Search

Transactions of the American Nuclear Society, 114, ISSN:0003-018X

We demonstrate that irradiation may enhance the plasticity in metallic glasses by increasing the free-volume content via micropillar compression experiments on an ion-irradiated bulk metallic glass (BMG). Results show that irradiation decreases the flow stress and enhances the shear band formation by lowering the magnitude of stress serrations in plastic flow regime. These results highlight that amorphous alloys can mitigate the deleterious affects of severe ion irradiation as compared to their crystalline counterparts. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

During the last two decades, silicon carbide based ceramic composites (SiC/SiC) have become a candidate material for nuclear fuel cladding, first for advanced nuclear systems, such as the GFR, and recently also as potential replacement material for nuclear fuel cladding in light water reactors. This work is contributing to the General Atomics/Westinghouse led CARAT project. In this frame, the thermal conductivity (TC) of cladding prototype sections are measured both before and after neutron irradiation with the purpose of better understanding the effects of neutron irradiation on the pyrolytic carbon interphase linking the SiC fibres to the SiC matrix.

During the last two decades, silicon carbide based ceramic composites (SiC/SiC) have become a candidate material for nuclear fuel cladding, first for advanced nuclear systems, such as the GFR, and recently also as potential replacement material for nuclear fuel cladding in light water reactors. This work is contributing to the General Atomics/Westinghouse led CARAT project. This paper focuses on the consequences of the microstructural changes occurring at the pyrolytic (PyC) carbon interphases linking silicon carbide matrix and fibres in the composite. The experimental approach is two-fold: a macro-scale approach deals with the measurement of the thermal conductivity of short sections of prototype SiC/SiC cladding tubes, whereas the effects of radiation damage on the PyC interlayer are studied by energy filtered transmission electron microscopy (EFTEM) as well as spectrum imaging in STEM (ESI). The results from both faces of the experimental work are brought to gether in an effective medium model based on the work of Markworth and Youngblood et al. A detailed outlining of the radial heat flow experimental apparatus developed to this end has been presented at the 2016 ANS meeting in New Orleans.

In this document a novel nuclear fuel production method is suggested. It combines previously developed approaches and improvements of the internal gelation process, which was used over decades to produce sphere-pac fuel for minor actinide transmutation. And modern 3D printing of ceramics. The improvements to the internal gelation, which were developed in two CCEM.CH projects in the recent years comprise the two following aspects • Change of the heating media from silicon oil to microwave fields • Application of in-situ preparation of the feed solution, with the sparing of any need in cooling The internal gelation together with these improvements can be used to implement a new 3D printing technique for ceramics, and especially nuclear fuels, with the following advantages: • Almost powder-less process up to the formation of the fuel body in complex shapes • Any local composition change in metal or enrichment can be achieved • Pore-formers can be introduced locally, allowing the formation of buffer regions/layers

Silicon carbide based composites are candidates for structural components and fuel claddings in nuclear power plants. In the frame of accident tolerant fuel research, the effective through-thickness thermal conductivity of SiC/SiC prototype claddings is investigated. This property, which is both material and geometry dependent, has been measured with a custom-made radial heat flow apparatus. Conductivities ranging between 0.5 and $\sim4$~W$\cdot$m$^{-1}\cdot$K$^{-1}$ have been measured, well below the literature values of 8 to 15~W$\cdot$m$^{-1}\cdot$K$^{-1}$. This significant difference is due to several factors. Firstly, with the method used here, the components of the conductivity, parallel, and perpendicular to the fibre weave, are fully separated. Secondly, the complex multilayered architectures of the tubes are detrimental, acting as barriers against heat transport. Indeed, in order to be water- and gas-tight, the tubes have to include dense ceramic or metallic layers. The poor adhesion between these and SiC/SiC results in a severely reduced heat transfer. Moreover, the matrix of the composite shows a heterogeneously distributed porosity --~ about 20 to 45\%. This leads to an inhomogeneous thermal conductivity, along both the length and the circumference of the cladding. Although the total porosity should be lessened in future development steps, it currently contributes to the transport of heat through Mie forward scattering of infrared radiation, an effect known as radiation thermal conductivity. In parallel to these activities, a specific component of SiC/SiC, the pyrolytic carbon interphase, has been investigated using analytical electron microscopy. This work showed that this layer, linking the fibres to the matrix, becomes partially amorphous after irradiation. In-situ ion irradiations evidenced radiation-induced dimensional changes of the said layer, the impact of which would mostly be on the mechanical properties of the composite. Using continuous medium modelling, the effect of a partial amorphisation of the PyC is estimated to a thermal conductivity loss of 20 to 25\%. In their current state, SiC/SiC clads do not have a thermal conductivity high enough for them to used in power plants. Indeed, a thermal conductivity locally as low as 1~W$\cdot$m$^{-1}\cdot$K$^{-1}$ results in fuel temperatures high enough for \ce{UO2} to reach its melting point.

