Nuclear Waste Containers Research Articles

Effect of carbon content on the low temperature sensitization of 304L SS and its corrosion resistance in simulated ground water

Abstract During vitrification of nuclear high level waste, the molten glass containing the waste oxides is poured into 304L SS canisters at 1323 K (1050 °C), which subsequently, cools through the sensitization temperature, wherein, nucleation of chromium carbides occur. Due to the radioactive decay heat, the canister is exposed to 373 K 573 K (100 °C- 300 °C) over long containment periods, leading to the growth of the carbides, resulting in low temperature sensitization (LTS). The sensitized microstructure is susceptible to intergranular corrosion (IGC) when it comes in contact with ground water. In the present work, AISI Type 304L SS containing 0.02% and 0.03% carbon were subjected to LTS, simulating 10 and 100 years of vitrified nuclear waste containment respectively, and subsequently, the corrosion resistance was evaluated in simulated ground water. The microstructural evaluation was carried out by optical, scanning electron microscope (SEM), transmission electron microscope (TEM) and electron backscatter diffraction (EBSD). Double loop electrochemical potentiokinetic reactivation (DL-EPR) experiments were conducted to quantify the degree of sensitization (DOS). The study revealed that 304L SS containing 0.02% C is resistant to LTS and IGC, while that containing 0.03% C is highly susceptible to LTS and IGC in ground water composition.

Transport of gadolinium in a cement composite

Development of cement based composites suitable for radionuclide immobilization is of great interest for secure containment of nuclear waste. This requires, in particular, detailed knowledge of how radionuclides are transported in such composites. In this paper we investigate the transport of gadolinium in a dry cement composite. Experimental data are obtained in a non-destructive way by neutron imaging. Their theoretical interpretation is based on a diffusion-advection model in which sorption is included.

First-of-a-kind muography for nuclear waste characterization.

In the last decade, there has been a surge in the number of academic research groups and commercial companies exploiting naturally occurring cosmic-ray muons for imaging purposes in a range of industrial and geological applications. Since 2009, researchers at the University of Glasgow and the UK National Nuclear Laboratory (NNL) have pioneered this technique for the characterization of shielded nuclear waste containers with significant investment from the UK Nuclear Decommissioning Authority and Sellafield Ltd. Lynkeos Technology Ltd. was formed in 2016 to commercialize the Muon Imaging System (MIS) technology that resulted from this industry-funded academic research. The design, construction and performance of the Lynkeos MIS is presented along with first experimental and commercial results. The high-resolution images include the identification of small fragments of uranium within a surrogate 500-litre intermediate level waste container and metal inclusions within thermally treated GeoMelt® R&D Product Samples. The latter of these are from Lynkeos' first commercial contract with the UK National Nuclear Laboratory. The Lynkeos MIS will be deployed at the NNL Central Laboratory facility on the Sellafield site in Summer 2018 where it will embark upon a series of industry trials.This article is part of the Theo Murphy meeting issue 'Cosmic-ray muography'.

Novel muon imaging techniques.

Owing to the high penetrating power of high-energy cosmic ray muons, muon imaging techniques can be used to image large bulky objects, especially objects with heavy shielding. Muon imaging systems work just like CT scanners in the medical imaging field-that is, they can reveal information inside of a target. There are two forms of muon imaging techniques: muon absorption imaging and muon multiple scattering imaging. The former is based on the flux attenuation of muons, and the latter is based on the multiple scattering of muons in matter. The muon absorption imaging technique is capable of imaging very large objects such as volcanoes and large buildings, and also smaller objects like spent fuel casks; the muon multiple scattering imaging technique is best suited to inspect smaller objects such as nuclear waste containers. Muon imaging techniques can be applied in a broad variety of fields, i.e. from measuring the magma thickness of volcanoes to searching for secret cavities in pyramids, and from monitoring the borders of countries checking for special nuclear materials to monitoring the spent fuel casks for nuclear safeguards applications. In this paper, the principles of muon imaging are reviewed. Image reconstruction algorithms such as Filtered Back Projection and Maximum Likelihood Expectation Maximization are discussed. The capability of muon imaging techniques is demonstrated through a Geant4 simulation study for imaging a nuclear spent fuel cask.This article is part of the Theo Murphy meeting issue 'Cosmic-ray muography'.

