Nuclear Waste Containers Research Articles

(Corrosion Division Morris Cohen Graduate Student Award) In-situ Monitoring of Ultra-slow Oxide Growth on Copper

The atmospheric corrosion of metals and alloys is a very complex process, which involves a wide range of chemical, electrochemical, and physical processes occurring in gas, liquid, and solid phases, as well as at the interfaces between them. Essentially all metals and alloys undergo corrosion in indoor or outdoor atmospheric conditions due to the presence of gaseous corrosion stimulators. However, the rate of the atmospheric corrosion might vary significantly depending on the nature of the metals or alloys, the concentration of corrosive species, as well as the exposure conditions. In some applications such as electrical contacts in microelectronics, military equipment or nuclear waste containers, even ultra-slow oxidation/corrosion may result in catastrophic consequences. In order to study such slow oxidation/corrosion processes, techniques involving ultra-high vacuum (UHV) are commonly used. Despite a very high surface sensitivity of these techniques they suffer from the drawback that the experiments are not performed under normal ambient conditions, but rather under vacuum. This has the consequence that the water adlayer which always exists on metal surfaces in atmospheric environments is removed under UHV conditions. Furthermore, in the case of ex-situ UHV studies, the structure of the oxidation/corrosion products might undergo vacuum induced changes. Hence, in-situ and highly surface sensitive techniques capable of working under atmospheric pressures are desired to monitor very slow oxidation and corrosion processes on metals and alloys at a molecular level. In the present study, several analytical techniques have been used to obtain a picture as complete as possible of an atmospheric corrosion process, focusing on both the formation of corrosion products and the evolution of the protective layer. Copper surfaces covered with protective organic monolayers have been exposed to various atmospheric environments with the aim of simulating typical indoor atmospheric corrosion processes. Infrared reflection/absorption spectroscopy (IRAS) was used to determine the nature of the corrosion products, quartz crystal microbalance (QCM) to measure the mass of them, and nano plasmonic sensing (NPS) to determine the corrosion rate as well as to evaluate how homogeneous the formation of the corrosion products was. In addition, an inherently surface sensitive technique, vibrational sum frequency spectroscopy (VSFS) was used to study induced changes in the molecular structure of the protective monolayers. VSFS studies of octadecanethiol (ODT) covered copper surfaces exposed to dry air allowed the detection of the formation of thin copper oxide layers and the observation of accompanying changes in the molecular structure of the hydrocarbon chains of ODT. VSFS was combined with QCM to quantify the amount of copper oxide formed on the surface. Since oxidation and corrosion processes often occur heterogeneously, the VSFS-QCM setup was integrated with NPS, which enabled us to distinguish homogeneous and heterogeneous corrosion under in-situ conditions with a time resolution on the order of seconds. This unique combination of three in-situ techniques facilitates the study of different corrosion processes where interfacial changes in electronic and optical properties as well as changes in mass occur. The sensitivity of the techniques under current exposure conditions corresponds to 5% of an oxide monolayer for VSFS, 2.5% for QCM, and 1.2% for NPS. As an extension of the above mentioned studies a combination of VSFS and IRAS was used to compare the corrosion protection efficiency of alkanethiol SAMs with different chain lengths and to compare SAMs with –S or –Se as their anchoring group when the samples were exposed to humidified air (HA) containing formic acid (FA). It was found that the corrosion inhibition efficiency of alkanethiol SAMs on copper was enhanced with an increased chain length. Furthermore, alkanethiol SAMs feature “selective hindrance” properties, i.e. they selectively allow the penetration of formic acid and oxygen and hinder the penetration of water resulting in the formation of copper formate and copper hydroxide. Interestingly, no formation of copper oxide was observed, which was in contrast to unprotected copper. VSFS revealed that SAMs of alkanethiols get more disordered over the course of sample exposure to HA+FA but remain attached to the substrate. Alkaneselenol SAMs initially behave similarly to their alkanethiol counterparts. However, after prolonged exposure to HA+FA they are partially removed from the surface, resulting in an accelerated corrosion of the copper substrate. Eventually, the corrosion products formed on alkaneselenol SAM covered samples consists of copper formate, copper hydroxide, and copper oxide. Figure 1

