High-level Waste Storage Research Articles

Effect of Heat Treatment on Corrosion Behavior of Alloy 690 and Alloy 693 in Simulated Nuclear High-Level Waste Medium

Nickel-based alloys are being considered as candidate materials for the storage of high-level waste generated from the reprocessing of spent nuclear fuel. In the present investigation, Alloy 690 (UNS N06690) and Alloy 693 (UNS N06693) were assessed using the potentiodynamic anodic polarization technique for the corrosion behavior in the solution-annealed and sensitized conditions, in 3 M nitric acid (HNO3) and simulated high-level waste (HLW) at 25°C and 50°C. From the results of the investigation, it was found that the polarization curves for the solution-annealed specimens were characterized by a long passive range and low passivation current density, at 25°C. Increasing solution temperature to 50°C led to a corresponding increase in corrosion potential as well as passivation current density. The solution-annealed specimen showed improved corrosion resistance compared to the sensitized specimen in both media, for both alloys. Double-loop electrochemical potentiokinetic reactivation (DL-EPR) test was carried out for both solution-annealed and sensitized Alloy 690 and Alloy 693 in 0.5 M sulfuric acid (H2SO4) containing 0.0001 M potassium thiocyanate (KSCN), to measure the degree of sensitization (DOS). Microstructural examination was carried out by optical microscopy and scanning electron microscopy (SEM) after electrolytic etching. The results of the present investigation are discussed in the paper.

From the Editors

From the Editors

The Research of Determining the Design Ground Motion Parameters for the High-Level Radioactive Waste Storage Structure

On the basis of analyzing the special requirements for security for the high-level radioactive waste storage structure, the seismic design parameters of this engineering are determined. There is no criterion that determines the design ground motion parameters of underground engineering. Based on the thought of the severest design ground motion, the basic methods and principles to determine the design ground motion parameters of underground structure are put forward. The characteristics of high-level radioactive waste storage structure are analyzed, and the frequencies corresponding to the maxim modal participation factors are gain. Combined with the analysis of the site geological safety assessment report, through the initial search and optimization with the method of numerical simulation, the design ground motion parameters of the high-level radioactive waste storage structure on the specific site is determined. The method is easy to operate, which can also be applied in other important deep underground engineerings.

Corrosion behavior of Alloy 690 and Alloy 693 in simulated nuclear high level waste medium

Nickel based alloys are candidate materials for the storage of high level waste (HLW) generated from reprocessing of spent nuclear fuel. In the present investigation Alloy 690 and Alloy 693 are assessed by potentiodynamic anodic polarization technique for their corrosion behavior in 3 M HNO 3, 3 M HNO 3 containing simulated HLW and in chloride medium. Both the alloys were found to possess good corrosion resistance in both the media at ambient condition. Microstructural examination was carried out by SEM for both the alloys after electrolytic etching. Compositional analysis of the passive film formed on the alloys in 3 M HNO 3 and 3 M HNO 3 with HLW was carried out by XPS. The surface of Alloy 690 and Alloy 693, both consists of a thin layer of oxide of Ni, Cr, and Fe under passivation in both the media. The results of investigation are presented in the paper.

End of the road for Yucca mountain

After almost 25 years in the planning and almost £90bn wasted, the United States has abandoned its plans for a high-level nuclear waste storage facility at Yucca Mountain in Nevada. There are four types of nuclear waste, ranging from the very low level waste that can be disposed of commercially to the high level waste primarily spent fuel, which is of the greatest concern. This announcement for a deep geological storage repository has only created an atmosphere of uncertainty. It seems that the US has seen the light and will look elsewhere for its long-term storage options.

