Marina du Toit - Academia.edu (original) (raw)

Papers by Marina du Toit

International Journal of Hydrogen Energy

Energy & Fuels, 2020

Passive autocatalytic recombiners (PARs) are used for the removal of accidentally released hydrog... more Passive autocatalytic recombiners (PARs) are used for the removal of accidentally released hydrogen inside confined spaces. The high catalyst surface temperature is an important safety issue to be considered in the use of a PAR. The ability to predict the catalyst surface temperature could be very useful in preventing the self-ignition and explosion of hydrogen inside the PAR. This study seeks to investigate the changes in temperature profiles of the PAR catalytic section upon variation of the inlet hydrogen concentration. Experiments were conducted on a small-scale test setup. The catalytic section comprised cylindrical ceramic elements arranged in parallel and held upright by a stainless-steel frame. The temperature profiles were measured with a high-resolution infrared camera. The commercial computational fluid dynamics (CFD) code STAR-CCM+ was used as numerical tool for modeling the gas mixture flow inside the experimental setup and the chemical reaction kinetics. Results of numerical studies are presented and compared with experimental results. The presented CFD-based approach and software offer an appropriate numerical tool for the investigation of hydrogen safety issues. Finally, the catalyst was subjected to a prolonged high temperature combustion fatigue procedure to determine its stability. The surface of the fatigued

International Journal of Hydrogen Energy, 2020

h i g h l i g h t s Computational fluid dynamics methods were used to analyze hydrogen leakage. S... more h i g h l i g h t s Computational fluid dynamics methods were used to analyze hydrogen leakage. STAR-CCMþ code was used to compare the simulation results with experimental results. Combinations of hydrogen release pressure and leak orifice size were considered. Efficiency of forced ventilation in semi-closed spaces was investigated. Numerical results could be useful in the analysis of safety issues in mining tunnels.

Nuclear Engineering and Design, 2018

A new approach to designing thorium based fuels for a European pressurized reactor (EPR) core is ... more A new approach to designing thorium based fuels for a European pressurized reactor (EPR) core is introduced by considering a three dimensional full core MCNP6 model. Existing information on designing thorium/uranium fuel for Generation II pressurized water reactors (PWRs) have been analyzed and the new fuel was proposed for the EPR (Gen III+). The newly designed fuel assembly (Th-B1) can reach 24-month fuel cycles without altering the geometry or disregarding any design limits. The Th-B1 was developed by reducing and ultimately removing Gd 2 O 3 and slightly reducing the overall initial fissile content compared to the uranium only EPR. The same overall initial fissile content of the U-EPR was maintained, with the exception for fuel pin sections where pure ThO 2 replaced the UO 2 and Gd 2 O 3 containing sections of the original U-EPR design. Results showed that there is no need to increase the initial fissile content or soluble boron enrichment as predicted by previous studies. Both xenon and samarium fission product yields for the Th-B1 are lower than for the U-B1, which contributes to a higher value for ∞ k and better breeding. Results also showed that the total fissile content for the Th-B1 shows a smaller decrease with time, which implies a smaller decrease for ∞ k with time and subsequently an extension of the fuel cycle. The innovative optimized fuel design of the EPR (five axial zones), enabled fuel enrichment savings and longer fuel cycles for Th-EPR. Not only does one save on fuel costs by keeping the overall initial fissile content similar, but this also results in reactor properties that are similar to those of the U-EPR and no serious changes need to be made. The only concerns are a lower delayed neutron fraction and boron worth for the Th-B1 at EOL. However, not all assemblies in a full core will be at the end of their life and if a three batch reloading scheme is assumed, the boron worth and delayed neutron fraction will most likely be within acceptable limits. Other differences between the Th-EPR and U-EPR such as the axial power profile and difference in boron letdown curve can be accounted for by amending the operating procedures during the design phase.

Journal of Energy in Southern Africa, 2016

In recent years, due to economic and social infrastructure development and growth, South Africa h... more In recent years, due to economic and social infrastructure development and growth, South Africa has been facing growth in energy demand. Addressing this demand includes building more coal power stations, however with attention paid in designing them to reduce greenhouse gas emissions. A second response is to deploy more power sources using renewable and nuclear energy. The South African government has plans to add about 9.6 GW of nuclear energy to the electricity grid. Accepting that South Africa will seek an international vendor or vendors to supply nuclear plants, a certain degree of localisation of manufacture and operation should be planned. One localisation task that can be actively pursued is reactor analysis, including criticality, burnup, shielding and accident analysis of the reactor. Such development of expertise will support both economic and safety aspects of building and running a nuclear reactor. With this in mind, neutronic analysis of the VVER 1000 reactor was initia...

