CBP [TASK 12] experimental study of the concrete salstone two-layer system (original) (raw)
Modeling approaches for concrete barriers used in low-level waste disposal
1993
A seriesof threeNUREGsand severalpapersaddressingdifferentaspectsof modelingperformanceof concretebarriersfor low-level radioactivewaste disposal havebeen preparedpreviouslyfor the Concrete BarriersResearch Project.This documentintegratesthe information fromthe previous documentsinto a generalsummaryof modelsand approachesthatcan beused in performanceassessmentsof concretebarriers. Models for concretedegradation,flow,and transport throughcrackedconcretebarriersare discussed. The modelsfor flow andtransportassumethatcracks have occurredand thusshouldonly be used forlater times in simulationsafterfully penetratingcracksareformed.Most of the modelshave been implemented in a computercode, CEMENT,thatwas developed concurrentlywith this document.User documentation forCEMF_NT is providedseparatefromthisreport.To avoid duplication, the readeris referredto the three previousNUREGsfor detailed discussionsof each of the mathematical models. ,Someadditionalinformation that was notpresentedi_ the previousdocumentsis also included. Sectionsdiscussinglessons learned from applicationsto actual performanceassessmentsof low-level waste disposal facilities are provided. Sensitivedesignparametersare emphasizedto identify criticalareas of performanceforconcrete barriers, andpotentialproblems in performanceassessments are also identifiedand discussed.
Magazine of Concrete Research, 2007
Within the French context of nuclear waste disposal in deep geological formation, concrete will be used as the building material for structures as well as for engineered barrier systems (EBSs). With regard to the durability that is necessary to keep the disposal facilities safe and secure, the long-term behaviour of cement-based materials has to be modelled. To assess the long-term evolution of physical and chemical properties of concrete, it is necessary to perform short-term experiments in conditions allowing extrapolations. The aim of this project was to determine the variation of transfer properties (diffusivity, permeation) with hydrolysis/decalcification. To simulate the two time phases of the structure life according to the external conditions, transfer properties were examined through two decalcification tests: a dynamic (temperature between 20 and 80°C, under a hydraulic pressure drop from 2 to 10 MPa) and a static test (NH4NO3 attack). The results could be used as input da...
Crystals
Details are presented of the development of a coupled modeling simulator for assessing the evolution in the near-field of a geological repository for radioactive waste disposal where concrete is used as a backfill. The simulator uses OpenMI, a standard for exchanging data between simulation software programs at run-time, to form a coupled chemical-mechanical-hydrogeological model of the system. The approach combines a tunnel scale stress analysis finite element model, a discrete element model for accurately modeling the patterns of emerging cracks in the concrete, and a finite element and finite volume model of the chemical processes and alteration in the porous matrix and cracks in the concrete, to produce a fully coupled model of the system. Combining existing detailed simulation software in this way with OpenMI has the benefit of not relying on simplifications that might be necessary to combine all of the modeled processes in a single piece of software.
Volume 1: Low/Intermediate-Level Radioactive Waste Management; Spent Fuel, Fissile Material, Transuranic and High-Level Radioactive Waste Management, 2013
The paper aims to highlight recent developments at the Belgian Nuclear Research Centre SCK•CEN in experimental and numerical study of the coupled physical-chemical behaviour of concrete subject to chemical degradation. The discussion mainly focusses on three interlinked research projects covering novel experimental methods to study the alteration of hydraulic and transport properties during carbonation and calcium leaching, a pore scale numerical model to capture microstructural changes due to the above degradation processes and a generic multiscale model aimed at determining evolution of the properties of a macrostructure over the long term. The paper also describes supplementary continuum scale numerical studies concerning concrete-clay interactions and geochemical impact on the physical structure of concrete. Preliminary findings from these studies show encouraging results such as the development of novel leaching, water permeability and diffusion apparatus, a robust pore scale model based on Lattice-Boltzmann method and a mesoscale study focused on the importance of interfacial transition zones on the effective diffusivity for linear and nonlinear diffusion problems.
