1000 resultados para Voisinage par cycles


Relevância:

20.00% 20.00%

Publicador:

Resumo:

The proliferation potential of the present light water reactor (LWR) fuel cycle is related primarily to the quantity and the quality of the residual Pu contained in the spent-fuel stockpile, although other potentially “weapons usable” materials are also a concern. Thorium-based nuclear fuel produces much smaller amounts of Pu in comparison with standard LWR fuel, and consequently, it is more proliferation resistant than conventional slightly enriched all-U fuel; the long-term toxicity of the spent-fuel stockpile is also reduced

Relevância:

20.00% 20.00%

Publicador:

Resumo:

Nowadays nuclear is the only greenhouse-free source that can appreciably respond to the increasing worldwide energy demand. The use of Thorium in the nuclear energy production may offer some advantages to accomplish this task. Extensive R&D on the thorium fuel cycle has been conducted in many countries around the world. Starting from the current nuclear waste policy, the EU-PUMA project focuses on the potential benefits of using the HTR core as a Pu/MA transmuter. In this paper the following aspects have been analysed: (1) the state-of-the-art of the studies on the use of Th in different reactors, (2) the use of Th in HTRs, with a particular emphasis on Th-Pu fuel cycles, (3) an original assessment of Th-Pu fuel cycles in HTR. Some aspects related to Thorium exploitation were outlined, particularly its suitability for working in pebble-bed HTR in a Th-Pu fuel cycle. The influence of the Th/Pu weight fraction at BOC in a typical HTR pebble was analysed as far as the reactivity trend versus burn-up, the energy produced per Pu mass, and the Pu isotopic composition at EOC are concerned. Although deeper investigations need to be performed in order to draw final conclusions, it is possible to state that some optimized Th percentage in the initial Pu/Th fuel could be suggested on the basis of the aim we are trying to reach. Copyright © 2009 Guido Mazzini et al.

Relevância:

20.00% 20.00%

Publicador:

Resumo:

Inflatable aerodynamic decelerators present potential advantages for planetary entry in missions of robotic and human exploration. The design of these structures face many engineering challenges, including complex deformable geometries, anisotropic material response, and coupled shockturbulence interactions. In this paper, we describe a comprehensive computational fluid-structure interaction study of an inflation cycle of a tension cone decelerator in supersonic flow and compare the simulations with earlier published experimental results. The aeroshell design and flow conditions closely match recent experiments conducted at Mach 2.5. The structural model is a 16-sided polygonal tension cone with seams between each segment. The computational model utilizes adaptive mesh refinement, large-eddy simulation, and shell mechanics with self-contact modeling to represent the flow and structure interaction. This study focuses on the dynamics of the structure as the inflation pressure varies gradually, and the behavior of forces experienced by the flexible and rigid (the payload capsule) structures. © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Relevância:

20.00% 20.00%

Publicador:

Resumo:

1. The importance of vertical mixing in modulating the impact of UVR on phytoplankton photosynthesis was assessed in a tropical, shallow lake in southern China from late winter to mid-spring of 2005. 2. Daily cycles of fluorescence measurements (i.e. photosynthetic quantum yield, Y) were performed on both 'static' and in situ samples. Static samples were of surface water incubated at the surface of the lake under three radiation treatments - PAB (PAR + UVR, 280-700 nm), PA (PAR + UV-A, 320-700 nm) and P (PAR, 400-700 nm). In situ samples were collected every hour at three different depths - 0, 0.5 and 1 m. 3. The general daily pattern was of a significant decrease in Y from early morning towards noon, with partial recovery in the afternoon. Samples incubated under static conditions always had lower Y than those under in situ conditions at the same time of the day. 4. Under stratified conditions, no overall impact of UVR impact could be detected in situ when compared with the static samples. Further rapid vertical mixing not only counteracted the impact of UVR but also stimulated photosynthetic efficiency. 5. Based on these measurements of fluorescence, the mixing speed of cells moving within the epilimnion was estimated to range between 0.53 and 6.5 cm min(-1). 6. These data show that mixing is very important in modulating the photosynthetic response of phytoplankton exposed to natural radiation and, hence, strongly conditions the overall impact of UVR on aquatic ecosystems.

Relevância:

20.00% 20.00%

Publicador:

Resumo:

Studies of Erebus volcano's active lava lake have shown that many of its observable properties (gas composition, surface motion and radiant heat output) exhibit cyclic behaviour with a period of ~10 min. We investigate the multi-year progression of the cycles in surface motion of the lake using an extended (but intermittent) dataset of thermal infrared images collected by the Mount Erebus Volcano Observatory between 2004 and 2011. Cycles with a period of ~5-18 min are found to be a persistent feature of the lake's behaviour and no obvious long-term change is observed despite variations in lake level and surface area. The times at which gas bubbles arrive at the lake's surface are found to be random with respect to the phase of the motion cycles, suggesting that the remarkable behaviour of the lake is governed by magma exchange rather than an intermittent flux of gases from the underlying magma reservoir. © 2014 The Authors.

Relevância:

20.00% 20.00%

Publicador:

Resumo:

The compensatory responses of juvenile gibel carp and Chinese longsnout catfish to four cycles of 1 part of a study designed to determine feeding regimes that would maximise growth rates. Both species showed compensatory growth in the re-feeding periods. The compensation was not sufficient for the deprived fish to match the growth trajectories of controls fed to satiation daily. The compensatory growth response was more clearly defined in the later cycles. The deprived fish showed hyperphagia during the 2-week periods of re-feeding and the hyperphagic response was clearer in the later cycles. The hyperphagia tended to persist for both weeks of the re-feeding period. The gibel carp showed no difference in gross growth efficiency between deprived and control fish. In the catfish, the gross growth efficiency of the deprived fish was marginally higher than that of control fish, but the efficiency varied erratically from week to week. Over the experiment, the deprived fish achieved growth rates 75-80% of those shown by control fish, although fed at a frequency of 66%. There was no evidence of growth over-compensation with the deprivation-re-feeding protocol used in this study. (C) 2004 Elsevier B.V. All rights reserved.

