Analysis of transient history of underground excavations for radioactive waste isolation (original) (raw)
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Thermal Modeling of High-Level Nuclear Waste Disposal in a Salt Repository
Salt formations have received recent attention for geologic disposal of heat-generating, high-level nuclear waste (HLW). Existing investigations are summarized and expanded upon using analytical and numerical models to investigate simulated temperatures in the salt after emplacement of HLW. Analytical modeling suggests that temperature variations near canisters will be smooth, indicating that the system can be approximated by a coarsely discretized numerical model. Two multidimensional parameter studies explore canister configuration using characteristics from (a) defense HLW and (b) spent nuclear fuel (SNF) waste. Numerical modeling was conducted for a disposal concept consisting of emplacement of waste canisters on the floor of drifts and covering each with salt backfill. Results indicate that waste forms with U.S. Department of Energy (DOE) waste characteristics can be easily configured to maintain simulated temperatures far below 200uC at spacings as close as 0.3 m (*1 ft), the minimum feasible spacing that could practically be achieved. For SNF waste packaged into canisters with heat loads of 1500 or 1000 W with canister spacing of 6 m (*20 ft) and 3 m (*10 ft), respectively, simulated temperatures can be maintained below 200uC; much higher maximum temperatures would result for designs with higher canister heat loads and smaller spacings. These results indicate that from a thermal loading perspective, in-drift disposal of HLW in salt deposits is feasible for DOE-managed waste as long as the maximum temperature is managed through proper selection of canister heat loads and spacings. The results will aid in the design of potential future field tests to confirm this conclusion.
Transient thermal response in nuclear waste repositories
Nuclear Engineering and Design, 2000
In this paper, it is shown that a previously reported non-linear, one-dimensional, theoretical approximation simplifies-from a computational point of view-the calculation of the time-decay temperature field in nuclear waste repositories (NWR). This conclusion has been reached after solving, by using the control volume numerical method, the full three dimensional, transient, non-linear heat diffusion equation. The transient thermal field in a rock salt repository, is analytically solved and numerically predicted, along 100 years, after the disposal of a high-level waste (HLW). The nuclear waste, with a half-life of 32.9 years, releases an exponentially time dependent heat flux with 12 W m − 2 as the initial thermal load. Two cases are studied, in the first one it is assumed that the conductivity (k) and the volumetric heat capacity zc p of the host rock (diffusion domain) remain constant (linear case), whereas in the second one, a more realistic situation is analysed. In this last case, the conductivity of the rock salt varies as a function of the temperature field and the product z× c p remains constant (non-linear case). In order to observe the effect of the salt conductivity (constant or variable) on the repository temperature distribution, a comparison of both cases is performed. It is concluded, that the theoretical model, which provides an analytical solution of the thermal fields may be a powerful low cost method for design purposes.
Thermohydrologic behavior at an underground nuclear waste repository
Water Resources Research, 2002
1] We present a multiscale model that simulates coupled thermal and hydrological behavior driven by radioactive decay heat from a potential nuclear waste repository at Yucca Mountain, Nevada. We use this model to evaluate repository performance for different designs with respect to major thermal design goals (e.g., keeping waste packages dry). A locally boiling or globally boiling design uses rock dry out to create dry (low relative humidity (RH)) conditions around waste packages for a long period of time. A subboiling design eliminates boiling in the host rock (possibly reducing uncertainty) but is less effective at maintaining dry conditions. The addition of backfill increases the duration of RH reduction significantly without added boiling in the host rock but at the cost of higher waste package temperatures. The interaction between engineering design variables and natural system factors that affect thermohydrologic behavior is highly nonlinear. As a consequence, designs that differ by seemingly small details can result in markedly different thermohydrologic behavior and consequently repository performance, whereas with regards to other details, large differences may have little or no effect.
A simplified methodology for nuclear waste repository thermal analysis
Annals of Nuclear Energy, 2011
A simplified model for repository thermal analysis is presented in this paper. The proposed model is to provide a general capability to efficiently calculate the time dependent temperature field in a geologic repository. The model analyzes both horizontal and vertical emplacement of nuclear waste packages. Verification of the code was performed based on the comparison with detailed numerical method-based standard models. The new model's utility was demonstrated through a case study where a large number of repository-scale thermal analysis calculations is needed.