Effect of 1-Hydroxyethane-1,1-diphosphonic Acid (HEDPA) on Partitioning of Np and Pu to Synthetic Boehmite

The effect of 1-hydroxyethane-1,1-diphosphonic acid (HEDPA) on the sorption of Np(V) and Pu(V) to synthetic boehmite (γ-AlOOH) was examined as a function of time and pH (between 4 to 11). The sorption of both elements in boehmite suspensions (1 M NaCl, 600 mg L−1 boehmite) increased with increasing pH. The sorption edges for neptunium and plutonium occurred at approximately pH 8.0 and 6.6, respectively. After steady state partitioning was reached, HEDPA was added to the neptunium-boehmite and plutonium-boehmite suspensions. Neptunium and plutonium partitioning appears to be primarily affected by the formation of soluble Np:HEDPA and Pu:HEDPA complexes, the dissolution of boehmite promoted by HEDPA, and the precipitation of Np:HEDPA and Pu:HEDPA colloids. The results are discussed in terms of applicability of HEDPA-promoted dissolution as a waste reduction method in the treatment of sludge phases contained within high-level nuclear waste storage tanks.

Soil–environment interactions in geotechnical engineering

The range of problems that geotechnical engineers must face is increasing in complexity and scope. Often, complexity arises from the interaction between the soil and the environment – the topic of this lecture. To deal with this type of problem, the classical soil mechanics formulation is progressively generalised in order to incorporate the effects of new phenomena and new variables on soil behaviour. Recent advances in unsaturated soil mechanics are presented first: it is shown that they provide a consistent framework for understanding the engineering behaviour of unsaturated soils, and the effects of suction and moisture changes. Building on those developments, soil behaviour is further explored by considering thermal effects for two opposite cases: high temperatures, associated with the problem of storage and disposal of high-level radioactive waste; and low temperatures in problems of freezing ground. Finally, the lecture examines some issues related to chemical effects on soils and rocks, focusing in part on the subject of tunnelling in sulphate-bearing rocks. In each case new environmental variables are identified, enhanced theoretical formulations are established, and new or extended constitutive laws are presented. Particular emphasis is placed on mechanical constitutive equations, as they are especially important in geotechnical engineering. The lecture includes summary accounts of a number of case histories that illustrate the relevance and implications of the developments described for geotechnical engineering practice.

From wasteland to waste site: the role of discourse in nuclear power's environmental injustices

The purpose of this essay is twofold. First, I examine interdisciplinary literature to reveal the environmental injustices associated with the front and back ends of nuclear power production in the USA – Uranium mining and high-level nuclear waste (HLW) storage. Second, I argue that the injustices associated with nuclear power are upheld, in part, through discourse. This essay examines how the term “wasteland” is invoked in relation to HLW waste storage in the USA and contributes to the discursive formation of nuclear colonialism. Examination of this discourse not only contributes to current literature on nuclear colonialism but also to environmental justice research by arguing for the importance of examining the discursive aspects of environmental injustices. Further, the essay adds to current scholarship in energy justice by highlighting the environmental injustices associated with nuclear power.

Sorption and desorption of 125I-, 137Cs+, 85Sr2+ and 152,154Eu3+ on disturbed soils under dynamic flow and static batch conditions

Sorption of radionuclides on homogenized soils (under 2.5 mm grain size) from synthetic groundwater of 8·10−3M ionic strength and pH 8.5 has been studied under dynamic (flow) and static (batch) conditions. The corresponding water-soluble compounds, as carriers in the 10−6 mol/dm3 concentration, were added into the SGW prior to the experiments. Soil samples were taken from several locations around the environment of the High Level Waste Storage Facility at Nuclear Research Institute Řež plc in 5–100 cm depth. The dynamic experiments were carried out in columns made of PP+PE injection syringes of 17.8 cm length and 2.1 cm in diameter. A multi-head peristaltic pump was used for pumping the water upward through the columns at a seepage velocity of about 0.06 cm/min in average. The radioactive nuclides were added into the water stream individually in a form of a short pulse in 0.1 cm3 of demineralized water. Dynamic desorption experiments were performed with the same experimental arrangement using a mixture of 10−2N H2SO4 and 10−2N HNO3 in a volume ratio of 2: 1. Retardation, distribution and hydrodynamic dispersion coefficients during transport of radionuclides were determined by the evaluation of the integral form of a simple advection-dispersion equation, used for fitting experimental data and modeling the theoretical sorption breakthrough and desorption displacement curves. The static experiments were realized in 100 cm3 plastic bottles stirring 5 g of soil samples with SGW occasionally in a soil to SGW ratio of 1: 10 (m/V). Kinetic parameters including equilibrium sorption activity, activity transfer rate constants and sorption half-times were also determined. The results of dynamic experiments were compared with static sorption experiments.

