Evaluation of long-term behavior of concretes in high level waste repositories. An accelerated leaching test (original) (raw)
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Radioactive Cementitious Waste Form Behavior Under Long-Term Field and Laboratory Test Conditions
Control leaching experiments with laboratory cement samples containing simulated radioactive RBMK operational waste were set up to compare the leaching parameters with the field test leach data of pilot-scale cemented waste forms containing real RBMK operational waste. The aim was to gain information on reliability of field test data and to validate the leach test methods. Average annual 137 Cs leach rate in deionized water was about thirty times greater then the measured value for the 1 st year of the repository test that may be accounted for by the lower average annual temperature in the repository, influence of leachant medium and some other factors. 137 Cs cumulative leached fraction (3,7% for 1 yr) was close to value reported in literature for a similar experiment in deionized water. It was more than two orders of magnitude higher then the 1 st-yr leached fraction in the repository test (0,01%). In the latter case sample volume was 630 times greater and sample surface to volume ratio, S/V, was 8.6 times less. To compare the field test results with the laboratory leach test, a scaling factor S/V multiplied by a temperature factor and a leach rate decrease coefficient related to the leachant media should be taken into consideration.
Long-term field and laboratory leaching tests of cemented radioactive wastes
Journal of Hazardous Materials, 2011
Experiments with real and simulated radioactive cementitious wasteforms were set up to compare the leaching behaviour of cementitious wasteforms containing nuclear power plant operational waste in field and laboratory test conditions. Experiments revealed that the average annual 137 Cs leach rate in deionised water was about thirty-five times greater compared with the measured average value for the 1st year of the field test. Cumulative leached fraction of 137 Cs for 1st year (3.74%) was close to values reported in literature for similar laboratory experiments in deionised water, however more than two orders of magnitude higher than the 1st year leached fraction of 137 Cs in the repository test (0.01%). Therefore, to compare field and laboratory test results, a scaling factor is required in order to account for surface to volume factor difference, multiplied by a temperature factor and a leach rate decrease coefficient related to the ground water composition.
Applied Geochemistry, 2014
Near-surface cement-based disposal systems for hazardous materials such as radioactive waste will undergo chemical alterations due to interaction with the surrounding environment. One of the most relevant long-term geochemical alteration processes is decalcification or leaching of the cement phases by percolating water. Consequently, the cementitious components of the disposal system will evolve through different chemical degradation states, also altering physical material parameters such as porosity and bulk density and chemical parameter relevant to solute migration such as the solidliquid partition coefficient or distribution ratio. This paper presents a novel approach in which geochemical modeling serves as a fundamental basis for assessing the evolution of geochemical conditions within a cement-based near-surface disposal facility. On one hand, geochemical modelling is used to quantify uncertainties related to the infiltrating water composition and C-S-H degradation model, both of which allow for various conceptualizations of the evolution of retardation factor. On the other hand, the concept of mixed tank reactor is used to represent cement degradation within the entire disposal system. This paves the way to establish a link between the evolution of the geochemical conditions and the evolution of the retardation factor via the knowledge of amount of percolated water through the system. The usefulness of the approach is demonstrated via a number of case studies concerning leaching of radionuclides ( 14 C and 94 Nb) from a cementitious near-surface disposal facility. The studies reveal that there is a large effect of the conceptualization on calculated fluxes from the disposal facility, depending on the type of radionuclide. A crucial factor is the amount of radionuclide mass present in the disposal system when large changes in the retardation factor occur, for instance, when different retardation factors exist in different chemical degradation states.
The Development of Durable Cementitious Materials for Use in a Nuclear Fuel Waste Disposal Facility
MRS Proceedings, 1985
This paper describes the work on cement paste development and short-term leaching tests in Standard Canadian Shield Saline Solution (SCSSS) in the presence of bentonite at 150°C. It has been found that:-supplementary cementing materials such as silica fume or fly ash could significantly improve the properties of sulphate resistant portland cement (SRPC), in particular, permeability to water and pore size distribution.-the addition of bentonite suppressed the normal tendency of the pH of groundwater to increase rapidly in the presence of cement.-the presence of bentonite increased the release of potassium ions from the cements.-– SRPC blended with 20% silica fume resulted in a groundwater pH lower than that of SRPC, with and without bentonite. Moreover, its cumulative fraction of release of potassium was significantly lower than that of SRPC.
