On the use of encapsulated phase change materials pebbles and pellets as freeze protection method for low-stature plants/crops

© 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 use of phase change materials (PCMs) in encapsulated pebbles or pellets as freeze protection method for low-stature plants/crops is proposed and discussed at the conceptual level. Utilizing a simplified geometrical model, it is shown that, for practical applications, small encapsulated-PCM pebbles/pellets of 1-cm sized could provide the necessary heat to prevent the freezing during typical freezing time. Taking into account the balance between the loss of energy on a typical frost night and the latent energy stored in the encapsulated PCMs-pebbles/pellets, the amount of such pebbles/pellets per unit of soil-area was calculated and resulting in the use of less than 2% of the soil-area. The preliminary results at conceptual level are encouraging, however, additional work and R&D is require before its real practical efficacy can be put to test. The choice of the suitable crops, the specific environmental conditions during the freezing time, the manufacture recyclability and environmental impact of the pcm-pebbles/pellets are some of the aspects which must be carefully addressed.Peer ReviewedPostprint (author's final draft

On the use of thermal conductive focusing for solar concentration enhancement

© 2016. This version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/We discuss the possibility for solar concentration enhancement via conductive heat transport. Here, we are concerned, as in orthodox approaches, about maximizing the solar concentration to obtain the highest receiver temperature possible, but with one important difference: In the proposed approach, the solar concentration enhancement is attained not by the use of lenses, mirrors, or funnels (i.e., by optical concentration based on radiative transport), but via thermal conduction, what we call thermal conductive focusing. Among the additional advantages of thermal conductive focusing is the capability to concentrate indistinct direct incidence as well as diffusive radiation. Thus, the concept is especially insensitive to cloudy days and particularly attractive in application to environments with important diffusive components of light. Utilizing a simplified geometrical model, an analytical expression for the temperature and concentration gain at the receiver was derived. The particular application for a parabolic solar trough was analysed. Additional research and development is required to explore the possibilities of solar flux enhancement by thermal conductive focusing as well as the optimization of several variables.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

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

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

Ocean thermal energy conversion by deliberate seawater salinization

This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.Consideration is given to the possibility of ocean thermal energy conversion (OTEC) by the deliberate salinization of surface seawater. The proposed technique is similar to traditional OTEC, with one important exception: rather than cold water being brought from the bottom to the surface, the warm surface water is circulated to the bottom, cooled there, and lifted back to the surface. The entire process is driven by the induced salinity gradient at the surface. As a result, there is no need for a pumping system to bring the cold bottom water to the surface. Two methods are explored for surface salinity enhancement, namely solar evaporation and the direct addition of salt to the seawater.Peer ReviewedPostprint (author's final draft

A negative reactivity feedback driven by induced buoyancy after a temperature transient in lead-cooled fast reactors

Consideration is given to the possibility to use changes in buoyancy as a negative reactivity feedback mechanism during temperature transients in heavy liquid metal fast reactors (HLMFRs). It is shown that by the proper use of heavy pellets in the fuel elements, fuel rods could be endowed with a passive self-ejection mechanism and then with a negative feedback. A first estimate of the feasibility of the mechanism is calculated by using a simplified geometry and model. If in addition, a neutron poison pellet is introduced in the bottom of the fuel, then when the fuel element is displaced upward by buoyancy force, the reactivity will be reduced not only by disassembly of the core but also by introducing the neutron poisson from the bottom. The use of induced buoyancy opens up the possibility of introducing greater amounts of actinides into the core, as well as providing a palliative solution to the problem of positive coolant temperature reactivity coefficients that could be featured by the HLMFRs.Peer ReviewedPostprint (author's final draft

On the use of a dedicated ballast pellet for a prompt self-ejection mechanism after a temperature transient in lead-cooled fast reactors

The potential use of changes in buoyancy as a reactivity feedback mechanism during temperature transients in heavy liquid metal fast reactors (HLMFRs) is discussed. It is shown that with the use of ballast pellets (~15% volume fraction) introduced in combination with fuel pellets, fuel rods will be endowed with a reliable self-ejection mechanism that is able to compensate temperature transients. Utilizing a simplified model, an estimate of the negative reactivity insertion expected from this mechanism is derived. The use of ballast pellets opens up the possibility of introducing greater amounts of actinides into the core, as well as providing a solution to the classical problem of positive coolant temperature reactivity coefficients in fast reactors.Peer ReviewedPostprint (author's final draft

On the feasibility of a thermodynamic combined heating-depressurization process and the direct use of surface wind energy for mining the Martian regolith for water

Deposited with Space Colonization Ltd permissionIn this paper a combined heating-depressurization process for mining water from the regolith of Mars is proposed and its feasibility discussed. It is shown that for water content in the regolith \leq 4wt% as found in latitudes between -45^0 and 45^0 a combined heating-depressurization process offers better energy usage performance, with savings up to 40% possible. Utilizing a simplified geometrical model, an analytical expression for the performance of such combined processes in comparison with traditional heating is derived. Finally, the direct use of surface wind energy as a driving force for depressurization-sublimation is assessed, showing that for any season and any water content in the regolith a rudimentary low-tech windmill could provide the water for the design reference missions for human exploration of Mars. These results can make an important contribution to the design of energy strategies for water mining from the regolith in, for example, the conception and design of new rover vehicles.Peer ReviewedPostprint (published version