CBP [TASK 12] experimental study of the concrete salstone two-layer system (original) (raw)
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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...
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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.