Becoming a Scholar Savva, Maria; Nygaard, Lynn P
UCL Press,
2021, 2021-03-01
eBook, Book
Odprti dostop
Becoming a Scholar provides a window into the lives of nine non-traditional doctoral students. As mature, part-time, international students enrolled in a professional doctorate programme, they ...reflect on the transformation process of becoming scholars, and their narratives provide breadth and depth to themes that represent a diverse cross-section of cultures, identities and communities. Recognising that the process of becoming a scholar is as much internal as it is external, the book provides an opportunity to engage with authentic personal stories that remain firmly rooted in academic literature. By bringing the ‘human face’ behind the doctoral journey to the forefront, the narratives draw much-needed attention to the personal journey that inevitably parallels and intersects with the academic journey. Although the narratives are drawn from a professional Doctor in Education (EdD) programme based in the UK, the struggles are sure to resonate with a much wider range of doctoral students and academics, sparking lively discussion, debate and reflection. A must-read for students preparing to embark on the doctoral journey, and essential reading for doctoral programmes that wish to equip students with important knowledge about the challenges ahead.
•Results of benchmarking TRIPOLI based on the VENUS-2 MOX benchmark are reported.•3-D TRIPOLI calculations were performed using ENDF/B-VI.4, ENDF/B-VII.0 and JEFF-3.•Results are compared with ...measured data and results of other benchmark participants.•TRIPOLI results agree well with experimental data and results from other participants.
The reliability and verification of numerical solutions derived from neutronic codes and the use of nuclear data libraries is a very important issue in nuclear technology. To this purpose, computational benchmarks based on well-defined problems with a complete set of input and a unique solution, are often used. The OECD/NEA VENUS-2 is a widely used MOX benchmark problem for the validation of numerical methods and nuclear data sets. In this paper, the results of benchmarking the TRIPOLI Monte Carlo code based on the VENUS-2 MOX benchmark problem, are reported. 3-D TRIPOLI calculations were performed using the ENDF/B-VI.4, ENDF/B-VII.0 and JEFF-3.1 nuclear data sets. The computational results are compared with measured data, as well as with the results of other benchmark participants. In general the TRIPOLI results agree well with both the experimental data as well as those of other participants.
The present work reports up-to-date information regarding the reaction mechanism of the catalytic hydrogenation of nitrates in water media. In the present mechanistic study, an attempt is made, for ...the first time, to elucidate the crucial role of several catalysts and reaction parameters in the mechanism of the NO(3)(-)/H(2) reaction. Steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with ex situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were performed on supported Pd-Cu catalysts for the NO(3)(-)/H(2) and NO(3)(-)/H(2)/O(2) reactions. The latter experiments revealed that the formation and surface coverage of various adsorbed active intermediate N-species on the support or Pd/Cu metal surface is significantly favored in the presence of TiO(2) in the support mixture and in the presence of oxygen in the reaction's gaseous feed stream. The differences in the reactivity of these adsorbed N-species, found in the present work, adequately explain the large effect of the chemical composition of the support and the gas feed composition on catalyst behaviour (activity and selectivity). The present study leads to solid mechanistic evidence concerning the presence of a hydrogen spillover process from the metal to the support. Moreover, this study shows that Cu clusters are active sites for the reduction of nitrates to nitrites.
A numerical analysis of an ammonothermal synthesis process for the bulk growth of nitride crystals was performed. The analysis includes the development of a thermal model for a lab-scale ...ammonothermal autoclave, which was validated by in situ temperature measurements and applied to tailor the temperature field inside the autoclave. Based on the results of the global thermal 2D simulations, a local 3D model was used to include convective phenomena in the analysis. Moreover, the influence of the baffle and different baffle shapes on the flow velocity was investigated. Fluctuations of the temperature as well as the flow velocities occur, indicating that 3D considerations are essential to accurately investigate the heat and mass transport in ammonothermal systems.
•Global thermal 2D simulations validated by in-situ temperature measurements.•First local numerical 3D model of an ammonothermal system for the bulk growth of nitrides.•Temperature/flow velocities fluctuations prove the necessity of 3D considerations.
