The relationships between the microstructure and the thermal conductivity of binderless WC have been quantified, considering crystal orientation, isotopic abundance, porosity, and grain size. A significantly higher conductivity is predicted in the out-of-plane (c-axis) direction vs. the in-plane (a-axis) direction, using first principles simulations. Isotopic enrichment of the tungsten sublattice is predicted to increase conductivity, e.g., by a factor of 4-5 in the absence of boundary scattering. The results suggest that for an isotopically pure single crystal a thermal conductivity exceeding 1000 W m(-1) K-1 may be achievable normal to the basal plane. The conductivity of samples with various porosities could be well fit by a minimum surface area (exponential) model, with a porosity exponent of b = 4.4. Experiment and simulation show a strong grain size dependence to conductivity below 1 mu m, with a saturation beyond similar to 10 mu m. The experimental plateau values for kappa were similar to 45 % lower than those of the simulations due to deviations from perfect stoichiometry. We also find a higher scattering coefficient in the experiments, likely due to effects of grain size distribution and elongation. Our study clarifies the physical origin of disagreeing literature reports as being predominantly due to grain boundary scattering and enables microstructural design for thermally demanding environments.

This study provides a green method for producing nanosized tantalum carbide (TaC) by pyrolyzing natural polymer (Gum karaya)/tantalum oxide hybrid composites, which is both sustainable and environmentally benign. The carbothermal reduction, carried out at 1500 °C, produces cubic TaC with a cell constant of ao=4.451Å. The synthesised TaC reveals a near spherical morphology with an average size of 200nm, far smaller than commercial TaC. The synthesised TaC displays a beginning oxidation temperature of 420 °C, as compared to the commercial powder (500 °C). Incorporating 5wt% as-synthesized TaC to commercial TaC results in an impressive relative density of 98%, demonstrating the TaC ability as an excellent sintering aid. This green synthesis not only reduces the environmental impact of conventional methods, but also provides improved characteristics of nanosized TaC for advanced applications, contributing to the field of green nanotechnology by emphasizing safer production methods.

Oxidation of uranium carbide (UC) small fragments from sintered pellets was experimentally tested to better understand UC safe-handling procedures given the renewed interest of non-oxide fuels for high temperature gas or liquid metal cooled reactors. Transformation from UC to U3O8 via a self-ignition reaction was observed at partial pressure of oxygen as low as 10 Pa. The heat output from UC self-ignition in fragments (not-free from UO2 contamination) previously stored in either air atmosphere or inert-atmosphere during a three months period was monitored at 973 K and 1073 K in air atmosphere in a TGA/DTA and no difference could be observed. Residual carbon content, measured as amorphous carbon, carbide or CO/CO2, decreased with exposition temperature in U3O8 resulting oxide, this is in contrast with zirconium carbide resulting oxide, ZrO2. Cracking and stresses accumulated in the oxide were highest for UC to U3O8 compared to UC to UO2 reactions and ZrC to ZrO2 reactions.

A combination of modeling methods has been used to analyze the configurational entropy of undoped and doped amorphous and crystalline ZrO2 systems. In this work, chromia, Cr2O3, has been analyzed as a dopant in crystalline and amorphous zirconia currently at 10 at.% chromium and at a range of temperatures between 300 and 2,000 K. These systems have been modeled using a range of techniques, including molecular dynamics and density functional theory. Modifications to the structures upon introducing chromium is observed. For the first time, the entropy of the amorphous system is modeled and compared to the crystalline system. Amorphous undoped systems have higher configurational entropies owing to greater disorder observed compared with the crystalline counterpart. Dopants act to modify the original crystalline lattice, inducing greater disorder compared with the amorphous counterpart. Further work will analyze whether this difference in entropy has an impact on the overall Gibbs free energy and stabilization of amorphous chromium-doped ZrO2 systems.

