A simplified methodology for nuclear waste repository thermal analysis (original) (raw)
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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.
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.
Nuclear Technology - NUCL TECHNOL, 2004
The MultiScale ThermoHydrologic Model is used to predict thermal-hydrologic conditions in emplacement drifts and the adjoining host-rock throughout a proposed nuclear waste repository. The presented modeling effort simulates a lower temperature operation mode with a different panel loading than the repository currently being considered for the Yucca Mountain license application. Simulations address the influence of repository-scale thermal-conductivity heterogeneity and the influence of pre-closure operational factors on thermal-loading conditions. MSTHM can accommodate a complex repository layout, a development that, along with other improvements, enables more rigorous analyses of preclosure operational factors. Differences in MSTHM output occurring with these new capabilities are noted for a new sequential waste-package loading technique compared to a standard simultaneous loading technique. Alternative approaches to modeling repository-scale thermal-conductivity heterogeneity in ...
The Evaluation of the Thermal Behaviour of an Underground Repository of the Spent Nuclear Fuel
Lecture Notes in Computer Science, 2008
The paper concerns the evaluation of the thermal behaviour of an underground repository of the spent nuclear fuel where the canisters are disposed at a vertical position in the horizontal tunnels. The formulation of thermo-elastic problems should regard the basic steps of the construction of the repository. We tested the influence of the distance between the deposition places on the thermo-elastic response of the rock massif. The problems are solved by the in-house GEM-FEM finite element software. One sided coupling allows a separate solution of the temperature evolution and the computation of elastic responses only in predefined time points as a post-processing to the solution of the heat equations. A parallel solution of the arising linear systems by the conjugate gradient method with a preconditioning based on the additive Schwarz methods is used.
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.
2008
As part of the ongoing international DECOVALEX project, four research teams used five different models to simulate coupled thermal, hydrological, and mechanical (THM) processes near waste emplacement drifts of geological nuclear waste repositories. The simulations were conducted for two generic repository types, one with open and the other with back-filled repository drifts, under higher and lower postclosure temperatures, respectively. In the completed first 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 is currently being resolved. In particular, good agreement in the basic thermal-mechanical responses was achieved for both repository types, even though some teams used relatively simplified thermal-elastic heat-conduction models that neglected complex near-field thermal-hydrological processes. The good agreement between the complex and simplified process models indicates that the basic thermalmechanical responses can be predicted with a relatively high confidence level.
Towards higher temperatures in nuclear waste repositories
2020
Recently, there is a tendency to explore the possibility of increasing the maximum design temperature in deep geological repositories for high-level nuclear waste and spent fuel. In the paper, a number of issues related to the use of higher temperatures are reviewed. Both bentonite barriers and argillaceous host rocks are addressed. An application involving the modelling of a large-scale field test conducted at a maximum temperature of 140oC is presented. It is shown that currently available theoretical formulations and computer codes are capable to deal with temperatures above 100oC and to reproduce satisfactorily the thermally-induced overpressures in the rock.
Linear and nonlinear transient heat conduction in nuclear waste repositories
Mathematical and Computer Modelling, 1988
Analytical solutions of thermal problems connected with the disposal of nuclear wastes are presented. Linear and nonlinear diffusion problems are analyzed considering time-dependent heat sources. Comparisons between the temperature distributions at a nuclear waste site are made. Greek symbols GL = Thermal diffusivity (m*/yr) I = Decay constant (jr-') y, = Expansion coefficient (m-l) p = Density (kg/m3)