The Long-Term Performance of Nuclear Waste Forms: Natural Materials - Three Case Studies
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THE LONG-TERM PERFORMANCE OF NUCLEAR WASTE FORMS: NATURAL MATERIALS - THREE CASE STUDIES RODNEY C. EWING Department of Earth & Planetary Sciences University of New Mexico, Albuquerque, New Mexico 87131 USA
ABSTRACT Natural materials may be used to advantage in the evaluation of the long-term performance of nuclear waste forms. Three case studies are presented: (I) radiation effects in ceramic waste forms; (II) corrosion products of U02 under oxic conditions; (Ill) corrosion rate of nuclear waste glasses. For each case, a natural phase which is structurally and chemically analogous to the waste form is identified and used to evaluate the long-term behavior of a nuclear waste form. Short-term experimental results are compared to the observations made of analogous natural phases. The three case studies illustrate that results may range between providing fundamental data needed for the long-term evaluation of a waste form to only providing qualitative data of limited use. Although in the most rigorous view the long-term behaviour of a phase cannot be predicted, the correspondence between short-term experimental results and observations made of natural phases provides confidence in the "predicted" behavior of the waste form. The strength of this approach rests with the degree to which a mechanistic understanding of the phenomenon is attained.
INTRODUCTION One of the unique and scientifically most challenging aspects of nuclear waste isolation is the extrapolation of short-term laboratory data (hours to years) to the long time periods (103 - 10 5 years) required by regulatory agencies for performance assessment. The direct verification of these extrapolations is not possible [1], but methods must be developed to demonstrate compliance with government regulations and to satisfy the public that there is a demonstrable and reasonable basis for accepting the long-term extrapolations. This paper emphasizes near-field interactions, specifically the role of the waste form (borosilicate glass, U02 in spent nuclear fuel, or crystalline ceramics) in the retention of radionuclides. The evaluation of the long-term behaviour of waste package materials has a long history, and first efforts were directed at evaluating the durability of nuclear waste form glasses by the study of natural glasses [2]. More recently, this need has resulted in a "codified" approach embodied in ASTM standard Cl174-91 [3] entitled, "Standard Practice for Prediction of the Long-Term Behavior of Waste Package Materials Including Waste Forms Used in the Geologic Disposal of High-level Nuclear Waste." Although the ASTM standard provides a "cookbook" approach to the problem (replete with definitions and flow diagrams), the thesis of this paper is that there is no standard approach. Rather, on a case-bycase basis, one may use the occurrence and properties of natural phases to obtain useful information. I should also emphasize, without extended discussion, that one cannot predict or verify performance [2]. However, we can build confidence in the understanding of t
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