Heat removal system for shutdown in nuclear thermal rockets and advanced concepts

© 2016 by the American Institute of Aeronautics and Astronautics, Inc.It is well-known that a nuclear thermal rocket (NTR) cannot be abruptly shut down. After a power manoeuver, the reactor has contaminated itself with fission products and the decay heat released must be removed by maintaining an adequate flow of hydrogen through its passages. The objective of this work was to derive a first estimate of how much hydrogen will be needed to prevent the core from overheating after shutdown, and, from this, be able to assess the advantages of using a dedicated decay heat removal system to reduce or eliminate the amount of hydrogen needed to prevent the core from overheating after shutdown. Furthermore, the use of such a heat removal system could be needed by certain special nuclear thermal propulsion concepts, such as the fission fragment rocket or the more recently proposed pulsed nuclear thermal rocket, where significant amplification of specific impulse, Isp, as well as thrust can be obtained by the direct use of fission fragments or by pulsing the nuclear core, respectively.Peer ReviewedPostprint (author's final draft

The behavior of radiogenic particles at solidification fronts

© 2016. This version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/The thermal behavior of insoluble radiogenic particles at the solid-liquid interface of an advancing solidification front and its significance with regard to environmental impact are discussed. It is shown that, unlike classical particles, where the most probable behavior is engulfing by the solidification front, radiogenic particles are more likely to be rejected by the solidification front. Utilizing a simplified physical model, an adaptation of classical theoretical models is performed, where it is shown that, unlike classical particles, for radiogenic particles the mechanism is thermally driven. An analytical expression for the critical velocity of the solidification front for engulfing/rejection to occur is derived. The study could be potentially important to several fields, e.g. in engineering applications where technological processes for the physical removal of radionuclide particles dispersed throughout another substance by inducing solidification could be envisaged, in planetary science where the occurrence of radiogenic concentration could result in the possibility of the eruption of primordial comet/planetoids, or, if specific conditions are suitable, particle ejection may result in an increase in concentration as the front moves, which can translate into the formation of hot spots.Peer ReviewedPostprint (author's final draft

A neutron diode for subcritical multistage multipliers with special reference in tritium breeding

This is a copy of the author 's final draft version of an article published in the Journal of fusion energy. The final publication is available at Springer via http://dx.doi.org/10.1007/s10894-015-0049-7In this paper the interaction between a magnetic field and the neutron spin magnetic moment is explored for use in the design of a neutron diode or valve that allows a neutron flux to pass in one direction, while preventing a neutron flux in the opposite direction. A neutron diode that ensures the unidirectional movement of neutrons could be used in the design of a subcritical multistage neutron multiplier, a device that has thus far not been realised. With a subcritical multistage neutron multiplier, an initial source of neutrons could be multiplied substantially in a very small area. Such a device could have potential applications in tritium breeding in a fusion reactor, in medicine, in space exploration, etc. Utilizing a simplified geometrical model, a first preliminary study is performed to assess the feasibility of this concept.Peer ReviewedPostprint (author's final draft

A linear mass spectrometer by induced Hall potential for electromagnetic isotopic separation working at high pressures

© 2016. This version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/In this paper a novel alternative for bulk electromagnetic separation working at high pressures is proposed. It is shown that if a self-induced Hall potential is stimulated in the boundaries, the system will be able to take advantage of the collisions process, boosting the isotopic separation and resulting in a linear-spectrometer with a higher spatial separation per unit length than a traditional calutron. Although originally the concept was devised for the production of medical isotopes where the minority isotope to be separated is produced by neutron capture and is the heavier isotope, if the Hall potential is replaced by an external electrical field, the concept is equally applicable for situations where the minority isotope is the lighter one, as for example in the enrichment of uranium. Additional R&D is required to explore further the possibilities of this concept and to identify optimal values for several of the system design variables.Peer ReviewedPostprint (author's final draft

Guido Bonarelli, tireless explorer and geologist: Pioneer in the prospection of hydrocarbons in Argentina