Single-phase Hf2Al4C5 ternary carbide was fabricated from Hf/Al/C powder mixtures by pressure assisted sintering techniques such as hot pressing and spark plasma sintering at 1900 °C for 3 h and 10 min, respectively. XRD confirmed that the ternary carbide started to form at temperatures as low as 1500 °C and with total formation of Hf2Al4C5 after reactive sintering for 1 h at 1900 °C. It is evident from HRTEM that two Hf-C layers were sandwiched with 4 Al-C layers (Al4C3) in the Hf2Al4C5 ternary carbide. Tight interlocking of grains, faceted grains and stacking faults were occasionally observed. Thermal conductivity of Hf2Al4C5 is measured to be 14 w m−1k−1 from room temperature to 1300 °C. The oxidation studies carried out at 1300 °C for 3 h reveal that the oxidation layer thickness is around 220 μm and it contains microcracks closer to sample surface whereas the interface looks seamless without any cracking or spallation of the oxide layer.

The structure and chemistry of planar defects in Nb2O5- and Bi2O3-doped BaTiO3 grains exhibiting "core-shell" microstructures has been examined. In addition to 90° ferroelectric domain boundaries within the core, twins and stacking faults were observed both with interfaces lying along {111}. While the twins bisected grains completely, stacking faults were observed only in the shell region and the latter are enriched in Nb and Bi relative to the surrounding matrix. A stacking fault structure is proposed based on a double BiO33- "layer with partial cation site occupancy by Ba2+ and charge compensation by Nb5+ substitution in adjacent octahedral sites.

In recent years, countries across the world have started developing small modular reactors (SMRs), nuclear reactors that generally produce around 300 megawatts of electricity (MWe). Many believe this type of reactor could be key in helping countries achieve their net-zero goals, as they are theoretically less expensive and safer than their larger counterparts, which usually produce more than 500 MWe. SMRs will be assembled in factories and operated in a mostly remote manner, raising concerns about cybersecurity. This paper attempts to analyze the cybersecurity of traditional nuclear reactors and the cyberphysical systems they rely on, analyze the novel ways in which SMRs will be developed and operated, and then highlight how and why SMRs could be particularly vulnerable to cyber-attacks. This paper finds that SMRs will be more susceptible to cyberattacks when compared to larger, more traditional reactors. Mitigations are offered that should increase the cyber-resilience of SMRs.

Preparation and characterization of a Simulated Spent Nuclear Fuel (SIMFuel), which replicates the chemical state and microstructure of Spent Nuclear Fuel (SNF) discharged from UK Advanced Gas-cooled Reactor (AGR) after a cooling time of 100 years is described. Thirteen stable elements were added to depleted UO 2 and sintered to simulate the composition of fuel pellets after burn-ups of 25 and 43 GWd/tU and, as a reference, pure UO 2 pellets were also investigated. The fission product distribution was calculated using the Fispin code provided by NNL. SIMFuel pellets exhibit a microstructure up to 92% TD. During the sintering process in H 2 atmosphere Mo-Ru-Rh-Pd metallic precipitates and grey-phase ((Ba, Sr)(Zr, RE)O 3 oxide precipitates) formed within the UO2 matrix. These secondary phases are present in real PWR and AGR SNF, although they are smaller in size than those examined in this study. The grain size of the produced SIMFuel is in good agreement with literature references.

Grain boundaries in ZrO2 may act as favourable pathways for species, such as oxygen and hydrogen, which play an important role in corrosion when compared with volume diffusion through the bulk of the crystalline material. It is known that segregation of impurity and alloying elements can lead to highly-doped grain boundaries with amorphous structure. Here, these amorphous structures are compared to crystalline materials of equivalent composition. Atomic scale modelling methods have been used to anal-yse diffusion in undoped systems and zirconia cells doped with 5.3 at.% and 11.0 at.% of trivalent lan-thanide species. Diffusion coefficients, pre-exponential factors and activation energies are reported. Oxy-gen diffusivity was markedly increased in amorphous doped and undoped ZrO2 systems compared to equivalent undoped crystalline systems. Similar diffusivities are reported between amorphous and crys-talline doped systems at the concentrations considered. (C) 2021 The Author(s). Published by Elsevier B.V.