Corrosion Monitoring of Steels Embedded in Swelled Bentonite Clay

An in-situ corrosion monitoring system of metals embedded in swelled bentonite clay was developed to predict corrosion rate of nuclear waste container in the repository environment. In the monitoring system three thin iron wires (0.2 mmf in diameter, 55 mm in length exposed to bentonite) were used to measure the corrosion rate by using resistometry, impedance and DC coupling current measurements. Three iron wires and Ag/AgCl-ink electrode were embedded in a laminate sheet and attached to the bottom of the test cell filled with bentonite powder. Bentonite powder was then swelled with purified water. The cell temperature was kept at 70°C during test period of ca. one month. All operation was conducted in a globe box filled with N2gas to reduce oxygen concentration in the test environment. Dissolved oxygen (DO) concentration in swelled bentonite clay was monitored by using a luminescent DO sensor embedded in a test cell and confirmed to be lower than the detection limit of sensor (0.1 ppm). Corrosion loss of corroding iron wire was estimated by resistometry in which electric resistance change of corroding wire was measured as a relative value against the non-corroded wire for temperature compensation. Small constant AC current was applied to wire electrodes and AC voltage response at wires was measured by using a digital lock-in amplifier to detect very small change of their resistance. Impedance spectra between two corroding wires was also measured by using a FRA under non-bias condition. Coupling current between two corroding iron wires was also measured to enable evaluation of anode-cathode distribution (non-uniform corrosion) on embedded wires. In the test period bentonite powder was getting wet with penetration of water and corrosion of iron wire initiated when water reached the metal surface. This process took ca. 1000-2000 ks, depending on the density of bentonite powder. Since the water penetrating bentonite powder reached a part of iron surface, coupling condition of corroding area with the other area may be formed. Time transition of data obtained by resistometry, impedance and coupling current revealed such changes in wetting environment and corrosion progress of iron wires. Such disproportionation of corroding surface may be considerable for a large container and may accelerate the early stage corrosion in the underground repository. After reaching steady states of corrosion progress, iron wire showed corrosion rate of 19~29 μm y–1 and tended to decrease with time. The corrosion rate was considerably lower than that measured in the test condition with insufficient removal of DO from the test cell. Impedance spectra between two corroding iron wires showed gradual decrease in impedance at the initial stage of corrosion and constant low resistance at the steady state of corrosion. Charge transfer resistance of corroding iron wire in the steady state estimated from impedance spectra was ca. 1 kW. This value was considerably lower than the value predicted from the actual corrosion rate obtained for resistometry assuming that the resistance corresponded to the simple dissolution reaction. Iron surface was actually covered with corrosion film and swelled bentonite and thus the corrosion reaction is more complicated. Raman spectrum of sample surface after the corrosion test included peaks of FeS, Fe3O4, and FeOOH. This research includes portions of the results of the “Study on Performance Evaluation for Engineering Components of HLW, FY2015 and FY2016” under a grant from the Agency for Natural Resources and Energy (ANRE) in the Ministry of Economy, Trade and Industry (METI) of Japan.

Radiation tolerance of iron phosphate: A study of amorphous and crystalline structures

Iron phosphate has been proposed as a candidate material for the encapsulation of nuclear waste. We studied amorphous and crystalline structures of Fe2+Fe3+(P2O7)22 and Fe3+(PO3)3. Amorphous Fe3+(P2O7)34, and two structures comparable with experimentally produced iron phosphate glass (IPG): 40 mol% Fe2O3 and 60 mol% P2O5, with 4% and 17% Fe2+ ion concentrations were also investigated.A series of constant volume molecular dynamics (MD) simulations of radiation cascades at 4 keV were performed. In the crystal structures, the cascades caused local amorphisation. For the glass structures, the cascades generally produced a rearrangement of atoms but did not significantly alter the overall atomic co-ordination or the ring statistics.Our work suggests that IPG, especially that with a low Fe2+ ion content, is resistant to radiation in the nuclear stopping regime. However, the crystalline material with its transition to a radiation tolerant glassy form, could also be a potential candidate for the containment of nuclear waste.

