Uranium alloyed with 50 wt% zirconium (U-50Zr) is a proposed light water reactor (LWR) nuclear fuel by Lightbridge Corporation (LTBR). The proposed method for making the U-50Zr alloy is to arc-melt master alloys and then remelt in a vacuum induction melter (VIM) to consolidate the material and cast into an intermediate shape. After casting, the material will eventually need to be formed into a desired fuel shape. The work discussed in this report resulted from a joint effort between Ltbr and Pacific Northwest National Laboratory (PNN) to investigate a 500g – 1kg scale casting process to produce the U-50Zr alloy in the desired δ-UZr2 phase and characterize the impurities and microstructure that result from the casting process. Master alloys were fabricated in an arc melter, then five castings were carried out in a VIM with multiple inert coating and crucible materials to find an appropriate combination. Both ZrO2 and graphite crucibles were used and different combinations of Y2O3, CaZrO3, and TiC to identify which would contain the molten metal with the least reaction. On each casting, the C, O, N, H impurities were analyzed as well as the phase by x-ray diffraction and microstructure. Of the five castings, two resulted in majority of δ-UZr2 phase-pure material and had impurity levels within acceptable ranges. The two most successful castings utilized a graphite crucible with a TiC undercoating and a Y2O3 overcoat. The O and N levels were below 1000 ppm and the C content was variable but did not result in measurable carbide formation. The highest success casting resulted in an average of 282 ppm C, 567 ppm O, 217 ppm N and 79 ppm H. This casting's crucible and inert coating material was repeated with slightly different casting parameters and resulted in higher C numbers but similar phase identification. The differences between each casting are discussed and recommendations are made for future experiments to better decide on a casting process to go forward with.

Uranium (U) enrichment measurements for the U.S. High Performance Research Reactor (USHPRR) conversion program are needed to certify the fuel is < 20% U-235 (tolerance range: 19.55%-19.95%). The main thrust of this work was to determine whether thermal ionization mass spectrometry (TIMS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) U isotopic measurements can be made without chemically separating U from molybdenum (Mo) and other trace metal impurities. Although the data TIMS produces is considered the most accurate and precise, it is not typically used for isotopic analysis of U alloyed with 10 wt% Mo (U-10Mo) materials because of the complex sample preparation required prior to analysis. Results indicate that relative uncertainties on measurements made on chemically separated and unseparated samples was less than 0.03% (2 standard deviations) for this sample set with < 0.06% trace metal impurities with no observable biases stemming from the presence of 10% Mo. This methodology provides relief from the bottleneck of separations chemistry, which discourages the use of TIMS and MC-ICP-MS in the case of U-235/U-238 ratios for U-10Mo, and allows for more rapid, highly accurate and precise determination of the U-235 content in final fuel foils by TIMS. Additionally, samples were isotopically characterized for laser ablation multi-collector inductively coupled mass spectrometry (LA-MC-ICP-MS) by both TIMS and MC-ICP-MS in order to utilize them as inhouse matrix matched reference materials in the absence of a standard.(c) 2021 Elsevier B.V. All rights reserved.

We present a method for analyzing rare earth elements (REEs) by isotope dilution without separation of individual elements. Utilizing a Nu Plasma 2 multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) equipped with an Aridus II desolvation nebulizer, sample introduction tuning parameters (torch position, gas flows) were investigated and used to minimize oxide formation (e.g., NdO/Nd <0.05%) and obviate the need to analyze the REEs as separated "cuts." Ratios for spike isotopes relative to reference in mixed elemental standards containing varying amounts of Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb were measured accurately within 1% of IUPAC values. La and Lu in the standards deviated more than 2% from expected values due to more pronounced interferences reducing the efficacy of corrections. A USGS (United States Geological Survey) BCR-2 rock standard was also processed with flux fusion, iron coprecipitation and bulk REE separation by a single column pass followed by analysis. The concentrations for Ce, Nd, Sm, Eu, Dy, Er, and Yb fell within 2% of consensus values. A USGS BIR-1A rock standard was also processed in triplicate by hydrofluoric and nitric acid digestion followed by iron coprecipitation and a single ion exchange column pass, yielding rare earth concentrations within 3% of consensus values with 0.3% relative standard deviation (n = 3). This demonstrates the ability to produce high quality REE patterns of comparable quality to traditional methods which require labor and time intensive REE separation, but with sample preparation and analytical times that are comparable to isotope dilution analyses by single-collector ICP-MS. (C) 2021 Elsevier B.V. All rights reserved.