The safe introduction of Generation IV (Gen IV) reactor concepts into operation will require extensive testing of their components. This must be performed under neutronic conditions representative of ...those expected to prevail inside the new reactor cores when in operation. In a thermal Material Testing Reactor (MTR) such neutronic conditions can be achieved by tailoring the prevailing neutron spectrum with the utilization of a device containing appropriate materials. In this work various materials are investigated as candidate components of a device that will be required in case that a thermal MTR neutron energy spectrum must be locally transformed, so as to imitate Sodium cooled Fast Reactor (SFR). Many nuclides have been examined with respect to only their neutronic behavior, providing thus a pool of neutronically appropriate materials for consideration in further investigation, such as regarding reactor safety and fabrication issues. The nuclides have been studied using the neutronics code TRIPOLI-4.8 while the reflector of the Jules Horowitz Reactor (JHR) was considered as the hosting environment of the transforming device. The results obtained suggest that elements with important inelastic neutron scattering could be chosen at a first level as being able to modify the prevailing neutron spectrum towards the desired direction. The factors which are important for an effective inelastic scatterer comprise density and inelastic microscopic cross section, as well as the energy ranges where inelastic scattering occurs. All the above factors have been separately examined in order to suggest potential device materials, able to locally produce SFR neutron spectrum imitation in a thermal MTR.
The presence of fast neutron spectra in new reactors is expected to induce a strong impact on the contained materials, including structural materials, nuclear fuels, neutron reflecting materials, and ...tritium breeding materials. Therefore, introduction of these reactors into operation will require extensive testing of their components, which must be performed under neutronic conditions representative of those expected to prevail inside the reactor cores when in operation. Due to limited availability of fast reactors, testing of future reactor materials will mostly take place in water cooled material test reactors (MTRs) by tailoring the neutron spectrum via neutron screens. The latter rely on the utilization of materials capable of absorbing neutrons at specific energy. A large but fragmented experience is available on that topic. In this work a comprehensive compilation of the existing neutron screen technology is attempted, focusing on neutron screens developed in order to locally enhance the fast over thermal neutron flux ratio in a reactor core.
•ANET is a new neutronics stochastic code.•Criticality calculations in both subcritical and critical nuclear systems of conventional design were conducted.•Simulations of thermal, lower epithermal ...and fast neutron fluence rates were performed.•Axial fission rate distributions in standard and MOX fuel pins were computed.
ANET (Advanced Neutronics with Evolution and Thermal hydraulic feedback) is an under development Monte Carlo code for simulating both GEN II/III reactors as well as innovative nuclear reactor designs, based on the high energy physics code GEANT3.21 of CERN. ANET is built through continuous GEANT3.21 applicability amplifications, comprising the simulation of particles’ transport and interaction in low energy along with the accessibility of user-provided libraries and tracking algorithms for energies below 20MeV, as well as the simulation of elastic and inelastic collision, capture and fission. Successive testing applications performed throughout the ANET development have been utilized to verify the new code capabilities. In this context the ANET reliability in simulating certain reactor parameters important to safety is here examined. More specifically the reactor criticality as well as the neutron fluence and fission rates are benchmarked and validated. The Portuguese Research Reactor (RPI) after its conversion to low enrichment in U-235 and the OECD/NEA VENUS-2 MOX international benchmark were considered appropriate for the present study, the former providing criticality and neutron flux data and the latter reaction rates. Concerning criticality benchmarking, the subcritical, Training Nuclear Reactor of the Aristotle University of Thessaloniki (TNR-AUTh) was also analyzed. The obtained results are compared with experimental data from the critical infrastructures and with computations performed by two different, well established stochastic neutronics codes, i.e. TRIPOLI-4.8 and MCNP5. Satisfactory agreement is found with both measurements and independent computations, verifying thus ANET’s ability to successfully simulate important parameters of critical and subcritical systems.
The present paper involves a detailed study of the selective catalytic reduction of nitrates in aqueous mediums by the use of H2 in the presence of O2 over monometallic and bimetallic supported ...catalysts. In this study, an attempt has been made to improve the denitrification efficiency (XNO3(-), SN2) of several catalysts by regulating some experimental parameters that are involved in the process. Therefore, the effects of the type of reactor (semi-batch reactor vs continuous flow reactor), the nature of the active phase (Pd, Cu, and Pd-Cu) and the particle size of γ-Al2O3 spheres (particle diameter = 1.8 mm and 3 mm) on catalytic activity and reaction selectivity, as well as the adsorption capacity of γ-Al2O3 spheres for nitrates, were examined. As the review indicates, most of the research has so far been conducted on batch or semi-batch reactors. This study successfully demonstrates the benefits of using a continuous flow reactor in terms of catalytic activity (XNO3(-), %) and reaction selectivity (SN2, %). Another important aspect of this study is the crucial role of bimetallic Pd-Cu clusters for the prevention of NH4(+) formation. Moreover, the use of 1.8 mm diameter γ-Al2O3 spheres as a support was proved to significantly enhance the catalytic performance of bimetallic Pd-Cu catalysts towards nitrate reduction compared to 3 mm diameter γ-Al2O3 spheres. This difference may be attributed to mass (NO3(-), OH(-)) transfer effects (external mass transfer phenomena).
