A full-scale in situ heating test for high-level nuclear waste disposal: observations, analysis and interpretation

Abstract

The paper describes the performance, observations and interpretation of a large-scale in situ heating test that simulates a disposal concept for heat-emitting, high-level nuclear waste. In the experiment, heaters are emplaced in the axis of a tunnel excavated in granite to simulate the heat production of radioactive waste. The test is fully instrumented, and attention is focused on the thermo-hydro-mechanical (THM) behaviour of the near-field region constituted by the compacted bentonite barrier surrounding the heater and the immediately adjacent rock. Interpretation of the test is assisted by the performance of a coupled numerical analysis based on a formulation that incorporates the relevant THM phenomena. Initial and boundary conditions for the analysis as well as material parameters are determined from an extensive programme of field and laboratory experiments. The paper presents and discusses the thermal, hydraulic and mechanical observations in the bentonite barrier and in the host rock. Special attention is paid to the progress of hydration in the barrier, to the effects of heating and vapour transport, and to the development of swelling pressures in the barrier. After five years of heating, one of the heaters was switched off and the experiment was partially dismantled, allowing the final state of the barrier to be observed directly. The numerical analysis performed has proved able to represent the progress of the experiment very satisfactorily. In addition, predictions concerning the final state of the clay barrier are very close to the observations obtained during dismantling. The performance and analysis of the in situ test have significantly enhanced the understanding of a complex THM problem and have proved the capability of the numerical formulation to provide adequate predictive capacity

Report on THMC Modeling of the Near Field Evolution of a Generic Clay Repository: Model Validation and Demonstration

“…In addition, we now add the material parameters for the BBM, which are given in Table 4-2. The parameters for the BBM used in this initial 3D THM model simulation are those derived for FEBEX bentonite by Gens et al (2009), and also applied in Rutqvist et al (2013) for the modeling of THM processes around a generic nuclear waste repository. A large number of material parameters are required to describe the THM material behavior, and those given in Table 4-2 have been determined from laboratory experiments.…”

Disposal systems evaluations and tool development : Engineered Barrier System (EBS) evaluation.

“…Over time, the expansion of the bentonite as it is being hydrated may displace the position of the waste canister enveloped in it, as well as the position of the buffer-backfill interface. Chemically within the bentonite buffer, there may be dissolution of buffer minerals and precipitation of chemical compounds (Castellanos, et al, 2008;Guimaraes et al, 2009;Seetharam et al, 2006).…”

Benchmarking selection of parameter values for the Barcelona basic model

“…Costa et al, 2008) and underground disposal of nuclear waste (e.g. Gens et al, 2009). Dissemination and use of the BBM outside the unsaturated soils research community has however been relatively limited, and possible contributory factors in this have been uncertainty in how best to select BBM model parameter values from laboratory test data and concerns on the robustness of such parameter value selection procedures.…”