Quantification of α-particle radiation damage in zircon

Analysis of radiation damage in natural mineral analogs such as zircon is important for the evaluation of the long-term behavior of nuclear waste forms and for geochronology. Here we present results of experiments to determine the partitioning of radiation damage due to the heavy nuclear recoil of uranium and thorium daughters and the α-particles ejected in an α-decay event in zircon. Synthetic polycrystalline zircon ceramics were doped with 10B and irradiated in a slow neutron flux for 1, 10, and 28 days to achieve the reaction 10B + n → 7Li + α (+2.79 MeV), creating an α event without a heavy nuclear recoil. The 7Li atoms produced in the nuclear reaction were directly detected by NMR “spin-counting”, providing a precise measurement of the α-dose applied to each sample. The amount of damage (number fraction and volume fraction) created by each α-event (one α-event being a 7Li + α-particle) has been quantified using radiological nuclear magnetic resonance and X-ray diffraction data. The number of permanently displaced atoms in the amorphous fraction was determined by 29Si NMR to be 252 ± 24 atoms for the 10B(n,α) event when the heavy recoil is absent, which is broadly in agreement with ballistic Monte Carlo calculations. The unit-cell swelling of the crystalline fraction, determined by X-ray diffraction, is small and anisotropic. The anisotropy is similar to that observed in ancient natural samples and implies an initial anisotropic swelling mechanism rather than an anisotropic recovery mechanism occurring over geological timescales. The small unit-cell volume swelling is only ~6% of the expansion frequently attributed to α-particles associated with an actinide α-decay event. The lattice parameters indicate a volume increase as α function of a dose of 0.21 A3/1018 α-events/g, which is significantly less than the increase of 3.55 A3/1018 α-events/g seen in Pu-doped zircon and 2.18 A3/1018 α-events/g seen in natural zircon. It is concluded that the heavy recoil plays a more important role in unit-cell swelling than previously predicted. The likely mechanism for such an effect is the rapid, and thus defect-rich, recrystallization of material initially displaced by the heavy recoil

Corrosion behaviour of AGR simulated fuels:evolution of the fuel surface

We have prepared a range of Advanced Gas-cooled Reactor (AGR) SIMFUELs at a range of simulated burn-ups and, using Raman spectroscopy, have studied the effect of the SIMFUEL dopants on the UO2 crystal structure. We have also studied the effect of exposure to hydrogen peroxide solutions on the SIMFUEL surface. The intensity of the fundamental U-O stretch (445 cm-1) decreases as the amount of dopant increases in each SIMFUEL burn-up composition. A simultaneous increase in the lattice damage (500 – 700 cm-1) peak is observed as the UO2 cubic fluorite lattice structure becomes more distressed and moves towards a tetragonal structure. Exposure to 100 µmol dm-3 H2O2 further decreases the fundamental U-O stretch and increases the lattice damage peak, suggesting that additional point defects are established as the concentration of interstitial oxygen is increased in the lattice via the H2O2-induced corrosion of the SIMFUEL

An investigation of the long-range and local structure of sub-stoichiometric zirconium carbide sintered at different temperatures

ZrC1−x (sub-stoichiometric zirconium carbide), a group IV transition metal carbide, is being considered for various high temperature applications. Departure from stoichiometry changes the thermo-physical response of the material. Reported thermo-physical properties exhibit, in some cases, a degree of scatter with one likely contributor to this being the uncertainty in the C/Zr ratio of the samples produced. Conventional, methods for assigning C/Zr to samples are determined either by nominal stochiometric ratios or combustion carbon analysis. In this study, a range of stoichiometries of hot-pressed ZrC1−x were examined by SEM, XRD, Raman spectroscopy and static 13C NMR spectroscopy and used as a basis to correct the C/Zr. Graphite, amorphous, and ZrC1−x carbon signatures are observed in the 13C NMR spectra of samples and are determined to vary in intensity with sintering temperature and stoichiometry. In this study a method is outlined to quantify the stoichiometry of ZrC1−x and free carbon phases, providing an improvement over the sole use and reliance of widely adopted bulk carbon combustion analysis. We report significantly lower C/Zr values determined by 13C NMR analysis compared with carbon analyser and nominal methods. Furthermore, the location of carbon disassociated from the ZrC1−x structure is analysed using SEM and Raman spectroscopy

