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99Mo is an essential medical isotope that comprises of at least 70% of radioactive procedures globally. Currently an essential component of 99Mo manufacturing is the uranium target from which 99Mo is produced by fission. As the world moves towards low enriched uranium (LEU) targets due to non-proliferation concerns it is becoming of interest to find methods to increase the efficiency of the LEU targets in order to reduce the ever increasing nuclear waste levels of which a long term solution for disposal or treatment has yet to be satisfactorily found. Advantages and disadvantages of various target designs are investigated and discussed along current disposal and reprocessing methods. The idea of a reusable target is introduced as a way forward in reducing the nuclear waste burden for future generations. • Fission based manufacturing of 99Mo is moving towards Low Enriched Uranium methods. • Low Enriched Uranium leads to increased radioactive waste volumes. • The majority of uranium targets are single use and highly inefficient. • Reusable targets have the potential to lower the nuclear waste burden. [ABSTRACT FROM AUTHOR]

Abstract The simple conversion of zirconium phosphonate sorbent materials, with known affinity for lanthanide elements, to durable ceramic waste forms via thermal treatment has been demonstrated. The use of zirconium phosphonate enables both removal of targeted elements from spent nuclear fuel and immobilisation into leach resistant solid products to be achieved using a single material. Thermal conversion was performed on the zirconium phosphonate both before and after loading with europium, which acted as a surrogate for the chemically similar minor actinides. Without europium loaded, the zirconium phosphonate sorbent formed predominantly KZr 2 (PO 4) 3 upon heating, independent of the processing conditions used. A maximum relative density of 87% was achieved with cold isostatic pressing (200 MPa) and sintering at 1200 °C for 12 h. When the zirconium phosphonate sorbent was loaded with europium, the phase composition formed upon thermal treatment was more complex. Specifically, mixtures of ZrP 2 O 7 , Eu 0.33 Zr 2 (PO 4) 3 , EuPO 4 and Zr 2 O(PO 4) 2 were formed, with phase compositions depending on the temperatures and pressures used. The simplest phase composition for the europium loaded material was achieved via uniaxial pressing (120 MPa) and sintering at 1300 °C for 1 h, although the ceramic pellet produced under these conditions had a relative density of only 53%. The loaded europium deported primarily to a EuPO 4 phase, which is known to be highly stable and leach resistant. As such, these zirconium phosphonate materials have potential utility for treatment of nuclear wastes. [ABSTRACT FROM AUTHOR]

The high temperature structural behaviour of the uranium perovskites Ba2CaUO6 and BaSrCaUO6 has been investigated using a combination of synchrotron X-ray and neutron powder diffraction. Ba2CaUO6 undergoes a complex sequence of structures associated with the progressive loss of cooperative octahedral tilting: P21/n→I2/m→I2/m→I4/m→Fm3̅m. The observation of the intermediate tetragonal structure, I4/m, in this, contrasts with the previously reported rhombohedral R3̅ intermediate formed by the Ba2SrUO6 oxide. The importance of π-bonding in determining the structural sequence is discussed. [ABSTRACT FROM AUTHOR]

The thermal expansion of NH 4 TcO 4 and NH 4 ReO 4 between 97 and 500 K has been established using variable temperature synchrotron X-ray diffraction. Both oxides display a tetragonal scheelite-type structure over the temperature range. The observed anomalous thermal expansion is associated with disorder in the orientation of the NH 4 groups and the presence of anisotropic, equatorial and axial, interactions between the H atoms and the B O 4 tetrahedra. The importance of the hydrogen interactions was verified by a study of the isostructural compound NaReO 4 over a similar temperature range. The differences in the details of the Negative Thermal Expansion (NTE) behaviour of NH 4 TcO 4 and NH 4 ReO 4 is a consequence of differences in the H B O 4 interactions due to changes in the B O bond lengths within the B O 4 groups. Both ammonium salts decompose when heated above ∼500 K. Variable temperature synchrotron X-ray diffraction revels that the thermal expansion of NH 4 TcO 4 and NH 4 ReO 4 between 97 and 500 K is highly anisotropic, including negative thermal expansion along the c -axis. Whereas NH 4 ReO 4 exhibits negative volume thermal expansion over a narrow temperature range NH 4 TcO 4 does not exhibit negative volume thermal expansion. Both oxides have the tetragonal Scheelite type structure and the observed anomalous thermal expansion is associated with disorder in the orientation of the NH 4 groups and the presence of both equatorial and axial H-bonding to the B O 4 tetrahedral. Image 1 • Structures of NH 4 TcO 4 and NH 4 ReO 4 determined using synchrotron X-ray powder diffraction. • Thermal expansion of NH 4 TcO 4 and NH 4 ReO 4 between 97 and 500 K determined. • Anisotropic thermal expansion, including NTE, related to disorder of NH 4 groups. [ABSTRACT FROM AUTHOR]