History and Operation of the Hanford High-Level Waste Storage Tanks

Abstract The Hanford Site is a 560-square-mile (1,450-km2) complex established by the U.S. government in 1943 to produce plutonium for nuclear weapons, ultimately bringing an end to World War II. Plutonium production activities continued after the war through 1991, at which point the site's mission changed from plutonium production to environmental cleanup and restoration. Production activities at the site resulted in a broad range of contaminated materials and facilities, including 57 million gallons of high-level (i.e., highly radioactive) nuclear waste in liquid and solid forms. The high-level waste was stored as it was created, first in single-shell tanks built between 1943 and 1964, then in more robust double-shell tanks constructed between 1968 and 1986. Due to waste leakage in a small number of single-shell tanks and the potential for additional single-shell tank failures, all single-shell tanks were removed from service by 1980. All pumpable liquid has been transferred to sound double-shell tanks....

X-ray absorption and photoelectron spectroscopy investigation of selenite reduction by Fe II-bearing minerals

The long-lived radionuclide 79Se is one of the elements of concern for the safe storage of high-level nuclear waste, since clay minerals in engineered barriers and natural aquifer sediments strongly adsorb cationic species, but to lesser extent anions like selenate (Se VIO 4 2−) and selenite (Se IVO 3 2−). Previous investigations have demonstrated, however, that Se IV and Se VI are reduced by surface-associated Fe II, thereby forming insoluble Se 0 and Fe selenides. Here we show that the mixed Fe II/ III (hydr)oxides green rust and magnetite, and the Fe II sulfide mackinawite reduce selenite rapidly (< 1 day) to FeSe, while the slightly slower reduction by the Fe II carbonate siderite produces elemental Se. In the case of mackinawite, both S −II and Fe II surface atoms are oxidized at a ratio of one to four by producing a defective mackinawite surface. Comparison of these spectroscopic results with thermodynamic equilibrium modeling provides evidence that the nature of reduction end product in these Fe II systems is controlled by the concentration of HSe −; Se 0 forms only at lower HSe − concentrations related to slower HSeO 3 − reduction kinetics. Even under thermodynamically unstable conditions, the initially formed Se solid phases may remain stable for longer periods since their low solubility prevents the dissolution required for a phase transformation into more stable solids. The reduction by Fe 2+-montmorillonite is generally much slower and restricted to a pH range, where selenite is adsorbed (pH < 7), stressing the importance of a heterogeneous, surface-enhanced electron transfer reaction. Although the solids precipitated by the redox reaction are nanocrystalline, their solubility remains below 6.3 × 10 − 8 M. No evidence for aqueous metal selenide colloids nor for Se sorption to colloidal phases was found. Since Fe II phases like the ones investigated here should be ubiquitous in the near field of nuclear waste disposals as well as in the surrounding aquifers, mobility of the fission product 79Se may be much lower than previously assumed.

Modeling Cation Diffusion in Compacted Water-Saturated Sodium Bentonite at Low Ionic Strength

Sodium bentonites are used as barrier materials for the isolation of landfills and are under consideration for a similar use in the subsurface storage of high-level radioactive waste. The performance of these barriers is determined in large part by molecular diffusion in the bentonite pore space. We tested two current models of cation diffusion in bentonite against experimental data on the relative apparent diffusion coefficients of two representative cations, sodium and strontium. On the "macropore/nanopore" model, solute molecules are divided into two categories, with unequal pore-scale diffusion coefficients, based on location: in macropores or in interlayer nanopores. On the "surface diffusion" model, solute molecules are divided into categories based on chemical speciation: dissolved or adsorbed. The macropore/nanopore model agrees with all experimental data at partial montmorillonite dry densities ranging from 0.2 (a dilute bentonite gel) to 1.7 kg dm(-3) (a highly compacted bentonite with most of its pore space located in interlayer nanopores), whereas the surface diffusion model fails at partial montmorillonite dry densities greater than about 1.3 kg dm(-3).