Impacts of composition and beta irradiation on phase separation in multiphase amorphous calcium borosilicates

Borosilicate glasses for nuclear waste applications are limited in waste loading by the precipitation of water-soluble molybdates. In order to increase storage efficiency, new compositions are sought out that trap molybdenum in a water-durable CaMoO4 crystalline phase. Factors affecting CaMoO4 combination and glass-in-glass phase separation in calcium borosilicate systems as a function of changing [MoO3] and [B2O3] are examined in this study in order to understand how competition for charge balancers affects phase separation. It further examines the influence of radiation damage on structural modifications using 0.77 to 1.34 GGy of 2.5 MeV electron radiation that replicates inelastic collisions predicted to occur over long-term storage. The resulting microstructure of separated phases and the defect structure were analyzed using electron microscopy, XRD, Raman and EPR spectroscopy prior to and post irradiation. Synthesized calcium borosilicates are observed to form an unusual heterogeneous microstructure composed of three embedded amorphous phases with a solubility limit ~ 2.5 mol% MoO3. Increasing [B2O3] increased the areas of immiscibility and order of (MoO4)2 − anions, while increasing [MoO3] increased both the phase separation and crystallization temperature resulting in phases closer to metastable equilibrium, and initiated clustered crystallization for [MoO3] > 2.5 mol%. β-irradiation was found to have favorable properties in amorphous systems by creating structural disorder and defect assisted ion migration that thus prevented crystallization. It also increased reticulation in the borosilicate network through 6-membered boroxyl ring and Si ring cleavage to form smaller rings and isolated units. This occurred alongside an increased reduction of Mo6 + with dose that can be correlated to molybdenum solubility. In compositions with existing CaMoO4 crystallites, radiation caused a scattering effect, though the crystal content remained unchanged. Therefore β-irradiation can preferentially prevent crystallization in calcium borosilicates for [MoO3] < 2.5 mol%, but has a smaller impact on systems with existing CaMoO4 crystallites

Synthesis and sintering of ZrC1-x powders with variable stoichiometry (0

ZrC is a potential candidate for high temperature nuclear applications, such as nuclear fuel cladding on TRISO fuels and as a coating on conventional clad in the third and fourth generation of fission nuclear power plants due to its refractoriness, chemical stability, high thermal conductivity, irradiation tolerance as well as low activation under neutron irradiation [1]. The variation in C content in ZrC1-x is known to produce a significant variation in physical properties, such as thermal and electrical conductivity over the range x=0.0-0.2, and a non-monotonic change in lattice parameter that peaks in the same range of stoichiometry (figure 1.) We have investigated the evolution of the distribution of C vacancies with the C content. Usually, ZrC is prepared from ZrO2 followed by a carbothermal reduction process at high temperatures under an inert atmosphere. Recently, a work in our group showed that ZrC1-x for stoichiometry x\u3c0.2 have a non-negligible amount of O, and could be therefore considered oxy-carbides rather than carbides [2]. This may indicate that the previously reported ZrC could also contain O, since that earlier work dates back to the 1970s. Please click Additional Files below to see the full abstract