Guido bonarelli is an important figure in the field of geology in Argentina and he constituted one of the most relevant geologists in the oil exploration in our country. of Italian origin, he was born in Ancona on July 25, 1871 and he died in montereano, January 11, 1951. He excelled in stratigraphy, geological survey, historical geology and paleontology, becoming a specialist in the taxonomy of Jurassic ammonites and the stratigraphy of the Central Apennines. His professional work began in Italy and continued in southeast Asia, south America, bolivia, Chile, Argentina and uruguay. During 1911-1918, he worked in the Dirección General de minas, Geología e Hidrografía of Argentina. He defined the sub-Andean ranges as an orographic complex and he pointed out the locations of oil and natural gas reservoirs. He developed his researches in several Argentinean provinces and in the pacific Chilean coast. During his second period of research in Argentina (1923 -1927), Guido bonarelli organized the Geological Division of YpF, and he intensified the oil exploration in the northwestern of Argentina, as well as in the provinces of mendoza, neuquén and the Comodoro rivadavia region. He developed a professional and academic team that dealt with the geology and the hydrocarbon exploration in the country. bonarelli discovered in 1891 a sedimentary horizon that has great importance and it serves as a baseline to characterize the global anoxic event known to the Cenomanian-turonian transition. this world-famous skyline has been named Livello Bonarelli as a posthumous tribute to his discoverer.Fil:Concheyro, A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Montenegro, T. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Ordering in the dilute weakly-anisotropic antiferromagnet Mn(0.35)Zn(0.65)F2

The highly diluted antiferromagnet Mn(0.35)Zn(0.65)F2 has been investigated by neutron scattering in zero field. The Bragg peaks observed below the Neel temperature TN (approximately 10.9 K) indicate stable antiferromagnetic long-range ordering at low temperature. The critical behavior is governed by random-exchange Ising model critical exponents (nu approximately 0.69 and gamma approximately 1.31), as reported for Mn(x)Zn(1-x)F2 with higher x and for the isostructural compound Fe(x)Zn(1-x)F2. However, in addition to the Bragg peaks, unusual scattering behavior appears for |q|>0 below a glassy temperature Tg approximately 7.0 K. The glassy region T<Tg corresponds to that of noticeable frequency dependence in earlier zero-field ac susceptibility measurements on this sample. These results indicate that long-range order coexists with short-range nonequilibrium clusters in this highly diluted magnet.Comment: 7 pages, 5 figure

Efficient Implementation of Elastohydrodynamics via Integral Operators

The dynamics of geometrically non-linear flexible filaments play an important role in a host of biological processes, from flagella-driven cell transport to the polymeric structure of complex fluids. Such problems have historically been computationally expensive due to numerical stiffness associated with the inextensibility constraint, as well as the often non-trivial boundary conditions on the governing high-order PDEs. Formulating the problem for the evolving shape of a filament via an integral equation in the tangent angle has recently been found to greatly alleviate this numerical stiffness. The contribution of the present manuscript is to enable the simulation of non-local interactions of multiple filaments in a computationally efficient manner using the method of regularized stokeslets within this framework. The proposed method is benchmarked against a non-local bead and link model, and recent code utilizing a local drag velocity law. Systems of multiple filaments (1) in a background fluid flow, (2) under a constant body force, and (3) undergoing active self-motility are modeled efficiently. Buckling instabilities are analyzed by examining the evolving filament curvature, as well as by coarse-graining the body frame tangent angles using a Chebyshev approximation for various choices of the relevant non-dimensional parameters. From these experiments, insight is gained into how filament-filament interactions can promote buckling, and further reveal the complex fluid dynamics resulting from arrays of these interacting fibers. By examining active moment-driven filaments, we investigate the speed of worm- and sperm-like swimmers for different governing parameters. The MATLAB(R) implementation is made available as an open-source library, enabling flexible extension for alternate discretizations and different surrounding flows.Comment: 37 pages, 17 figure

Chemistry in isolation: High CCH/HCO+ line ratio in the AMIGA galaxy CIG 638

Multi-molecule observations towards an increasing variety of galaxies have been showing that the relative molecular abundances are affected by the type of activity. However, these studies are biased towards bright active galaxies, which are typically in interaction. We study the molecular composition of one of the most isolated galaxies in the local Universe where the physical and chemical properties of their molecular clouds have been determined by intrinsic mechanisms. We present 3 mm broad band observations of the galaxy CIG 638, extracted from the AMIGA sample of isolated galaxies. The emission of the J=1-0 transitions of CCH, HCN, HCO+, and HNC are detected. Integrated intensity ratios between these line are compared with similar observations from the literature towards active galaxies including starburst galaxies (SB), active galactic nuclei (AGN), luminous infrared galaxies (LIRG), and GMCs in M33. A significantly high ratio of CCH with respect to HCN, HCO+, and HNC is found towards CIG 638 when compared with all other galaxies where these species have been detected. This points to either an overabundance of CCH or to a relative lack of dense molecular gas as supported by the low HCN/CO ratio, or both. The data suggest that the CIG 638 is naturally a less perturbed galaxy where a lower fraction of dense molecular gas, as well as a more even distribution could explain the measured ratios. In this scenario the dense gas tracers would be naturally dimmer, while the UV enhanced CCH, would be overproduced in a less shielded medium.Comment: Letter accepted for publication in A&