Geological Repository For High-level Waste Research Articles

Gas migration modelling in geological repository module in clay formation and sensitivity analysis

Corrosion of steel canisters disposed of in a geological repository for high-level waste leads to generation and accumulation of hydrogen gas which may significantly affect long-term safety of the repository. Numerical modelling can be used to predict the likely hydraulic evolution of such a disposal facility and to estimate the influence of generated gas on repository safety.This article presents a two-phase flow (not taking into account mechanical coupling) gas migration model developed in the original version of the computer code TOUGH2-MP (USA). An important and new aspect of this model is its capability to introduce a network of very thin interfaces in three-dimensional analysis of gas migration. This model was verified in the international benchmark exercises in the recent European Commission project FORGE (Fate Of Repository GasEs). The analysis was focused on the results of a second benchmark exercise, i.e. hydrogen gas behaviour in a part of a geological repository (module) situated in clay formation, and sensitivity analysis. The results showed that the interfaces, backfill and excavation disturbed zone are the main migration and accumulation media for gaseous hydrogen in the repository module. The peak pressure in the module did not exceed the geological environment pressure at a depth of 500m, and is not sufficient to disturb mechanical stability and functionality of the system of engineered and natural barriers.

Measuring hydraulic conductivity and swelling pressure under high hydraulic gradients

Abstract The concept of a deep geological repository for high-level waste in the Czech Republic is based on disposal in a granite environment. The construction of an engineered barrier between the rock and canister is considered, and compacted swelling clay (bentonite) is chosen as the most suitable material for such a barrier. The most important geotechnical properties of bentonite in this context are low hydraulic conductivity and high swelling pressure. Measuring these parameters in such a low-permeable material requires much time. This study focuses on combined measurements of hydraulic conductivity and swelling pressure and on the impact of using high hydraulic gradients that are consistent with possible porewater pressures in a deep repository. High hydraulic gradients may also quicken the determination of these parameters. Results show a decrease in hydraulic conductivity with increasing saturation pressure up to a threshold dry density, with no further change at higher dry densities. The influence of saturation pressure on measured pressure decreases with dry density until a certain value is reached. For samples with lower density, the slope of the increase is close to unity.

Destruction rate analysis of transuranic targets in sodium-cooled fast reactor (SFR) assemblies using MCNPX and SCALE 6.0

The purpose of this research was to model destruction rates of potential transuranic (TRU) targets made of americium and other inert matrix materials in the core of a sodium-cooled fast reactor (SFR) using depletion analysis in MCNPX and SCALE 6.0. The research is concentrated on the destruction of long-lived transuranics in an effort to reduce the overall volume and activity of spent nuclear fuel as proposed by the Global Nuclear Energy Partnership (GNEP) Technology Development Plan ( GNEP/TIO, 2007). The model uses a once-through approach in a single assembly of a sodium-cooled fast reactor based off of the Clinch River reactor design. Analysis was performed on both unmoderated and moderated targets in the fast reactor assembly. Optimization of the transmutation scheme could result in a 24-percent reduction in mass of long-lived transuranic actinides. Further iterations and optimization of the target material and geometry could greatly increase the actinide destruction rate. The overall reduction of TRU in spent nuclear fuel wastes will increase the usable life of the geological repository for high-level waste.

Nuclear Energy in a Global Context

This paper is a brief overview of the perspectives of development on nuclear power and nuclear wastes management in a global context. In contrast to a few years ago, nuclear energy is now considered by several countries as an important part of their future energy portfolio. Effective technical solutions for the long-term management of the radioactive wastes produced by current reactors are an industrial reality. Although practical experience in building and operating geologic repositories for high-level waste is still mainly limited to a few pilot-scale facilities, there is today a high level of confidence within the scientific and technical community that the geologic repository approach is capable of safely isolating the waste from the biosphere for as long as it poses significant risks.

