[en] The U(VI) solid phases schoepite, metaschoepite, and dehydrated schoepite are important reservoirs of mobile uranium in the environment. These simple uranyl oxide hydrates result from weathering of uranium minerals and the corrosion of anthropogenic uranium solids. The authors have studied the role of hydrational water among these phases and in subsequent transformation to other secondary metal-U(VI) oxide hydrates. Synthetic metaschoepite (MS, UO₃·2.0H₂O), its dehydrated phases, and its secondary alteration products were characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), and high-resolution thermogravimetric analysis (HRTGA). Drying MS at 105 C resulted in the formation of a dehydrated phase (UO₃·0.9H₂O) that was structurally distinct from natural dehydrated schoepite (DS, UO₃·0.75H₂O) reported by others. Unlike natural DS, their dehydrated material was easily rehydrated, although crystallinity of the rehydrated phase was reduced. The rates of transformation of synthetic MS and dehydrated MS in the presence of Ca²⁺ to form becquerelite (Ca[(UO₂)₆O₄(OH)₆]·8H₂O) were determined. Alteration rates were significantly faster when the starting material had been dehydrated. These results are explained in the context of structural aspects of U(VI) solid phases, and the possible impact of hydration on long-term stability of U(VI) oxide hydrates in environmental systems is discussed

[en] There is consensus within the global research and development (R&D) community that the barriers to deployment of accident tolerant fuel (ATF) for commercial use in the near-future are too high and carry too much risk for any one organization to succeed alone. International collaboration is needed to leverage existing and new resources and expertise. Efforts are now underway to bring key entities together to share experiences and identify gaps and opportunities to leverage resources. In the wake of Fukushima Daiichi, momentum and funding currently exist in many countries for R&D targeting enhanced accident tolerance fuel (and other non-fuel reactor components) for Generation II/III/III+ light-water reactors (LWRs) with the goal of fundamentally changing severe accident outcomes while also maintaining or even improving fuel and reactor system performance under normal operations. While funding and interest are relatively high at present, the long time frames required for implementing substantial changes to in-core components and fuel designs demand a stable and sustained R&D focus. Likewise, the geographic dispersion and scarcity of key experimental and test facilities further highlight the need for coordination of experimental programmes and testing whenever possible and appropriate. Success in ATF development will come with the investment by, engagement of, and collaboration among the many key entities involved in the arduous path from early research through commercial deployment. As utilities are the ultimate customer for any new technology targeting enhanced performance and accident tolerance for LWRs, a clear understanding of nuclear plant operator needs and constraints is essential for the success of the global ATF R&D enterprise. Ultimately, the safety and performance benefits from ATF related investment will be realized only to the extent that new technologies are widely adopted and deployed in operating reactors. (author)

[en] Synthetic metaschoepite was altered at 70 C, for 30 days in silicate solutions of varying compositions to study alteration reactions and to compare experimental observations with a thermodynamic model of stability of the uranyl minerals metaschoepite ([(UO₂)₈O₂(OH)₁₂](H₂O)₁₀), becquerelite (Ca[(UO₂)₃O₂(OH)₃]₂(H₂O)₈), α-uranophane (Ca[(UO₂)(SiO₃OH)]₂(H₂O)₅) and soddyite ((UO₂)₂(SiO₄).(H₂O)₂). Powder X-ray diffraction analysis, scanning electron microscopy and solution chemistry were used to monitor the alteration of the initial metaschoepite. In general, the secondary phases predicted by the thermodynamic model formed. However, an unexpected mineral (a calcium uranate) was observed above pH 8. Also, within the timeframe of these experiments, U(VI) silicate phases only formed at Si concentrations much higher than the lower field boundaries for uranophane and soddyite. (orig.)