CORROSION OF COPPER-COATED STEEL HIGH LEVEL NUCLEAR WASTE CONTAINERS UNDER PERMANENT DISPOSAL CONDITIONS

The corrosion of high level nuclear waste containers under permanent disposal conditions can occur via a number of processes which change in importance as the environment within a deep geologic repository evolves from oxic to anoxic. The container design involves a copper coating on a steel vessel. The characteristics of these coatings are being assessed using standard analytical methods, including microscopy and electron backscatter diffraction. The corrosion processes on the surface of the coatings and at through-coating defects are also being investigated electrochemically. Although minor differences exist, the surface corrosion of cold sprayed and electrodeposited coatings generally exhibited similar corrosion behaviour to standard wrought copper. Corrosion at the base of a simulated through-coating defect in a cold-sprayed coating was found to proceed via galvanic coupling to oxygen reduction on the coating surface. Besides accumulating damage at the base of the defect, corrosion propagated along the copper/steel interface, most likely a consequence of the damage inflicted during the deposition process.

Mineralogy and thermodynamic properties of magnesium phyllosilicates formed during the alteration of a simplified nuclear glass

The precipitation of crystallized magnesium phyllosilicates generally sustains the alteration rate of nuclear waste containment glass. However, glass alteration slows down to a residual rate as soon as Mg disappears from the solution. The identification of the phyllosilicates formed is therefore crucial for modeling the long-term behavior of nuclear glass. This study deals with batch alteration of the simplified nuclear glass ISG in presence of magnesium, and the characterization of the secondary phases. Morphological, chemical and structural analyses (MET, EDX, XRD) were performed to determine the nature and structure of the precipitated phases identified as trioctahedral smectites. Analyses conducted on the secondary phases proved the presence of Al, Na and Ca in the Mg-phyllosilicate phases. Such elements had been suspected but never quantitatively measured. The experimental results were then used to determine the thermodynamic solubility constants for each precipitated secondary phase at various temperatures. The calculated values were consistent with those available for sodium and magnesium saponites in the existing thermodynamic databases.

Passive 3D imaging of nuclear waste containers with Muon Scattering Tomography

The non-invasive imaging of dense objects is of particular interest in the context of nuclear waste management, where it is important to know the contents of waste containers without opening them. Using Muon Scattering Tomography (MST), it is possible to obtain a detailed 3D image of the contents of a waste container on reasonable timescales, showing both the high and low density materials inside. We show the performance of such a method on a Monte Carlo simulation of a dummy waste drum object containing objects of different shapes and materials. The simulation has been tuned with our MST prototype detector performance. In particular, we show that both a tungsten penny of 2 cm radius and 1 cm thickness, and a uranium sheet of 0.5 cm thickness can be clearly identified. We also show the performance of a novel edge finding technique, by which the edges of embedded objects can be identified more precisely than by solely using the imaging method.

Predicting Geologic Corrosion with Electrodes

Ever since humans discovered how to separate metal from its ore mineral, preserving its metallic luster has been a driving force in the advancement of materials science. In modern times, developing materials that will contain and isolate nuclear waste has pushed corrosion science to new limits. We must now predict corrosion rates over geologic time scales, upwards of a million years. This article reviews the electrochemical basics that underpin metal and mineral corrosion and uses that to understand the case study of copper corrosion in nuclear-waste containers. Electrochemistry can also explain electron-transfer processes on mineral surfaces and so offer insight into weathering and environmentally relevant natural redox processes, such as those forming supergene metal deposits.

Localized Corrosion of Carbon Steel in Simulated Concrete Pore Solution: Influence of Chloride Ion and Temperature

Passivity breakdown on carbon steel, which is the reference candidate material for fabricating high-level nuclear waste containers in Belgium, has been studied and the data are interpreted in terms of the point defect model (PDM). The near normal distribution in breakdown potential, measured in deaerated simulated concrete pore water solution (pH 12.5) at different chloride concentrations and temperatures are in satisfactory agreement with prediction of the PDM. An increase in temperature from 25°C to 85°C results in a decrease in localized corrosion resistance according to a linear relationship between the critical breakdown voltage, Eb , and log(chloride activity) for all chloride concentrations. The linear dependence of the passivity breakdown potential on the square root of potential scan rate (ν1/2), as predicted by the PDM, yields an estimate of the critical areal concentration of condensed cation vacancies at the metal/film interface (ξ<=5.45×1014cm-2 at 85oC and <=1.07×1014cm-2 at 55oC ) that leads to the passivity breakdown. The experimental distributions in the breakdown potential for carbon steel at 55oC and 85oC are in satisfactory agreement with the relevant calculated distributions. Acknowledgment: The authors gratefully acknowledge the support of this work by ONDRAF-NIRAS of Belgium.