Conceptual design of ultra-high performance fiber reinforced concrete nuclear waste container

Abstract This research presents a structural design of high-level waste (HLW) container using ultra-high performance fiber reinforced concrete (UHP-FRC) material. The proposed design aims to overcome the drawbacks of the existing concrete containers which are heavy, difficult to fabricate, and expensive. In this study, the dry storage container (DSC) that commonly used at Canadian Nuclear facilities is selected to present the proposed design. The design has been performed such that the new UHP-FRC alternative has a structural stiffness equivalent to the existing steel-concrete-steel container under various loading scenarios. Size optimization technique is used with the aim of maximizing stiffness, and minimizing the cost while satisfying both the design stresses and construction requirements. Then, the integrity of the new design has been evaluated against accidental drop-impact events based on realistic drop scenarios. The optimization results showed: the stiffness of the UHP-FRC container (300 mm wall thick) is being in the range of 1.35–1.75 times the stiffness of existing DSC (550 mm wall thick). The use of UHP-FRC leads to decrease the container weight by more than 60%. The UHP-FRC container showed a significant enhancement in performance in comparison to the existing DSC design under considered accidental drop impact scenarios.

Effect of Microstructure on the Morphology of Atmospheric Corrosion Pits in Type 304L Stainless Steel

Atmospheric corrosion of stainless steel is of concern for intermediate level nuclear waste (ILW) containers. The effect of microstructure on the morphology of atmospheric corrosion pits in Type 30...

Recent Advances in Cast Iron Structure and Properties Ultrasonic Testing and Flaw Detection

The advantage of ultrasonic testing of iron castings prior to destructive control consists of an impressive reduction in time and costs, as well as the ability to assess the quality of the metal anywhere on the castings. According to the acoustic characteristics of cast iron it is possible to determine the form of graphite in it and its strength. The presence of chill in the metallic base and the thickness of the chilled layer is also determined by ultrasonic method. The use of electronic signal processing allows to distinguish it from high structural noise, which makes possible the testing of cast iron for a thickness of several meters. The use of transducers with phased array is the basis of the flaw detection of containers for nuclear waste from cast iron with globular graphite with a thickness of 500 mm.

Iron Vacancies Accommodate Uranyl Incorporation into Hematite.

Radiotoxic uranium contamination in natural systems and nuclear waste containment can be sequestered by incorporation into naturally abundant iron (oxyhydr)oxides such as hematite (α-Fe2O3) during mineral growth. The stability and properties of the resulting uranium-doped material are impacted by the local coordination environment of incorporated uranium. While measurements of uranium coordination in hematite have been attempted using extended X-ray absorption fine structure (EXAFS) analysis, traditional shell-by-shell EXAFS fitting yields ambiguous results. We used hybrid functional ab initio molecular dynamics (AIMD) simulations for various defect configurations to generate synthetic EXAFS spectra which were combined with adsorbed uranyl spectra to fit experimental U L3-edge EXAFS for U6+-doped hematite. We discovered that the hematite crystal structure accommodates a trans-dioxo uranyl-like configuration for U6+ that substitutes for structural Fe3+, which requires two partially protonated Fe vacancies situated at opposing corner-sharing sites. Surprisingly, the best match to experiment included significant proportions of vacancy configurations other than the minimum-energy configuration, pointing to the importance of incorporation mechanisms and kinetics in determining the state of an impurity incorporated into a host phase under low temperature hydrothermal conditions.

A novel technique for finding gas bubbles in the nuclear waste containers using Muon Scattering Tomography

Nuclear waste is deposited for many years in the concrete or bitumen-filled containers. With time hydrogen gas is produced, which can accumulate in bubbles. These pockets of gas may result in bitumen overflowing out of the waste containers and could result in spread of radioactivity. Muon Scattering Tomography is a non-invasive scanning method developed to examine the unknown content of nuclear waste drums. Here we present a method which allows us to successfully detect bubbles larger than 2 litres and determine their size with a relative uncertainty resolution of 1.55 ± 0.77%. Furthermore, the method allows to make a distinction between a conglomeration of bubbles and a few smaller gas volumes in different locations.