11th International Conference on Environmental Remediation and Radioactive Waste Management, Parts A and B, 2007
Gas generation and gas transport phenomena occur in geological repositories of radioactive waste. This has been extensively studied over the past ten years, usually within the framework of international projects (PEGASUS, MEGAS, PROGRESS, etc.). These studies indicate that the production of hydrogen by anaerobic corrosion of metals is the most important source for gas generation. Laboratory and in situ experiments carried out at SCK•CEN indicate that, in the presence of Boom Clay (the reference geologic formation for deep disposal studies in Belgium), carbon steel suffers generalised corrosion estimated conservatively at 1 µm y -1 . Simulations with the finite difference multi-phase flow code TOUGH2 were carried out in an attempt to quantify the effects of hydrogen gas generation on desaturation of initially saturated concrete components of the disposal gallery and the concomitant expulsion of cementitious porewater into the surrounding host formation. Several simulation cases were considered and addressed differences in (1) initial concrete saturation degree, (2) hydrogen gas generation rate, and (3) porosity. Several conceptual models have been developed to better understand the phenomena at work in the transport of gas in the cementitious engineered barriers and Boom Clay. Multi-phase flow modelling was found to be helpful to get insight into the phenomenology of coupled water-gas flow in the cementitious engineered barriers. However, discontinuous variation in the conductivity of the clay relative to the gas (creation of preferential pathways) makes it impossible to use conventional models based on the laws of two-phase flow.
Journal of Nuclear Fuel Cycle and Waste Technology(JNFCWT)
Long-term experiments have been conducted on two important safety issues: long-term durability of a concrete barrier with the steel reinforcements and gas generation from low-and intermediate-level wastes in an underground research tunnel of a radioactive waste disposal facility. The gas generation and microbial communities were monitored from waste packages (200 L and 320 L) containing simulated dry active wastes. In the concrete experiment, corrosion sensors were installed on the steel reinforcements which were embedded 10 cm below the surface of concrete in a concrete mock-up, and groundwater was fed into the mock-up at a pressure of 2.1 bars to accelerate groundwater infiltration. No clear evidence was observed with respect to corrosion initiation of the steel reinforcement for 4 years of operation. This is attributed to the high integrity and low hydraulic conductivity of the concrete. In the gas generation experiment, significant levels of gas generation were not measured for 4 years. These experiments are expected to be conducted for a period of more than 10 years.
Volume 1: Low/Intermediate-Level Radioactive Waste Management; Spent Fuel, Fissile Material, Transuranic and High-Level Radioactive Waste Management, 2013
In large cement-based structures such as a near surface disposal facility for radioactive waste voids and cracks are inevitable. However, the pattern and nature of cracks are very difficult to predict reliably. Cracks facilitate preferential water flow through the facility because their saturated hydraulic conductivity is generally higher than the conductivity of the cementitious matrix. Moreover, sorption within the crack is expected to be lower than in the matrix and hence cracks in engineered barriers can act as a bypass for radionuclides. Consequently, understanding the effects of crack characteristics on contaminant fluxes from the facility is of utmost importance in a safety assessment. In this paper we numerically studied radionuclide leaching from a crack-containing cementitious containment system. First, the effect of cracks on radionuclide fluxes is assessed for a single repository component which contains a radionuclide source (i.e. conditioned radwaste). These analyses reveal the influence of cracks on radionuclide release from the source. The second set of calculations deals with the safety assessment results for the planned near-surface disposal facility for low-level radioactive waste in Dessel (Belgium); our focus is on the analysis of total system behaviour in regards to release of radionuclide fluxes from the facility. Simulation results are interpreted through a complementary safety indicator (radiotoxicity flux). We discuss the possible consequences from different scenarios of cracks and voids.
