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

Efficient optimisation of structures using tabu search

This paper presents a novel approach to the optimisation of structures using a Tabu search (TS) method. TS is a metaheuristic which is used to guide local search methods towards a globally optimal solution by using flexible memory cycles of differing time spans. Results are presented for the well established ten bar truss problem and compared to results published in the literature. In the first example a truss is optimised to minimise mass and the results compared to results obtained using an alternative TS implementation. In the second example, the problem has multiple objectives that are compounded into a single objective function value using game theory. In general the results demonstrate that the TS method is capable of solving structural optimisation problems at least as efficiently as other numerical optimisation approaches

Reactor physics modelling of accident tolerant fuel for LWRs using ANSWERS codes

The majority of nuclear reactors operating in the world today and similarly the majority of near-term new build reactors will be LWRs. These currently accommodate traditional Zr clad UO2/ PuO2 fuel designs which have an excellent performance record for normal operation and most transients. However, the events at Fukushima culminated in significant hydrogen production and hydrogen explosions, resulting from high temperature Zr/steam interaction following core uncovering for an extended period. These events have resulted in increased emphasis towards developing more accident tolerant fuels (ATFs)-clad systems, particularly for current and near-term build LWRs. R&D programmes are underway in the US and elsewhere to develop ATFs and the UK is engaging in these international programmes. Candidate advanced fuel materials include uranium nitride (UN) and uranium silicide (U3Si2). Candidate cladding materials include advanced stainless steel (FeCrAl) and silicon carbide. The UK has a long history in industrial fuel manufacture and fabrication for a wide range of reactor systems including LWRs. This is supported by a national infrastructure to perform experimental and theoretical R&D in fuel performance, fuel transient behaviour and reactor physics. In this paper, an analysis of the Integral Inherently Safe LW R design (I2S-LWR), a reactor concept developed by an international collaboration led by the Georgia Institute of Technology, within a U.S. DOE Nuclear Energy University Program (NEUP) Integrated Research Project (IRP) is considered. The analysis is performed using the ANSWERS reactor physics code WIMS and the EDF Energy core simulator PANTHER by researchers at the University of Cambridge. The I2S-LWR is an advanced 2850 MWt integral PWR with inherent safety features. In order to enhance the safety features, the baseline fuel and cladding materials that were chosen for the I2S- LWR design are U3Si2 and advanced stainless steel respectively. In addition, the I S-LWR design adopts an integral configuration and a fully passive emergency decay heat removal system to provide indefinite cooling capability for a class of accidents. This paper presents the equilibrium cycle core design and reactor physics behaviour of the I2S-LWR with U3Si2 and the advanced steel cladding. The results were obtained using the traditional two-stage approach, in which homogenized macroscopic cross-section sets were generated by WIMS and applied in a full 3D core solution with PANTHER. The results obtained with WIMS/PANTHER were compared against the Monte Carlo Serpent code developed by VTT and previously reported results for the I2S-LWR. The results were found to be in a good agreement (e.g. < 200 pcm in reactivity) among the compared codes, giving confidence that the WIMS/PANTHER reactor physics package can be reliably used in modelling LWRs with ATFs.This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1051/epjn/201601

Correction to: Numerical evaluation of entropy generation in isolated airfoils and Wells turbines

n/

Dimension Reduction via Gaussian Ridge Functions

Ridge functions have recently emerged as a powerful set of ideas for subspace-based dimension reduction. In this paper we begin by drawing parallels between ridge subspaces, sufficient dimension reduction and active subspaces, contrasting between techniques rooted in statistical regression and those rooted in approximation theory. This sets the stage for our new algorithm that approximates what we call a Gaussian ridge function---the posterior mean of a Gaussian process on a dimension-reducing subspace---suitable for both regression and approximation problems. To compute this subspace we develop an iterative algorithm that alternates between optimizing over the Stiefel manifold to compute the subspace and optimizing the hyperparameters of the Gaussian process. We demonstrate the utility of the algorithm on two analytical functions, where we obtain near exact ridge recovery, and a turbomachinery case study, where we compare the efficacy of our approach with three well-known sufficient dimension reduction methods: SIR, SAVE, and CR. The comparisons motivate the use of the posterior variance as a heuristic for identifying the suitability of a dimension-reducing subspace.The first author's work was supported by a Rolls-Royce fellowship. The second author's work was supported by Magdalene College, Cambridg