A UK National Thermal-Hydraulics Facility (NTHF) dedicated to supporting new reactor and other relevant business is being developed, one of the purposes being to deliver on the government’s carbon emission reduction commitments. The facility site is foreseen to be at Menai Science Park on the isle of Anglesey in North Wales, a region expected to see significant low carbon energy deployment in coming years. The UK NTHF is envisioned to cater for the needs of emerging nuclear in the UK – but also to serve as a hardware platform for international thermal-hydraulics research collaboration. Plans are to construct a platform capable of maintaining several test loops including support for the UK’s on-going, conventional nuclear new build programme as well as Gen-IV systems and associated materials like molten salt and liquid metal coolant media. Motivations are given for NTHF expected capabilities and requirements, which form the basis for its current design and planning state.

Hafnium diboride (HfB2) is a highly refractory (melting above 3000°C) ceramic with many potential applications at high temperatures. To enable its use at temperature for extended periods its high-temperature plasticity must be known. This paper examines the mechanical response at temperatures between 900°C and 2000°C in air and in a reducing atmosphere, interpreting the data in the frame of classical models for the plasticity of compact-packed metals at low temperatures. In particular, the Friedel law and the principle of similitude for dislocation patterning are assessed. This reveals that HfB2 is a singular example of a ceramic material with "metal" mechanical behaviour.

The dependencies of the enhanced thermomechanical properties of zirconium carbide (ZrCx) with sample purity and stoichiometry are still not understood due to discrepancies in the literature. Multiple researchers have recently reported a linear relation between the carbon to zirconium atomic ratio (C/Zr) and the lattice parameter, in contrast with a more established relationship that suggests that the lattice parameter value attains a maximum value at a C/Zr ~ 0.83. In this study, the relationship between C/Zr atomic ratio and the lattice parameter is critically assessed: it is found that recent studies reporting the thermophysical properties of ZrCx have unintentionally produced and characterised samples containing zirconium oxycarbide. To avoid such erroneous characterization of ZrCx thermophysical properties in the future, we propose a method for the accurate measurement of the stoichiometry of ZrCx using three independent experimental techniques, namely: elemental analysis, thermogravimetric analysis and nuclear magnetic resonance spectroscopy. Although a large scatter in the results (ΔC/Zr = 0.07) from these different techniques was found when used independently, when combining the techniques together consistent values of x in ZrCx were obtained.

Ceramics have played a vital role in the rise of nuclear power since its inception in the mid-20th century. Their development and improved understanding are still driving improvements in the safety, efficiency, and reliability of nuclear power (where it remains the leading source of low carbon energy) and these are having beneficial knock-on effects related to nuclear power's economic benefits. In this chapter, we discuss the leading fuel concepts starting with uranium dioxide (discussing its manufacture and operation), as well as considering the potential advanced fuels including the so-called accident tolerant fuel candidate materials. Ceramics used to improve fuel cycle costs (e.g., burnable absorbers), ceramics formed or applied as protective barriers reducing corrosion and moderating ceramics are also discussed. Finally, a section on nuclear waste management is provided highlighting the pivotal role of ceramic and glassy waste forms.

High temperature compressive creep tests have been performed at 1650 -1750 degrees C under applied stresses of 50 - 150 MPa on sintered boron carbide samples exhibiting high relative density and a mean grain size of 0.5 mu m. The creep behaviour of two types of materials, sintered by spark plasma sintering from both raw and heat-treated powders, are characterized. For both materials, the identification of creep parameters (i.e. apparent activation energy and stress exponent values) coupled with TEM structural observations suggest a power law creep regime controlled by dislocation glide, which is limited by the presence of twins. However, the TP material exhibits lower stationary strain rates. This improved creep resistance seems to be directly correlated to the stoichiometry modification of the carbide induced by the powder pre-heat treatment, i.e. increase of structural carbon content and slight decrease of oxygen amount.