THCM numerical simulations of the engineered barrier system for radioactive waste disposal
Environmental Geotechnics, 2020
The final disposal of high-level radioactive waste in geological repositories envisages an engineered barrier system with a bentonite buffer, which will be subjected to strongly coupled thermal, hydrodynamic, geochemical and mechanical (THCM) processes. This paper presents coupled THCM numerical simulations of laboratory and in situ tests performed with compacted Full-scale Engineered Barrier Experiment (Febex) bentonite having different space and time scales. The simulations of the heating and hydration tests fit the measured temperature, water content and water intake data and reproduce the trends of the geochemical data. Although simulation results of the tests display similar trends, they show differences due to geometry and initial and confining conditions. The changes in porosity due to mineral dissolution/precipitation are not relevant in these tests but become relevant in long-term simulations, which show that the precipitation of corrosion products reduces significantly the...
Waste Management and the Environment IV, 2008
A modular system for containment of nuclear waste packages based on two barriers of cementitious materials was designed for a repository of radioactive substances, and then investigated in order to test leachability in a period of time of 300 years. Cement agglomerates designed for the isolation of dangerous waste exhibit low permeability, hence a favourable durability, that appoints them as proper material for construction of hazardous waste in general, i.e. toxic, medical, or radioactive waste. Characterisation of suitable concrete mix design has been attained through leakage tests, carried out on a concrete slab 300×300 mm wide. Four full-scale prototypes of the system were then built from the same concrete mix, and were submitted to a test of waterproofing at a laboratory. Experimental evidence on two weak points has been collected. One, of a general character, is related to the almost unavoidable presence of "construction joints", a second weak point is related to non-homogeneity of concrete. Then two different design procedures were suggested.
For many decades, radioactive wastes related to the reprocessing of nuclear fuel have been stored in underground storage tanks at former nuclear materials production sites at Aiken, South Carolina (Savannah River Site); Hanford, Washington; and Idaho Falls, Idaho (Idaho National Laboratory). The U.S. Department of Energy (DOE) plans to remove highly radioactive or key radionuclides from the tanks to the maximum extent practical and close the emptied tanks by filling the tanks, pipework, and concrete vaults with grout. This cement-based material is expected to provide structural support, encapsulate and stabilize the residual tank waste and tank heel, act as a physical barrier to inhibit the flow of groundwater through the waste, and serve as a barrier to plant roots or to inadvertent intrusion by burrowing animals or humans drilling or excavating at the site. DOE performance assessments demonstrating that disposal actions at the sites will meet performance objectives have included quantitatively evaluating the influence of cement-based barriers in mitigating radionuclide release to the environment. In these performance assessments, assumptions are made regarding the physical integrity (e.g., hydraulic conductivity) and chemical condition (e.g., pH and Eh affecting radionuclide solubility and sorption) of the cement-based engineered barrier and the changes to these properties that occur with time. Although much research has been done on the use of cement-based materials for immobilizing and stabilizing toxic metals and radioactive wastes, the ability of these materials to maintain the low permeability and other properties necessary to retain radionuclides for the long time periods-up to 10,000 years-required for nuclear waste disposal is uncertain.
Modeling the long-term durability of concrete barriers in the context of low-activity waste storage
Epj Web of Conferences, 2013
The paper investigates the long-term durability of concrete barriers in contact with a cementitious wasteform designed to immobilize low-activity nuclear waste. The high-pH pore solution of the wasteform contains high concentration level of sulfate, nitrate, nitrite and alkalis. The multilayer concrete/wasteform system was modeled using a multiionic reactive transport model accounting for coupling between species, dissolution/ precipitation reactions, and feedback effect. One of the primary objectives was to investigate the risk associated with the presence of sulfate in the wasteform on the durability of concrete. Simulation results showed that formation of expansive phases, such as gypsum and ettringite, into the concrete barrier was not extensive. Based on those results, it was not possible to conclude that concrete would be severely damaged, even after 5,000 years. Lab work was performed to provide data to validate the modeling results. Paste samples were immersed in sulfate contact solutions and analyzed to measure the impact of the aggressive environment on the material. The results obtained so far tend to confirm the numerical simulations.