A B S T R A C T Uranium alloyed with 10 wt percent molybdenum (U-10Mo) is the proposed fuel for use in the United States' high-performance research reactors. The U-10Mo fuel is fabricated by a two-step casting process that downblends highly enriched uranium with depleted uranium (DU) and/or natural uranium (NU) and subsequently alloys the resulting high-assay low-enriched uranium (HALEU) with Mo. Currently, a twostep casting process is needed to meet the ingots' U-235 enrichment homogeneity specifications. This work demonstrates the ability to provide more homogeneous U-235 distributions during downblend casting by using zirconia for crucibles rather than graphite. When graphite is used as a crucible, the electromagnetic field produced by the induction heater couples directly with the graphite. Zirconia is nonconducting in this study, and therefore does not couple with the field. The induction field couples directly with the metal, and causes electromagnetic stirring (EMS) in the molten metal pool. Eight downblend casting experiments were carried out in this work. Four were performed with zirconia crucibles and four with graphite crucibles. The U-235 enrichment was measured at nine discrete points in each casting using laser ablation multi-collector inductively coupled mass spectrometry (LA-MC-ICP-MS). The enrichment homogeneity was approximately eight times better in zirconia crucible castings than in graphite crucible castings as measured by the plate enrichment range and enrichment coefficient of variation. The results show a single casting step could be used to meet USHPRR enrichment specifications if electromagnetic stirring is present during casting. A related investigation was carried out to determine whether it is appropriate to measure enrichment on as-cast ingots or whether heat treated specimens should be used to accurately characterize enrichment. The results show that the as-cast microstructure plays a significant role in enrichment homogeneity. It is recommended that all enrichment measurements be done after the homogenization heat treatment typical of U-10Mo processing. (c) 2021 Elsevier B.V. All rights reserved.

Accurate dating of young (<10 ka) volcanic rocks poses numerous challenges yet determining eruption ages is critical to understanding magmatic evolution and hazards associated with active volcanic centers. C-14 dating is the preferred technique for dating young geologic materials, but a few studies have suggested that volcanic degassing can result in erroneously old C-14 dates (e.g. Pasquier-Cardin et al., 1999). Finding an isotopic system with which to accurately constrain eruptive recurrence intervals at recently active volcanic centers is critical to assessing future volcanic hazards. In this study, we have obtained C-14 data from modern terrestrial gastropods on Sao Miguel island, Azores to assess the current impact of volcanic degassing on C-14 ages near active and extinct volcanic centers, and for one active volcanic center, we have compared C-14 of paleosols with Ra-226-Th-230 maximum eruptive ages for pumices immediately overlying the respective paleosols. C-14 ages of the modem terrestrial gastropod Cornu aspersum demonstrate the effects of dilution from a range of current outgassing levels at three active trachytic stratovolcanoes on Sao Miguel island: Sete Cidades, Fogo, Furnas, as well as the extinct Povoacao volcano. A modem gastropod sampled at Furnas, where there is evidence of extensive present-day degassing, has a C-14 age of 394 +/- 97 cal BP. At Fogo volcano, also a location with active degassing, the C-14 age of a modem gastropod is 1324 +/- 60 cal BP. Both of these ages clearly show effects of dilution of atmospheric C-14 due to volcanic degassing. In contrast, there is currently no apparent outgassing occurring at Sete Cidades or Povoacao, and modem gastropod ages from these volcanoes are post-bomb. We also obtained C-14 data from paleosols and Ra-226-Th-230 ages associated pumices from the youngest eruptive sequence of Sete Cidades volcano, the Pepom P1-P17 deposits, the ages of which are poorly constrained but thought to be <5000 years (Queiroz, 1997). Ra-226-Th-230 disequilibrium in a P1 glass separate indicates a maximum eruptive age of 4440 +/- 195 years. C-14 ages of paleosols below P1 and P3 are older than those derived from Ra-226-Th-230 dating, at 7293 +/- 141 cal BP and 7698 +/- 172 cal BP, respectively. We interpret the C-14 ages as being erroneously old due to abundant volcanic degassing at the time of eruption. Similarly, but to a lesser extent, the Ra-226-Th-230 maximum eruptive age of glass separated from the P8 eruption is 2659 +/- 55 y, whereas the age of the paleosol sampled directly beneath P8 is older (3054 +/- 194 y BP). In contrast, paleosols sampled at multiple field localities from below the most recent eruptive deposit (P17) are within error of each other (476 +/- 56 cal y BP and 585 +/- 68 cal y BP), while Ra-226-Th-230 maximum eruptive ages in glass separates from two different P17 sampling sites are 762 +/- 14 y and 730 y +/- 15 y. This is consistent with the observation that Sete Cidades volcano is experiencing limited hydrothermal activity at the present time and likely had similarly low levels of degassing just prior to the last eruption. The average eruptive recurrence interval for the P1-P17 deposits, based on Ra-226-Th-230 maximum eruptive ages from P1 and P8 and C-14 ages from P17, is similar to 230 years. This recurrence interval is similar to the similar to 200 y eruptive recurrence interval at Furnas volcano and much shorter than the similar to 1200 y eruptive interval at Fogo volcano. This has important implications with respect to volcanic hazards and should be considered in any future volcanic hazard planning. (C) 2019 Elsevier B.V. All rights reserved.

