Repository performance assessment

Site characterization studies include a surface-based testing program, potential environmental impact, and societal aspects of the repository. Performance assessment considers both the engineered barriers and the geologic environment. Among features being studied are the normal water flow, some release of carbon-14, and abnormal events such as volcanic activity and human intmsion. The expected date for operation of the repository is 2013. 

It has been the underlying theme of this paper that the theoretical approach to solubility and speciation of key radionuclides adopted here can be helpful in identifying the general types of species likely to be dominant in aqueous solutions. At present, these estimates are useful in identifying those radionuclides that require special attention in repository performance assessment studies. However, sophisticated experimental studies are necessary to provide important confirmatory data for nuclides of critical importance to the safe isolation of nuclear waste in a repository in basalt. 

Wolery, T. J., C. E. Palmer, and K. G. Knauss. 1995. The neptunium solubility problem in repository performance assessment A white paper. Unpublished paper. Yucca Mtn. Site Characterization Project, Lawrence Livermore Natl. Lab. 

In addition to analysis of these BMTs and TCs. particular topics were selected for discussion and review among project participants. These range from the state-of-the-art of constitutive relations of rock fractures to the current international treatment of THM issues in repository performance assessment. 

Natural systems that may be analogous in certain respects to a geologic repository for nuclear wastes provide a useful basis for testing assumptions, models and concepts used in repository performance assessments. The present study demonstrates that maximum U concentrations in groundwaters of the Tono Uranium Deposit appear to be limited by the solubility of the amorphous, hydrous oxide, U02(am). This conclusion, drawn from observations of a geologic system that has evolved over long periods of time, supports the... 

Banwart, S. (ed.) 1995. The Aspo Redox Investigations in Block Scale. Project Summary and Implications for Repository Performance Assessment. Technical Report 95-26. The Swedish Nuclear Fuel and Waste Management Co., Stockholm. 

OCWRM, 1998. Viabilily Assessment of a Repository at YuccaMountain, Volume 3 Total System Performance Assessment, DOE/RW-0508/V3, U.S. Department of Energy, Office ofCivilian Radioactive Waste Management, North Las Vegas. 

Bonotto, D. M. 1998. Generic performance assessment for a deep repository for low and intermediate level waste in the UK - a case study in assessing radiological impacts on the natural environment. Journal of Environmental Radioactivity, 66, 89-119. 

Grambow, B. 1991. What do we know about nuclear waste glass performance in the repository near field In Sellin, P., Apted, M. Gago, J. (eds) Proceedings Technical Workshop on Near-Field Performance Assessment for High-Level Wastes, Madrid, Spain. SKB Technical Report 91-59. Stockholm, Sweden, 25-49. 

The interactions between the waste matrix and the contacting fluids in the near field constitute the source term of the ulterior complex models used in the performance assessment of spent fuel repositories. 

Fig. 6. The Nagra/PSI TDB 01/01 was used in this modelling chain for the performance assessment of a proposed Swiss repository for spent fuel, vitrified high-level waste, and long-lived intermediate-level waste.

Bradbury, M. H. Baeyens, B. 2002a. Near-Field Sorption Data Bases for Compacted MX-80 Bentonite for Performance Assessment of a High Level Radioactive Waste Repository in Opalinus Clay Host Rock. Nagra Technical Report NTB 02-18, Nagra, Wettingen, Switzerland. Also issued as PSI Bericht Nr. 03-07, Paul Scherrer Institut, Villigen, Switzerland. 

Currently proposed licensing regulations for geologic nuclear waste repositories require a performance assessment involving long-term predictive capabilities. Previous work (J- 5) has shown the importance of solubility controls for modeling maximum actinide concentrations in repository groundwaters. However, until reliable data are available on the actinide solid phases that may be present or that may precipitate in the environment, the solubility of solid phases such as hydrous oxides that have fast precipitation kinetics can be used to initially set maximum solution concentration limits. 

Because of the presence of Fe and Fe(II), Pu(VI) is not expected to be stable under WIPP repository conditions. Plutonium(IV) is expected to be the dominant oxidation state, although Pu(III) was also considered to be a possibility in the WIPP CCA. In the WIPP performance assessment speciation and solubility calculations, Th(IV) was used as an analogue for Pu(IV) (US DOE, 1996). Wall et al. (2002) evaluated the appropriateness of the analogy and found that this assumption was highly conservative and that predicted solubilities for Pu(IV) in Salado and Castile brines were 10-11 orders of magnitude lower than those for Th(IV). Similarly, Am(III) was used to estimate the solubility of Pu(III). 

Performance assessment calculations of actinide speciation and solubility, and of the potential releases that could result if the repository is breached, were carried out as part of the CCA) for the waste isolation pilot plant (WIPP) (US DOE, 1996 US EPA, 1998a,b,c,d). The calculations modeled actinide behavior in a reference Salado brine and a less magnesium-rich brine from the Castile Formation as described previously (see Tables 6 and 8). The performance assessment calculations will be periodically repeated with updated parameter sets as part of site recertification. 

Siegel M. D. and Erickson K. E. (1986) Geochemical sensitivity analysis for performance assessment of HEW repositories effects of speciation and matrix diffusion. Proceedings of the Symposium on Groundwater Flow and Transport Modeling for Performance Assessment of Deep Geologic Disposal of Radioactive Waste A Critical Evaluation of the State of the Art. Sandia National Eaboratories, Albuquerque, NM, pp. 465-488. 

Andrews, R. A. et al. 1995. Total system performance assessment-J 995 An evaluation of the potential Yucca Mountain repository. Las Vegas, NV Civilian Radioactive Waste Management System Management and Operating Contractor. 

Geochemical modeling has also been widely used in performance assessment of high-level nuclear waste repositories in other countries, with which we are not as familiar. Therefore, our book has a strong bias toward using examples in the USA. 

The performance of a geological repository for nuclear waste is influenced by many processes. Determination of whether the total system will comply with regulatory requirements necessitates consideration of all components of the systems and the effects of linking all the components together. This linkage is important because it allows each component to be viewed in the context of the behavior of the entire system. Hence, the concept and methodology of Total System Performance Assessment (TSPA) is widely used in nuclear waste disposal (Figure 2.2). 

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