In order to obtain insight into the hardening mechanism of Cr(N,O) thin films with respect to increase in oxygen content, the microstructure of Cr(N,O) grains in thin films was studied. High resolution transmission electron microscopy provided evidence that the grains in Cr(N,O) thin films prepared by pulsed laser deposition, which have an oxygen content of > 27 mol%, contained stacking faults. Phase identification via electron diffraction, indicated that the grains consisted of two phases, which had a NaCl-type and corundum-type structure and these phases had the same orientation as that of the Cr 0.67 O thin film. The estimated stacking fault energy of Cr(N,O) with an oxygen content of 34 mol% was 20 mJ/m 2 . Our results revealed the Cr(N,O) grains had the same nano-lamellar morphology as that of the Cr 0.67 O thin film which supports the possibility that the nano-lamellar morphology was formed by the introduction of extended dislocations. It is also possible that the hardening of Cr(N,O) was caused by dislocation pinning at the boundaries of the nano-lamellar morphology. [ABSTRACT FROM AUTHOR]

A new target material combination was modelled to replace the existing uranium-aluminium design used for 99Mo manufacture to increase the sustainability of the production process. Previous efforts to develop a more sustainable uranium target for 99Mo production, resulted in the levels of 239Pu in the target after irradiation being elevated due to the increase in 238U present. MCNP6.2 was used to model 4 different cylindrical targets based on 4–7 days irradiation to further understand this effect. To reduce the resultant 239Pu levels, ratios of 0–99% of Ce were used as a replacement for 238U. The results show that the addition of 140Ce and the removal of 238U reduced the 239Pu levels in the target significantly thus increasing the sustainability of the target and giving a slight increase to the 99Mo output of the targets. [ABSTRACT FROM AUTHOR]

The crystal structures of seven samples of orthorhombic ( Pnma ) Ln 2 TiO 5 compounds with Ln =La, Pr, Nd, Sm, Gd, Tb and Dy were refined by Rietveld analysis of synchrotron X-ray powder diffraction (S-XRD) data. With increasing size of the lanthanide cation, the lattice parameters increase systematically: c by only ~1.5% whereas both a and b by ~6% from Dy 2 TiO 5 to La 2 TiO 5 . The mean Ti–O bond length only increases by ~1% with increasing radius of the Ln cation from Gd to La, primarily due to expansion of the pair of Ti–O 3 bonds to opposite corners of the Ti–O 5 square based pyramid polyhedra. For Dy 2 TiO 5 and Tb 2 TiO 5 , a significant variation in Ti–O 1 and Ti–O 4 bond lengths results in an increased deformation of the Ti–O 5 base. The particular configuration consists of large rhombic shaped tunnels and smaller triangular tunnels along the b axis, which have implications for defect formation and migration caused by radiation damage or the ionic conductivity. [ABSTRACT FROM AUTHOR]

The incorporation of uranium into gadolinium zirconate (Gd2Zr2O7) is investigated by synchrotron X-ray powder diffraction and X-ray absorption near-edge structure (XANES) spectroscopy. The results suggest that the uranium cation is largely located on the pyrochlore B-site instead of the targeted A-site. Cation disorder in Gd2Zr2O7 and U-doped Gd2Zr2O7 is investigated by positron annihilation lifetime spectroscopy (PALS) which demonstrates the absence of cation vacancies in these systems. This work provides direct evidence for cation antisite (A- and B-site mixing) disorder in U-doped and off-stoichiometric Gd2Zr2O7 pyrochlore. [ABSTRACT FROM AUTHOR]