The Effects of Cathodic Reagent Concentration and Small Solution Volumes on the Corrosion of Copper in Dilute Nitric Acid Solutions

The exposure conditions experienced by copper-coated high-level nuclear waste containers in a deep geologic repository will evolve with time. An early exposure period involving the gamma irradiation of aerated humid vapor could lead to the formation of nitric acid condensed in limited volumes of water on the container surface. The evolution of the corrosion processes under these conditions have been studied using pH measurements in limited volumes of water containing various concentrations of nitric acid. The extent and morphology of corrosion was examined using scanning electron microscopy on surfaces and on focused ion beam cut cross sections. The composition of corrosion products was determined by energy dispersive x-ray analyses and Raman spectroscopy. In the absence of dissolved oxygen only minor corrosion was observed with the reduction of nitric acid inhibited by the formation of either chemisorbed nitrate and nitrite species or the formation of a thin cuprite (Cu2O) layer. When the solution was aerated, both oxygen and nitric acid acted as cathodic reagents. After extensive exposure periods corrosion was stifled by the formation of corrosion product deposits of Cu2O, CuO (tenorite), and Cu2NO3(OH)3 (rouaite).

Feasibility of using ultra-high performance fiber reinforced concrete for radioactive waste containers: Drop test simulation

A comparative numerical study has been conducted to investigate the advantage of using ultra-high performance fiber reinforced concrete (UHP-FRC) material as an alternative to replace traditional cross sections of nuclear waste container. Three different finite element (FE) models of a typical waste container with identical dimensions and steel reinforcement detailing are developed. The cross section (HSC, HSC with steel liners, UHP-FRC) is varied parametrically to study its effect on mass, principal stresses, damage distribution, and plastic deformation. The current investigation considers the worst load case scenario by simulating accidental drops of the containers from a full stack height of 5.00m. Two different accidental drop scenarios are considered (flat on-base and corner drop). The detailed 3D-FE models have been built considering nonlinear material properties and the simulations are performed using ABAQUS/Explicit. The material constitutive models have been calibrated based experimental tests.Based on the results of numerical comparative study, the UHP-FRC container suffered the least damage level and distribution among the three designs under considered loading cases. Both HSC, HSC with steel liners containers suffered high damage levels aligned between lid and container’s body which might lead to an opening of the container. On the other hand, stresses and damage levels at the lid to body interface are substantially improved in case of using UHP-FRC.

Real time, in-situ deuteriding of uranium encapsulated in grout; effects of temperature on the uranium-deuterium reaction

To accurately predict the initiation and evolution of uranium hydride potentially present in nuclear waste containers, studies of simulated conditions are required. Here, for the first time, the uranium-deuterium reaction was examined in-situ, in real time, whilst within grouted media. A deuterium gas control rig and stainless steel-quartz glass reaction cell were configured on a synchrotron beam line to collect X-ray diffraction and X-ray tomography data. It was found that deuteride formation was limited by the uranium and grout thermal conductivities and deuteride initiation only commenced above a threshold temperature. Strong adherence between uranium oxide and grout was also observed.

Corrosion of Copper as a Nuclear Waste Container Material in Simulated Anoxic Granitic Groundwater

Copper (Cu) is a candidate material for waste packages in geological disposal systems for high-level radioactive waste in Switzerland, Sweden, Finland, Japan, and Canada. This paper reports experimental tests of Cu in the context of radioactive waste disposal applications. Experimental tests of Cu general corrosion and hydrogen evolution were conducted under anoxic conditions (less than 10 ppb of O2) using synthetic saline groundwater based on reference compositions of deep groundwaters in crystalline rock of the Canadian Shield. The results indicate that the Cu open-circuit potential and corrosion rates in anoxic waters were very sensitive to the residual O2 concentration in solution. The corrosion rates ranged from submicrometer to micrometer per year, depending on the residual O2 concentration level. The corrosion products were predominantly cuprous oxide (Cu2O). Chlorine was present in corrosion products for tests exposed to synthetic saline groundwater, but more work is needed to assess its role in the corrosion process. Minute amounts of hydrogen were detected from the autoclave as test cell, however, they cannot be simply correlated to Cu corrosion because of complication of the autoclave material corrosion.

