Nuclear waste from small modular reactors.

SignificanceSmall modular reactors (SMRs), proposed as the future of nuclear energy, have purported cost and safety advantages over existing gigawatt-scale light water reactors (LWRs). However, few studies have assessed the implications of SMRs for the back end of the nuclear fuel cycle. The low-, intermediate-, and high-level waste stream characterization presented here reveals that SMRs will produce more voluminous and chemically/physically reactive waste than LWRs, which will impact options for the management and disposal of this waste. Although the analysis focuses on only three of dozens of proposed SMR designs, the intrinsically higher neutron leakage associated with SMRs suggests that most designs are inferior to LWRs with respect to the generation, management, and final disposal of key radionuclides in nuclear waste.

The Role of Water and Hydroxyl Groups in the Structures of Stetindite and Coffinite, MSiO4 (M = Ce, U).

Orthosilicates adopt the zircon structure types (I41/amd), consisting of isolated SiO4 tetrahedra joined by A-site metal cations, such as Ce and U. They are of significant interest in the fields of geochemistry, mineralogy, nuclear waste form development, and material science. Stetindite (CeSiO4) and coffinite (USiO4) can be formed under hydrothermal conditions despite both being thermodynamically metastable. Water has been hypothesized to play a significant role in stabilizing and forming these orthosilicate phases, though little experimental evidence exists. To understand the effects of hydration or hydroxylation on these orthosilicates, in situ high-temperature synchrotron and laboratory-based X-ray diffraction was conducted from 25 to ∼850 °C. Stetindite maintains its I41/amd symmetry with increasing temperature but exhibits a discontinuous expansion along the a-axis during heating, presumably due to the removal of water confined in the [001] channels, which shrink against thermal expansion along the a-axis. Additional in situ high-temperature Raman and Fourier transform infrared spectroscopy also confirmed the presence of the confined water. Coffinite was also found to expand nonlinearly up to 600 °C and then thermally decompose into a mixture of UO2 and SiO2. A combination of dehydration and dehydroxylation is proposed for explaining the thermal behavior of coffinite synthesized hydrothermally. Additionally, we investigated high-temperature structures of two coffinite-thorite solid solutions, uranothorite (UxTh1-xSiO4), which displayed complex variations in composition during heating that was attributed to the negative enthalpy of mixing. Lastly, for the first time, the coefficients of thermal expansion of CeSiO4, USiO4, U0.46Th0.54SiO4, and U0.9Th0.1SiO4 were determined to be αV = 14.49 × 10-6, 14.29 × 10-6, 17.21 × 10-6, and 17.23 × 10-6 °C-1, respectively.

Thermodynamics of CeSiO4: Implications for Actinide Orthosilicates.

Zircon (ZrSiO4, I41/amd) can accommodate actinides, such as thorium, uranium, and plutonium. The zircon structure has been determined for several of the end-member compositions of other actinides, such as plutonium and neptunium. However, the thermodynamic properties of these actinide zircon structure types are largely unknown due to the difficulties in synthesizing these materials and handling transuranium actinides. Thus, we have completed a thermodynamic study of cerium orthosilicate, stetindite (CeSiO4), a surrogate of PuSiO4. For the first time, the standard enthalpy of formation of CeSiO4 was obtained by high temperature oxide melt solution calorimetry to be -1971.9 ± 3.6 kJ/mol. Stetindite is energetically metastable with respect to CeO2 and SiO2 by 27.5 ± 3.1 kJ/mol. The metastability explains the rarity of the natural occurrence of stetindite and the difficulty of its synthesis. Applying the obtained enthalpy of formation of CeSiO4 from this work, along with those previously reported for USiO4 and ThSiO4, we developed an empirical energetic relation for actinide orthosilicates. The predicted enthalpies of formation of AnSiO4 are then determined with a discussion of future strategies for efficiently immobilizing Pu or minor actinides in the zircon structure.

A Critical Review of Existing Criteria for the Prediction of Pyrochlore Formation and Stability.

