I am a PhD Researcher that focuses on characterising the microstructural behaviour of superalloys.
Providing an insight to how we currently deal with nuclear waste
In 10 Seconds? How we deal with nuclear waste can be a very sensitive subject – I mean, who wants nuclear waste to be stored near their backyard? One of the most common methods of storage is to contain the nuclear waste in cylindrical concrete cylinders and bury it underground. However, this causes much debate as you can imagine concerning environmental factors. That being said, the spent nuclear waste could be kept in storage pools or kept in secure engineering facilities.
What materials are used for these storage containers? Since this nuclear waste is still highly radioactive, the containers need to be manufactured from a material that can prevent emitting gamma x-rays. Therefore, the chosen material must be very dense! The most common shielding materials for internals of the containment are lead and steel. Respective densities are 11340kg/m3 and 8000kg/m3. Due to their high densities materials such as lead have a relatively high number of protons with corresponding electrons – the electrons within the shielding material blocks the gamma and x-ray particles. The shielding ability of the material can also be improved by increasing the wall thickness.
Improving the safety of these containers? Due to the extremely long length of time these containers are left in underground repositories, one major concern has been the corrosion resistance of the metallic containers. Therefore, research has been carried out in the United States showing the benefits that can be obtained by applying High Velocity oxy fuel copper coatings to enhance the corrosion resistance greatly –
What are High Velocity Oxygen Fuel Coatings (HVOF)? HVOF process is a form of thermal spray coatings. The deposition material is in a molten or semi-molten state and is mixed into the gas stream and accelerated towards the substrate, thus providing a dense coating which improves the metallic substrate properties such as; erosion, wear and corrosion resistance. This deposition process is extensively used within the oil and gas industry to improve the life cycle of metallic offshore structures i.e. oil-rig platforms – in the most extreme corrosive environments.
Observing the research that has been done in regard to material behaviour under nuclear conditions, I would say that underground storage is a suitable and safe option for now, until such a time we can reuse the spent fuel without having to wait ‘x’ amount of years.
Abstract: Copper specimens were exposed to oxygen-deficient artificial groundwater in the presence and absence of micro-organisms enriched from the deep bedrock of the planned nuclear waste repository site at Olkiluoto island on the western coast of Finland. During the exposure periods of 4 and 10 months, the copper specimens were subjected to electrochemical measurements. The biofilm developed on the specimens and the water used in the exposures were subjected to microbiological analyses. Changes in the water chemistry were also determined and surfaces of the copper specimens were characterized with respect to the morphology and composition of the formed corrosion products. The results showed that under biotic conditions, redox of the water and open circuit potential (OCP) of the copper specimens were generally negative and resulted in the build-up of a copper sulphide, Cu2S, layer due to the activity of sulphate-reducing bacteria (SRB) that were included in the system. In the 4-month test, the electrochemical behaviour of the specimens changed during the exposure and alphaproteobactria Rhizobiales were the dominant bacterial group in the biofilm where the highest corrosion rate was observed. In the 10-month test, however, deltaproteobacteria SRB flourished and the initial electrochemical behaviour and the low corrosion rate of the copper were retained until the end of the test period. Under abiotic conditions, the positive water redox potential and specimen OCP correlated with the formation of copper oxide, Cu2O. Furthermore, in the absence of SRB, Cu2O provided slightly inferior protection against corrosion compared to that by Cu2S in the presence of SRB. The obtained results show that the presence of microorganisms may enhance the passivity of copper. In addition, the identification of key microbial species, such as SRB thriving on copper for long time periods, is important for successful prediction of the behaviour of copper.
