For the past 30 years, the development of durable materials for radionuclide immobilization has been driven by efforts to dispose of wastes generated by the nuclear fuel cycle [National Research Council, ‘Waste Forms Technology and Performance: Final Report’, the National Academies Press, Washington D.C., 2011]. Many materials have been developed, but there still exist large gaps in the knowledge of fundamental modes of waste form degradation in repository environments. An important aspect of waste form science is the behavior of the materials under intense irradiation from decaying actinides and fission products. This irradiation induces a wide range of defects and disorder, the details of which depend on the specific waste form material. At the present time, it is not fully explained how radiation effects will influence the performance of nuclear waste forms and their long-term retention of fission products and actinides under operational conditions. The complex defect behavior and radiation damage must be understood over a range of length scales, from the initial atomic-scale defect structure to the long-range observable material modification. This is particularly challenging and requires advanced characterization techniques. This contribution describes how pair distribution function (PDF) analysis obtained from neutron total scattering experiments can be applied in the research field of waste form science to uniquely characterize radiation effects in a wide range of materials, including crystalline complex oxides and waste glasses. Neutron scattering strength does not have an explicit Z-dependence; this allows access to many low-Z elements, such as oxygen, that cannot be accurately studied with X-rays. In many cases, this can permit a detailed analysis of both cation (often high-Z) and anion (often low-Z) defect behavior. In contrast to traditional crystallography, which relies on long-range order, PDF analysis probes the local defect structure, including changes in site occupation, coordination, and bond distance. This is particularly important when characterizing aperiodic waste glasses with no long-range order at all. In contrast to X-ray characterization which requires very little sample mass (∼0.1 mg), neutron characterization (even at state-of-the-art spallation facilities) requires relatively large sample mass (∼50 - 100 mg). Obtaining this quantity is challenging for studies of irradiated materials, but by tailoring our experimental approach to use high-energy ions (GeV) with very high penetration depth, we are able to produce the required mass.

Understanding the precipitation of brittle hydride phases is crucial in establishing a failure criterion for various zirconium alloy nuclear fuel cladding. Accordingly, it is important to quantify the sensitivity of hydride precipitation to the component microstructure. This experimental investigation focuses on two microstructural characteristics and their role as hydride nucleation sites: The grain size and the alloy chemical composition. Samples of commercially pure zirconium (Zr-702) and Zircaloy-4, each with a wide range of grain sizes, were hydrided to 100 ppm and micrographs of the hydride distribution were optically analyzed for inter-granular and intra-granular precipitate sites. For most grain sizes, it was found that a significantly lower fraction of the precipitated hydrides nucleated at grain boundaries in Zircaloy-4 than in Zr-702, suggesting that a higher SPP content encourages the formation of intra-granular hydrides. Moreover, this effect became more prominent as the grain size increased; large-grain specimens contained a higher fraction of intra-granular hydrides than small-grain specimens of both Zr-702 and Zircaloy-4, highlighting the potency of grain boundaries as nucleation sites and how SPPs can influence the hydride distribution profile.

Atomistic simulations are employed to investigate the dynamical behavior of atoms in cubic silicon carbide (SiC) following a 5 keV radiation knock. Specifically, we have computed the time-resolved van Hove self-correlation function, Gs(r,t), separately for the silicon and carbon sub-lattices. Our goal is to probe the early radiation damage mechanisms using a dynamical methodology. The simulation results show that the carbon atoms engage in a dynamic hopping mechanism as the system recovers from the radiation knock. The silicon atoms, however, exhibit a strikingly different behaviour: the time variation of 4πr2Gs(r,t) indicates a dynamic tension between the crystalline and disordered regions of the Si sub-lattice. The power-law tail of the 4πr2Gs(r,t) correlation for silicon atoms suggests a scale-free self-organized critical (SOC) state – a possible precursor to the collapse of the Si sub-lattice.

The role of oxide nanoparticles in cavity formation of a nano-oxide-dispersed ferritic steel subjected to (Fe + He) dual-ion and (Fe + He + H) triple-ion irradiations has been studied using transmission electron microscopy to elucidate the synergistic effects of helium and hydrogen on radiation tolerance of nano-oxide-dispersed ferritic steel for fusion energy systems. The effect of oxide nanoparticles on suppressing radiation-induced void swelling is clearly revealed from the observation of preferred trapping of helium bubbles at oxide nanoparticles, which results in a unimodal distribution of cavities in the (Fe + He) dual-ion irradiated specimen. An adverse effect of hydrogen implantation, however, is revealed from the observation of a bimodal distribution of cavities with large and facetted voids in association with the formation of HFe5O8-based hydroxide in local regions of the (Fe + He + H) triple-ion irradiated specimen.

