On December 5, 2022, an indirect drive fusion implosion on the National Ignition Facility (NIF) achieved a target gain G_{target} of 1.5. This is the first laboratory demonstration of exceeding "scientific breakeven" (or G_{target}>1) where 2.05 MJ of 351 nm laser light produced 3.1 MJ of total fusion yield, a result which significantly exceeds the Lawson criterion for fusion ignition as reported in a previous NIF implosion [H. Abu-Shawareb et al. (Indirect Drive ICF Collaboration), Phys. Rev. Lett. 129, 075001 (2022)PRLTAO0031-900710.1103/PhysRevLett.129.075001]. This achievement is the culmination of more than five decades of research and gives proof that laboratory fusion, based on fundamental physics principles, is possible. This Letter reports on the target, laser, design, and experimental advancements that led to this result.

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. As recently reported, a burning plasma state, where the alpha-heating in the plasma is the primary source of heating, was achieved in laboratory experiments. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin ``burn propagation'' into surrounding cold fuel, enabling the possibility of high energy gain. While ``scientific breakeven'' (i.e. unity target gain) has not yet been achieved, this talk reports the first controlled fusion experiment on the National Ignition Facility to produce capsule gain greater than unity (here 5.8) and reach ignition by many different formulations of the Lawson criterion. In the talk, we will discuss some key basic physics inertial confinement fusion (ICF) principles behind the burning plasma and ignition results as well as discuss future challenges.

An approach to transform and accelerate nuclear fuel development and qualification is outlined. The approach exploits advanced modeling and simulation at the outset to inform constituent and system selection and to enable integral fuel performance analyses. Analyses using these tools identify and prioritize the most important fuel performance parameters and phenomena for subsequent targeted characterization with separate-effects tests. Separate-effects testing spans out-of-pile and in-pile tests and is meant to iterate with and inform engineering-scale integral fuel performance analyses throughout the development process. Exercising this cycle in an agile fashion will increase confidence in the integral fuel performance predictions while reducing uncertainties. This process sets the stage for executing a much more limited set of well-defined integral irradiation tests designed to validate engineering-scale fuel performance codes and to confirm the performance and safety of the fuel system under prototypic conditions. This approach will reduce the time for development and qualification of a new fuel system, and it will also reduce associated costs.

Mo nanopowders are synthesized by ball milling and the following hydrogen reduction of MoO3 powders. The densification mechanism of Mo nanopowders during spark plasma sintering (SPS) is analyzed by the combination of the regression of the experimental data on regular SPS and on the SPS multistep pressure dilatometry based on the continuum theory of the sintering. The Mo mirror with 400 nm average grain size and 93% relative density is fabricated by the SPS. The performance of the mirrors made from a single crystal Mo and from the hydrogen-reduced sintered Mo nanopowder is discussed. The microstructure and optical properties of the mirrors are characterized before and after plasma exposure, and no substantial degradation of the reflectivity was observed. (C) 2019 Elsevier B.V. All rights reserved.

The first AFQ workshop was held on Friday, May 31, 2019, in Washington, DC, with over 30 expert participants from industry (General Atomics, Framatome, Lightbridge, TerraPower, Westinghouse), national labs (ANL, INL, LANL, ORNL), DOE-NE, NRC and academia (University of Florida and University of Tennessee). The overall goal of the workshop was to discuss how modeling and experiments can be simultaneously exploited to markedly reduce the years of data (and associated costs) that are currently required for deployment of new nuclear fuels utilizing the new methodology of Accelerated Fuel Qualification (AFQ). There were nine presentations given during the workshop, which included an overview presentation, five case studies, summary of relevant national lab capabilities and on-going DOE programs, as well as perspective from the NRC.

Surface-oxidized molybdenum nanopowders are compacted by spark plasma sintering (SPS). The oxide impurity behavior is analyzed under various sintering temperatures. The densification mechanism of the nanopowders with a melted oxide phase is identified in situ by regression analysis of the experimental data on the temperature-dependent porosity change and on the SPS multistep pressure dilatometry. To increase the density of the compacted pellets, the nanopowders with the oxide phase are consolidated by SPS using the two in situ oxide removal methods: carbothermic reduction and particle surface cleaning by the electric current flow through the powders. The advantages and disadvantages of these methods in terms of the density, grain size, and mechanical properties of the final products are discussed.

The net-shaping capability in spark plasma sintering (SPS) of ultrahigh-temperature ceramics has been explored in this study. The annular or ring-like shape is considered here as the first step to a more complex geometry compared to a solid cylinder or disk. ZrC powders have been SPS processed in specially designed graphite tooling to achieve the annular shape geometry. Experimental runs have been carried out to determine the optimal processing parameters for producing highly dense ZrC specimens in this geometry. Finite-element modeling framework has been constructed to determine the internal stress evolution, as well as densification, during the SPS of the annular-shaped ZrC. The formulated processing schemes for ZrC have been adapted to process SiC/ZrB2 composite powder with the purpose of making tooling components for SPS applications. The applicability of the obtained composite SPS tooling has been evaluated at high temperature and high pressure associated with SPS regimes.