In this study, in-situ micro-tensile testing technique was used to investigate the mechanical properties of ultrafine grained Ni-3wt% SiC composite the size effect on the mechanical properties of the ultrafine grained Ni-3wt% SiC composite, and to further reveal the reasons for the low ductility of the bulk Ni-3wt%SiC composite. Dog-bone micro-tensile samples were manufactured using a Focused Ion Beam (FIB) milling machine to 15 μm length with a cross sectional area of 5 μm by 5 μm. The micro-tensile samples are pulled in tension at a quasi-static strain rate of 0.000087/s (LSR) and a relatively faster strain rate of 0.011/s (HSR). Analysis of experimental stress-strain plots for the LSR tests measured yield stress, ultimate tensile stress and modulus values that approach values previously reported for bulk/macro-level tensile tests. However, the elongation and fracture energy at the micro-level is approximately half that at the bulk scale. This discrepancy is attributed to the presence of unwanted carbon and silicon oxide impurities ∼1.5 μm in diameter which act as stress concentrators especially given their large size relative to the width of the tensile specimens. The composition of these impurities was validated by transmission electron microscopy, and they seem to be the most likely cause of low ductility of the Ni-3wt% SiC composite. In all, the study undertaken here was able to replicate mechanical properties observed at the macro scale as well as reproduce a strain rate effect. Furthermore, the failure mode of Ni-3wt% SiC composite was identified and analysed in detail. Image 1 • Micro-tensile Ni–SiC samples exhibited strain rate sensitivity comparable with literature values. • The yield stress and ultimate tensile stress of Ni–SiC micro-tensile samples closely match macro scale tests. • Presence of impurity particles within the matrix bring about premature failure of Ni–SiC composite. [ABSTRACT FROM AUTHOR]

CuI is a candidate wasteform for the immobilization of the fission product 129 I. CuI can be made simply by the addition of CuCl to an I − bearing solution such that exchange of Cl − with I − takes place. The CuI material can then be consolidated into a wasteform by sintering at approximately 550 °C in argon or by hot isostatically pressing at 550 °C with 100 MPa of pressure. A waste loading of greater than 60 wt.% is achievable with good water leach resistance, in keeping with the low solubility product of CuI. However, like the well known wasteform candidate AgI, CuI decomposes in water containing metallic Fe. To compensate this deficiency, the sintered CuI wasteform can be further protected by surrounding it by Sn powder and HIPing at the low temperature of 200 °C. [ABSTRACT FROM AUTHOR]

This work presents the results for identification of chemical phases obtained by several laboratories as a part of an international nuclear forensic round-robin exercise. In this work powder X-ray diffraction (p-XRD) is regarded as the reference technique. Neutron diffraction produced a superior high-angle diffraction pattern relative to p-XRD. Requiring only small amounts of sample, µ-Raman spectroscopy was used for the first time in this context as a potentially complementary technique to p-XRD. The chemical phases were identified as pure UO2 in two materials, and as a mixture of UO2, U3O8 and an intermediate species U3O7 in the third material. [ABSTRACT FROM AUTHOR]

The Sr1-xBaxTcO3 (x=0, 0.1, 0.2) oxides were prepared and their solid-state and magnetic structure studied as a function of temperature by x-ray and neutron powder diffraction. The refined Tc moments at room temperature and Néel temperatures for Ba0.1Sr0.9TcO3 and Ba0.2Sr0.8TcO3 were 2.32(14)μβ and 2.11(13)μβ and 714*C and 702*C, respectively. In contrast to expectations, the Néel temperature in the series Sr1-xAxTcO3 decreases with increasing Ba content. This observation is consistent with previous experimental measurements for the two series AMO3 (M=Ru, Mn; A=Ca, Sr, Ba) where the maximum magnetic ordering temperature was observed for A=Sr. Taken with these previous results the current work demonstrates the critical role of the A-site cation in the broadening of the π* bandwidth and ultimately the magnetic ordering temperature. [ABSTRACT FROM AUTHOR]

