Near-field repositories

The nature of a BMT study is well demonstrated with BMT3 of the DECOVALEX I project. It was a problem associated with a near-field repository model, set up as a two-dimensional plane-strain problem in which a tunnel with a deposition hole was located in a fractured rock mass. The model is 50 X 50 m in size, and situated at 500 m below the ground level (Figure 1). The fracture network is a two-dimensional realization of 6,580 fractures from a realistic three-dimensional fracture network model of the Stripa Mine, Sweden (Figure 2). The problem is set up as a fully coupled THM near-field repository problem, with thermal effects caused by heat release from radioactive waste in the deposition hole (the heater). Heat output decreases... 

Grenthe, I. Ferri, D. Proc. OECD/NEA Workshop on Near-field Phenomena In Geologic Repositories for Radioactive Waste OECD/NEA Paris 1981. 

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 near field of the repository includes the engineered barrier system (EBS, i.e., canister and buffer) and the waste form. Also included in the near field is the interface between the buffer and the host rock, denoted as excavation disturbed zone (EDZ). In terms of waste/water interactions, the geochemical evolution of the near field is essential as it controls the composition of the fluids that will eventually contact the waste. 

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. 

There are inherent scale limitations in the time and space dimensions covered by laboratory studies. The applicability of the near field geochemical models derived from laboratory observations have to be applied to long-term, large-scale situations like the ones involved in the safety assessment of nuclear waste repositories. Hence, there is a need to test the models developed from laboratory investigations in field situations that are related to the ones to be encountered in repository systems. 

Fig. 1. Potential colloid generation processes in a nuclear waste repository near field (for explanation, see text) (HLW, high-level waste EBS, engineered barrier system).

Th-oxyhydroxide species readily dissolve upon dilution below the solubility limit, it is not veiy likely that such actinide(IV) colloids play a role away from the source in the far field of a repository. In the near field of a repository, however, they may be predominant species controlling the solubility of tetravalent actinide species such as U(IV) and Pu(IV) and thus the source term. Unusual stability at high ionic strength has been also reported for amorphous SiOz colloids (Iler 1979 Healy 1994) which also cannot be explained solely by electrostatic repulsion. Formation of oligomeric or polymeric silicate species at the colloid-water interface are thought to exert additional steric stabilization by preventing close approach of those particles. 

Berner, U. 2002a. Project Opalinus Clay Radionuclide Concentration Limits in the Near-Field of a Repository for Spent Fuel and Vitrified High-Level Waste. Nagra Technical Report NTB 02-10, Nagra, Wettingen, Switzerland. Also issued as PSI Bericht Nr. 02-22, Paul Scherrer Institut, Villi-gen, Switzerland. 

Rai, Dhanpat Strickert, R. G. Swanson, J. L., "Actinide Solubilities in the Near-Field of a Nuclear Waste Repository," In Workshop on Near-Field Phenomena in Geologic Repositories, (August 31 - September 3, 1981, Seattle, Washington), Nuclear Energy Agency of OECD, Paris, France, 1981 pp. 13-20. 

Goodwin, B. W. Johnson, L. H. Wuschke, D. M. Proc. NEA Workshop, OECD, Near Field Phenomena in Geologic Repositories for Radioactive Waste, 1981, 33. 

Rutqvist, J M. Chijimatsu, L. Jing, A. Millard. T.S. Nguyen, A. Rejeb, Y.Sugita and C.F. Tsang, Evaluation of the impact of thermal-hydrological-mechanical couplings in bentonite and near-field rock barriers of a nuclear waste repository in sparsely fractured hard rock, this volume, 2004. 

Jing, L and Nguyen, T.S. (eds.). 2003. Report of BMTIA/WP2 - Implications of T-H-M coupling on the near-field safety of a nuclear waste repository. SKI (in Press). 

From these comparative studies, and in the particular case considered here, it has been possible to evaluate the importance of the various T-H-M couplings on the safety assessment of typical repository, in the near field. The major findings are summarized in table 4. 

EVALUATION OF THE IMPACT OF THERMAL-HYDROLOGICAL-MECHANICAL COUPLINGS IN BENTONITE AND NEAR-FIELD ROCK BARRIERS OF A NUCLEAR WASTE REPOSITORY IN SPARSELY FRACTURED HARD ROCK... 

The DECOY ALEX project is an international co-operative project, initiated by SKI, the Swedish Nuclear Power Inspectorate, to support the development of mathematical models of coupled THM processes in the host rock for potential nuclear fuel waste repositories. The DECOY ALEX project has been running for more than a decade the third phase of DECOY ALEX called DECOY ALEX III is now being finalized with two main objectives. The first objective is the validation of codes by simulating large-scale experiments. The second objective is to determine the relevance of THM processes on the safety of a repository. To achieve the second objective, benchmark tests are proposed, where typical repository designs, engineered barriers and host rocks are examined. The benchmark problem called BMTl defined in this paper looks at the implications of coupled THM processes on the near-field performance of a repository. The performance of a nuclear waste repository is dependent on two main components ... 

Figure 4. Conceptual representation of repository near-field...

In this paper, we provided the rationale and definition of a benchmark test called BMTl to look at the implications of THM couplings on safety parameters in the near field of a hypothetical repository. This hypothetical repository possesses composite features since it is based on a Japanese design, with a Japanese bentonite used as buffer material and the heat output characteristics of Japanese spent fuel. However, the permeability and strength characteristics of the rock mass are based on typical properties of granites of the Canadian Shield. 

The work presented here is being performed in the framework of FEBEX (Full-scale Engineered Barriers Experiment in Crystalline Host Rock), which is a project for the study of the near field for a HLW repository in crystalline rock according to the Spanish concept the waste canisters are surrounded by a clay barrier constructed from... 

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