Browsing by Author "Aschman, David"
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- ItemMetadata onlyExploring the spectroscopy of vibrational levels in the 160 mass region(2016) Majola, Siyabonga Ntokozo Thandoluhle; Aschman, David; Bark, R A; Jones, PVibrational levels are well known in atomic nuclei but despite many decades of research, some of their properties still remain elusive. In particular, low-lying rotational bands based on the first excited 0+ state, which are traditionally understood as β vibrational bands nevertheless show properties at odds with this interpretation, more especially in the transitional rare earth region with N~ 90 [Gar01]. An alternative is that they can be better be described as a "second vacuum", or coexisting minimum in the pairing degree of freedom [Sch11a]. In order to produce a complete and definitive microscopic picture of the so-called β and γ bands, an extensive systematic review is performed for nuclides in the 160 mass region, between N = 88 and 92 and Sm to Yb. The data are explained using a five dimensional collective Hamiltonian for quadrupole rotational and vibrational degrees of freedom [Li09, Nik09]. A good qualitative agreement is obtained between measured energies and electromagnetic transition rates across the entire A ~ 160 mass region. The implication of these findings on the interpretation the first excited 0+ states is there from discussed. In this study, the γ-ray spectroscopy of 156Dy and 157Dy are also revised. Three experiments with different reactions to produce 156Dy have been analyzed. The data were acquired using the 155Gd (α, 3n) 156Dy and 148Nd (12C, 4n) 156Dy and 124Sn (36S, 4n) 156Dy reactions at beam energies of 25, 65 and 165 MeV, respectively. The 155Gd (α, 3n) 156Dy experiment was performed with aid of the JUROGAM II array. The experiments that used the 148Nd (12C, 4n) 156Dy and 124Sn (36S, 4n) reactions were both conducted using the GAMMASPHERE array. In total, the current work reports the observation of about 30 bands in 156Dy. The spins and parities for the majority of new rotational structures have been successfully assigned using the Directional Correlation from Oriented States (DCO ratios) and polarization anisotropy (Ap). In order to meaningfully describe the quantum behavior of the newly established structures, the Nilsson and Cranked Shell Models have been used. The results also test the latest and most sophisticated quantum theories of nuclear structure and also point to inadequacies of these models, where they exist. Amongst important spectroscopic features, is the identification of the γ wobbling candidate, which implies in some way that 156Dy is γ deformed. Extensions have been made on the γ bands and they are now observed to spins 31+ and 32+, which marks the highest spin to be ever observed for both the odd and even spin members of the γ bands in any nucleus. The current study also reports the first candidate for the aligned γ bands based on the two-quasiparticle configurations (S-band). This is the first time "γ bands" have been identified on both the ground state band (continuation) and on the yrast aligned S-band. Finally, an odd spin negative parity sequence, namely band 9, has been identified as a candidate of the K = 1- octupole excitation. [Please note: the full text of the thesis is deferred until January 2018]
- ItemOpen AccessLong-term reactivity transient analysis in an extended loss of all AC power at the Koeberg nuclear plant(2022) Foster, Neil; Aschman, DavidThis study investigates, through neutronics and thermodynamic modelling of the Koeberg Nuclear Station, a 900 MW pressurized water reactor (PWR), whether extended loss of all AC power (ELAP) scenarios will result in an uncontrolled return to criticality. The purpose is to determine whether recriticality can be avoided without performing plant modifications, or significantly adjusting the existing procedures to depressurize the primary system. The concern is that many currently proposed solutions introduce the risk of further complications, such as a loss of reactor coolant accident (LOCA). The Fukushima Daiichi accident in 2011 was caused by a tsunami that resulted in an extended loss of all AC power at the power station, causing a major nuclear accident, highlighting the importance of introducing measures to deal with ELAP scenarios over and above the short-term AC power loss scenarios previously catered for. The immediate concern in an ELAP is to ensure sufficient removal of heat from the primary system. This is achieved by providing adequate secondary side feed-water and steam-evacuation capability from the steam generators, and maintaining an adequate volume of borated water in the primary system, to prevent overheating of the reactor core. However, in the ELAP scenario, a long-term reactivity concern develops as additional reactivity is introduced into the core by the cooling of the moderator (during the course of recovery actions) which is compounded by the decay of neutron poisons in the core. The xenon isotope 135Xe is a significant neutron poison which accumulates in the fuel of an operating nuclear reactor. Its presence in the fuel of a recently-tripped reactor initially helps to maintain subcriticality, but this eventually decays away. This depletion starts at about 