Relevância:

20.00% 20.00%

Publicador:

Resumo:

Individual juvenile three-spined sticklebacks Gasterosteus aculeatus and European minnow Phoxinus phoxinus, from sympatric populations, were subjected to four cycles of I week of food deprivation and 2 weeks of ad libitum feeding. Mean specific growth rate during the weeks of deprivation was negative and did not differ between species. The three-spined stickleback showed sufficient growth compensation to recover to the growth trajectory shown by control fish daily fed ad libitum. The compensation was generated by hyperphagia during the re-feeding periods, and in the last two periods of re-feeding, the gross growth efficiencies of deprived three-spined sticklebacks were greater than in control fish. The expression of the compensatory changes in growth and food consumption became clearer over the successive periods of re-feeding. The European minnow developed only a weak compensatory growth response and the mass trajectory of the deprived fish deviated more and more from the control trajectory During re-feeding periods, there were no significant differences in food consumption or gross growth efficiency between control and deprived European minnows. The differences between the two species are discussed in terms of the possible costs of compensatory growth, the control of growth and differences in feeding biology (C) 2003 The Fisheries Society of the British Isles.

Relevância:

20.00% 20.00%

Publicador:

Resumo:

算法被誉为计算的“灵魂”(spirit of computing),算法程序的可靠性和正确性对软件系统的可信度起着至关重要的作用。动态规划、贪心、分支限界等传统的算法设计策略缺乏有效的选择标准,一些算法形式化开发方法和工具的应用水平和范围也很有限,因此算法的可靠性已成为高可信软件系统的一个主要瓶颈。 作为军事运筹学的一个重要分支,装备保障计算主要研究军事活动中装备领域的决策优化问题,保障计算软件特别是算法的可靠性在很大程度上决定了装备保障的效率乃至军事行动的成败。利用形式化软件工程的研究成果、特别是引入算法程序的形式化开发方法,对我军通用装备保障领域中的大量算法类问题进行有效求解,有利于优化保障结构、统筹保障资源、整合保障力量、提高保障时效,促进装备整体战斗力水平的提升。 本文以高可信软件开发方法PAR为基础,面向装备保障计算的领域需求,提出了一类离散最优化问题(discrete optimization problem, DOP)的结构模型和算法推演技术,并成功推演了一系列典型的装备保障算法。本论文的主要创新性贡献如下: (1)定义了DOP的结构模型,提出了基于单点结构(Singleton)的问题分划递推策略,进而通过PAR算法推演生成问题的高效求解算法,涵盖了多种传统算法设计策略,显著提高了算法程序设计的机械化水平。 (2)提出了描述DOP分划递推过程的问题简约图(problem reduction graph, PRG)模型,并针对典型DOP结构推演得到了一组PRG构造算法模式,其中每个算法模式都涵盖了满足特定代数结构性质的一大类具体问题。 (3)阐述了PAR算法推演的范畴模型,为算法设计和重用提供了抽象而有效的方法指导。 (4)对我军通用装备保障领域具有代表性的48个问题进行了形式化的算法推演,构建了领域算法库。 (5)在PAR平台的基础上设计了装备保障算法开发平台的原型COPALM,支持DOP算法推演和重用,进而提高了相关应用软件的开发效率。

Relevância:

20.00% 20.00%

Publicador:

Resumo:

形式化方法是构建可信软件的重要途径.基于对算法问题的分析,针对形式化方法PAR开发算法的特征,刻划了问题分划、递推关系构造方面的规律.从一类问题的形式化功能规约出发,可机械地完成问题的分划及规约的变换,自然地揭示出求解问题的算法思想,在相关工具的支持下自动生成算法程序.研究结果将算法设计中尽可能多的创造性劳动转化为非创造性劳动,降低了形式化求解算法问题的难度,提高了算法程序的可靠性和形式化开发效率.

Relevância:

20.00% 20.00%

Publicador:

Resumo:

范畴论对理解程序规约及程序设计和正确性证明十分有用.PAR方法则是建立在严格的数学基础之上的一种统一的算法程序设计方法.循环不变式在循环算法程序的设计中至关重要.使用格理论和范畴论作为工具对PAR方法建立一个理论框架,并对其用范畴论的概念加以解释,从而使得PAR有更强的理论基础.在此基础上引入不动点原理深入刻划循环不变式的含义,循环不变式可以表示为谓词泛函的最小不动点,并从范畴论的角度解释该过程.

Relevância:

20.00% 20.00%

Publicador:

Resumo:

The objectives of this paper are to study the thermodynamic cycles in an inertance tube pulse tube refrigerator (ITPTR) by means of CFD method The simulation results show that gas parcels working in different parts of ITPTR undergo different thermodynamic cycles The net effects of those thermodynamic cycles are pumping heat from the low temperature part to the high temperature part of the system The simulation results also show that under different frequencies of piston movement the gas parcels working in the same part of the system will undergo the same type of thermodynamic cycles The simulated thermal cycles are compared with those thermodynamic analysis results from a reference Comparisons show that both CFD simulations and theoretical analysis predict the same type of thermal cycles at the same location However only CFD simulation can give the quantitative results while the thermodynamic analysis is still remaining in quality (C) 2010 Elsevier Ltd All rights reserved

Relevância:

20.00% 20.00%

Publicador:

Resumo:

IEECAS SKLLQG