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[en] The era of near-room-temperature superconductivity started after experimental discovery by Drozdov et al (2015 Nature 525 73) who found that compressed H3S exhibits superconducting transition at T c = 203 K. To date, the record near-room-temperature superconductivity stands with another hydrogen-rich highly compressed compound, LaH10 (Somayazulu et al 2019 Phys. Rev. Lett. 122 027001), which has critical temperature of In this paper, we analyse available upper critical field, B c2(T), data for LaH10 (Drozdov et al 2019 Nature 569 528) and report that this compound in all considered scenarios has the ratio of T c to the Fermi temperature, T F, 0.009 < T c/T F < 0.038, which is typical range for unconventional superconductors. In attempt to extend our finding, we examined experimental B c2(T) data for superconductors in the palladium-hydrogen system and surprisingly find that PdHx compounds have the ratio of 0.008 < T c/T F < 0.012. Taking in account that H3S has the ratio of 0.012 < T c/T F < 0.039 (Talantsev 2019 Modern Phys. Lett. B 33 1950195) we come to conclusion that in the Uemura plot all discovered to date hydrogen-rich superconductors, i.e. PdHx, H3S and LaH10, lie in same band as all unconventional superconductors, particularly heavy fermions, fullerenes, pnictides, and cuprates, and former should be classified as a new class of unconventional superconductors. (paper)
[en] This article presents an extensive study of the calculations performed in the configuration where the criticality safety is achieved by both controlling the mass of fissile material and the moderation (for example water) of a single unit. This case often occurs when the control of the fissile mass alone is not sufficient to economically or practically operate a process. This method is often used for the fuel fabrication where an important quantity of powder need to be handled but may also be met in other nuclear facilities and transportation. In this context, from a calculation point of view, a homogeneous distribution of the moderation within the fissile material is generally not a penalizing configuration. So a heterogeneous repartition of the moderation is then considered: it currently consists in a given part of the fissile material uniformly moderated by the entire quantity of the moderator, this system being surrounded by the rest of the dry fissile material and eventually another reflector (water, concrete, lead, etc.). This paper will firstly briefly discuss how to calculate safety limits for this kind of configuration, in the past and nowadays, using state-of-the-art algorithms. Then, new results will be presented for this kind of configuration with various enrichments and densities. These calculations show that the penalizing configuration depends on the maximum dry density of the fissile material considered. The new results obtained with high maximum dry densities are more restrictive than those previously presented.
[en] Iron-based superconductors (IBSs) with ultrahigh upper critical fields and low anisotropies have attracted significant attention in terms of the novel mechanism of superconductivity and high-field applications. A major concern for practical research is the fabrication of long wires with enhanced critical current density and low cost. In this paper, Cu/Ag composited sheathed Ba0.6K0.4Fe2As2 (Ba122) tapes were fabricated through a hot isostatic pressing method, which is feasible for long-wire manufacturing. The Cu/Ag composite sheath can lower the tape cost by reducing the use of expensive Ag. A high-transport critical current density (J c) up to 5.8 × 104 A cm−2 under 10 T at 4.2 K was achieved in our tapes. Evidence has shown that these tapes have pure Ba122 phase, homogeneous element distribution, orientated grains and good grain connectivity. Our work shows that low-cost Cu/Ag-sheathed IBS tapes have great promise for practical applications in the future. (paper)
[en] This article reports the influence of isostatic pressure (from 0.1 MPa to 1.1 GPa), low annealing temperature of 570 ∘C, and annealing time for the formation of high-field pinning centers in 2% C-doped MgB2 wires. Measurements indicate that 1.1 GPa pressure significantly increases the density of high-field pinning centers below 20 K. However, lower pressure (0.6 GPa) slightly increases the density of high-field pinning centers. Increasing the annealing time from 120 to 210 min leads to a reduction of critical temperature (Tc), irreversibility field (Birr), critical current density (Jc), and upper critical field (Bc2), suggesting that a long annealing time leads to a reduction of high-field pinning center density and the number of connections between superconducting grains. The high pressures and low annealing temperature lead to a high critical current density of 1000 A/mm2 in 7.2 T and 100 A/mm2 in 12.5 T at 4.2 K in MgB2 wires.
