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[en] The physics parameters of a bare sphere of metallic 237Np have been investigated. The calculations indicate that a chain reaction can be sustained with fast neutrons. The predicted critical radius is 8.86 cm, and the corresponding critical mass is 59.7 kg. Kinetic parameters such as the prompt neutron lifetime, l, the prompt neutron decay constant at delayed criticality, α/sub c/, and the effective delayed neutron fraction, β/sub eff/, were also calculated. With respect to the last quantity, the phenomenon of the so-called vanishing dollar in systems consisting of even-neutron nuclei is discussed. Some remarks concern the possible utilization of 237Np, which is produced in nuclear power reactors as a by-product
[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] The critical mass of the reactor RP-0 of the Peruvian Institute of Nuclear Energy is determined by using the Fermi model of diffusion and age in connection with an experimental subcritical neutron population. A desk computer HP9825 is used with a program in HP4. The observed discrepancy between the calculated and the measured mass is 5%
[en] The results of various conventional tests with this 1000kW experimental reactor (KSTR) are shown in diagrams. The comparatively high negative temperature coefficient of the reactivity was found to be a great advantage. Some inexplicable temperature differences and temperature changes in the reactor vessel were found. (J.S.)
[en] Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O, F) several families of superconductors based on Fe layers (1111, 122, 11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length and unconventional pairing. On the other hand, the Fe-based superconductors have metallic parent compounds and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, and the supposed order parameter symmetry is s-wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviors and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest Tc, huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates. On the other hand, the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the Tc of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, and intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families.
[en] We consider the superconducting transition in fermionic quantum critical systems. Assuming the validity of Migdal theorem, the gap equation can be written in terms of the retarded pair susceptibility. Instead of the usual BCS form, the pair susceptibility is now subject to scale invariance. The gap and transition temperature is thus of the algebraic form, totally different from the exponential behavior in BCS theory. Consequently, with reasonably small glue strength, we can get very large gap and transition temperature comparable to those discovered in cuprates. The ratio of the gap to retardation gets boosted by increasing retardation. We also find the upper critical field has a different scaling with the critical temperature. With a non-Lorentzian dynamical exponent, the upper critical field is greatly enhanced when approaching the critical point, though the critical temperature only changes modestly, in agreement with recent experiments on heavy fermions.
[en] Three-group diffusion theory has been applied to a computation of the 231Pa mass required to sustain a fast-neutron chain reaction. The method was tested by preliminary calculation of several 235U--238U systems and of a single 239Pu--238Pu system. The best agreement between predictions and measurements was found for high-enrichment 235U--238U systems. In the case of 231Pa, where many of the necessary data do not exist, use was made of the characteristics of 237Np as a substitute. The predicted critical radius for 231Pa is 22.67 +- 1.81 cm, and the corresponding critical mass is 750 +- 180 kg
[en] The preliminary results of development of a new heat transfer correlation for supercritical water flowing in vertical heated tubes have been reported. It has been shown through a comparison with experimental data that the suggested method of allowance for variations of water thermophysical properties near the critical point provides a satisfactory agreement of the predicted and experimental results
[ru]Подведены предварительные итоги исследования, основной целью которого является получение нового уравнения для инженерных расчетов теплоотдачи от поверхности обогреваемых вертикальных труб при движении в них воды сверхкритического давления. Путем сравнения с большим массивом экспериментальных данных показано, что предлагаемый метод учета сильного изменения теплофизических свойств теплоносителя вблизи критической точки обеспечивает удовлетворительное согласование расчетных и экспериментальных результатов