Experimental constraints on Li isotope fractionation during clay formation

Knowledge of the lithium (Li) isotope fractionation factor during clay mineral formation is a key parameter for Earth sys-tem models. This study refines our understanding of isotope fractionation during clay formation with essential implicationsfor the interpretation of field data and the global geochemical cycle of Li. We synthesised Mg-rich layer silicates (stevensiteand saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffrac-tion (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product.Bulk solid samples were treated with ammonium chloride to remove exchangeable Li in order to distinguish the Li isotopicfractionation between these sites and structural (octahedral) sites. Bulk solids, residual solids and exchangeable solutions wereall enriched in6Li compared to the initial solution. On average, the exchangeable solutions hadd7Li values 7?lower than theinitial solution. The average difference between the residual solid and initial solutiond7Li values (D7Liresidue-solution) for the syn-thesised layer silicates was?16.6 $\pm$ 1.7?at 20?C, in agreement with modelling studies, extrapolations from high tempera-ture experimental data and field observations. Three bonding environments were identified from7Li-NMR spectra which werepresent in both bulk and residual solid7Li-NMR spectra, implying that some exchangeable Li remains after treatment withammonium chloride. The7Li-NMR peaks were assigned to octahedral, outer-sphere (interlayer and adsorbed) and pseudo-hexagonal (ditrigonal cavity) Li. By combining the7Li-NMR data with mass balance constraints we calculated a fractionationfactor, based on a Monte Carlo minimum misfit method, for each bonding environment. The calculated values are?21.5$\pm$ 1.1?,?0.2 $\pm$ 1.9?and 15.0 $\pm$ 12.3?for octahedral, outer-sphere and pseudo-hexagonal sites respectively (errors 1r).The bulk fractionation factor (D7Libulk-solution) is dependent on the chemistry of the initial solution. The higher the Na concen-tration in the initial solution the lower the bulkd7Li value. We suggest this is due to Na outcompeting Li for interlayer sitesand as interlayer Li has a highd7Li value relative to octahedral Li, increased Na serves to lower the bulkd7Li value. Threeexperiments conducted at higher pH exhibited lowerd7Li values in the residual solid. This could either be a kinetic effect,resulting from the higher reaction rate at high pH, or an equilibrium effect resulting from reduced Li incorporation in theresidual solid and/or a change in Li speciation in solution.This study highlights the power of7Li-NMR in experimental studies of clay synthesis to target site specific Li isotope frac-tionation factors which can then be used to provide much needed constraints on field processes

Mechanism of powellite crystallite expansion within nano-phase separated amorphous matrices under Au-irradiation.

This is the final version of the article. It first appeared from Royal Society of Chemistry via https://doi.org/10.1039/D0CP02447CA fundamental approach was taken to understand the implications of increased nuclear waste loading in the search for new materials for long-term radioisotope encapsulation. This study focused on the formation and radiation tolerance of glass ceramics with selectively induced CaMoO4 as a form to trap the problematic fission product molybdenum. Several samples were synthesised with up to 10 mol% MoO3 within a soda lime borosilicate matrix, exhibiting phase separation on the nano scale according to thermal analysis, which detected two glass transition temperatures. It is predicted that these two phases are a result of spinodal decomposition with Si-O-Ca-O-Si and Si-O-Ca-O-B units, with the latter phase acting as a carrier for MoO3. The solubility limit of molybdenum within this matrix was 1 mol%, after which crystallisation of CaMoO4 occurred, with crystallite size (CS) increasing and cell parameters decreasing as a function of [MoO3]. These materials were then subjected to irradiation with 7 MeV Au3+ ions to replicate the nuclear interactions resulting from α-decay. A dose of 3 × 1014 ions per cm2 was achieved, resulting in 1 dpa of damage within a depth of ∼1.5 μm, according to TRIM calculations. Glasses and glass ceramics were then analysed using BSE imaging, XRD refinement, and Raman spectroscopy to monitor changes induced by accumulated damage. Irradiation was not observed to cause any significant changes to the residual amorphous network, nor did it cause amorphisation of CaMoO4 based on the relative changes to particle size and density. Furthermore, the substitution of Ca2+ to form water-soluble Na2/NaGd-MoO4 assemblages did not occur, indicating that CaMoO4 is resilient to chemical modification following ion interactions. Au-irradiation did however cause CaMoO4 lattice parameter expansion, concurrent to growth in CS. This is predicted to be a dual parameter mechanism of alteration based on thermal expansion from electronic coupling, and the accumulation of defects arising from atomic displacements.This work was funded by the University of Cambridge, Department of Earth Sciences and EPSRC (Grant No. EP/K007882/1) for an IDS. Additional financial support provided by FfWG and the Cambridge Philosophical Society