Tritiated water detection in the 2.17 µM spectral region by cavity ring down spectroscopy

Nuclear waste containers are intended to be stored in dedicated disposal sites. For the inside and environmental safety of the disposal sites, the tiny outgassing rates leaking out the containers are measured. Presently, the radioactive HT gas is measured by liquid scintillation. However an alternative method—cavity ring down spectroscopy, an isotopically selective laser technique based on molecular spectroscopy—is currently developed and evaluated for tritium measurement in its oxidized form, HTO. Applying this method, the number density of the gaseous HTO sample hold in the optical cavity cell, is derived from the laser beam absorption by vibrational symmetric stretching 2ν1 (R) HTO lines in the 4590 and 4600cm−1 spectral range. To ensure a future accurate HTO measurement, the theoretical line intensities are confronted to the experiment: two tritiated water standards are measured with a dedicated CRDS set-up. Compared to the theoretical database, the line positions are correct (−0.067 to −0.128cm−1), their relative intensities are in agreement with the database, but their absolute intensities are 20% weaker. Among the seven intense lines, the 4596.485cm−1 line (intensity 8.22 10−22cm/molecule) and the 4592.407cm−1 line (intensity 9.83 10−22cm/molecule) are isolated and intense for a sensitive detection. The HTO detection limit with the present set-up is 3kBq (10min), equivalent to 1.8 1012 molecules in the 111cm3 CRDS cell. This detection limit could be improved by a factor 3 by reducing the detection noise.

Characterising encapsulated nuclear waste using cosmic-ray muon tomography

Tomographic imaging techniques using the Coulomb scattering of cosmic-ray muons have been shown previously to successfully identify and characterise low- and high-Z materials within an air matrix using a prototype scintillating-fibre tracker system. Those studies were performed as the first in a series to assess the feasibility of this technology and image reconstruction techniques in characterising the potential high-Z contents of legacy nuclear waste containers for the U.K. Nuclear Industry. The present work continues the feasibility study and presents the first images reconstructed from experimental data collected using this small-scale prototype system of low- and high-Z materials encapsulated within a concrete-filled stainless-steel container. Clear discrimination is observed between the thick steel casing, the concrete matrix and the sample materials assayed. These reconstructed objects are presented and discussed in detail alongside the implications for future industrial scenarios.

Detection and evaluation of cracks in the concrete buffer of the Belgian Nuclear Waste container using combined NDT techniques

The long term management of high-level and heat emitting radioactive waste is a worldwide concern, as it directly influences the environment and future generations. To address this issue, the Belgian Agency for Radioactive Waste and Enriched Fissile Materials (ONDRAF/NIRAS) has come up with a reference design for the geological disposal of this waste. In this design the wastes are first post-conditioned in a massive concrete structure called Supercontainer (SC) before being transported to an underground repository. The feasibility to construct this structure is being evaluated through a number of scaled models that are tested using various measurement techniques. This paper reports on the onset and evolution of the observed cracking of the concrete structure obtained by simultaneously applying Digital Image Correlation (DIC), Acoustic Emission (AE) and Ultrasonic Pulse Velocity (UPV) nondestructive testing techniques. In particular, the results highlight the time of onset and location of crack initiation as well as the width and depth of the cracks. Finally, emphasis is given to AE indices which indicate the severity of the cracks.

Initiation and propagation of a single pit on stainless steel using a local probe technique.

Experiments about single pit initiation and propagation were performed on 316L stainless steel with the aim to determine the pitting corrosion behaviour for nuclear waste containers. The experimental setup permits to control the pit development at will and to create reproducible single pits in three dimensions. Radial and deep evolutions of a disc shaped pit were studied for propagation times of up to 10 hours. These evolutions were used to determine what limiting mechanism takes place during long-term pit propagation. Special attention has been paid to the chloride ion action on pitting. A minimum chloride concentration was found to be necessary in the bulk electrolyte to support pit propagation. The existence of a critical pit depth of 230 μm was also underlined.