[en] Nowadays, in situ- and ex situ-processed MgB2 superconducting wires are commercially available. A premix process is a hybrid of the in situ and ex situ processes, in which a mixture of magnesium, boron, and MgB2 powders is used as a precursor. Hence, the premix process should cause less trouble in the manufacturing of practical wires. In this study, to clarify the potential regarding the critical current density of the premix process, the mono-filamentary MgB2 wires are prepared at premix ratios, x p, of 0.25, 0.50, and 0.75 by drawings with a large total area reduction ratio of 99.92%, and their electromagnetic properties and microstructures are investigated. Here, x p is defined as the mass ratio of MgB2 to the total mixture; that is, the composition of the mixture is Mg: B: MgB2 = 1 − x p: 2(1 − x p): x p on a molar basis. Although a high MgB2 packing factor is obtained by the premix process, it does not lead to an increase in electrical connectivity of MgB2 filaments because of the poor sinterability of the premixed MgB2 powder. On the other hand, strong electron scattering, which is introduced to the premixed MgB2 particles in the pulverisation process, leads to the enhancement of the upper critical field in the low temperature region. As a result, the critical current density in a low temperature and high magnetic field region is improved in wires prepared at x p = 0.50. (paper)
[en] The purpose of this paper is to discuss the application of Sandia National Laboratories' (Sandia's) generic standard set of ICSBEP benchmarks, the set of benchmarks shipped with the Whisper-1.1 software (LANL), and the combined set of benchmarks in support of criticality calculations with MCNP-6.2. Sensitivity and uncertainty (S/U) analysis was performed with Whisper-1.1 software to determine similarity between the benchmarks and the computational models of spherical fissile metal-water mixtures that are formulated to compose the critical mass curves of 235U and 239Pu. Relationships between the chosen benchmark set and the observed results from the Whisper S/U software are discussed, including discussions of the correlation coefficient, ck, software weighting factors, and the upper subcritical limit (USL) of the effective neutron multiplication factor, keff. The results indicate that all studied benchmark suites are sufficient to support reasonable criticality limits along the critical mass curves for 235U and 239Pu. The Sandia benchmarks are strongly correlated to the models that compose the critical mass curves in most of the thermal range and near the fast/metal region for both nuclides. As expected, the number of benchmarks with very high similarity decreases near the minimum concentration necessary to achieve criticality and in the very poorly moderated region. This trend is shown for 235U in Figures 3-5, with the contribution from various benchmark system types detailed. To improve similarity in regions that have lower values of ck (e.g., <0.9), a greater number of applicable benchmarks is required. Figure 6 shows the average ck of Whisper-1.1 benchmarks when using the generic set of Sandia ICSBEP benchmarks, the ICSBEP benchmarks shipped with Whisper-1.1 (LANL), and a combined suite of benchmarks from both sources, with and without specific benchmarks that fail statistical tests (e.g., chi-squared) excluded for both 235U and 239Pu curves. Figure 6 shows the incremental improvements in average ck that occur when benchmark suites are combined. These improvements are most noticeable in regions with lower values of ck. While ck improves with the addition of more benchmarks, the Whisper-calculated USL value does not necessarily improve, as shown in Figure 7 for 235U and Figure 8 for 239Pu. In addition to providing the Whisper-calculated USL for different benchmark suites, Figure 8 highlights which individual benchmarks have the largest effect on the calculated baseline USL. A noticeable increase in USL is observed when these highlighted benchmarks are removed from the suite. (authors)
[en] Due to their high upper critical field (H c2) and small anisotropy, iron-based superconductors have a great potential in high magnetic field applications. In this study, Cu/Ag composite sheathed (Ba,K)Fe2As2 (Ba-122) superconducting tapes were fabricated by an ex-situ powder-in-tube method. Moreover, an ambient pressure sintering process was adopted aiming at reducing the manufacturing cost and allowing for mass production. However, fabricating Cu/Ag composite sheathed Ba-122 tapes with a large transport J c by an ambient pressure producing process is fairly challenging due to the absence of the over pressure. By optimizing sintering temperatures, sintering times and the composition of the raw materials, a transport J c as high as 36 000 A cm−2 (4.2 K, 10 T) was obtained in tapes sintered at 740 °C for 3 h. The mechanism of the J c improvement was systematically analyzed. This result is of great significance for the fabrication of low cost and long-length practical iron-based superconducting tapes. (paper)