8 hours after a reactor trip, gradually adding more reactivity to the core. If borated water cannot be injected into the primary system to compensate for this, the reactor could return to criticality in an uncontrolled manner. Although this would be self-limiting (due to re-heating of the moderator and fuel), and not catastrophic in itself, the undesired generation of additional nuclear power in the reactor would, by consuming already limited supplies of cooling water, decrease the time available for recovery before the fuel in the reactor core overheats and melts. Utilities have made proposals to maintain sub-criticality in an ELAP scenario, which do not require AC power. They involve introducing boron into the primary system. They are often costly, and some proposals require the operator to open the primary system relief valves, introducing an additional risk of a failure of the relief valves to close, leading to a loss of coolant accident, and a core melt. The primary aim of the study was to determine, in an ELAP scenario, whether it is necessary to provide additional boron injection (over and above the existing accumulator inventory) to maintain the reactor sub-criticality. This was achieved using neutronics and thermodynamic modelling of Koeberg Nuclear Power Station. Another area of focus was to confirm that the existing cooldown strategies to mitigate ELAP events, are sufficient to maintain sub-criticality. After modelling and assessing the ELAP scenario over four different stages of the fuel cycle, it was concluded that, with best estimate assumptions, the selected reactivity acceptance criteria were met. However, with assumptions that envelope the majority of fuel cycle variances and code uncertainty (i.e. greater than 97.5% of cases), meeting the acceptance criteria for the latter part of the fuel cycle could not be demonstrated. Some potential solutions to ensure long-term sub-criticality are proposed.
- ItemOpen AccessMeasurements of Gamow-Teller strength in medium mass nuclei using (p,n) reactions at intermediate energies(1997) Steyn, Douw; Aschman, DavidThe simplicity of the beta decay process makes it a powerful tool for the investigation of the weak interaction and of nuclear structure. The two main operators involved, the Fermi (F) and Gamow-Teller (GT) operators, change just the isospin projection, and the isospin and spin projections of a nucleon respectively. Beta decay studies enable the extraction of the transition matrix elements to high precision. However, most GT transition rates deduced from beta decay measurements turn out to be smaller than the calculated single particle rates [1, 2, 3], a phenomenon that has become known as the quenching of GT strength. Beta decay studies are limited io radioactive nuclei in which the transitions are energetically possible. These are invariably between states of low excitation energy, and are also often relatively weak transitions. In addition, the calculation of the beta decay strengths is model-dependent. The model-dependence and uncertainties would be reduced if larger fractions of the total strength were analysed [4]. It is, however, possible to do so with the use of other probes of spin-isospin strength and to compare the results of these to those of beta decay. One such probe is the zero degree (p,n) reaction at intermediate energies [2, 5, 6]. Such a reaction is not subject to some of the limitations of beta decay in that any desired target nucleus may be probed and that the GT strength function may be investigated up to high excitation energies in the final nucleus. The essential similarity of the transition matrix elements of the two processes allows the measured (p,n) strengths to be converted to beta decay strengths.
- ItemOpen AccessNumerical simulation of nuclear reactor isotope depletion(2018) Keyser, Tinus; Aschman, DavidA program was written in Python to simulate nuclide reactions and burnup in a thermal fission reactor numerically. The program focused on the depletion calculations and used a simplified neutron flux equation. Nuclide data like cross-sections and fission products were read in from ENDF format files that have undergone pre-processing. To solve the more than 500 simultaneous differential equations that describe the varying isotopic concentrations, short-lived decay isotopes and their decay chains were identified and solved with a modified Bateman solution and then the long-lived isotopes concentrations were solved with matrix exponentiation. The flux was calculated to keep the heat output of the reactions constant. The simulation calculations were validated by comparing the output of decay chains with known analytical solutions. The output of the reactor burnup simulation was compared to that of ORIGEN (The Oak Ridge National Laboratory Isotope Generation And Depletion Code) for a Light Water Reactor at constant load to a burnup of 33GWd/ton. The output of the simulation was relatively similar to that of ORIGEN, but differed in some marked ways, e.g. plutonium breeding, which suggested that the neutron flux calculations and neutron absorption by U238 was not similarly modelled as in ORIGEN. By slightly adjusting the neutron absorption of U238 in the simulation, the correspondence between the simulation and the reference output was improved.