Kinetics of AgI precipitation from AgCl as affected by background electrolyte

Iodide retention by AgCl, a potential sorbent in high-level waste (HLW) storage systems, was determined. The kinetics and steady state sorption of iodide were determined in single and mixed electrolytes of NaNO3, and NaCl at ionic strengths of 25 and 50 mM. Iodide retention involved the conversion of AgCl to AgI. This conversion increased rapidly within 0.02 hours, and retention maxima of 0.92 and 1.0 mol·l·mol−1 Ag occurred for low and high ionic strengths, respectively. These short-term studies indicated that AgCl would be an effective scavenger of I− in HLW containment systems.

Transformation of magnetite to goethite under alkaline pH conditions

AbstractMagnetite is a redox active Fe oxide common in most soil and sedimentary environments. In this study, we investigated magnetite transformations under extreme alkaline conditions (0.1–2 mol l–1 NaOH) similar to those found under high-level radioactive waste storage tanks at the Hanford site in the State of Washington, USA. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results showed that magnetite was transformed into maghemite and goethite, and the XRD peak intensities for goethite increased with NaOH concentration. Goethite was presumably formed through reconstructive dissolution/crystallization reactions that can be written as: Fe3O4 + OH– + H2O = γ-Fe2O3 + Fe(OH)3–; 2Fe(OH)3– + 1/2O2 + 2H+ = 2α-FeOOH + 3H2O. Some of the newly formed maghemite may also redissolve to form goethite at greater NaOH concentrations.

Use of massively parallel molecular dynamics simulations for radiation damage in pyrochlores

DL_POLY_3 is a general purpose molecular dynamics (MD) simulation package designed to simulate systems of the order of tens of millions of particles and beyond by efficiently harnessing the power of modern computer clusters. Here we discuss the package design, functionality and report on performance and capability limits. We then report the application of DL_POLY_3 to study radiation cascades in Gd2Ti2O7 and Gd2Zr2O7, potential materials for high-level radioactive waste storage and discuss problems associated with the analysis of the cascades. We see little direct amorphisation but rather the start of a transition to the fluorite structure which is more pronounced for the Zr than the Ti compound.

NMR Study of Strontium Binding by a Micaceous Mineral

The nature of strontium binding by soil minerals directly affects the transport and sequestration/remediation of radioactive strontium species released from leaking high-level nuclear waste storage tanks. However, the molecular-level structure of strontium binding sites has seldom been explored in phyllosilicate minerals by direct spectroscopic means and is not well-understood. In this work, we use solid-state NMR to analyze strontium directly and indirectly in a fully strontium-exchanged synthetic mica of nominal composition Na(4)Mg(6)Al(4)Si(4)O(20)F(4). Thermogravimetric analysis, X-ray diffraction analysis, and NMR evidence supports that heat treatment at 500 degrees C for 4 h fully dehydrates the mica, creating a hydrogen-free interlayer. Analysis of the strontium NMR spectrum of the heat-treated mica shows a single strontium environment with a quadrupolar coupling constant of 9.02 MHz and a quadrupolar asymmetry parameter of 1.0. These quadrupolar parameters are consistent with a highly distorted and asymmetric coordination environment that would be produced by strontium cations without water in the coordination sphere bound deep within the ditrigonal holes. Evidence for at least one additional strontium environment, where proton-strontium couplings may occur, was found via a (1)H-(87)Sr transfer of populations by double resonance NMR experiment. We conclude that the strontium cations in the proton-free interlayer are observable by (87)Sr NMR and bound through electrostatic interactions as nine coordinate inner-sphere complexes sitting in the ditrigonal holes. Partially hydrated strontium cations invisible to direct (87)Sr NMR are also present and located on the external mica surfaces, which are known to hydrate upon exposure to atmospheric moisture. These results demonstrate that modern pulsed NMR techniques and high fields can be used effectively to provide structural details of strontium binding by phyllosilicate minerals.