Lecture Notes in Civil Engineering, 2017
Nuclear power, to most of us, is mystic and somehow scary, and despite its drawbacks, is still playing an important role in the world wide energy supply. However concrete, without mystery as the most widely used materials in construction, is used as a major constituent for nuclear facilities such as radioactive waste repositories and nuclear power plants. Concrete is the only practical material offering a number of advantages including sufficient shielding against the dangers of radiation, good compressive strength, low cost, easy building, and retention of radionuclides limiting their dissipation. The assessment of the long-term durability of such concrete structures is of utmost importance and urgently needed as our knowledge on concrete durability beyond the basis of an expected several decade service life is limited. Within its service environment, these structures undergo chemical degradation processes which are very slow but they significantly change the physical integrity and the chemical conditions of the structures with the passage of time. Current issues on durability of these concrete structures include alkali-silica reaction, delayed ettringite formation, leaching, carbonation, etc. which might be magnified under severe/accelerated conditions (high temperature, radiation, moisture, cyclic loading, and acidic environments). These degradations induce an evolution of the microstructure, cracking and changes in transport properties of concrete which are still unclear due to the limited experimental timeframe available to capture these processes. This paper presents an overview on these concerns with the focus on the long-term chemical degradation aspect and presenting a case study on Ca-leaching.
Applied Sciences, 2022
Bentonite clays have unique properties that determine their use as the main component of engineered barrier systems (EBS) for the isolation of radioactive waste. At present, the Russian Federation is elaborating the concept of deep geological disposal of radioactive waste in crystalline rocks of the Yeniseisky site, where bentonite clays play an important role in ensuring the safety of the repository for a long period of time. This work demonstrates the first results of short-term laboratory experiments (1 and 3 months) on the thermochemical interaction of bentonite and concrete in the presence of synthetic water at an elevated temperature. These experiments will help predict the effect of EBS materials on montmorillonite. Bentonite from the 10th Khutor deposit (Russia) and Portland cement were used in the experiments. At the first stage of the experiments, solutions were obtained after leaching the concrete with a synthetic groundwater solution for 1 month at 90 °C. At the second s...
Radionuclide Retention in Concrete Waste Forms
2010
Assessing long-term performance of Category 3 waste cement grouts for radionuclide encasement requires knowledge of the radionuclide-cement interactions and mechanisms of retention (i.e., sorption or precipitation); the mechanism of contaminant release; the significance of contaminant release pathways; how waste form performance is affected by the full range of environmental conditions within the disposal facility; the process of waste form aging under conditions that are representative of processes occurring in response to changing environmental conditions within the disposal facility; the effect of waste form aging on chemical, physical, and radiological properties; and the associated impact on contaminant release. This knowledge will enable accurate prediction of radionuclide fate when the waste forms come in contact with groundwater. The information presented in the report provides data that 1) quantify radionuclide retention within concrete waste form materials similar to those used to encapsulate waste in the Low-Level Waste Burial Grounds (LLBG); 2) measure the effect of concrete waste form properties likely to influence radionuclide migration; and 3) quantify the stability of uranium-bearing solid phases of limited solubility in concrete.
Journal of Porous Media, 2006
To assess the safety of disposal of radioactive waste material in cement, curing conditions and time of leaching radionuclides 60 Co have been studied. Leaching tests in cement-ion exchange resins-bentonite matrix, were carried out in accordance with a method recommended by IAEA. Curing conditions and curing time prior to commencing the leaching test are critically important in leach studies since the extent of hydration of the cement materials determines how much hydration product develops and whether it is available to block the pore network, thereby reducing leaching. Incremental leaching rates R n (cm/d) of 60 Co from cement-ion exchange resins-bentonite matrix after 180 days were measured. The results presented in this paper are the examples of results obtained in a 20-year concrete testing project which will influence the design of the engineer trenches system for future central Serbian radioactive waste disposal center.