Considerable effort is being devoted to development and regulatory qualification of low enriched fuels for research and test reactors by many agencies worldwide. One promising fuel configuration ...being examined for United States higher power research and test reactors (USHPRRs) are plate-type fuels composed of a metallic uranium-molybdenum foil clad in an aluminum alloy. The two pieces of aluminum alloy cladding are bonded using a hot isostatic pressing method. The mechanical properties of the resulting bond line in the aluminum alloy cladding will vary by the HIP'ing parameters, requiring a need to characterize the bond line. Small scale mechanical testing can provide a path for evaluating the mechanical properties and deformation behavior of the bond line both prior to and following irradiation. In this research, room temperature micro-tensile specimens of non-irradiated and irradiated samples containing an Al alloy (AA 6061) bond line were tested to evaluate its strength and deformation behavior. Observations indicated that the strain rate did not affect the deformation behavior or strength and most of the micro-tensile specimens failed in a ductile mode in grains around the bond line. There was no indication that the microstructural features from the bond line affected the mechanical properties of the micro-tensile specimens. An initial examination was performed on irradiated material but further systematic studies of the effects of irradiation can be performed in the future.
The objective of this paper is to analyze the effect of different heat treatments on the fuel microstructure and texture in laboratory scale monolithic U-Mo fuel plates. Such analyses are relevant as ...they could possibly inform future fabrication methods for the optimization of microstructure and of material properties to ultimately improve fuel performance during irradiation. For this reason, detail characterization including Energy x-ray dispersive spectroscopy (EDS) and electron backscattered diffraction (EBSD) techniques were applied on U-Mo plates fabricated with different heat treatments to understand the influence of fabrication on fuel microstructure. U-Mo fresh fuel specimens with or without annealing treatments (homogenization and stress release annealing) after rolling were analyzed in this work. In this study for the first time EBSD was systematically used to analyze the heat treatments influence on texture present in fresh fuel monolithic U-Mo fuel plates. Such changes were connected to other known microstructural and chemical changes. In this work minimization of molybdenum concentration variation and gamma phase decomposition of the U-Mo fuel core after homogenization was observed. Also, an increase in the interaction layer of the U-Mo fuel core with the Zr interlayer diffusion barrier was observed with the UZr2 growing up to 1 µm in thickness. Such microstructural changes were aligned to the changes in texture and grain structure. Indeed, common texture observed in body-centered cubic (BCC) metal after cold rolling (e.g., α, ξ, γ fibers) were minimized by the annealing/homogenization process. Moreover, grain structure was influenced by the fabrication route with elongated grains formed in the rolled sample and equiaxed grains found after annealing. Such features are relevant since the formation of microstructural features formed during fuel fabrication can impact fuel performance in reactor.
Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and focused ion beam (FIB) milling were performed on an irradiated U–10Mo monolithic fuel to understand its irradiation ...microstructure. This is the first reported TEM work of an irradiated fuel sample prepared using a FIB. Advantages and disadvantages of using the FIB to create TEM samples from this irradiated fuel will be presented along with some results from the work. Sample preparation techniques used to create SEM and FIB samples from the brittle irradiated monolithic sample will also be discussed.
This paper will discuss the results of transmission electron microscopy (TEM) analysis performed on two samples taken from the low-flux and high-flux sides of the same fuel plate with U–7Mo fuel ...particles dispersed in Al–2Si matrix. The corresponding local fission density of the fuel particles and the peak fuel-plate centerline temperature between the low-flux and high-flux samples are 3.32×1027f/m3 and 90°C, and 6.31×1027f/m3 and 120°C, respectively. The results showed the presence of a bubble superlattice within the U–7Mo grains that accommodated fission gases (e.g., Xe). The presence of this structure helps the U–7Mo exhibit a stable swelling behavior during irradiation. The change in bubble distribution at the high fission density suggests that the bubble superlattice is stable as the U–7Mo matrix remains crystalline. It appears that there is a threshold Si content in the fuel particle above which the U–Mo turns to amorphous under irradiation. The threshold Si content is approximately 8at.% and 4at.% for low-flux and high-flux conditions, respectively.
Purpose
Past studies have paid extensive attention to investigate learning issues in individual, organizational and team contexts. Learning activities in the workplace, however, often occur in the ...interdisciplinary dyadic context. Hence, the purpose of this paper is to clarify what factors make interdisciplinary dyads lead to better learning effects. The authors attempted to clarify two major agendas: What knowledge factors (intellectual capitals) owned by the parties of the dyads can induce better learning effects? What contextual factors (learning tasks) can make better learning effects during the dyadic learning process?
Design/methodology/approach
To examine the previous agendas, the authors first conducted in-depth interviews and an exploratory survey so that a four-element dyadic intellectual capital (DIC) architecture was circumscribed: knowledge interdependency, expertise similarity, collaborative routines and mutual trust. Dyadic learning tasks were classified as exploitative and exploratory learning. The authors then sampled 248 respondents for the formal empirical survey to examine the relationship between DIC, dyadic learning tasks and knowledge sharing.
Findings
The statistical evidences confirmed the positive relationships between DIC and knowledge sharing, and the results also signified the previous relationships with a mediating effect from exploratory learning, while the effect of exploitative learning was not supported. Thus, only dyads (the two heads) with knowledge interdependency and mutual trust can make the exploratory task better than that of one individual (the one head).
Originality/value
This study provides a new insight into the learning issue with an interdisciplinary dyadic perspective to supplement the existing gap between academic efforts and learning practices in the workplace.
•Microstructural evolution of irradiated monolithic U-Mo fuel plate was analyzed.•BCC rolled texture was developed after fabrication.•After irradiation, a loss of the induced texture was ...observed.•Evidence for polygonization as the grain subdivision mechanism was detected.
