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[en] An examination is made of the various parameters acting on the neutron balance: mass, moderation, density, geometry, interactions, absorption and poisoning. The monitoring methods are deduced from this. Using two examples, it is shown how the various parameters act together and in different ways so that it is difficult to predict the results and that it is necessary to have calculation codes well qualified on experience
[fr]On examine les differents parametres agissant sur le bilan neutronique: masse, moderation, densite, geometrie, interactions, absorption et empoisonnement. On en deduit les modes de controle. On montre par deux exemples que les differents parametres agissent ensemble et de facons differentes de sorte qu'il est difficile de prevoir le resultat et qu'il est necessaire d'avoir des codes de calcul bien qualifies sur l'experience
[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] 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] This paper presents the results of an analysis of both published and unpublished critical current data given as a function of both field and temperature. Simple formulas have been obtained for (1) the critical temperature as a function of field that is needed to obtain an estimate of the current sharing temperature and hence temperature margin, (2) the critical current density for constant temperature as a function of field, and (3) the critical current density for constant field as a function of temperature
[en] Technique of critical current density measurement (Jc) of HTc bulk ceramic superconductor has been performed by using linear extrapolation with four-point probes method. The measurement of critical current density HTc bulk ceramic superconductor usually causes damage in contact resistance. In order to decrease this damage factor, we introduce extrapolation method. The extrapolating data show that the critical current density Jc for YBCO (123) and BSCCO (2212) at 77 K are 10,85(6) Amp.cm-2 and 14,46(6) Amp.cm-2, respectively. This technique is easier, simpler, and the use of the current flow is low, so it will not damage the contact resistance of the sample. We expect that the method can give a better solution for bulk superconductor application. Key words. : superconductor, critical temperature, and critical current density