A new generation of alloys, which rely on a combination of various strengthening mechanisms, has been developed for application in molten salt nuclear reactors. In the current study, a battery of dispersion and precipitation-strengthened (DPS) NiMo-based alloys containing varying amounts of SiC (0.5-2.5 wt %) were prepared from Ni-Mo-SiC powder mixture via a mechanical alloying (MA) route followed by spark plasma sintering (SPS) and rapid cooling. Neutron Powder Diffraction (NPD), Electron Back Scattering Diffraction (EBSD), and Transmission Electron Microscopy (TEM) were employed in the characterization of the microstructural properties of these in-house prepared NiMo-SiC DPS alloys. The study showed that uniformly-dispersed SiC particles provide dispersion strengthening, the precipitation of nano-scale Ni3Si particles provides precipitation strengthening, and the solid-solution of Mo in the Ni matrix provides solid-solution strengthening. It was further shown that the milling time has significant effects on the microstructural characteristics of these alloys. Increasedmilling time seems to limit the grain growth of theNiMomatrix by producing well-dispersed Mo2C particles during sintering. The amount of grain boundaries greatly increases the Hall-Petch strengthening, resulting in significantly higher strength in the case of 48-h-milled NiMo-SiC DPS alloys compared with the 8-h-milled alloys. However, it was also shown that the total elongation is considerably reduced in the 48-h-milled NiMo-SiC DPS alloy due to high porosity. The porosity is a result of cold welding of the powder mixture during the extended milling process. [ABSTRACT FROM AUTHOR]

A new class of materials, which rely on the dispersion strengthening of SiC particles in addition to precipitation strengthening by nano-precipitates is being developed for the application in molten salt nuclear reactors. A battery of dispersion and precipitation strengthened (DPS) NiMo-based alloys containing varying amount of SiC (0.5–2.5 wt.%) was prepared via a mechanical alloying (MA) route followed by spark plasma sintering (SPS), rapid cooling, high-temperature annealing and water quenching. Lab X-ray Diffraction (XRD), Electron Back Scattering Diffraction (EBSD), and Transmission Electron Microscopy (TEM) were employed in the microstructural characterization of this new type of alloys. It is shown that the NiMo matrix of these alloys is effectively reinforced by dispersion of SiC from the initial powder mixture and nano-Ni 3 Si precipitates, which precipitated during the sintering/annealing process. Furthermore, the matrix is strengthened by solid-solution of Mo in Ni. As a result, these newly developed NiMo alloys take advantage of dispersion, precipitation and solid solution strengthening, which leads to their superior mechanical properties. [ABSTRACT FROM AUTHOR]

A combination of S-XRD and NPD demonstrate the structure of Ba 2 Y 0.879 UO 6+x to be monoclinic in space group I 2/ m . That the U is hexavalent is evident from the U L 2 -edge XANES measurements. This appears to be a rare example of a double perovskite containing vacancies at the octahedral B -sites and interstitial oxygen defects, which combine to stabilise hexavalent U and appears to be a consequence of the preparation of the sample in air. The Y vacancies, coupled with anion disorder, results in a distortion of the B O 6 octahedra. [ABSTRACT FROM AUTHOR]

The structures of some A UO 4 ( A =Ca, Sr, or Ba) oxides have been determined using a combination of neutron and synchrotron X-ray diffraction, supported by X-ray absorption spectroscopic measurements at the U L 3 -edge. The smaller Ca cation favours a rhombohedral A UO 4 structure with 8-coordinate UO 8 moieties whilst an orthorhombic structure based on UO 6 groups is found for BaUO 4 . Both the rhombohedral and orthorhombic structures can be stabilised for SrUO 4 . The structural studies suggest that the bonding requirements of the A site cation play a significant role in determining which structure is favoured. In the rhombohedral structure, Bond Valence Sums demonstrate the A site is invariably overbonded, which, in the case of rhombohedral α-SrUO 4 , is compensated for by the formation of vacancies in the oxygen sub-lattice. The uranium cation, with its flexible oxidation state, is able to accommodate this by inducing vacancies along its equatorial coordination site as demonstrated by neutron powder diffraction. [ABSTRACT FROM AUTHOR]

Copper-doped nanosized zeolite A crystals were synthesized by an in situ templating approach using [Cu(EDTA)] 2− -complex. The structural properties of the copper containing zeolite crystals were characterized by a suite of different techniques including SEM–EDX, ESR, mid-IR and Far-IR, Raman, in situ XRD and non-ambient neutron powder diffraction. The SEM investigations on the morphology show spheroidal zeolite A crystals with average size ∼200 nm. The asymmetric ESR spectrum shows that the Cu 2+ ion is in a tetragonal-distorted octahedral crystal field. FT-IR and Raman spectroscopies provide information on coordination environment of the copper ion. The band due to stretching vibration of C–N bond, where N is coordinated to the copper ion (C–N–Cu), was observed at 1109 cm −1 in the mid-infrared region. The Raman band due to the Cu–O bond is present at 630 cm −1 indicating the coordination of the Cu 2+ -cation to COO − -group of the EDTA-ion. The XRD data shows an enlarged d -spacing between the adjacent zeolite lattice planes due to the presence of the [Cu(EDTA)] 2− -complex in comparison to template-free LTA zeolite structure. LeBail fitting approach on temperature-dependent in situ X-ray and neutron diffraction profiles have demonstrated the expansion of the zeolite cell during the thermal treatment followed by subsequent contraction with the decomposition of the organic template. [ABSTRACT FROM AUTHOR]