Study of the contaminant transport into granite microfractures using nuclear ion beam techniques

Hydrated bentonite is a very plastic material and it is expected to enter in the rock microfractures at the granite/bentonite boundary of a deep geological high-level waste repository. This process is enhanced by the high swelling pressure of the clay. Since bentonite has a very good sorption capability for many radionuclides, the displacement of the clay might lead to a “clay-mediated” contaminant transport into the rock. The aim of this work is to study the contaminant transport into granite microfractures using nuclear ion beam techniques, and to determine to what extent the clay can favour it. To do so, bentonite previously doped with uranium, cesium and europium was put in contact with the surface of granite sheets. Granite sheets contacted with non-doped bentonite and with radionuclide solutions were also prepared as references. This allowed analysing the differences in the diffusion behaviour of the three systems: clay, radionuclides and clay plus radionuclides. A combination of Rutherford backscattering spectrometry (RBS) and other nuclear ion-beam techniques such as particle-induced X-ray emission (PIXE) and microPIXE was used to study the depth and lateral distribution of clay and contaminants inside granite. It was also tried to evaluate not only the diffusion depth and diffusion coefficients but also the different areas of the granite where the diffusants have a preferential access.

Effect of heterogeneity on radionuclide retardation in the alluvial aquifer near Yucca Mountain, Nevada.

The U.S. Department of Energy is currently studying Yucca Mountain, Nevada, as a potential site for a geological high-level waste repository. In the current conceptual models of radionuclide transport at Yucca Mountain, part of the transport path to pumping locations would be through an alluvial aquifer. Interactions with minerals in the alluvium are expected to retard the downstream migration of radionuclides, thereby delaying arrival times and reducing ground water concentrations. We evaluate the effectiveness of the alluvial aquifer as a transport barrier using the stochastic Lagrangian framework. A transport model is developed to account for physical and chemical heterogeneities and rate-limited mass transfer between mobile and immobile zones. The latter process is caused by small-scale heterogeneity and is thought to control the macroscopic-scale retardation in some field experiments. A geostatistical model for the spatially varying sorption parameters is developed from a site-specific database created from hydrochemical measurements and a calibrated modeling approach (Turner and Pabalan 1999). Transport of neptunium is considered as an example. The results are sensitive to the rate of transfer between mobile and immobile zones, and to spatial variability in the hydraulic conductivity. Chemical heterogeneity has only a small effect, as does correlation between hydraulic conductivity and the neptunium distribution coefficient. These results illustrate how general sensitivities can be explored with modest effort within the Lagrangian framework. Such studies complement and guide the application of more detailed numerical simulations.

Solute transport properties of compacted Ca-bentonite used in FEBEX project

The present Spanish concept of a deep geological high level waste repository includes an engineered clay barrier around the canister. The clay presents a very high sorption capability for radionuclides and a very small hydraulic conductivity, so that the migration process of solutes is limited by sorption and diffusion processes. Therefore, diffusion and distribution coefficients in compacted bentonite (i.e. in “realistic” liquid to solid ratio conditions) are the main parameters that have to be obtained in order to characterise solute transport that could be produced after the canister breakdown. Through-Diffusion (TD) and In-Diffusion (ID) experiments with HTO, Sr, Cs and Se were carried out using compacted FEBEX bentonite, which is the reference material for the Spanish concept of radioactive waste disposal. Experiments were interpreted by means of available analytical solutions that allow the estimation of diffusion coefficients and, in some cases, distribution coefficients. Analytical solutions are simple to use, but rely on hypotheses that do not hold in all the experiments. These experiments were interpreted also using an automatic parameter estimation code that overcomes the limitations of analytical solutions. Numerical interpretation allows the simultaneous estimation of porosity, diffusion and distribution coefficients, accounts for the role of porous sinters and time-varying boundary concentrations, and can use different types of raw concentration data.

An Ocean Island Geological Repository—A Second-Generation Option for Disposal of Spent Fuel and High-Level Waste

The concept of an ocean subseabed geological high-level waste repository with access via an ocean island is discussed. The technical advantages include, in addition to geologic waste isolation, geographical isolation, near-zero groundwater flow through the disposal site, and near-infinite ocean dilution as a backup in the event of a failure of the repository geological waste isolation system. The institutional advantages may include reduced siting problems and the potential of creating an international waste repository. Establishment of a repository accepting wastes from many countries would allow cost sharing, aid international nonproliferation goals, and ensure proper disposal of spent fuel from developing countries. Major uncertainties that are identified in this concept are the uncertainties in rock conditions at waste disposal depths, costs, and ill-defined institutional issues.