Depending on intrinsic (e.g., radius ratio rule rLn/ rZr) and extrinsic factors (e.g., processing conditions), pyrochlore-type Ln2Zr2O7 oxides achieve variable degrees of structural disorder. We report on a systematic study of the structural and microstructural characteristics of the Gd2- xLn xZr2O7 system, exploring the effect of replacing Gd with a wide range of homovalent lanthanide ions (Ln = Nd, Sm, Dy, Ho, Y, and Er; x = 0.20 and 0.80). All compositions were prepared via a mechanochemical reaction between the corresponding oxides and characterized by X-ray diffraction (standard and synchrotron sources) using the Rietveld method, as well as by Raman spectroscopy. Irrespective of chemical composition, this study reveals that all compositions exhibit a fluorite-like structure. Furthermore, by firing each sample at 800 and 1400 °C, we are able to analyze the transition to pyrochlore-like structures, featuring different degrees of disorder, in all but Gd1.20Y0.80Zr2O7, which retains the fluorite structure even after heating. The structural data are used to assess the existing criteria for predicting the formation and stability of the pyrochlore structure; according to this analysis, the simple radius ratio rule ( rLn/ rZr), provides a useful and sufficiently robust criterion. Because the pyrochlore structure has a strong tendency to disorder, it is not possible to define an empirical index similar to the Goldschmidt tolerance factor for perovskite.

A2TiO5 (A = Dy, Gd, Er, Yb) at High Pressure.

The structural evolution of lanthanide A2TiO5 (A = Dy, Gd, Yb, Er) at high pressure is investigated using synchrotron X-ray diffraction. The effects of A-site cation size and of the initial structure are systematically examined by varying the composition of the isostructural lanthanide titanates and the structure of dysprosium titanate polymorphs (orthorhombic, hexagonal, and cubic), respectively. All samples undergo irreversible high-pressure phase transformations, but with different onset pressures depending on the initial structure. While each individual phase exhibits different phase transformation histories, all samples commonly experience a sluggish transformation to a defect cotunnite-like (Pnma) phase for a certain pressure range. Orthorhombic Dy2TiO5 and Gd2TiO5 form P21am at pressures below 9 GPa and Pnma above 13 GPa. Pyrochlore-type Dy2TiO5 and Er2TiO5 as well as defect-fluorite-type Yb2TiO5 form Pnma at ∼21 GPa, followed by Im3̅m. Hexagonal Dy2TiO5 forms Pnma directly, although a small amount of remnants of hexagonal Dy2TiO5 is observed even at the highest pressure (∼55 GPa) reached, indicating kinetic limitations in the hexagonal Dy2TiO5 phase transformations at high pressure. Decompression of these materials leads to different metastable phases. Most interestingly, a high-pressure cubic X-type phase (Im3̅m) is confirmed using high-resolution transmission electron microscopy on recovered pyrochlore-type Er2TiO5. The kinetic constraints on this metastable phase yield a mixture of both the X-type phase and amorphous domains upon pressure release. This is the first observation of an X-type phase for an A2BO5 composition at high pressure.

Uranium Dioxides and Debris Fragments Released to the Environment with Cesium-Rich Microparticles from the Fukushima Daiichi Nuclear Power Plant.

Trace U was released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) during the meltdowns, but the speciation of the released components of the nuclear fuel remains unknown. We report, for the first time, the atomic-scale characteristics of nanofragments of the nuclear fuels that were released from the FDNPP into the environment. Nanofragments of an intrinsic U-phase were discovered to be closely associated with radioactive cesium-rich microparticles (CsMPs) in paddy soils collected ∼4 km from the FDNPP. The nanoscale fuel fragments were either encapsulated by or attached to CsMPs and occurred in two different forms: (i) UO2+X nanocrystals of ∼70 nm size, which are embedded into magnetite associated with Tc and Mo on the surface and (ii) Isometric (U,Zr)O2+X nanocrystals of ∼200 nm size, with the U/(U+Zr) molar ratio ranging from 0.14 to 0.91, with intrinsic pores (∼6 nm), indicating the entrapment of vapors or fission-product gases during crystallization. These results document the heterogeneous physical and chemical properties of debris at the nanoscale, which is a mixture of melted fuel and reactor materials, reflecting the complex thermal processes within the FDNPP reactor during meltdown. Still CsMPs are an important medium for the transport of debris fragments into the environment in a respirable form.

Novel Method of Quantifying Radioactive Cesium-Rich Microparticles (CsMPs) in the Environment from the Fukushima Daiichi Nuclear Power Plant.