We have investigated the use of tree bark and lichen as tracers of the source, nature, and extent of environmental contamination from the nuclear industry using two test cases in southwest Ohio: the former Miamisburg Mound Laboratories site (MML) and the former Fernald Feed Materials Production Center (FFMPC). Although these facilities were primarily dedicated to the nuclear weapons industry during the Cold War, similar sampling and analytical approaches could be applied towards environmental monitoring associated with the nuclear energy industry. Results from tree bark transects in the vicinity of, and emanating from, the FFMPC show that tree bark serves as a long-term archive of particulate matter leaked to the atmosphere from the nuclear industry. Increasing uranium concentrations, and the increasing presence of non-natural (depleted and enriched) uranium (DU and EU, respectively) in tree bark as the FFMPC is approached, preserves a time-integrated record of the source, nature, and extent of uranium contamination to the environment. Furthermore, our results show that the presence of U, an essentially non-naturally occurring isotope of U that is produced in nuclear reactors, is a highly sensitive tracer that can be diagnostic of environmental contamination even in cases for which perturbations in the relative abundances of the major isotopes of uranium, U and U, cannot be resolved. These data are consistent with historically documented major releases of uranium dust to the environment from the FFMPC (Conte et al., in review). The MML, in contrast, represents a site for which there is no previously documented evidence for off-site contamination. Nevertheless, results from both uranium and thorium isotopic analysis of lichens from the vicinity of the MML reveal the presence of actinide contamination in the environment surrounding the former MML site. Despite natural U/U ratios, the presence of measureable U provides unequivocal evidence for a component of anthropogenic uranium related to the nuclear industry. Furthermore, a positive correlation between U/U and Th/Th in the lichen samples suggests the presence in the environment of contaminant Th, an isotope previously identified as one of several significant on-site contaminants (McHugh et al., in prep). Given the projected increase in demand for electricity and expansion of nuclear power capacity globally (IAEA 2016), and associated increase in nuclear fuel production and reactors, there is a need for monitoring programs that evaluate present day background levels of actinides in the environment, as well as past, present, and future contamination of the environment in the vicinity of facilities related to nuclear energy production. We propose that techniques similar to those demonstrated in our FFMPC and MML studies can be used to monitor baseline actinide levels in the environment, and to monitor both inevitable, and often “stealth” leaks to the environment, as well as to evaluate the degree and areal extent of contamination resulting from catastrophic events.