- ItemOpen AccessPHY2014F - Waves & Electromagnetism (mainstream physics 2nd year)(2014-09-18) Aschman, David; Buffler, AndyPHY2014F is a second-year half course, aimed primarily at students who are majoring in physics. VIBRATIONS AND WAVES: Harmonic oscillations, damped and forced oscillations, resonance, Fourier analysis, harmonic chains, waves, dispersion, interference, diffraction. ELECTROMAGNETISM: Vector calculus (div, grad, curl), electrostatics, special techniques for potentials, electric fields in matter, magnetostatics, magnetic fields in matter, current, Ohm's law, circuits, electromagnetic induction, electrodynamics, Maxwell's equations.
- ItemOpen AccessSearch for non-yrast states in ¹⁶°Yb(2008) Singo, Thifhelimbilu Daphney; Aschman, David; Bark, RobertThe search for the theoretically predicted tetrahedral states has been the source of much interest in recent years with studies of the nuclear structure at low spin states. In this work we have looked for low spin non-yrast states of the even-even nucleus ¹⁶⁰Yb populated through the electron capture or β⁺-decay of ¹⁶⁰Lu. The radioactive ¹⁶⁰Lu source was created through the heavy-io fusion evaporation reaction ¹⁴⁴Sm(¹⁹F,3n) at a beam energy of 85 MeV. Both ϒ-ϒ coincidences and single ϒ rays were detected using the AFRODITE Ge detector array at iThemba LABS. The computer program EONS was developed to measure the half-life of single ϒ rays for identification. The level scheme constructed from the ϒ-ϒ coincidence and single ϒ ray measurements confirms the existing level structure of ¹⁶⁰Yb. Population of low spin states was success and two new levels have been found, the 1755 ke V level decays by a 1512 keV ϒ ray and 1825 kev level decays through 1582 keV.
- ItemOpen AccessStudy of unsuspected gamma ray energy peaks observed during the determination of the half-life of activated aluminium(2018) Nel, Andre Pierre; Aschman, DavidIn the experimental determination of the half-life of the decay of a radioactive element, an aluminium cylinder is activated through immersion in a thermal neutron bath. The decay of the irradiated aluminium cylinder is then measured to determine the half-life of the resultant 28Al. During the execution of the experiment, unsuspected gamma ray peaks other than the expected 1.78 MeV from the decay of the activated 28Al are however observed. Two potential hypotheses are considered in the study. The first hypothesis is that the 27Al from which the aluminium cylinder is manufactured is not pure and contains certain impurities which could be activated resulting in the measurement of the decay of materials other than the intended 28Al. The second hypothesis is the inadequate moderation of the neutrons resulting in the interaction of fast neutrons with the aluminium cylinder and hence the creation of something other than the intended 28Al. Based on the results of the two experiments conducted, it has been demonstrated that there is some evidence, though not very convincing, that both postulated hypotheses potentially hold true. Considering the relatively small amounts of alloying elements in a typical aluminium sample and the poor correlation between published half-lives with the derived half-lives from the experiment, the hypothesis that the impurities in the aluminium from which the aluminium cylinder is manufactured are activated when immersed in the neutron bath resulting in the measurement of the decay of materials other than the intended 28Al, is considered possible but improbable. On the other hand, considering the pertinence of the decay signature of 27Mg in both experiments, the observed increase in the 27Mg activity in the unmoderated activation, the abundance of the target nuclei of 27Al, as well as the increase in the 27Al (n,p) 27Mg reaction with increased neutron energy, hypotheses 2 is deemed both possible and probable.
- ItemOpen AccessThe Relevance Of Load Following Capability Of Nuclear Power Plant For The South African Grid(2020) Qwemesha, Mbuyisi; Aschman, DavidThe main focus of this study is to evaluate whether load following operation can be performed using the existing South African nuclear power plant. Of which, knowing that will assist in determining whether the addition of load following capabilities on the planned nuclear fleet is justifiable or not. In this report the relevance of Koeberg Nuclear Power Plant to adapt to the demand is examined and the effects on plant operation simulated. The report analyses the operation of the existing nuclear power plant (Koeberg units) in South Africa and describes the regulations that govern safe operation of the plant. The Koeberg plant is analyzed based on the current design i.e. operating as a baseload station. This allows a prediction of the Koeberg plant response to big load variations. The simulation results of the load variation are analyzed and the results used to make the conclusion that the Koeberg units are not capable of load following safely. Modifying the Koeberg units from being base load station to load following will require changing the Safety Analysis Report and therefore affect the Koeberg license NIL-01.