Measurement and modelling of reactive transport in geological barriers for nuclear waste containment.

Compacted clays are considered as excellent candidates for barriers to radionuclide transport in future repositories for nuclear waste due to their very low hydraulic permeability. Diffusion is the dominant transport mechanism, controlled by a nano-scale pore system. Assessment of the clays' long-term containment function requires adequate modelling of such pore systems and their evolution. Existing characterisation techniques do not provide complete pore space information for effective modelling, such as pore and throat size distributions and connectivity. Special network models for reactive transport are proposed here using the complimentary character of the pore space and the solid phase. This balances the insufficient characterisation information and provides the means for future mechanical-physical-chemical coupling. The anisotropy and heterogeneity of clays is represented using different length parameters and percentage of pores in different directions. Resulting networks are described as mathematical graphs with efficient discrete calculus formulation of transport. Opalinus Clay (OPA) is chosen as an example. Experimental data for the tritiated water (HTO) and U(vi) diffusion through OPA are presented. Calculated diffusion coefficients of HTO and uranium species are within the ranges of the experimentally determined data in different clay directions. This verifies the proposed pore network model and validates that uranium complexes are diffusing as neutral species in OPA. In the case of U(vi) diffusion the method is extended to account for sorption and convection. Rather than changing pore radii by coarse grained mathematical formula, physical sorption is simulated in each pore, which is more accurate and realistic.

Intergranular Corrosion Fatigue Fracture Surface Analysis of Nickel Alloy

The paper presents a systematic investigation of the effects of a corrosive condition on grain boundary corrosion and other deleterious phenomena in a nickel alloys commonly used in nuclear waste containers, chemical refining plants and steam generator tubes. Grain boundary corrosion is investigated with the aid of microscopy. The results show that a concentrated corrosive condition greatly increases the susceptibility to crack initiation and crack propagation along grain boundaries, i.e. intergranular fracture.

Assessing the risk of under-deposit chloride-induced stress corrosion cracking in austenitic stainless steel nuclear waste containers

This paper provides a short summary of investigations that have focused on the occurrence of the phenomenon that is commonly referred to as atmospheric chloride-induced stress corrosion cracking (AISCC) in types 304L and 316L austenitic stainless steels; the materials from which storage containers for the UK’s intermediate level nuclear waste are currently fabricated. A brief overview of the relationship between localised corrosion phenomena (pitting or crevice corrosion) and stress corrosion cracking from observations derived under conditions of immersion in chloride-containing environments is provided. The impact of environmental variables such as the relative humidity, temperature and chloride deposition density on the development of AISCC and potential difficulties in extrapolating data derived from laboratory investigations to an in-store situation are also discussed. Proposals for the development of laboratory protocols that may more accurately reflect behaviour in an actual storage situation, including the possibility of employing accelerated testing methods, are also briefly addressed.

Modelling development of acidification within corroding sites on spent fuel surfaces

A model has been developed to predict whether the development of acidity is feasible within actively corroding sites on spent nuclear fuel (UO2) surfaces inside a failed nuclear waste container. The model simulations demonstrate that the build-up of acidity is possible within flaws and pores in a corroded UO2 surface, providing the separation of anodes and cathodes occurs. The extent to which the pH can be depressed is determined by the dissolution rate of the fuel, the dimensions of the defect, the local redox conditions which determine the corrosion potential, and the fraction of the fuel surface that is reactive. Based on the anticipated redox conditions established radiolytically in a failed container it is shown that a suppression of the pH sufficient to accelerate dissolution (pH≤5) is very unlikely.

The design and performance of a scintillating-fibre tracker for the cosmic-ray muon tomography of legacy nuclear waste containers