Internal gelation and 3D printing processes were proposed to combine in a process that is promising to produce nuclear fuels with simplified production route and enhanced in-core performance [1]. The process relies on in-situ mixing of feed solutions that bear actinides and uranium nitrates with the methenamine and urea mixture. These feed solutions should be thoroughly mixed and kept at low temperature up to an inkjet printing head, where an inkjet printing process is used to deposit controlled volumes of the mixed solution on the pellet under-fabrication. Microwave and/or laser-heating are then used to trigger the gelation reaction and to solidify these printed dots and later to drive out the excess water and pre-sinter the gelled oxides. The current study is an initial approach to optimize the upfront parameters of the 3D printing process. The feed solutions need be mixed in a passive microfluidic mixer, where the first optimization parameter is the minimum mixing time in the microfluidic mixer that will yield an acceptable mixing level. To do so, CFD modelling using OpenFOAM for different designs of microfluidic mixers was conducted to select the most efficient conception. To validate the CFD results, simulation of microfluidic mixers similar to some literature cases were performed and compared. The uncertainty of selected parameters in the CFD models was used to evaluate a conservative retention time and length. Secondly, the next printing step was modeled with OpenFOAM, which is the droplet generation process. The objective from these models was to correlate the feed parameters like the flow rate and the usage of an acoustic mechanical vibrator to the printed droplets size, generation rate, and the required stand-off distance of the printing head. To perform these CFD simulations with the right solution parameters, rheology and diffusivity measurements were conducted. A proposed empirical approach was investigated for the evaluation of the diffusivity of multi-species solutions. The resultant parameters from the current study are the mixing times and lengths for given microfluidic mixers that are industrially feasible, also, the rate and volume at which the droplets of the broth mixture are generated. These results can be used as guidelines for designing the inkjet printing head and to highlight areas where further investigations are required for an efficient production process.