Guest Editorial: Nuclear Power and Public Health

Guest Editorial: Nuclear Power and Public Health

A Combined Nonfertile and UO2 PWR Fuel Assembly for Actinide Waste Minimization

A new COmbined NonFertile and Uranium (CONFU) fuel assembly is proposed to limit the actinides that need long-term high-level waste storage from the pressurized water reactor (PWR) fuel cycle. In the CONFU assembly concept, ~20% of the UO2 fuel pins are replaced with fertile free fuel hosting the transuranic elements (TRUs) generated in the previous cycle. This leads to a fuel cycle sustainable with respect to net TRU generation, and the amount and radiotoxicity of the nuclear waste can be significantly reduced in comparison with the conventional once-through UO2 fuel cycle. It is shown that under the constraints of acceptable power peaking limits, the CONFU assembly exhibits negative reactivity feedback coefficients comparable in values to those of the reference UO2 fuel. Feasibility of the PWR core operation and control with complete TRU recycle has been shown based on full-core three-dimensional neutronic simulation. However, gradual buildup of small amounts of Cm and Cf challenges fuel reprocessing and fabrication due to the high spontaneous fission rates of these nuclides and heat generation by some Pu, Am, and Cm isotopes. Feasibility of the processing steps becomes more attainable if the time between discharge and reprocessing is 20 yr or longer.

Microstructure of 24-1928 Ma concordant monazites; implications for geochronology and nuclear waste deposits

The microstructure of monazite was studied using scanning electron microscopy (SEM), electron microprobe analysis (EMP), X-ray diffraction patterns (XRD), and transmission electron microscopy (TEM). Four well-characterized monazites were investigated, having very different concordant U-Pb ages (24 to 1928 Ma), and up to ∼15 wt.% ThO 2, and ∼0.94 wt. % UO 2. The SEM and EMP analyses of polished single crystal fragments reveal the absence of significant chemical zoning. XRD and TEM investigations show that the monazites are not metamict, despite their old ages, very high abundances of radionuclides, and hence, high time-integrated radiation doses. Except for the youngest one, the monazite crystals are composed of a mosaic of crystalline but slightly distorted domains. This structure is responsible for the presence of (1) mottled diffraction contrasts on the TEM, and (2) a second structural phase (B), with very broad reflections in the XRD patterns. Older monazites receive higher self-irradiation doses, and hence, they contain higher amounts of this B-phase. For the 1928 Ma monazite, XRD reveals only the broad reflections of phase B, implying that the whole monazite was affected by radiation damage that resulted in total distortion of the lattice. It is concluded that radiation damage in the form of amorphous domains does not accumulate in monazite because self-annealing heals the defects as they are produced by α-decay damage. The only memory of irradiation-induced defects is the presence of distorted domains. As the diffusion rate of Pb in an undisturbed monazite lattice is extremely low, Pb loss due to volume diffusion out of the monazite lattice is virtually impossible. This is considered as one reason why almost all monazites have concordant U-Th-Pb ages. Moreover, as long-term self-irradiation effects are limited in monazite, we consider this phase as a good candidate for the storage of high-level nuclear waste under the aspect of its high resistance to irradiation.

Remote Inspection of a 46-Year-Old Buried High-Level Waste Storage Tank

This paper describes the remote ultrasonic (UT) examinations of a high-level radioactive waste (HLW) storage tank at the Savannah River Site in South Carolina. The inspections, carried out by E.R. Holland, R.W. Vande Kamp, and J.B. Elder, were performed from the contaminated, annular space of the 46-year-old, inactive, 1.03 million gallon waste storage tank. A steerable, magnetic wheel wall crawler was inserted into the annular space through small (6 in., or 150 mm, diameter) holes/risers in the tank top. The crawler carried the equipment used to simultaneously collect data with up to four UT transducers and two cameras. The purpose of this inspection was to verify corrosion models and to investigate the possibility of previously unidentified corrosion sites or mechanisms. The inspections included evaluation of previously identified leak sites, thickness mapping, and crack detection scans on specified areas of the tank. No indications of reportable wall loss or pitting were detected. All thickness readings were above minimum design tank-wall thickness, although several small indications of thinning were noted. The crack detection and sizing examinations revealed five previously undetected indications, four of which were only partially through-wall. The cracks that were examined were found to be slightly longer than expected but still well within the flaw size criteria used to evaluate tank structural integrity.