Highly radioactive cesium-rich microparticles (CsMPs) were released from the Fukushima Daiichi nuclear power plant (FDNPP) to the surrounding environment at an early stage of the nuclear disaster in March of 2011; however, the quantity of released CsMPs remains undetermined. Here, we report a novel method to quantify the number of CsMPs in surface soils at or around Fukushima and the fraction of radioactivity they contribute, which we call "quantification of CsMPs" (QCP) and is based on autoradiography. Here, photostimulated luminescence (PSL) is linearly correlated to the radioactivity of various microparticles, with a regression coefficient of 0.0523 becquerel/PSL/h (Bq/PSL/h). In soil collected from Nagadoro, Fukushima, Japan, CsMPs were detected in soil sieved with a 114 µm mesh. There was no overlap between the radioactivities of CsMPs and clay particles adsorbing Cs. Based on the distribution of radioactivity of CsMPs, the threshold radioactivity of CsMPs in the size fraction of <114 µm was determined to be 0.06 Bq. Based on this method, the number and radioactivity fraction of CsMPs in four surface soils collected from the vicinity of the FDNPP were determined to be 48-318 particles per gram and 8.53-31.8%, respectively. The QCP method is applicable to soils with a total radioactivity as high as ∼106 Bq/kg. This novel method is critically important and can be used to quantitatively understand the distribution and migration of the highly radioactive CsMPs in near-surface environments surrounding Fukushima.

Radiation-induced disorder in compressed lanthanide zirconates.

The effects of swift heavy ion irradiation-induced disordering on the behavior of lanthanide zirconate compounds (Ln2Zr2O7 where Ln = Sm, Er, or Nd) at high pressures are investigated. After irradiation with 2.2 GeV 197Au ions, the initial ordered pyrochlore structure (Fd3[combining macron]m) transformed to a defect-fluorite structure (Fm3[combining macron]m) in Sm2Zr2O7 and Nd2Zr2O7. For irradiated Er2Zr2O7, which has a defect-fluorite structure, ion irradiation induces local disordering by introducing Frenkel defects despite retention of the initial structure. When subjected to high pressures (>29 GPa) in the absence of irradiation, all of these compounds transform to a cotunnite-like (Pnma) phase, followed by sluggish amorphization with further compression. However, if these compounds are irradiated prior to compression, the high pressure cotunnite-like phase is not formed. Rather, they transform directly from their post-irradiation defect-fluorite structure to an amorphous structure upon compression (>25 GPa). Defects and disordering induced by swift heavy ion irradiation alter the transformation pathways by raising the energetic barriers for the transformation to the high pressure cotunnite-like phase, rendering it inaccessible. As a result, the high pressure stability field of the amorphous phase is expanded to lower pressures when irradiation is coupled with compression. The responses of materials in the lanthanide zirconate system to irradiation and compression, both individually and in tandem, are strongly influenced by the specific lanthanide composition, which governs the defect energetics at extreme conditions.

Thermodynamics of formation of coffinite, USiO4.

Coffinite, USiO4, is an important U(IV) mineral, but its thermodynamic properties are not well-constrained. In this work, two different coffinite samples were synthesized under hydrothermal conditions and purified from a mixture of products. The enthalpy of formation was obtained by high-temperature oxide melt solution calorimetry. Coffinite is energetically metastable with respect to a mixture of UO2 (uraninite) and SiO2 (quartz) by 25.6 ± 3.9 kJ/mol. Its standard enthalpy of formation from the elements at 25 °C is -1,970.0 ± 4.2 kJ/mol. Decomposition of the two samples was characterized by X-ray diffraction and by thermogravimetry and differential scanning calorimetry coupled with mass spectrometric analysis of evolved gases. Coffinite slowly decomposes to U3O8 and SiO2 starting around 450 °C in air and thus has poor thermal stability in the ambient environment. The energetic metastability explains why coffinite cannot be synthesized directly from uraninite and quartz but can be made by low-temperature precipitation in aqueous and hydrothermal environments. These thermochemical constraints are in accord with observations of the occurrence of coffinite in nature and are relevant to spent nuclear fuel corrosion.

Inversion in Mg1-xNixAl2O4 Spinel: New Insight into Local Structure.

A wide variety of compositions adopt the isometric spinel structure (AB2O4), in which the atomic-scale ordering is conventionally described according to only three structural degrees of freedom. One, the inversion parameter, is traditionally defined as the degree of cation exchange between the A- and B-sites. This exchange, a measure of intrinsic disorder, is fundamental to understanding the variation in the physical properties of different spinel compositions. Based on neutron total scattering experiments, we have determined that the local structure of Mg1-xNixAl2O4 spinel cannot be understood as simply being due to cation disorder. Rather, cation inversion creates a local tetragonal symmetry that extends over sub-nanometer domains. Consequently, the simple spinel structure is more complicated than previously thought, as more than three parameters are needed to fully describe the structure. This new insight provides a framework by which the behavior of spinel can be more accurately modeled under the extreme environments important for many geophysics and energy-related applications, including prediction of deep seismic activity and immobilization of nuclear waste in oxides.

Probing disorder in isometric pyrochlore and related complex oxides.