A new Additive-Manufacturing (AM) or 3D printing concept is proposed to improve the printing resolution for metal additive manufacturing in the frame of the SFA-AM project, Powder Focusing for Beam-Induced Laser 3D Printing. The project aims to transport small powder particles (largest diameter < 10 um) at a high throughput to a spot smaller than 20 µm. To realize the objective, a metal ion included liquid is suggested as a source medium similar to the inkjet process, and the droplet is employed as a container for the metal ion transport. The key challenges of achieving this goal are 1) droplet size control and 2) efficient microwave heating that can dry the liquid portion of the dynamic droplet at a high-speed (3 m/s). The most demanding part is the development of a miniaturized, very concentrated field density microwave resonator where the droplet is passing through. This follows from the extremely short interaction time between the microwave and the ejected droplets (research: 0.5 ms) which results from the requirement for a small system size for its integration. The primary medium examined for the experiment is water due to 1) a high dielectric loss proportional to the drying efficiency and 2) avoiding explosion risks at the laser sintering stage compared to organic solutions. Various droplet generation conditions were investigated to determine the optimum conditions for the custom droplet generator, in particular, two key parameters were focused on: frequency and flow rate. The droplets obtained were analyzed with respect to size, gap distance, and linearity affecting the uniformity of the AM process. Moreover, Navier-Stokes-based fluid dynamics simulations enabled understanding of droplets behaviors, especially break-up formation. Eventually, the optimal condition for producing the smallest droplets with a diameter of 100 µm is determined to be 15 kHz and 0.5 ml/min. Such droplets loaded with metallic particles could result in smaller than 20 um of remaining metal powders aggregate after the drying process. A new TEM (Transverse ElectroMagnetic) resonator has been developed for drying and sensing applications. A full-wave electromagnetic simulation software, CST Microwave Studio, supported the working mode of the device. Firstly, it is used for the dielectric characterizations of different water-ethanol mixtures contained in a micro capillary. The sensing relies on electric field perturbation due to the inserted sample, and is measured based on the change of both resonance frequency and Q-factor. These sample-dependent variations were analyzed to estimate the complex permittivity using different methods such as the Perturbation Method (PM), Least-Square Model (LSM), and Log-Linear Model (LLM). This work demonstrates that the proposed microwave module is capable of sensing nano-liter volume ranges even though the samples only partially influence the sensing area (error < 13 % in permittivity). In microwave heating, the developed resonator achieved a temperature increase of 28 K in the microwave exposal time of 0.5 ms when the microwave power of 45 W was applied. Finally, numerical models have been proposed to estimate the maximum temperature of the initial droplet before cooling starts attributable to heat and mass transfer. This model well corresponds to the microwave electromagnetic simulation resulting in similar temperature deviations. It helps to determine the net heating efficiency of the microwave device.

Zirconium alloys used in the nuclear industry are exposed to extreme conditions undergoing high levels of irradiation damage and corrosion. Zircaloy-2 is used as nuclear fuel cladding in boiling water reactors and for the encapsulation of the spallation target in the neutron source SINQ at the Paul Scherrer Institute. As a result of oxidation, hydrogen is produced and partially taken up into the zirconium matrix. As the hydrogen reaches the solubility limit, it precipitates in the form of a zirconium hydride. A number of deleterious effects of hydrides have been extensively studied, including the embrittlement of the material and a unique cracking phenomenon, namely delayed hydride cracking (DHC). DHC occurs as hydrogen in solid solution diffuses towards locations of higher tensile stress, where it subsequently precipitates as the local solvus limit is reached. Once the hydride grows to a critical size, it fractures, which can continue to propagate in repeated steps by the same mechanism. Several studies have been performed in order to understand properties of DHC based on parameters like, alloy type, cracking temperature, hydrogen content, and cracking direction. The most important mechanical properties include the fracture toughness and cracking velocity, which governs the material failure conditions. Crystallographic properties have been investigated in order to understand how complex hydride precipitation develops within the context of DHC conditions. In this project, radial DHC is explored in irradiated and unirradiated thin-walled Zircaloy-2 nuclear fuel cladding with and without an inner liner, in addition to spallation target rod material. A unique three-point bending test, applicable also for irradiated samples in the hotlab, was developed to induce a consistent radially propagating outside-in crack. Results have provided insight into the effects of the inner liner on cracking velocity, showing correlations with the hydrogen depletion. The combined DHC and creep mechanisms, which cannot be decoupled, is coined as ‘creep-delayed hydride cracking’. As the initiating conditions for DHC are of great technical importance, the threshold stress intensity factor has been investigated, under ideal DHC conditions, where a minimum value occurs around 6 MPa√m. Quantitative analysis of the hydrogen distribution around the tip of an arrested DHC crack tip has been performed with neutron radiography at the SINQ. Trends show that the hydrogen concentration around the crack tip increases with temperature resulting likely from the larger hydrogen diffusion, and that the liner reduces the hydrogen available for DHC. Crystallographic analysis has provided insight on the hydride phases responsible for DHC with synchrotron micro X-ray diffraction, performed at the Swiss Light Source at PSI. The results directly suggest that the γ-hydride is a stable phase precipitated at low temperatures. Testing of irradiated material provided insight into the crack initiation of DHC as a result of the strongly irregular outer surface generated from the oxidation. The propagation pattern is highly irregular compared to the unirradiated material due to the embrittled nature of the cladding from irradiation damage.