Engineered cladding consisting of layers of monolithic silicon carbide (SiC) and SiC fiber reinforced SiC matrix (SiC-SiC) composite has potential to offer significant performance benefits and enhanced accident tolerance compared to existing light water reactor fuel rod designs. General Atomics is developing a multilayered cladding design that offers toughness, hermeticity, stability under irradiation, and retention of strength and performance at high temperatures and in potential accident conditions. Recent progress has focused on both the fabrication and characterization of this enhanced accident tolerant fuel cladding. Fabrication efforts have addressed scale-up in cladding length while meeting demanding dimensional tolerances and surface roughness requirements. In addition, a robust, SiC-based joining and sealing process has been demonstrated which is capable of fabricating cladding samples containing a pressurized gas backfill necessary for fuel rod applications. Mechanical and thermal properties including strength and thermal conductivity have been measured over a range of temperatures corresponding to normal operation and accident conditions. The hermeticity of the engineered cladding (including the tube and end plug joints) has been measured using high sensitivity helium leak detection. Cladding corrosion performance has been measured outof-pile (in flowing loop autoclaves under pressurized water reactor chemistry conditions), and sealed cladding samples retained hermeticity and showed extremely low corrosion mass loss.

The densification mechanism of conductive powders is revealed by comparing the electric current-assisted spark plasma sintering (SPS) of ZrN powder with conventional hot pressing (HP) carried out with the same powder and under the same pressure and temperature. To determine the actual temperature inside ZrN powder, a sacrificial thermocouple is directly inserted into the powder during the SPS process. The spatial distribution of the electric current passing through the powder is calculated using the finite element modeling. The porosity-interparticle neck area geometrical relationship is applied to estimate the electric current density inside the powder volume subjected to SPS. For the first time, by taking into account the explicit influence of the electric current effect on the SPS densification mechanism, the constitutive equations describing the electric current-assisted hot pressing of powders are developed. The densification mechanism of ZrN is determined by the inverse regression of the new SPS constitutive equations and by utilizing the experimental results on ZrN powder consolidation with and without the participation of the electric current effect.

Theoretical studies on the densification kinetics of the new spark plasma sinter-forging (SPS-forging) consolidation technique and of the regular SPS have been carried out based on the continuum theory of sintering. Both modelling and verifying experimental results indicate that the loading modes play important roles in the densification efficiency of SPS of porous ZrC specimens. Compared to regular SPS, SPS-forging is shown to be able to enhance the densification more significantly during later sintering stages. The derived analytical constitutive equations are utilised to evaluate the high-temperature creep parameters of ZrC under SPS conditions. SPS-forging and regular SPS setups are combined to form a new SPS hybrid loading mode with the purpose of reducing shape irregularity in the SPS-forged specimens. Loading control is imposed to secure the geometry as well as the densification of ZrC specimens during hybrid SPS process.

This paper reports on a nuclear-grade joining material for bonding of silicon carbide-based components. The joint material is fabricated via a hybrid preceramic polymer, chemical vapor infiltration process. The joint is comprised entirely of beta-SiC and results in excellent mechanical and permeability performance. The joint strength, composition, and microstructure have been characterized before and after irradiation to 4.5 dpa at 730 degrees C in the High Flux Isotope Reactor. The hybrid preceramic polymer-chemical vapor infiltrated joint exhibited complete retention of shear strength and no evidence of microstructural evolution or damage was detected following irradiation. (C) 2017 Elsevier B.V. All rights reserved.

Monolithic silicon carbide (SiC) to SiC plate joints were fabricated and irradiated with neutrons at 270 -310 degrees C to 8.7 dpa for SiC. The joining methods included solid state diffusion bonding using titanium and molybdenum interlayers, SiC nanopowder sintering, reaction sintering with a Ti-Si-C system, and hybrid processing of polymer pyrolysis and chemical vapor infiltration (CVI). All the irradiated joints exhibited apparent shear strength of more than 84 MPa on average. Significant irradiation-induced cracking was found in the bonding layers of the Ti and Mo diffusion bonds and Ti-Si-C reaction sintered bond. The SiC-based bonding layers of the SiC nanopowder sintered and hybrid polymer pyrolysis and CVI joints all showed stable microstructure following the irradiation. (C) 2017 Elsevier B.V. All rights reserved.