A cobalt-free tungsten carbide cermet (WC-FeNi) has been subjected to oxyacetylene flame tests to simulate extreme operating conditions such as a worst-case fusion reactor accident. In such an accident, air-ingress to the reactor may impinge on components operating at surface temperatures in excess of 1000 degrees C, leading to tungsten oxide formation and its subsequent hazardous volatilisation. Here, the most challenging accident stage has been simulated, where the initial air-ingress could lead to extremely rapid air-flow rates. These conditions were simulated using an oxidising oxyacetylene flame. The separation between flame nozzle and sample was varied to permit peak surface temperatures of similar to 950-1400 degrees C. When the peak temperature was below 1300 degrees C, the cermet gained mass due to the dominance of oxide scale formation. Above 1300 degrees C, the samples transitioned into a mass loss regime. The mass loss regime was dominated by liquid-phase ablation of the scale rather than its volatili-sation, which was confirmed by performing a systematic thermogravimetric kinetic analysis. The result was unexpected as in other candidate shielding materials, e.g. metallic tungsten, volatilisation is considered the primary dispersion mechanism. The unusual behaviour of the cermet scale is explained by its relatively low melting point and by the lower volatility of its FeWO4 scale compared to tungsten's WO3 scale. The substantially lower volatility of the WC cermet scale compared to metallic W scales indicates it may have a superior accident tolerance.

High emissivity coatings which aim to help the cement industry reduce heat loss in its production process have been developed with different CeO2 and AlH6O12P3 ratios (1:3, 1:5, and 1:12 by volume). The coating slurries were shear thinning and after heat treatment in air at 1300 degrees C, 1 degrees C/min, dwell 3 hours, XRD revealed that CePO4 forms more easily as the Ce/P ratio decreases. The composition with a 1:5 ratio of CeO2:AlH6O12P3 was gun sprayed on basic refractory bricks, then heat treated under the same conditions as the slurries. SEM, (S)TEM and EDX were used to study thickness, microstructure, and chemical composition of the coatings which revealed that the coating was composed of pores, CeO2 grains, CePO4 grains, and M-P-O glass. SEM images show that CePO4 was nucleated from a reaction between CeO2 and AlH6O12P3. Consequently, CePO4 grains (similar to 2 mu m diameter) were smaller than CeO2 (similar to 10 mu m diameter). The emissivities of un-coated and coated basic refractory bricks were measured at 1100 and 1300 degrees C over the wave number range of 700-12 000 cm(-1). At both temperatures, the emissivity of the coated bricks was higher than the uncoated bricks and the emissivity was measured to be higher at a higher temperature for both samples. The coated bricks gave the highest emissivity of 0.81 from 1050 to 11 000 cm(-1) which is about twice the un-coated bricks for the same conditions. This demonstrates that the developed high emissivity coating has potential to be used with basic refractory brick.

Synroc-Z is a new wasteform being developed to be specifically suited to waste from the advanced EURO-GANEX reprocessing route. This type of waste contains nuclides that release significant amounts of heat during their decay. Here, predictions for radiogenic heat generation from EURO-GANEX wasteforms based on Synroc-Z ceramic and R7T7 glass materials are presented. During reprocessing, any actinides present in the waste-stream are removed to leave only fission products. Simulation results are given demonstrating that, although initially classified as high level waste, the effects of decay mean the wasteforms effectively become intermediate level after 140 years of storage. Furthermore, many of the issues arising from alpha decay will be avoided. During the first 600 years, it is shown that the main source of self heating comes from the decay of Cs-137 and Sr-90. Our calculations show how the significant radiogenic heating by these wasteforms must be considered when defining package sizes and waste loadings. These effects are discussed for both ceramic and glass wasteforms and demonstrate that Synroc-Z provides more flexibility allowing larger packages even at higher waste loadings than R7T7.