Inappropriate handling of radioactive waste at nuclear facilities can introduce non-natural uranium (U) into the environment via the air or groundwater, leading to anthropogenic increases in U concentrations. Uranium isotopic analyses of natural materials (e.g. soil, plants or water) provide a means to distinguish between natural and anthropogenic U in areas near sources of radionuclides to the environment. This study examines the utility of two different tree bark transects for resolving the areal extent of U atmospheric contamination using several locations in southwest Ohio that historically processed U. This study is the first to utilize tree bark sampling transects to assess environmental contamination emanating from a nuclear facility. The former Fernald Feed Materials Production Center (FFMPC; Ross, Ohio) produced U metal from natural U ores and recycled nuclear materials from 1951 to 1989. Alba Craft Laboratory (Oxford, Ohio) machined several hundred tons of natural U metal from the FFMPC between 1952 and 1957. The Herring-Hall-Marvin Safe Company (HHM; Hamilton, Ohio) intermittently fabricated slugs rolled from natural U metal stock for use in nuclear reactors from 1943 to 1951. We have measured U concentrations and isotope signatures in tree bark sampled along an similar to 35 km SSE-NNW transect from the former FFMPC to the vicinity of the former Alba Craft laboratories (transect #1) and an similar to 20 km SW NE (prevailing local wind direction) transect from the FFMPC to the vicinity of the former HHM (transect #2), with a focus on old trees with thick, persistent bark that could potentially record a time-integrated signature of environmental releases of U related to anthropogenic activity. Our results demonstrate the presence of anthropogenic U contamination in tree bark from the entire study area in both transects, with U concentrations within 1 km of the FFMPC up to similar to 400 times local background levels of 0.066 ppm. Tree bark samples from the Alba Craft and HHM transects exhibit increasing U concentrations within similar to 5 and similar to 10 km, respectively of the FFMPC. The U-236/U-238 isotopic ratios in tree bark from both transects increase progressively towards the FFMPC with values as high as 2.00 x 10(-4) at the FFMPC. Tree bark sampled within 1 km of the FFMPC exhibits clear evidence for both enriched and depleted uranium with U-235/U-238 values from 0.00461 to 0.00736, with U-234/U-238 activity ratio ranging from 0.53 to 0.96, and U-236/U-238 from 6.05 x 10(-5) to 1.05 x 10(-4). Tree bark from transect #1 between I and 30 km from the FFMPC exhibits depleted and natural U-235/U-238 values ranging from 0.00552 to 0.00726 [U-234/U-238 activity ratio: 0.69-1.04; U-236/U-238: 2.49 x 10(-6) - 2.00 x 10(-4)]. Tree bark from transect #2 sampled between 1 and similar to 20 km away from the FFMPC exhibits evidence of enriched and depleted U in the environment with U-235/U-238 ranging from 0.00635 to 0.00738 [U-234/U-238 activity ratio: 0.83-0.98; U-236/U-238: 1.43 x 10(-5) - 2.00 x 10(-4)]. Results from scanning electron microscopy with energy dispersive spectrometry provides evidence for U-rich particles as the source of contamination found in tree bark growing within 1-3 km of the former FFMPC. Such observations are consistent with the previously observed 14 mu m U-rich particle identified in tree bark sampled within 1 km of the FFMPC (Conte et al., 2015). Overall, this study shows the usefulness of a tree bark sample transect to assess the areal extent of atmospheric contaminant U stemming from nuclear facilities. (C) 2017 Elsevier Ltd. All rights reserved.

Non-natural uranium (U) isotopic compositions have been reported in tree bark from southwest Ohio. Atmospheric releases of U from the nearby former Fernald Feed Materials Processing Center are thought to be the source (GSA Abstr progr 45:78612, 2013). This study employed scanning electron microscopy equipped with backscatter detection and energy dispersive absorption X-ray spectrometry to identify and chemically characterize a 14 µm U-rich particle found in tree bark growing within 1 km of the FFMPC. Simple atmospheric dispersion calculations demonstrate that a ~5 µm diameter U-rich particle can be transported up to 38 km by wind.