Tomographic imaging techniques using the Coulomb scattering of cosmic-ray muons are increasingly being exploited for the non-destructive assay of shielded containers in a wide range of applications. One such application is the characterisation of legacy nuclear waste materials stored within industrial containers. The design, assembly and performance of a prototype muon tomography system developed for this purpose are detailed in this work. This muon tracker comprises four detection modules, each containing orthogonal layers of Saint-Gobain BCF-10 2mm-pitch plastic scintillating fibres. Identification of the two struck fibres per module allows the reconstruction of a space point, and subsequently, the incoming and Coulomb-scattered muon trajectories. These allow the container content, with respect to the atomic number Z of the scattering material, to be determined through reconstruction of the scattering location and magnitude. On each detection layer, the light emitted by the fibre is detected by a single Hamamatsu H8500 MAPMT with two fibres coupled to each pixel via dedicated pairing schemes developed to ensure the identification of the struck fibre. The PMT signals are read out to standard charge-to-digital converters and interpreted via custom data acquisition and analysis software.The design and assembly of the detector system are detailed and presented alongside results from performance studies with data collected after construction. These results reveal high stability during extended collection periods with detection efficiencies in the region of 80% per layer. Minor misalignments of millimetre order have been identified and corrected in software. A first image reconstructed from a test configuration of materials has been obtained using software based on the Maximum Likelihood Expectation Maximisation algorithm. The results highlight the high spatial resolution provided by the detector system. Clear discrimination between the low, medium and high-Z materials assayed is also observed.

Evolution of corrosion parameters in a buried pilot nuclear waste container in el Cabril

ABSTRACTModern concrete has a record of good performance of around 120 years although there are structures in perfect conservation made with roman concrete (mixture of lime and natural pozzolans). El Cabril repository has a design life of 300-500 years and therefore, it should keep its integrity much longer than the back experience we have on reinforced concrete structures, which makes necessary a closer monitoring with time on the aging of concrete in real conditions. With this purpose, Enresa has designed in collaboration with IETcc and Geocisa the installation of permanent sensors in a pilot nuclear waste container in buried conditions. The sensors were installed in 1995 for monitoring corrosion parameters and have been working until present. The non-destructive tests (NDT) applied are based in electrochemical measurements (corrosion rate, corrosion potential, electrical resistivity, concrete strains, oxygen availability). Relations between the climatic influence, the buried depth and the corrosion parameters are also presented. The results indicate that temperature is a very relevant variable influencing the measurements. All the other parameters evolve according to seasonal changes. Values of activation energies of the resistivity changes are given although it seems more adequate to model the evolution with time by simply plotting the values registered at 20 ± 2 °C.

A Prototype Scintillating-Fibre Tracker for the Cosmic-ray Muon Tomography of Legacy Nuclear Waste Containers

Cosmic-ray muons are highly penetrative charged particles observed at sea level with a flux of approximately 1 cm−2 min−1. They interact with matter primarily through Coulomb scattering which can be exploited in muon tomography to image objects within industrial nuclear waste containers. A prototype scintillating-fibre detector has been developed for this application, consisting of two tracking modules above and below the volume to be assayed. Each module comprises two orthogonal planes of 2 mm fibres. The modular configuration allows the reconstruction of the initial and scattered muon trajectories which enable the container content, with respect to atomic number Z, to be determined. Fibre signals are read out by Hamamatsu H8500 MAPMTs with two fibres coupled to each pixel via dedicated pairing schemes developed to avoid space point ambiguities and retain the high spatial resolution of the fibres. A likelihood-based image reconstruction algorithm was developed and tested using a GEANT4 simulation of the prototype system. Images reconstructed from this simulation are presented in comparison with experimental results taken with test objects. These results verify the simulation and show discrimination between the low, medium and high-Z materials imaged.

A prototype scintillating-fibre tracker for the cosmic-ray muon tomography of legacy nuclear waste containers

Cosmic-ray muons are highly penetrative charged particles observed at sea level with a flux of approximately 1cm−2min−1. They interact with matter primarily through Coulomb scattering which can be exploited in muon tomography to image objects within industrial nuclear waste containers. A prototype scintillating-fibre detector has been developed for this application, consisting of two tracking modules above and below the volume to be assayed. Each module comprises two orthogonal planes of 2mm fibres. The modular configuration allows the reconstruction of the initial and scattered muon trajectories which enable the container content, with respect to atomic number Z, to be determined. Fibre signals are read out by Hamamatsu H8500 MAPMTs with two fibres coupled to each pixel via dedicated pairing schemes developed to avoid space point ambiguities and retain the high spatial resolution of the fibres. A likelihood-based image reconstruction algorithm was developed and tested using a GEANT4 simulation of the prototype system. Images reconstructed from this simulation are presented in comparison with experimental results taken with test objects. These results verify the simulation and show discrimination between the low, medium and high-Z materials imaged.