A 0.1 wt% Pt supported on La0.7Sr0.2Ce0.1FeO3 solid (mixed oxide containing LaFeO3, SrFeO3−x, CeO2, and Fe2O3 phases) has been studied for the NO/H2/O2 reaction in the 100–400°C range. For a critical ...comparison, 0.1 wt% Pt was supported on SiO2, CeO2, and Fe2O3 and tested under the same reaction conditions. For the Pt/La0.7Sr0.2Ce0.1FeO3 catalyst a maximum in the NO conversion (83%) has been observed at 150°C with a N2 selectivity value of 93%, while for the Pt/SiO2 catalyst at 120°C (82% conversion) with a N2 selectivity value of 65% using a GHSV of 80,000 h−1. Low N2 selectivity values, less than 45%, were obtained with the Pt/CeO2 and Pt/Fe2O3 catalysts in the 100–400°C range. For the Pt/La0.7Sr0.2Ce0.1FeO3 catalyst, addition of 5% H2O in the feed stream at 140°C resulted in a widening of the operating temperature window with appreciable NO conversion and no negative effect on the stability of the catalyst during 20 h on stream. In addition, a remarkable N2 yield (93%) after 20 h on 0.25% NO/1% H2/5% O2/5% H2O/He gas stream at 140°C has been observed. Remarkable N2 selectivity values in the range of 80–90% have also been observed in the 100–200°C low-temperature range either in the absence or in the presence of water in the feed stream. A maximum specific integral reaction rate of 443.5 μmol N2/s·g of Pt metal was measured at 160°C during reaction with a 0.25% NO/1% H2/5% O2/5% H2O/He gas mixture. This value is higher by 90% than the corresponding one observed on the 0.1 wt% Pt/SiO2 catalyst at 120°C and it is the highest value ever reported for the reaction at hand in the 100–200°C low-temperature range on Pt-based catalysts. A TOF value of 13.4×10−2 s−1 for N2 formation was calculated at 110°C for the Pt/La0.7Sr0.2Ce0.1FeO3 catalyst. Temperature-programmed desorption (TPD) of NO and transient titration experiments of the catalyst surface following NO/H2/O2 reaction have revealed important information concerning the amount and chemical composition of active and inactive (spectator) adsorbed N-containing species present under reaction conditions.
▶ Diffusion and MC calculations for rod worth dependence on burnup and Xe in reactors. ▶ One-step rod withdrawal/insertion are used for rod worth estimation. ▶ The study showed that when Xe is ...present the rods worth is significantly reduced. ▶ Rod worth variation with burnup depends on rod position in core. ▶ Rod worth obtained with MC code is higher than that obtained from deterministic.
One important parameter in the design and the analysis of a nuclear reactor core is the reactivity worth of the control rods, i.e. their efficiency to absorb excess reactivity. The control rod worth is affected by parameters such as the fuel burnup in the rod vicinity, the Xe concentration in the core, the operational time of the rod and its position in the core. In the present work, two different computational approaches, a deterministic and a stochastic one, were used for the determination of the rods worth dependence on the fuel burnup level and the Xe concentration level in a conceptual, symmetric reactor core, based on the MTR fuel assemblies used in the Greek Research Reactor (GRR-1). For the deterministic approach the neutronics code system composed by the SCALE modules NITAWL and XSDRN and the diffusion code CITATION was used, while for the stochastic one the Monte Carlo code TRIPOLI was applied. The study showed that when Xe is present in the core, the rods worth is significantly reduced, while the rod worth variation with increasing burnup depends on the rods position in the core grid. The rod worth obtained with the use of the Monte Carlo code is higher than the one obtained from the deterministic code.
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