[en] The purpose of this paper is to present and discuss the methodology, results, and conclusions of an analysis of spherical, homogenous metal-water mixtures in MCNP-6.2 with ENDF/BVII.1 data. The parametric analysis reproduced curves of critical fissile mass as a function of metal-water mixture density (i.e., concentration, H/X) that have been published in criticality safety handbooks. Curves are provided for highly-enriched uranium (HEU) and plutonium-239. Furthermore, the sensitivity of the effective neutron multiplication factor to changes in mass or cross-section data is plotted and discussed. Further evaluation of these curves and the data used to generate them reveals that the reactivity worth of incremental unit of mass in systems that are critical or nearly critical is worth less than the average reactivity worth per unit of existing mass in the system. This also means that, in order to obtain a given margin in keff (e.g., 5% or a USL of 0.95), a greater reduction in the corresponding mass limit must be made (e.g., a ∼25% reduction from the critical mass value)
[en] Highlights: • Manuscript derived from an ASME V&V Workshop paper (2018). • ‘C’ implies a substantial change of current V&V. • All available knowledge shall be part of ‘C’. • ‘C’ may require huge Investments. - Abstract: V&V constitutes a powerful framework to demonstrate the capability of computational tools in several technological areas. Passing V&V requirements is a needed step before applications. Let's focus hereafter to the area of (transient) Nuclear Thermal-hydraulic (NTH) and let's identify V1 and V2 as acronyms for Verification and Validation, respectively. V1 is performed within NTH according to the best available techniques and may not suffer of in-principle deficiencies. This is not the case of V2; understood limits are: 1) Validation implies comparison with experimental data: available experimental data cover a (very) small fraction of the parameter range space expected in applications of the codes; this can be easily seen if one considers data in large diameter pipe, high velocity and high pressure or high power and power density; scaling issue must be addressed which may result in controversial findings. 2) Water is at the center of the attention: the physical properties of water are known to a reasonable extent as well as large variations in values of quantities like density or various derivatives are expected within the range of variation of pressure inside application fields. Although not needed for current validation purposes (e.g. validation ranges may not include a situation of critical pressure and large heat flux) physically inconsistent values predicted by empirical correlations outside validation ranges, shall not be tolerated. 3) Occurrence of complex situations like transition from two-phase critical flow to ‘Bernoulli-flow’ (e.g. towards the end of blow-down) and from film boiling to nucleate boiling, possibly crossing the minimum film boiling temperature (e.g. during reflood). Therefore, whatever can be mentioned as classical V2 is not or cannot be performed in NTH. So the idea of the present paper is to add a component to the V&V. This component, or step in the process, is called ‘Consistency with Reality’, or with the expected phenomenological evidence. The new component may need to be characterized in some cases and is indicated by the letter ‘C’. Then, the V&V becomes V&V&C. The motivations at the bases of the V&V&C are clarified in the paper.
[en] Superconductivity in single crystalline Mo3P has been investigated by the measurement of electrical resistivity, magnetic susceptibility, and specific heat capacity. Its space group is I 4-2 m with lattice parameters a = b = 9.808 4 Å and c = 4.838 2 Å. A bulk superconductivity with a critical temperature of T C = 5.6 ± 0.1 K, an upper critical field of = 13.7 ± 0.1 kOe, and a lower critical field of = 190 ± 2 Oe for ρ ab were confirmed. A large anisotropic behavior was discovered in the resistivity and upper critical fields. A clear upward curvature in H c2 was observed, suggesting a multiband superconductivity in Mo3P. The value of the Ginzburg–Landau parameter κ GL indicates that Mo3P is a type-II superconductor. ΔC(T c)/γ T c = 1.50 and 2Δ/k B T c = 3.48 from the specific heat analyses indicate that Mo3P is a weak-coupling Bardeen–Cooper–Schrieffer superconductor with fully gapped superconductivity. (paper)