Energy & Fuels, 2018

This article presents a review of past and current research and development on H 2 combustion in ... more This article presents a review of past and current research and development on H 2 combustion in terms of power generation in gas turbines and safety in passive autocatalytic recombiners (PARs). The drive towards reducing greenhouse gas emissions has forced researchers to look at carbon-free alternatives for power generation. Fortunately, H 2 is one such fuel source and has been proposed as a fuel in gas turbines for large-scale power production. The effects of H 2 on the heating values, flame speed, burning velocity, flammability range, flashback, blow-off, ignition delay, and emissions have been reported, and the trends and gaps in R&D identified. Properties such as flame speed, burning velocities, and flammability limits at typical gas turbine conditions (high pressures, high temperatures, lean equivalence ratios, and turbulent conditions) still need to be determined experimentally for H 2 fuel mixtures. Especially for mixtures with higher H 2 concentrations and fuel mixtures with other fuels (such as biogas) as an evolutionary step towards adapting to a hydrogen economy. Much work has been done on premixed dry low emissions and diffusion combustion with dilution as means of accommodating high hydrogen content fuels. Staged combustion, vortex-stabilized combustion, multiple injection combustion,

Nuclear Science and Engineering

The European pressurized reactor (EPR) is classified as a Generation III+ reactor. It differs fro... more The European pressurized reactor (EPR) is classified as a Generation III+ reactor. It differs from a conventional pressurized water reactor in many aspects, one of which is the core design. This evolutionary reactor lends itself to new fuel designs, such as thorium-based fuels. To perform new design calculations, a base case model needs to be established because the detailed models that are currently available are either proprietary or regulated. This paper therefore presents such a model based on the Monte Carlo method. This method is a valuable component of reactor neutronic calculations because geometry and materials can be accurately modeled. We modeled a full core of the EPR using MCNP6, in which the individual fuel pin geometry and material definitions were used together with radial and axial temperature characterization based on fuel assemblies considered as nodes. Data for both the neutronic and thermal-hydraulic models were mainly obtained from the U.S. EPR Final Safety Analysis Report (FSAR) [Rev. 5, AREVA (2013)]. The neutronic and some thermal-hydraulic results were compared with data from the EPR FSAR. The following core neutronic parameters compared well with the FSAR data: the boron worth, axial flux distribution, neutron flux spectrum, reactivity coefficients, and control rod worth. However, the delayed neutron fraction showed a somewhat larger difference compared to the FSAR. Given this verification with the FSAR, confidence in the MCNP6 EPR model was therefore established. The model that we have developed serves as the basis for the follow-on study of introducing thorium in the EPR core.

Journal of Energy in Southern Africa, Apr 13, 2017

A systematic and strategic nuclear power reactor deployment roadmap has been developed for South ... more A systematic and strategic nuclear power reactor deployment roadmap has been developed for South Africa within the national strategic plan, utilizing thorium-based fuel. The roadmap was developed through analysis of economical, strategic and historical aspects. The accumulated advantages of thorium-based fuels are summarized, which could form the initiative to implement thorium-based nuclear fuels in South Africa. A timeline (which forms the basis of the roadmap) was constructed and consists of three different phases. Phase 1 starts in 2015 and extends to 2030. Phase 2 starts in 2031 and ends in 2044 whilst Phase 3 is from 2045 to 2060. Each phase is discussed with regard to construction, implementation and research activities. This roadmap starts at current pressurized water reactors (PWRs) and advances to future reactor technologies, using an evolutionary approach. In addition to the results reported in this paper, the economic advantages to introducing thorium as a fertile component in PWR fuels as compared to once-through conventional uranium-only cycles is explored (Du Toit & Cilliers, 2014). The economic evaluation compares uranium fuel to thorium-uranium fuel in terms of the fuel cycle costs, reactor downtime costs due to refuelling and income derived from electricity sales.

Nuclear Technology, 2014

Many studies have proven that thorium-based fuel cycles are more expensive than current uranium f... more Many studies have proven that thorium-based fuel cycles are more expensive than current uranium fuel cycles, which is confirmed in this paper as well. This research, however, goes beyond the conventional nuclear plant refueling cycle, aiming to optimize the refueling cycle in line with specific advantages realized from the thorium-based refueling cycle. These benefits are the good thermal neutronic characteristics of fertile 232Th and fissile 233U, resulting in longer refueling cycles, higher capacity factors, and reduced volumes of spent fuel. This paper focuses on once-through, homogeneously mixed, thorium-uranium fuel cycles and explores the economic advantages to introducing thorium as a fertile component in pressurized water reactor fuels as compared to once-through conventional uranium-only cycles. The economic evaluation compares the operational savings incurred as a result of longer fuel cycles and reduced reactor downtime with increased fuel expenses, due to higher initial ...

Sustainable Energy Technologies and Assessments, 2020

Abstract Worldwide energy supply is rapidly shifting towards more sustainable and cleaner power g... more Abstract Worldwide energy supply is rapidly shifting towards more sustainable and cleaner power generation technologies including solar PV, wind turbines and natural gas. The co-combustion of fuel blends, consisting of renewably-generated H2 and CH4, offer numerous operational advantages of gas turbines, including reduced turbine CO and NOx emissions. In this study, the combustion of H2 and CH4 was investigated experimentally, in a combined heat and power micro turbine (1–3 kWe). Three fuel scenarios were considered: 100 vol% CH4, a 15 vol% H2/CH4 blend, and a 20 vol% H2/biogas blend. These blends were used to demonstrate the micro turbine’s flexibility, simulating the addition of renewable H2 to natural gas and biogas. For the 20 vol% H2/biogas blend, CO and NOx emissions showed respective energy-equivalent reductions of 29.8% and 47.1% (compared with the 100 vol% CH4 fuel). Our findings contribute to the operational knowledge of micro gas turbines, obtaining cleaner combustion characteristics via co-combustion, and ultimately provide a solution for off-grid power applications and emergency back-up power systems.