Effects of Various Factors on Durability Prediction of Nuclear Waste Containment Structures-11546
A numerical methodology is presented in this paper to simulate the degradation of concrete vaults exposed to aggressive sulfate-containing pore solution of the low activity nuclear wastes. The methodology incorporates (i) transport of ions, (ii) chemical reactions, and (iii) damage accumulation due to cracking. The required parameters and boundary conditions for simulating structural damage are generally obtained from literature and experiments. Parameters that cannot be directly measured in experiments are often estimated using measurable quantities and empirical relations. Uncertainty in inputs, model parameters and boundary conditions due to inherent randomness, lack of data and incomplete knowledge of the physics introduces uncertainty in the model prediction, which is evaluated using sensitivity analysis and Monte Carlo simulation. This paper investigates the effects of three primary factors on the model prediction. These are-(1) chemical equilibrium models comprised of different combinations of mineral phases and initial concentrations of ionic species, (2) prescribed boundary conditions, and (3) a mechanical parameter required for initiation and propagation of cracking. The numerical model is then used to assess durability of a concrete containment structure considering uncertainty in the selected physical and chemical parameters. Various approaches for statistical representation of the uncertainties are incorporated in the durability assessment framework. Finally, the probability distribution of time to structural damage is evaluated using a single loop Monte Carlo simulation technique.
2020
The Cementitious Barriers Partnership (CBP) Project is a multi-disciplinary, multi-institutional collaboration supported by the U.S. Department of Energy (US DOE) Office of Tank Waste and Nuclear Materials Management. The CBP program has developed a set of integrated tools (based on state-of-the-art models and leaching test methods) that help improve understanding and predictions of the long-term structural, hydraulic and chemical performance of cementitious barriers used in nuclear applications. Tools selected for and developed under this program have been used to evaluate and predict the behavior of cementitious barriers used in near-surface engineered waste disposal systems for periods of performance up to 100 years and longer for operating facilities and longer than 1000 years for waste disposal. The CBP Software Toolbox has produced tangible benefits to the DOE Performance Assessment (PA) community. A review of prior DOE PAs has provided a list of potential opportunities for im...
Concrete Degradation Modeling in the Evaluation of Entombment as a Decommissioning Option | NIST
IEEE Spectrum, 2002
For entombment to be a viable option for the decommissioning of nuclear structures, the effectiveness of available engineered barriers needs to be assured. Barrier performance should be estimated with the aid of computer models that can accurately predict the response of the barrier to foreseeable physical and chemical conditions. For concrete barriers, virtually all degradation mechanisms are controlled by the transport of water and ionic species within the pore space. These, in turn, are controlled by the appropriate transport coefficients. For sound concrete, the transport coefficients are sufficiently small enough that isolation is expected. It is the presence of cracks within the concrete that compromises the barrier by increasing the transport coefficients dramatically. Therefore, additional efforts to characterize the performance of concrete barriers must focus on quantifying the existing cracks and flaws within the concrete. This characterization would include sampling (when...
Behavior of HPC nuclear waste disposal structures in leaching environment
Nuclear Engineering and Design, 2011
Nuclear waste disposal is a subject of concern for the French national agency for nuclear waste management (Andra). One of the solutions envisaged by Andra (in accordance with the French law (June 28th 2006)) to take care of nuclear wastes is long-term underground storage. The storage structures would be high-performance concrete tunnels. The safety level of these structures for the next 1000 years has to be demonstrated by Andra. To do this, scenarios compatible with the geological and hydrological environment of the tunnel will be considered including chemical and physical evolutions. Among them is water saturation of the disposal structure leading to a concrete leaching process combined with a ground convergence towards the concrete vault, which is studied here. The corresponding research program is presented; it includes chemical and chemo-mechanical aspects and a simulation of the long-term behavior of disposal cell.