Pub.: 14 Nov '16, Pinned: 08 Jun '17
Abstract: Publication date: 1 March 2017 Source:Applied Clay Science, Volume 137 Author(s): N. Finck, M. Bouby, K. Dardenne, T. Yokosawa The Y(III) binding mechanism(s) by coprecipitation with or by adsorption on hectorite, a mineral phase frequently detected in nuclear waste glass alteration experiments, was investigated by polarized EXAFS (P-EXAFS) spectroscopy. The novelty in this study is the use of yttrium to take advantage of the higher angular dependence of the absorption coefficient at the K-edge for P-EXAFS measurements. In the coprecipitation experiment, a brucite precursor was prepared in the presence of Y and subsequently aged to produce hectorite. In the adsorption experiment, Y(III) ions contacted pre-formed hectorite in dispersion. The coprecipitated hectorite and brucite and the hectorite from adsorption experiment were each prepared as textured samples and the Y(III) local environment was probed by P-EXAFS spectroscopy. P-EXAFS analysis indicated that Y(III) is 6-fold coordinated by O atoms in both the coprecipitated brucite and the coprecipitated hectorite, and surrounded by next-nearest Mg/Si shells. The angular dependences of the coordination numbers strongly point to Y(III) substituting for Mg(II) in brucite layers. Upon hectorite crystallization the local environment evolved. Mg and Si shells were detected at distances suggesting an octahedral clay-like environment in the coprecipitated hectorite, and this finding was corroborated by the angular dependence of the coordination numbers. In the adsorption sample, Y(III) forms inner-sphere surface complexes at the platelet edges (i.e., (0 1 0) plane), slightly tilted off the median clay plane. The presence of such surface complexes in the coprecipitation sample could not be evidenced. Finally, the supernatant of the dispersion containing the coprecipitated hectorite was analyzed by the asymmetrical flow field-flow fractionation (AsFlFFF) technique coupled to ICP-MS to obtain information on the smallest sized particles. The AsFlFFF data indicate that nanoparticulate hectorite of various sizes (50–75nm, 125–140nm and >450nm) can be separated from the bulk dispersion and this finding was corroborated by TEM experiments on the same supernatant. Furthermore, AsFlFFF data also indicate that Y(III) behaves like Mg, used as fingerprint of the presence of hectorite. This finding suggests random substitution for octahedral cation within hectorite nanoparticles. Trivalent yttrium was used as proxy for trivalent actinides (An(III)). Consequently, this study supports the incorporation of An(III) into hectorite forming in the nuclear waste glass alteration layer in deep disposal sites, as already suggested in previous studies.
Pub.: 14 Dec '16, Pinned: 08 Jun '17
Abstract: Ceramic materials provide an innovative opportunity for corrosion-resistant coatings for nuclear waste containers. Their suitability can be derived from the fully oxidized state for selected metal oxides. Ceramic coatings applied to plain carbon steel substrates by several thermal spray techniques have been exposed to 90 °C simulated ground water (at 10 times typical concentration) for nearly 6 years. Thermal spray processes examined in this work included plasma spray, high-velocity oxy fuel (HVOF), and detonation gun. Some thermal spray coatings have demonstrated superior corrosion protection for the plain carbon steel substrate. In particular, the HVOF and detonation gun thermal spray processes produced coatings with low connected porosity, which limited the growth rate of corrosion products. It was also demonstrated that these coatings resisted spallation of the coating even when an intentional flaw (which allowed for corrosion of the carbon steel substrate underneath the ceramic coating) was placed in the coating. An approach for a theoretical basis for prediction of the corrosion protection provided by ceramic coatings is also presented. The theoretical development includes the effect of the morphology and amount of the porosity within the thermal spray coating and provides a prediction of the exposure time needed to produce a crack in the ceramic coating.
Pub.: 01 May '05, Pinned: 08 Jun '17
Abstract: Secure disposal or storage of nuclear waste within stable geologic environments hinges on the effectiveness of artificial and natural radiation barriers. Fractures in the bedrock are viewed as the most likely passage for the transport of radioactive waste away from a disposal site. We utilize ground penetrating radar (GPR) to map fractures in the tunnel walls of an underground research tunnel at the Korea Atomic Energy Research Institute (KAERI). GPR experiments within the KAERI Underground Research Tunnel (KURT) were carried out by using 200 MHz, 500 MHz, and 1000 MHz antennas. By using the high-frequency antennas, we were able to identify small-scale fractures, which were previously unidentified during the tunnel excavation process. Then, through 3-D visualization of the grid survey data, we reconstructed the spatial distribution and interconnectivity of the multi-scale fractures within the wall. We found that a multi-frequency GPR approach provided more details of the complex fracture network, including deep structures. Furthermore, temporal changes in reflection polarity between the GPR surveys enabled us to infer the hydraulic characteristics of the discrete fracture network developed behind the surveyed wall. We hypothesized that the fractures exhibiting polarity change may be due to a combination of air-filled and mineralogical boundaries. Simulated GPR scans for the considered case were consistent with the observed GPR data. If our assumption is correct, the groundwater flow into these near-surface fractures may form the water-filled fractures along the existing air-filled ones and hence cause the changes in reflection polarity over the given time interval (i.e., 7 days). Our results show that the GPR survey is an efficient tool to determine fractures at various scales. Time-lapse GPR data may be essential to characterize the hydraulic behavior of discrete fracture networks in underground disposal facilities.