Spark plasma sintering (SPS) has been employed to consolidate a micron-sized zirconium carbide (ZrC) powder. ZrC pellets with a variety of relative densities are obtained under different processing parameters. The densification kinetics of ZrC powders subjected to conventional hot pressing and SPS are comparatively studied by applying similar heating and loading profiles. Due to the lack of electric current assistance, the conventional hot pressing appears to impose lower strain rate sensitivity and higher activation energy values than those which correspond to the SPS processing. A finite element simulation is used to analyze the temperature evolution within the volume of ZrC specimens subjected to SPS. The control mechanism for grain growth during the final SPS stage is studied via a recently modified model, in which the grain growth rate dependence on porosity is incorporated. The constant pressure specific heat and thermal conductivity of the SPS-processed ZrC are determined to be higher than those reported for the hot-pressed ZrC and the benefits of applying SPS are indicated accordingly.

The flash method is a commonly used technique to measure the thermal diffusivity of solid samples. In this work, the classical flash method for planar specimen geometries was extended to measure cylindrically curved plates. The temperature evolution of the back surface of the cylindrical plate was calculated and a measurement method incorporating a geometrical correction concept was evaluated. Experimental measurements of the thermal diffusivity on a set of planar and curved lead samples were used to confirm the validity of the method and determine the geometrical correction factor. Additional thermal diffusivity measurements on a set of stainless steel samples at temperatures from room temperature to 800°C demonstrated that the geometric factor is not a function of sample temperature. This method was then applied to obtain thermal diffusivity measurements for curved composite materials. Silicon carbide fiber reinforced, silicon carbide matrix composites (SiC-SiC) are being developed for nuclear fuel cladding applications, where thermal conductivity is an important design parameter. Due to the fabrication process, the structure and fiber geometry of curved composite samples may differ from planar samples, and as a consequence, data from planar samples may not be representative and direct measurement of curved composite specimens is critical. Composite structure was found to have a strong effect on the thermal diffusivity measurements for prototypical SiC-SiC cladding tubes.

Silicon carbide (SiC) is being investigated for accident tolerant fuel cladding applications due to its high temperature strength, exceptional stability under irradiation, and reduced oxidation compared to Zircaloy under accident conditions. An engineered cladding design combining monolithic SiC and SiC–SiC composite layers could offer a tough, hermetic structure to provide improved performance and safety, with a failure rate comparable to current Zircaloy cladding. Modeling and design efforts require a thorough understanding of the properties and structure of SiC-based cladding. Furthermore, both fabrication and characterization of long, thin-walled SiC–SiC tubes to meet application requirements are challenging. In this work, mechanical and thermal properties of unirradiated, as-fabricated SiC-based cladding structures were measured, and permeability and dimensional control were assessed. In order to account for the tubular geometry of the cladding designs, development and modification of several characterization methods were required.

Nuclear grade silicon carbide fiber (SiCf) reinforced silicon carbide matrix (SiCm) composite is a promising candidate material for accident tolerance fuel (ATF) cladding. A major challenge is ensuring the mechanical robustness of the ceramic cladding under accident conditions. In this work the high temperature mechanical response of a SiCf-SiCm composite tubing is studied using a novel thermo-mechanical test method. A solid surrogate tube is placed within and bonded to the SiCf-SiCm sample tube using a ceramic adhesive. The bonded tube pair is heated from the center using a ceramic glower. During testing, the outer surface temperature of the SiC sample tube rises up to 1274 K, and a steep temperature gradient develops through the thickness of the tube pair. Due to CTE mismatch and the temperature gradient, the solid surrogate tube induces high tensile stress in the SiC sample. During testing, 3D digital image correlation (DIC) method is used to map the strains on the outer surface of the SiC-composite, and acoustic emissions (AE) are monitored to detect the onset and progress of material damage. The thermo-mechanical behavior of SiC-composite sample is compared with that of monolithic SiC samples. Finite element models are developed to estimate stress-strain distribution within the tube assembly. Model predicted surface strain matches the measured surface strain using the DIC method. AE activities indicated a progressive damage process for SiCf-SiCm composite samples. For the composites tested in this study, the threshold mechanical hoop strain for matrix micro-cracking to initiate in SiCf -SiCm sample is found to be -300 microstrain. (C) 2015 Elsevier B.V. All rights reserved.

Silicon carbide fiber reinforced silicon carbide matrix (SiCf-SiCm) composite tube is being developed as an accident tolerant fuel cladding material for light water reactors. The mechanical robustness of the material is of critical importance to ensure that the nuclear fuel and fission products are contained during both normal operation and accident conditions, such as the loss of coolant accident (LOCA) encountered in Fukushima Daiichi Plant. We report the development and application of a high pressure burst testing method to evaluate the mechanical robustness of SiCf-SiCm composite cladding. The internal high pressure is generated using a rubber tubing placed within the SiCf-SiCm sample tube. A unique test rig was designed to seal the ends of the rubber tubing while pressurizing it up to 2,000 bar by hydraulic oil. The expanding rubber tubing confined by the SiCf-SiCm sample tube thus exerts a controlled uniform internal pressure to the SiCf-SiCm sample. The full-field strain distribution of the outside surface of the sample was captured by 3D digital image correlation (DIC) method. The acoustic emission (AE) technique was used to detect damage events during the high pressure burst testing.