Tc L3-edge XANES spectra have been collected on powder samples of SrTcO3 (octahedral Tc4+) and NH4TcO4 (tetrahedral Tc7+) immobilized in an epoxy resin. Features in the Tc L3-edge XANES spectra are compared with the preedge feature of the Tc K-edge as well as other 4d transition metal L3-edges. Evidence of crystal field splitting is obvious in the Tc L3-edge, which is sensitive to the coordination number and oxidation state of the Tc cation. The Tc L3 absorption edge energy difference between SrTcO3 (Tc4+) and NH4TcO4 (Tc7+) shows that the energy shift at the Tc L3-edge is an effective tool for studying changes in the oxidation states of technetium compounds. The Tc L3-edge spectra are compared with those obtained fromMo and Ru oxide standards with various oxidation states and coordination environments. Most importantly, fitting the Tc L3-edge to component peaks can provide direct evidence of crystal field splitting that cannot be obtained from the Tc K-edge. [ABSTRACT FROM AUTHOR]

The β -phase of Au 7 Cu 5 Al 4 undergoes a reversible shape-memory phase transformation for which several conflicting martensite phases have been reported. Here we show the significance of the cooling temperature used to obtain the martensite. If Au 7 Cu 5 Al 4 is cooled from the parent phase condition to cryogenic temperatures, e.g. below 200 K, the martensitic phase is orthorhombic (space group Pcmn , a = 4.4841 Å, b = 5.8996 Å, c = 17.8130 Å); however, when this composition is cooled to only ∼260 K it will in general consist of a mixture of orthorhombic and monoclinic phase (the latter has space group P 2 1 / m , a = 4.4742 Å, b = 5.9265 Å, c = 13.3370 Å, β = 91.425°). In contrast, a sample with decreased Al content (Au 7 Cu 5.7 Al 3.3 ) transforms fully to monoclinic phase if cooled to ∼260 K. [ABSTRACT FROM AUTHOR]

Current fission-based methods of 99Mo production require single use uranium targets which leads to spent uranium waste. This waste could be reduced if a target is developed that does not require dissolution so that it can be reused for multiple production runs. MCNP6.2 was used to model reusable targets of 20% and 1% enrichment for activity produced, target efficiency and burnup. The 1% enriched target was found to be much more efficient but had a lower activity produced compared to the 20% enriched target. The ideal target design for 99Mo production that optimises efficiency and reusability and reduces the self-shielding effect of UO 2 was found to be a target that is made from 1% enriched UO 2 with density as high as allowable for sufficient yields, efficient 99Mo extraction and having an irradiation time of 5 days, with the target able to be re-irradiated and re-processed 2–4 times. • Current fission based manufacturing of 99Mo using uranium targets is based on target destruction. • Target destruction leads to increased radioactive waste volumes. • MCNP6 modelling of uranium targets show that higher density targets have more self-shielding effects and are less efficient. • Low density uranium targets with 5 days irradiation maximise 99Mo output and minimising radioactive waste. [ABSTRACT FROM AUTHOR]

The structure and magnetic properties of the mixed technetium-ruthenium perovskites SrTc x Ru 1-x O 3 (x ​= ​0.25 and 0.5) have been investigated using variable temperature synchrotron X-ray diffraction, magnetic susceptibility measurements, and DFT calculations. Both oxides are isostructural with SrRuO 3 and SrTcO 3 , and there is no evidence of long-range cation order. Magnetostriction is observed along the c -axis for both oxides. The inverse susceptibility and DFT calculations suggest a cation-disordered ferrimagnetic ground state for SrTc0.5Ru0.5O3. Image 1 • Mixed technetium-ruthenium perovskites SrTc x Ru 1-x O 3 oxides prepared. • Structures determined using temperature synchrotron X-ray diffraction. • Magnetic susceptibility measurements suggest a ferrimagnetic ground state. • DFT calculations suggest a cation-disordered ferrimagnetic ground. [ABSTRACT FROM AUTHOR]