Computers and Geotechnics, 2016
The Thermal Simulation for Drift Emplacement heater test is modeled using two simulators for coupled thermal-hydraulic-mechanical processes. Results from the two simulators are in very good agreement. The comparison between measurements and numerical results is also very satisfactory, regarding temperature, drift closure and rock deformation. Concerning backfill compaction, a parameter calibration through inverse modeling was performed due to insufficient data on crushed salt reconsolidation, particularly at high temperatures. We conclude that the two simulators investigated have the capabilities to reproduce the data available, which increases confidence in their use to reliably investigate disposal of heat-generating nuclear waste in saliferous geosystems.
Water, Vapor, and Salt Dynamics in a Hot Repository
MRS Proceedings, 2006
The purpose of this paper is to report the results of a new model study critically examining the high temperature nuclear waste disposal concept at Yucca Mountain using MULTIFLUX, an integrated in-drift- and mountain-scale thermal-hydrologic model. In addition to new results the paper summarizes results of a previous study. The results show that a large amount of vapor flow into the drift is expected during the period of above-boiling temperatures in the emplacement drift. This phenomenon makes the emplacement drift a water/moisture attractor for thousands of years during the above-boiling temperature operation.The evaporation of the percolation water into the drift gives rise to salt accumulation in the rock wall, especially in the crown of the drift for about 1500 years in the example. The deposited salts over the drift footprint, almost entirely present in the fractures, may enter the drift either by rock fall or by water drippage. During the high temperature operation mode the b...
Nuclear Engineering and Design, 1982
An approximate, semi-analytical heat conduction model for predicting the time-dependent temperature distribution in the region of a high-level waste repository has been developed. The model provides the basis for a systematic, inexpensive examination of the impact of several independent thermal design constraints on key repository design parameters and for determining the optimal set of design parameters which satisfy these constraints. Illustrative calculations have been carried out for conceptual repository designs for spent pressurized water reactor (PWR) fuel and reprocessed PWR high-level waste in salt and granite media. .ii.
Engineering Geology, 2015
A modelling effort has been undertaken to investigate the long-term response of a generic salt repository for heat-generating nuclear waste, including processes that could affect the geological (natural salt host rock) and geotechnical (backfill) barriers. For this purpose, the TOUGH-FLAC sequential simulator for coupled thermal-hydraulic-mechanical processes modelling has recently been provided with a capability for large strains and creep. The responses of the saliferous host rock and the crushed salt backfill are modelled using dedicated constitutive relationships. Similarly, the coupling between the geomechanics and the flow subproblems is performed on the basis of theoretical and experimental studies. The repository investigated in this work considers in-drift emplacement of the waste packages and subsequent backfill of the drifts with run-of-mine salt. Using the updated TOUGH-FLAC, the compaction of the backfill and the evolution of its properties as porosity decreases can be modelled. Additionally, different processes that may influence the initial tightness of the host rock can be investigated. On the basis of state-of-the-art phenomenological models, our simulation results show that, in order to evaluate the barriers integrity, it is necessary to consider full coupling between thermal, hydraulic and mechanical processes. A base case scenario that accounts for these coupled processes is presented and compared to a case in which the mechanical processes are disregarded. Also, we investigate the sensitivity of the coupled numerical predictions to two factors: the initial saturation within the host rock and the capillary forces. Although the outcome of these simulations is preliminary and will be improved as the understanding of relevant salt processes moves forward, the numerical tools required to perform the target predictions have been significantly improved.
2006
As part of the ongoing international code comparison project DECOVALEX, four research teams usedfive dzflerent models to simulate coupled thermal, hydrological, and mechanical (THM) processes near underground waste emplacement drifts. The simulations were conducted for two generic repository types with open or back-filled repository drifts under higher and lower post-closure temperature, respectively. In the completedfirst model inception phase of the project, a good agreement was achieved between the research teams in calculating THM responses for both repository types, although some disagreement in hydrological responses are currently being resolved. Good agreement in the basic thermalmechanical responses was achieved for both repository types, even with some teams using relatively simplified thermalelastic heat-conduction models that neglect complex near-field thermal-hydrological processes. The good agreement between the complex and simplified (and well-known) process models indicates that the basic thermal-mechanical responses can be predicted with a relatively high confidence level. The research teams have now moved on to the second phase of the project, the analysis of THM-induced permanent (irreversible) changes and the impact of those changes on the fluidflowfield near an emplacement drift.