[en] Laser-induced kinetic phosphorimetry is an accurate, sensitive and rapid alternative to radiometric determination of natural and depleted uranium in aqueous solutions. This method offers detection limits below 10 ng/l U (2.5 x 10^-4 Bq/l natural U) and a broad analytical range to 5 mg/l U (130 Bq/l natural U). For many samples, dilution is the only sample preparation required. However, because this technique infers uranyl concentrations from time-resolved phosphorescence intensities, results are dependent upon sample matrix constituents that affect the phosphorescence of the uranyl cation. This study examines the influence of cations, anions and ligands common to natural water, process and bioassay samples on the quenching of uranyl phosphorescence and the consequences for lower limits of detection and accuracy of measurements. (author)

[en] The dehydration of uranyl minerals can affect phase structure and stability. Synthetic autunite hydrates, Ca[(UO₂)(PO₄)]₂.xH₂O, were studied by X-ray powder diffractometry (XRD) and thermogravimetric analysis (TGA) to address ambiguous or contradictory reports in the literature. Structurally, XRD analysis supported the three well-defined phases commonly reported in the literature, i.e. autunite, metaautunite I, and metaautunite II. In addition, a fourth phase with a basal plane spacing between that of autunite and metaautunite I, designated metaautunite Ia, was identified as an apparent metastable intermediate. TGA analysis confirmed that water loss or accumulation is tolerated to different degrees among the autunite hydrates. Loss of low temperature water appears to initiate collapse of the interlayer spacing from 10 to 9 A to form metaautunite I and/or Ia, while the lower hydrates accommodate minor water loss and accumulation without significant structural alteration. Our results support previous research indicating the reversibility of the autunite to metaautunite I conversion. The complex dehydration pattern of autunite is not observed in all the 1:1 uranyl phosphates, such as chernikovite (H[(UO₂)(PO₄)].4H₂O). (orig.)

[en] The influence of silica and phosphate on schoepite weathering in calcium bearing systems has been examined. The presence of Si (10^-3 M) retarded the transformation of schoepite to becquerelite in 10^-2 and 10^-3 M Ca systems as compared to silica free systems. The formation of uranyl silicates was not observed. The presence of phosphate (10^-2 M) in a 10^-2 M Ca system led to the transformation of schoepite into autunite via several intermediate products, including becquerelite and one or more uranyl phosphates. These results indicate the importance of silica and phosphate in the transformation of secondary uranyl phases in natural systems. (orig.)

[en] Kinetic dissolution studies were conducted on four prominent U-Ca-PO₄ minerals (metaschoepite, becquerelite, chernikovite and metaautunite). Synthetic samples were contacted with four extractants (acetic acid, deionized water, EDTA and sodium bicarbonate) at room temperature at two concentrations, 100 mM and 1mM. Dissolution progress was monitored by periodic sampling for dissolved U, and dissolution rates were obtained from fits to a three term exponential model. Significant variations were observed in the rate and extent of dissolution among the minerals examined. The uranyl phosphates chernikovite and metaautunite proved resistant to dissolution in non-carbonate systems, with dissolution half-times of days to weeks in 100 mM systems and weeks to years in 1mM systems. In contrast, the uranyl oxide hydrates schoepite and becquerelite were solubilized over much shorter time scales. While 100 mM bicarbonate was successful in dissolving U in all forms, dissolution rates varied among the four minerals. Overall, EDTA was the least sensitive to a 100 to 1 mM drop in its concentration in its solubilization of all four mineral phases, underscoring the importance of organic complexation for the environmental mobility of uranium. (author)

[en] To address challenges and gaps in nuclear fuel cycle option assessment and to support research, develop and demonstration programs oriented toward commercial deployment, EPRI (Electric Power Research Institute) is seeking to develop and maintain an independent analysis and assessment capability by building a suite of assessment tools based on a platform of software, simplified relationships, and explicit decision-making and evaluation guidelines. As a demonstration of the decision-support framework, EPRI examines a relatively near-term fuel cycle option, i.e., use of reactor-grade mixed-oxide fuel (MOX) in U.S. light water reactors. The results appear as a list of significant concerns (like cooling of spent fuels, criticality risk...) that have to be taken into account for the final decision