Pub.: 02 Apr '17, Pinned: 08 Jun '17
Abstract: MgO/SiO2 cements are materials potentially very useful for radioactive waste disposal, but knowledge about their physico-chemical properties is still lacking. In this paper we investigated the hydration kinetics of cementitious formulations prepared by mixing MgO/SiO2 and Portland cement in different proportions and the structural properties of the hydrated phases formed in the first month of hydration. In particular, the hydration kinetics was investigated by measuring the free water index on pastes by means of differential scanning calorimetry, while the structural characterization was carried out by combining thermal (DTA), diffractometric (XRD), and spectroscopic (FTIR, 29Si solid state NMR) techniques. It was found that calcium silicate hydrate (C-S-H) and magnesium silicate hydrate (M-S-H) gels mainly form as separate phases, their relative amount and structural characteristics depending on the composition of the hydrated mixture. Moreover, the composition of the mixtures strongly affects the kinetics of hydration and the pH of the aqueous phase in contact with the cementitious materials. The results here reported show that suitable mixtures of Portland cement and MgO/SiO2 could be used to modify the properties of hydrated phases with potential application in the storage of nuclear waste in clayey disposal.
Pub.: 02 Feb '17, Pinned: 08 Jun '17
Abstract: This work demonstrates the use of synchrotron-based, transmission X-ray microscopy (TXM) and scanning electron microscopy to image the 3-D morphologies and spatial distributions of Ga-doped phases within model, single- and two-phase waste form material systems. Gallium doping levels consistent with those commonly used for nuclear waste immobilization (e.g., Ba1.04Cs0.24Ga2.32Ti5.68O16) could be readily imaged. The analysis suggests that a minority phase with different stoichiometry/composition from the primary hollandite phase can be formed by the solid-state ceramic processing route with varying morphology (globular vs. cylindrical) as a function of Cs content. The results presented in this work represent a crucial step in developing the tools necessary to gain an improved understanding of the microstructural and chemical properties of waste form materials that influence their resistance to aqueous corrosion. This understanding will aid in the future design of higher durability waste form materials.
Pub.: 21 Apr '17, Pinned: 08 Jun '17
Abstract: For the first time, a system for specific consideration of radiological environmental protection has been applied in a major license application in Sweden. In 2011 the Swedish Nuclear Fuel & Waste Management Co. (SKB) submitted a license application for construction of a geological repository for spent nuclear fuel at the Forsmark site. The license application is supported by a post-closure safety assessment, which in accordance with regulatory requirements includes an assessment of environmental consequences. SKB's environmental risk assessment uses the freely available ERICA Tool. Environmental media activity concentrations needed as input to the tool are calculated by means of complex biosphere modelling based on site-specific information gathered from site investigations, as well as from supporting modelling studies and projections of future biosphere conditions in response to climate change and land rise due to glacial rebound. SKB's application is currently being reviewed by the Swedish Radiation Safety Authority (SSM). In addition to a traditional document review with an aim to determine whether SKB's models are relevant, correctly implemented and adequately parametrized, SSM has performed independent modelling in order to gain confidence in the robustness of SKB's assessment. Thus, SSM has used alternative stylized reference biosphere models to calculate environmental activity concentrations for use in subsequent exposure calculations. Secondly, an alternative dose model (RESRAD-BIOTA) is used to calculate doses to biota that are compared with SKB's calculations with the ERICA tool. SSM's experience from this review is that existing tools for environmental dose assessment are possible to use in order to show compliance with Swedish legislation. However, care is needed when site representative species are assessed with the aim to contrast them to generic reference organism. The alternative modelling of environmental concentrations resulted in much lower concentrations compared to SKB's results. However, SSM judges that SKB's in this part conservative approach is relevant for a screening assessment. SSM also concludes that there are big differences in dose rates calculated to different organisms depending on which tool that is used, although not systematically higher for either of them. Finally, independent regulatory modelling has proven valuable for SSM's review in gaining understanding and confidence in SKB's assessment presented in the license application.
Pub.: 22 Apr '17, Pinned: 08 Jun '17
Abstract: Long-term polarisation and immersion for copper corrosion were investigated in chloride-containing sulphide solutions. Results showed that no active state exists for copper in sulphide-containing solutions, whereas a typical active-to-passive transition exists in the second passivation region, indicating a hydroxyl activation effect on copper. After the sulphide passive film broke down, the pitting area was re-passivated by hydroxide ions; however, if the chloride and sulphide concentrations were both high, then there was no second passivation and the copper would dissolve rapidly. The corrosion resistance and the film thickness both increased with increasing immersion time and an average corrosion rate of 51.2 µm/year was obtained in aerobic solution, which was much higher than in bentonite or under deoxygenated conditions. Moreover, several local corrosion pits appeared after 1-year immersion and this would occur at the beginning of storage.
Pub.: 26 May '17, Pinned: 08 Jun '17