Transport Models for Performance Assessment: The WIPP Case by W. D. Weart
-Confidence in Geosphere Performance Assessment - The Canadian Nuclear Fuel Waste Disposal Programme 1981-1999: A Retrospective by M. R. Jensen and B. W. Goodwin
-Radionuclide Transport-Related Aspects of the Radiological Evaluation of the Mururoa and Fangataufa Atolls by G. de Marsily and J. Hadermann
-Regulatory Views and Experiences Regarding Confidence Building in Geosphere-Transport Models by B. Stromberg, B. Dverstorp, and J.Geier
Session II. Develoment of, and Confidence in, Geosphere-Transport Models Used in Performance Assessment by J. Hadermann and P. Lebon
-Confidence Building in the Nagra Approach to Geosphere-Transport Modeling in Performance Assessment by P.A. Smith, M. Mazurek, A. Gautschi, J. W.Schneider and P. Zuidema
-The Posiva/VTT Approach to Simplification of Geosphere-Transport Models, and the Role and Assessment of Conservatism by T. Vieno, H. Nordman, A. Poteri, A. Hautojarvi, and J. Vira
-The Transport Model for the Safety Case of the Konrad Repository and Supporting Investigations by E. Fein, R. Storck, H. Klinge, K. Schelkes, and J. Wollrath
-Regulatory Experience with Confidence-Building in Geosphere-Transport Models by N.A. Eisenberg, R. B. Codell, L. S. Hamdan, T. J. Nicholson, and B. Sagar
Session III. Approaches to Confidence in Models that Support Geosphere-Transport Models Used in Performance Assessment
-The Role of Matrix Diffusion in Transport Modeling in a Site-Specific Performance Assessment: Nirex 97 by S. Norris and J. L. Knight
-The Effect of Onsager Processes on Radionuclide Transportation in the Opalinus Clay by J. M. Soler
-Uranium Migration in Glaciated Terrain: Implications of the Palmottu Study, Southern Finland by D. Read, R. Blomqvist, T. Ruskeeniemi, K. Rasilainen and C. Ayora
Session IV. Expected Impact of Future Site Investigation Strategies and Approaches on Confidence in Transport Models
-An Unsaturated Zone Transport Field Text in Fractured Tuff by G. Y. Bussod
-The Impact of Performance Assessment