Investigations on Repository Near-Field Thermal Modeling - Repository Science/Thermal Load Management & Design Concepts (M41UF033302)
The various layers of material from the waste package (such as components of the engineered barrier system and the host rock surface) to a given distance within the rock wall at a given distance can be described as concentric circles with varying thermal properties (see Figure 5.1-1). The selected model approach examines the contributions of the waste package, axial waste package neighbors and lateral neighboring emplacement drifts (see Section 5.2.1 and Appendix H, Section 2). In clay and deep borehole media, the peak temperature is driven by the central waste package whereas, in granite and salt, the contribution to the temperature rise by adjacent (lateral) waste packages in drift or emplacement borehole lines is dominant at the time of the peak temperature. Mathematical models generated using Mathcad software provide insight into the effects of changing waste package spacing for six waste forms, namely UOX, MOX, co-extraction, new extraction, E-Chem ceramic and E-Chem metal in four different geologic media (granite, clay, salt and deep borehole). Each scenario includes thermal conductivity and diffusivity for each layer between the waste package and the host rock, dimensions of representative repository designs (such as waste package spacing, drift or emplacement borehole spacing, waste package dimensions and layer thickness), and decay heat curves generated from knowledge of the contents of a given waste form after 10, 50, 100 and 200 years of surface storage. Key results generated for each scenario include rock temperature at a given time calculated at a given radius from the central waste package (Section 5.2.1 and Appendix H, Section 3), the corresponding temperature at the interface of the waste package and EBS material, and at each EBS layer in between (Section 5.2.2 and Appendix H, Section 4). This information is vital to understand the implications of repository design (waste package capacity, surface storage time, waste package spacing, and emplacement drift or borehole spacing) by comparing the peak temperature to the thermal limits of the concentric layers surrounding the waste package; specifically 100 C for the bentonite buffer in granite and clay repositories, 100 C for rock wall in a clay repository and 200 C at the rock wall for a salt repository. These thermal limits are both preliminary and approximate, and serve as a means to evaluate design options rather than determining compliance for licensing situations. The thermal behavior of a salt repository is more difficult to model because it is not a concentric geometry and because the crushed salt backfill initially has a much higher thermal resistance than intact salt. Three models were investigated, namely a waste package in complete contact with crushed salt, secondly a waste package in contact with intact salt, and thirdly a waste package in contact with 75% intact and 25% crushed salt. The latter model best depicts emplacement of a waste package in the corner of an intact salt alcove and subsequently covered with crushed salt backfill to the angle of repose. The most conservative model (crushed salt) had temperatures much higher than the other models and although bounding, is too conservative to use. The most realistic model (75/25) had only a small temperature difference from the simplest (non-conservative, intact salt) model, and is the one chosen in this report (see Section 5.2.3). A trade-study investigating three key variables (surface storage time, waste package capacity and waste package spacing) is important to understand and design a repository. Waste package heat can be reduced by storing for longer periods prior to emplacement, or by reducing the number of assemblies or canisters within that waste package. Waste package spacing can be altered to optimize the thermal load without exceeding the thermal limits of the host rock or EBS components. By examining each of these variables, repository footprint (and therefore cost) can be optimized. For this report, the layout was fixed for each geologic medium based on prior published designs in the international community, but it will be varied in future work. Section 5.2.4 summarizes the conclusions based on varying two of the three parameters (storage time and waste package capacity), and the results are shown in Appendix H, Sections 5 and 6.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 1031294
- Report Number(s):
- LLNL-TR-491099; TRN: US1200106
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
12 MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
36 MATERIALS SCIENCE
42 ENGINEERING
97 MATHEMATICAL METHODS AND COMPUTING
BENTONITE
BOREHOLES
BUFFERS
CAPACITY
CERAMICS
CLAYS
COMPLIANCE
CONTAINERS
DESIGN
DIMENSIONS
GEOMETRY
GRANITES
LOAD MANAGEMENT
MATHEMATICAL MODELS
STORAGE
THERMAL CONDUCTIVITY
THERMODYNAMIC PROPERTIES
THICKNESS
WASTE FORMS
WASTES