This work describes the microstructural evolution of prototypical monolithic U-Mo fuel plates analyzed via scanning electron microscopy and electron backscattered diffraction (EBSD). The understanding of the microstructural and textural evolution of nuclear fuel from as-fabricated to post-irradiation is important in assessing changes in material properties during irradiation. In our work it was observed that the typical fabrication techniques applied in U-10Mo monolithic fuel plates develop features associated with a cold-rolled body-centered cubic (bcc) texture and development of α and γ fiber (parallel to the rolling and normal direction). After irradiation, a loss of the fabrication-induced preferred orientation was observed with an increased spread of grain-boundary misorientation as burnup increases. Grain subdivision was observed in the irradiated samples with the formation of submicron grains (200–500 nm). Evidence for polygonization as the mechanism leading to grain subdivision was detected. This has been observed for the first time for U-Mo monolithic fuel via EBSD and has been associated to formation of low-angle grain boundaries (<15 degrees) at the site of the submicron grains. Such analyses of the microstructural and textural evolution of fuel (from fabrication to after irradiation) have the potential to help develop and validate microstructural physics-based models and provide a key feedback loop to further understand the interplay between fabrication processes and fuel performance.
Monolithic U-10Mo alloy fuel plates with Al-6061 cladding are being developed for use in research and test reactors as low enrichment fuel (<20% U-235 enrichment) as a result of its high uranium ...loading capacity compared to that of U-7Mo dispersion fuel. These fuel plates contain a Zr diffusion barrier between the U-10Mo fuel and Al-6061 cladding that suppresses the interaction between the U-Mo fuel foil and Al alloy cladding that is known to be problematic under irradiation. Different methods have been employed to fabricate monolithic fuel plates, including hot-rolling with no cold-rolling. L1P09T is a hot-rolled fuel plate irradiated to high fission density in the RERTR-9B experiment. This paper discusses the TEM characterization results for this U-10Mo/Zr/Al6061 monolithic fuel plate (∼59% U-235 enrichment) irradiated in Advanced Test Reactor at Idaho National Laboratory with an unprecedented high local fission density of 9.8E+21 fissions/cm3. The calculated fuel foil centerline temperature at the beginning of life and the end of life is 141 and 194 °C, respectively. TEM lamellas were prepared using focus ion beam lift-out technique. The estimated U-Mo fuel swelling, based on the fuel foil thickness change from SEM, is approximately 76%. Large bubbles (>1 μm) are distributed evenly in U-Mo and interlink of these bubbles is evident. The average size of subdivided grains at this fission density appears similar to that at 5.2E+21 fissions/cm3. The measured average Mo and Zr content in the fuel matrix is ∼30 at% and ∼7 at%, respectively, in general agreement with the calculated Mo and Zr from fission density.
Phase constituents and microstructure changes i n RERTR fuel plate assemblies as functions of temperature and duration of hot-isostatic pressing (HIP) during fabrication were examined. The HIP ...process was carried out as functions of temperature (520, 540, 560 and 580 degreesC for 90 min) and time (45-345 min at 560 degreesC) to bond 6061 Al-alloy to the Zr diffusion barrier that had been co-rolled with U-10 wt.% Mo (U10Mo) fuel monolith prior to the HIP process. Scanning and transmission electron microscopies were employed to examine the phase constituents, microstructure and layer thickness of interaction products from interdiffusion At the interface between the U10Mo and Zr, following the co-rolling, the UZr sub(2) phase was observed to develop adjacent to Zr, and the alpha-U phase was found between the UZr sub(2) and U10Mo, while the Mo sub(2)Zr was found as precipitates mostly within the alpha-U phase. The phase constituents and thickness of the interaction layer at the U10Mo-Zr interface remained unchanged regardless of HIP processing variation. Observable growth due to HIP was only observed for the (Al, Si) sub(3)Zr phase found at the Zr/AA6061 interface, however, with a large activation energy of 457 + or - 28 kj/mole. Thus, HIP can be carried to improve the adhesion quality of fuel plate without concern for the excessive growth of the interaction layer, particularly at the U10Mo-Zr interface with the alpha-U, Mo sub(2)Zr, and UZr sub(2) phases.
Hematopoietic stem cells (HSCs) sustain long-term reconstitution of hematopoiesis in transplantation recipients, yet their role in the endogenous steady-state hematopoiesis remains unclear. In ...particular, recent studies suggested that HSCs provide a relatively minor contribution to immune cell development in adults. We directed transgene expression in a fraction of HSCs that maintained reconstituting activity during serial transplantations. Inducible genetic labeling showed that transgene-expressing HSCs gave rise to other phenotypic HSCs, confirming their top position in the differentiation hierarchy. The labeled HSCs rapidly contributed to committed progenitors of all lineages and to mature myeloid cells and lymphocytes, but not to B-1a cells or tissue macrophages. Importantly, labeled HSCs gave rise to more than two-thirds of all myeloid cells and platelets in adult mice, and this contribution could be accelerated by an induced interferon response. Thus, classically defined HSCs maintain immune cell development in the steady state and during systemic cytokine responses.
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•A subset of self-renewing HSCs in adult bone marrow was genetically marked•HSCs provide a major sustained contribution to endogenous hematopoiesis•HSCs give rise to all immune cell types except B-1a cells and tissue macrophages•Multilineage contribution of HSCs is accelerated by interferon response
The role of hematopoietic stem cells (HSCs) in steady-state hematopoiesis remains controversial. Here, Sawai et al. use lineage tracing to reveal a major contribution of HSCs to all blood cell lineages, including myeloid cells and lymphocytes, throughout adult life.
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