There has been an increased focus on understanding the energetics of structures with unconventional ordering (for example, correlated disorder that is heterogeneous across different length scales). In particular, compounds with the isometric pyrochlore structure, A2B2O7, can adopt a disordered, isometric fluorite-type structure, (A, B)4O7, under extreme conditions. Despite the importance of the disordering process there exists only a limited understanding of the role of local ordering on the energy landscape. We have used neutron total scattering to show that disordered fluorite (induced intrinsically by composition/stoichiometry or at far-from-equilibrium conditions produced by high-energy radiation) consists of a local orthorhombic structural unit that is repeated by a pseudo-translational symmetry, such that orthorhombic and isometric arrays coexist at different length scales. We also show that inversion in isometric spinel occurs by a similar process. This insight provides a new basis for understanding order-to-disorder transformations important for applications such as plutonium immobilization, fast ion conduction, and thermal barrier coatings.

Coffinite, USiO4, Is Abundant in Nature: So Why Is It So Difficult To Synthesize?

Coffinite, USiO4, is the second most abundant U(4+) mineral on Earth, and its formation by the alteration of the UO2 in spent nuclear fuel in a geologic repository may control the release of radionuclides to the environment. Despite its abundance in nature, the synthesis and characterization of coffinite have eluded researchers for decades. On the basis of the recent synthesis of USiO4, we can now define the experimental conditions under which coffinite is most efficiently formed. Optimal formation conditions are defined for four parameters: pH, T, heating time, and U/Si molar ratio. The adjustment of pH between 10 and 12 leads probably to the formation of a uranium(IV) hydroxo-silicate complex that acts as a precursor of uranium(IV) silicate colloids and then of coffinite. Moreover, in this pH range, the largest yield of coffinite formation (as compared with those of the two competing byproduct phases, nanometer-scale UO2 and amorphous SiO2) is obtained for 250 °C, 7 days, and 100% excess silica.

Three new silver uranyl diphosphonates: structures and properties.

The hydrothermal reaction of uranium trioxide and methylenediphosphonic acid in the presence of silver nitrate resulted in the formation of three new uranyl coordination polymers: AgUO2[CH2(PO3)(PO3H)] (Ag-1), [Ag2(H2O)1.5]{(UO2)2[CH2(PO3)2]F2}·(H2O)0.5 (Ag-2), and Ag2UO2[CH2(PO3)2] (Ag-3). All consist of uranyl pentagonal bipyramids that form two-dimensional layered structures. Ag-1 and Ag-3 possess the same structural building unit, but the structures are different; Ag-3 is formed through edge-sharing of F atoms to form UO5F2 dimers. The pH and silver cation have significant effects on the structure that is synthesized. Raman spectra of single crystals of Ag-1, Ag-2, and Ag-3 reveal v1 UO2(2+) symmetric stretches of 816 and 829, 822, and 802 cm(-1), respectively. Electronic structure calculations were performed using the projector augmented wave (PAW) method with density functional theory (DFT) to gain insight into the nature of bonding and electronic characteristics of the synthesized compounds. Herein, we report the syntheses, crystal structures, Raman spectroscopy, and luminescent behavior of these three compounds.

"Invisible" radioactive cesium atoms revealed: Pollucite inclusion in cesium-rich microparticles (CsMPs) from the Fukushima Daiichi Nuclear Power Plant.

Understanding radioactive Cs contamination has been a central issue at Fukushima Daiichi and other nuclear legacy sites; however, atomic-scale characterization of radioactive Cs in environmental samples has never been achieved. Here we report, for the first time, the direct imaging of radioactive Cs atoms using high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). In Cs-rich microparticles collected from Japan, we document inclusions that contain 27 - 36 wt% of Cs (reported as Cs2O) in a zeolite: pollucite. The compositions of three pollucite inclusions are (Cs1.86K0.11Rb0.19Ba0.22)2.4(Fe0.85Zn0.84X0.31)2.0Si4.1O12, (Cs1.19K0.05Rb0.19Ba0.22)1.7(Fe0.66Zn0.32X0.41)1.4Si4.6O12, and (Cs1.27K0.21Rb0.29Ba0.15)1.9(Fe0.60Zn0.32X0.69)1.6Si4.4O12 (X includes other cations). HAADF-STEM imaging of pollucite, viewed along the [111] zone axis, revealed an array of Cs atoms, which is consistent with a simulated image using the multi-slice method. The occurrence of pollucite indicates that locally enriched Cs reacted with siliceous substances during the Fukushima meltdowns, presumably through volatilization and condensation. Beta radiation doses from the incorporated Cs are estimated to reach 106 - 107 Gy, which is more than three orders of magnitude less than typical amorphization dose of zeolite. The atomic-resolution imaging of radioactive Cs is an important advance for better understanding the fate of radioactive Cs inside and outside of nuclear reactors damaged by meltdown events.