Corrosion behaviour of carbon steel, titanium and titanium alloy in simulated underground water in Beishan area preselected for nuclear waste repository in China

ABSTRACTThe groundwater will completely infiltrate to the surface of nuclear waste container after the closure of its deep geological disposal. Therefore, the corrosion behaviour of Q235 carbon steel, titanium and titanium alloy, which were the candidates as the container materials for high-level nuclear waste disposal in simulated groundwater solution of Beishan to be served as the preselected high-level nuclear waste disposal area in China at different temperatures, was studied through electrochemical methods including open circuit potential, electrochemical impedance spectroscopy and potentiodynamic polarisation curve measurements. The results show that the corrosion rate of titanium and titanium alloy is lower than that of carbon steel at all temperatures, and they are more promising as container materials. Another phenomenon is that higher temperatures facilitate the protection performance of corrosion products compared with lower temperatures.

Generalized Approach for Determination of Thermal Conductivity of Buffer Materials

Determination of thermal conductivity of buffer materials is an important aspect in the design and characterization of engineered barrier systems (EBS) in deep geological repositories (DGRs) for safe containment of high-level nuclear waste. Several factors, viz. compaction state, particle size distribution, and mineralogical characteristics of buffer materials, influence the thermal conductivity of composite buffer materials. Therefore, it is essential to give due regards to the influence of these factors while estimating thermal conductivity of buffer materials. In view of this, the present study pertains to an extensive laboratory scale determination of thermal conductivity of a wide range of sand-bentonite based buffer materials employing a thermal needle probe. Precision and accuracy of the thermal needle probe are established in relation to a contemporary and widely endorsed thermal property analyzer. Further, the efficacy of several predictive models available in the literature is evaluated to appraise the thermal conductivity of composite buffer materials in relation to the experimental data. Realizing the need of a generalized approach considering the influence of clay mineralogy and particle size fraction present in the geomaterial, the manuscript proposes generalized thermal conductivity prediction models meant for the buffer materials. The developed models are found to deliver satisfactory performance and accuracy in appraising thermal conductivity of sand-bentonite buffers used in DGRs.

Towards understanding the microstructural and structural changes in natural hierarchical materials for energy recovery: In-operando multi-scale X-ray scattering characterization of Na- and Ca-montmorillonite on heating to 1150 °C

Understanding the changes in the microstructures and structures of clays with varying intercalated metal ions at elevated temperatures is of importance for many applications ranging from the recovery of shale gas from unconventional formations to developing effective nuclear waste containment technologies, and engineering materials such as ceramics for fuel cell applications. In this study, synchrotron-based in-operando multi-scale X-ray scattering analyses are used to determine dynamic microstructural and crystal structural changes in Na- and Ca-montmorillonite on heating from 30°C to 1150°C. Larger cations such as Ca2+ confer more defined morphological regimes compared to Na+ ions in compacted clays, as evident from the ultra-small-angle X-ray scattering results. The hierarchical morphology of clays is characterized to distinguish between nano-scale interlayer swelling porosity, meso-scale porosity, and intergranular pore spaces between powdered clay grains. On heating from ambient temperature to 200°C, the removal of interlayer water reduced the basal distances to 9.6Å. On further heating to 800°C, gradual dehydroxylation of the clay sheets is evident from the structural changes. The effects of sintering at temperatures greater than 800°C are evident from significant reductions in the intrinsic porosities of the clay sheets, and the formation of newer phases such as mullite. By connecting the in-operando microstructural and structural changes across spatial scales ranging from micrometers to Angstroms, the possibility of engineering high temperature processes for achieving morphologies and chemical compositions of interest is presented.