The structural integrity of nuclear fission and fusion power plant components is the focus of this research. The state of the art is using micro scale specimens milled with a focussed ion beam (FIB). Because of their very low volume such specimens can be lab tested, even when irradiated to low or medium level of activity. This offers a possibility of testing multiple specimens to investigate stochastic effects, e.g. effects of irradiation on the shift of the ductile to brittle transition. However, FIB milled specimens suffer from Ga contamination, to the degree that the validity of fracture data obtained on such specimens is questionable. We propose to use nano-additive manufacturing as an alternative to FIB for making micro scale fracture specimens. A combination of two-photon polymerization and electrodeposition and sputtering was used to manufacture micro-scale Brazilian disk fracture specimens (CBD), which are free from Ga and thus better suited for the study of irradiation effects on structural integrity. In this study Ni CBD specimens were made with 30 µm diameter and up to 13 µm thickness. The slot width varied between 1 µm to 2.9 µm width the corresponding slot length of between 7.5 µm and 8 µm. Consecutive FIB characterization shows that the specimens have polycrystalline microstructure with sub-µm grains. The work is ongoing making W CBD specimens and on reducing the slot width and using chemical vapor deposition fabrication.

The oxygen ions in the high temperature superionic state of uranium dioxide (UO2) are known to be in an arrested or jammed state, exhibiting characteristic features of jammed kinetics such as low dimensional string-like ion hopping and dynamical heterogeneity (DH). This thermally-jammed state entails a configurational entropic cost. Using atomistic simulations and the 2PT method, we compute the solid-like (vibrational) and hard sphere-like (configurational) contributions to the total entropy across a temperature range of 1500 K to 2800 K that envelop both the onset of superionic conduction (2000 K) and the second order λ-transition (2610 K). To properly account for the thermally jammed state of the ions, we use an equation of state that is appropriate for the metastable fluid branch. Our simulation results are in excellent agreement with the entropy data extracted from specific heat experiments with a mean error of less than 2%.

(Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300 single crystals were successfully grown with the aim of exploring their potential for laser induced third harmonic generation (THG) and piezo-optical applications. Their Raman and luminescence spectra in the 150-300 K temperature range were studied. Influence of different gamma ray doses on the nonlinear optical properties were also explored. The optical properties of these crystals exhibit strong variation with temperature. It is concluded that (Ga54.59In44.66Er0.75)2S300 may be applied as promising materials for dosimetry applications in γ - radiation and optical temperature sensors.

Polymer-non-leaded metal (tin, bismuth-tin (BiSn) alloy, and tungsten) composites were prepared to investigate the effectiveness of their X-ray shielding. Films of the composites were found to exhibit excellent X-ray shielding due to the uniform dispersion of metal particles in the polymer matrix, as were fabrics impregnated with the BiSn alloy. The fabricated composites effectively absorb penetrating photons. The preparation of composites with a uniform dispersion and a multilayered structure can limit the formation of pin holes. Multilayered BiSn composites exhibit significantly enhanced shielding. The lamination of a tungsten composite film or a BiSn composite film onto a BiSn-coated layered sheet results in better and more uniform shielding. Thus non-leaded BiSn metal-polymer composites can be used in X-ray shielding applications instead of the lead. One advantage that metal BiSn has over tungsten is its low melting temperature, which means that it can be processed in a normal extruder or in internal mixers such as those used in polymer processing.

200 μm thick solution annealed AISI 316L stainless steel foils were implanted with Ar ions to produce a 0.25 at. % concentration-depth plateau extending from the near surface to a depth of ≈ 250 nm, and then annealed at 550°C for 2 hours to form small Ar bubbles and Ar-vacancy clusters. Distinct sets of samples (including control ones without Ar) were irradiated at the temperature of 550 °C with Au ions accelerated at 5 MeV to produce an average damage content about ≈36 dpa at the region containing the Ar plateau. These samples were investigated by transmission electron microscopy using plan-view specimens prepared by ion milling. In contrast with the control samples where the irradiation causes the formation of a high concentration of extended defects and large cavities, carbonite precipitation of 1:1 metal-carbon (MC) content with a cubic structure occurs only in the samples containing the Ar bubbles. This precipitation phenomenon is not commonly observed in the literature. The results are interpreted considering that the precipitate growth process requires the emission of vacancies which are synergistically absorbed by the growth of the Ar bubbles.