A comparative study of the modelling of cement hydration and cement–rock laboratory experiments
Applied Geochemistry, 2011
The use of cement and concrete as fracture grouting or as tunnel seals in a geological disposal facility for radioactive wastes creates potential issues concerning chemical reactivity. From a long-term safety perspective, it is desirable to be able model these interactions and changes quantitatively. The 'Long-term Cement Studies' (LCS) project was formulated with an emphasis on in situ field experiments with more realistic boundary conditions and longer time scales compared with former experiments. As part of the project programme, a modelling inter-comparison has been conducted, involving the modelling of two experiments describing cement hydration on one hand and cement-rock reaction on the other, with teams representing the NDA (UK), Posiva (Finland), and JAEA (Japan). This modelling exercise showed that the dominant reaction pathways in the two experiments are fairly well understood and are consistent between the different modelling teams, although significant differences existed amongst the precise parameterisation (e.g. reactive surface areas, dependences of rate upon pH, types of secondary minerals), and in some instances, processes (e.g. partition of alkali elements between solids and liquid during cement hydration; kinetic models of cement hydration). It was not conclusive if certain processes such as surface complexation (preferred by some modellers, but not by others) played a role in the cement-rock experiment or not. These processes appear to be more relevant at early times in the experiment and the evolution at longer timescales was not affected. The observed permeability profile with time could not be matched. The fact that no secondary minerals could be observed and that the precipitated mass calculated during the simulations is minor might suggest that the permeability reduction does not have a chemical origin, although a small amount of precipitates at pore throats could have a large impact on permeability. The modelling exercises showed that there is an interest in keeping the numerical models as simple as possible and trying to obtain a reasonable fit with a minimum of processes, minerals and parameters. However, up-scaling processes and model parameterisation to the timescales appropriate to repository safety assessment are of considerable concern. Future modelling exercises of this type should focus on a suitable natural or industrial analogue that might aid assessing mineral-fluid reactions at these longer timescales.
Physics and Chemistry of the Earth, Parts A/B/C, 2007
In order to evaluate the long term waste package integrity in a geological repository for radioactive waste, simulations of the geochemical interactions between a concrete engineered barrier and a mudrock were conducted in 1-D geometry and on time periods of up to 10 6 y with the reactive transport code Hytec. Scenarios involving sulfate attack are shown to potentially alter strongly a concrete engineered barrier based on pure Portland based cement. Spatial extension of chemical degradation of the host rock due to high pH fluids is restricted to a radial distance of less than 2 m of the tunnel border in 100 000 y. Results suggest that illite and quartz destabilization rates are key parameters governing the geochemical evolution of the degraded interface. Results also suggest that controls on Mg availability and speciation at the border of the altered concrete are important for a proper understanding of this system. Another key process is the progressive localized cementation of the altered mudrock. Defining a conservative and robust modelling of the effects of cementation is not an easy task, as both porosity opening and closing occurs in this reactive system. Results obtained here suggest that coupling between pH dependence of mineral stability and feedback of mineral precipitation on pH sharpen the cementation front.
Procedia Materials Science, 2012
The National Atomic Energy Commission of the Argentine Republic is developing a nuclear waste disposal management programme that contemplates the design and construction of a facility for the final disposal of intermediate-level radioactive wastes. The repository is based on the use of multiple, independent and redundant barriers. The major components are made in reinforced concrete so, the durability of these structures is an important aspect for the facility integrity. This work presents an investigation performed on a reinforced concrete specifically designed for this purpose, to predict the service life of the intermediate level radioactive waste disposal facility from data obtained with several techniques. Results obtained with corrosion sensors embedded in a concrete prototype are also included. The information obtained will be used for the final design of the facility in order to guarantee a service life more or equal than the foreseen durability for this type of facilities.