A versatile multicomponent assembly via ß-cyclodextrin host-guest chemistry on graphene for biomedical applications.

A multi-component nanosystem based on graphene and comprising individual cyclodextrins at its surface is assembled, creating hybrid structures enabling new and important functionalities: optical imaging, drug storage, and cell targeting for medical diagnosis and treatment. These nanohybrids are part of a universal system of interchangeable units, capable of mutilple functionalities. The surface components, made of individual ß-cyclodextrin molecules, are the "hosts" for functional units, which may be used as imaging agents, for anti-cancer drug delivery, and as tumor-specific ligands. Specifically, individual ß-cyclodextrin (ß-CD), with a known capability to host various molecules, is considered a module unit that is assembled onto graphene nanosheet (GNS). The cyclodextrin-functionalized graphene nanosheet (GNS/ß-CD) enables "host-guest" chemistry between the nanohybrid and functional "payloads". The structure, composition, and morphology of the graphene nanosheet hybrid have been investigated. The nanohybrid, GNS/ß-CD, is highly dispersive in various physiological solutions, reflecting the high biostability of cyclodextrin. Regarding the host capability, the nanohybrid is fully capable of selectively accommodating various biological and functional agents in a controlled fashion, including the antivirus drug amantadine, fluorescent dye [5(6)-carboxyfluorescein], and Arg-Gly-Asp (RGD) peptide-targeting ligands assisted by an adamantine linker. The loading ratio of 5(6)-carboxyfluorescein is as high as 110% with a drug concentration of 0.45 mg mL(-1). The cyclic RGD-functionalized nanohybrid exhibits remarkable targeting for HeLa cells.

Multilayered YSZ/GZO films with greatly enhanced ionic conduction for low temperature solid oxide fuel cells.

Strain confinement in heterostructured films significantly affects ionic conductivity of the electrolytes for solid oxide fuel cells based on a multi-layered design strategy. Nearly ideal tensile strain can be achieved by a dedicated manipulation of the lattice mismatch between adjacent layers and fine control of the layer thicknesses to minimize the formation of dislocations and thus to achieve optimized ionic conduction. This strategy was demonstrated by a model system of multilayered 8 mol%Y(2)O(3) stabilized ZrO(2) (YSZ) with Gd(2)Zr(2)O(7) (GZO) films, which were epitaxially grown on Al(2)O(3) (0001) substrates by pulsed laser deposition (PLD) with the {111} planes of YSZ/GZO along the Al(2)O(3) [0 1 -1 0] direction. The tensile strain (3%) resulting from the lattice mismatch can be confined in individual YSZ layers with the formation of a coherent, dislocation-free interface upon the manipulation of the layer thickness below a critical value, e.g., down to 5 nm. The strained heterostructure displays a two order-of-magnitude increase in oxide-ion conductivity as compared with bulk YSZ, and a high ionic conductivity of 0.01 S cm(-1) at 475 °C can be achieved, five times greater than that of Gd-doped ceria/zirconia. The approach of strain confinement by fine control of lattice mismatch and layer thickness represents a promising strategy in developing advanced electrolytes enabling the miniaturization of solid-state ionic devices that can be operated at low temperatures below 500 °C.

Effect of Dipole Interactions on Blocking Temperature and Relaxation Dynamics of Superparamagnetic Iron-Oxide (Fe3O4) Nanoparticle Systems.

The effects of dipole interactions on magnetic nanoparticle magnetization and relaxation dynamics were investigated using five nanoparticle (NP) systems with different surfactants, carrier liquids, size distributions, inter-particle spacing, and NP confinement. Dipole interactions were found to play a crucial role in modifying the blocking temperature behavior of the superparamagnetic nanoparticles, where stronger interactions were found to increase the blocking temperatures. Consequently, the blocking temperature of a densely packed nanoparticle system with stronger dipolar interactions was found to be substantially higher than those of the discrete nanoparticle systems. The frequencies of the dominant relaxation mechanisms were determined by magnetic susceptibility measurements in the frequency range of 100 Hz-7 GHz. The loss mechanisms were identified in terms of Brownian relaxation (1 kHz-10 kHz) and gyromagnetic resonance of Fe3O4 (~1.12 GHz). It was observed that the microwave absorption of the Fe3O4 nanoparticles depend on the local environment surrounding the NPs, as well as the long-range dipole-dipole interactions. These significant findings will be profoundly important in magnetic hyperthermia medical therapeutics and energy applications.