Crevice Corrosion of Grade-2 Titanium in Saline Solutions at Different Temperatures and Oxygen Concentrations

Titanium alloys are candidate materials for the fabrication of high level nuclear waste containers but may be susceptible to crevice corrosion in the initial warm oxidizing conditions anticipated in a deep geological repository. The crevice corrosion of Grade-2 titanium has been studied in 1.0 mol/L NaCl at various temperatures (120°C and 150°C) and oxygen concentrations using a galvanic coupling technique. The current due to propagation supported by oxygen reduction on the surface external to the crevice was monitored electrochemically and the total propagation determined from weight change measurements. The redistribution of impurities (Fe, V, Mo) during corrosion was determined by dynamic secondary ion mass spectrometry. Propagation was dominantly driven by the reduction of protons created inside the crevice by cation hydrolysis. Damage accumulated at the locations of TixFe intermetallic particles, which are both anodically reactive and function as catalytic cathodes for proton reduction. The compact TiO2·xH2O deposits at active sites led to stifling of corrosion when only between 9 and 56% of the available oxygen had been consumed. While propagation was more rapid at 150°C than at 120°C, it was more readily stifled at the higher temperature, leading to the accumulation of less damage.

Chemical ordering in substituted fluorite oxides: a computational investigation of Ho2Zr2O7 and RE2Th2O7 (RE=Ho, Y, Gd, Nd, La).

Fluorite-structured oxides find widespread use for applications spanning nuclear energy and waste containment, energy conversion, and sensing. In such applications the host tetravalent cation is often partially substituted by trivalent cations, with an associated formation of charge-compensating oxygen vacancies. The stability and properties of such materials are known to be influenced strongly by chemical ordering of the cations and vacancies, and the nature of such ordering and associated energetics are thus of considerable interest. Here we employ density-functional theory (DFT) calculations to study the structure and energetics of cation and oxygen-vacancy ordering in Ho2Zr2O7. In a recent neutron total scattering study, solid solutions in this system were reported to feature local chemical ordering based on the fluorite-derivative weberite structure. The calculations show a preferred chemical ordering qualitatively consistent with these findings, and yield values for the ordering energy of 9.5 kJ/mol-cation. Similar DFT calculations are applied to additional RE2Th2O7 fluorite compounds, spanning a range of values for the ratio of the tetravalent and trivalent (RE) cation radii. The results demonstrate that weberite-type order becomes destabilized with increasing values of this size ratio, consistent with an increasing energetic preference for the tetravalent cations to have higher oxygen coordination.

Roles of Radiolytic and Externally Generated H2 in the Corrosion of Fractured Spent Nuclear Fuel.

A 2-D model for the corrosion of spent nuclear fuel inside a failed nuclear waste container has been modified to determine the influence of various redox processes occurring within fractures in the fuel. The corrosion process is driven by reaction of the fuel with the dominant α radiolysis product, H2O2. A number of reactions are shown to moderate or suppress the corrosion rate, including H2O2 decomposition and a number of reactions involving dissolved H2 produced either by α radiolysis or by the corrosion of the steel container vessel. Both sources of H2 lead to the suppression of fuel corrosion, with their relative importance being determined by the radiation dose rate, the steel corrosion rate, and the dimensions of the fractures in the fuel. The combination of H2 from these two sources can effectively prevent corrosion when only micromolar quantities of H2 are present.

Corrosion behaviour of steel CSN 422707.9 in concentrated synthetic bentonite pore water

Abstract Concentrating of pore bentonite water as a result of water evaporation at the hot container surface is expected when the bentonite cover of the permanent nuclear waste container is being gradually saturated. The study assesses the influence of an extent of the pore water enrichment by chloride and sulphate ions up to a multiple of a hundred of their equilibrium concentration. An increase of concentration of these ions does not imply an increase of the electrolyte aggressivity automatically. A minimum of corrosion resistance was observed at triplicate concentration at all temperatures, 40, 70 and 90°C. Even more significant impact on corrosion behaviour was recognized for composition of anoxic atmosphere above the electrolyte. Contrary to nitrogen, the mixture of nitrogen and carbon dioxide remarkably elevates the electrolyte’s corrosion aggressivity.