Defended Dissertations in 2009

Darius Lukauskas
Author: Darius Lukauskas
Dissertation title: Study of the Radionuclide Composition of Radioactive Waste Streams in the Nuclear Power Plant
Fields of science: Physical sciences, Physics (02P)
Scientific supervisor: Prof. Dr. Vidmantas Remeikis
Defence of the dissertation: 2009-03-05
Continually increasing consumption of energy, expansion of new industrial branches and technology unavoidably cause problems related to the harmful impact of industrial waste on the environment and people. Due to the increasing technogenic load and declining self-regulation capabilities of the environment, the society is obliged to invest in the environment preserving development ways. Nuclear energy has no alternatives from this point of view. However, it is essential to ensure high nuclear safety and radiation protection level, resolve technological tasks of radioactive waste management, understand mechanisms of radionuclide migration  in  the environment and better conceive aspects of ionizing radiation impact on the environment and people. It is relevant to optimisation of occupational exposure and radiation protection of public during operation of nuclear facilities as well as processing, storing and disposal of radioactive waste. These issues are particularly relevant to decommissioning of nuclear facilities,  because many new technological and radiation safety aspects concerning large radioactive waste streams are not fully clear.
The work is closely related to the main task of energy in Lithuania during this decade - safe decommissioning of the Ignalina NPP. Modern radioactive waste management, utilization and disposal technologies shall be used, ensuring  long term safety and minimum impact of ionizing radiation on the environment and people.
During operation of nuclear power plants and their decommissioning, dismantling of installations and buildings, tens or more thousands cubic meters of radioactive wastes are generated. Those wastes are not equally hazardous from radiation safety and nuclear safety viewpoint due to different specific activities of radionuclide and other physicochemical characteristics. Hence, the first step in the assessment of potential radiological impact of radioactive waste is the estimation of the nuclide composition. It is an essential characteristic of operational radioactive waste and contamination of buildings and equipment for dividing radioactive waste to the streams for subsequent processing and disposal.  Lists of radionuclides, indicating significant radionuclides in assessment of ionizing radiation impact on the environment and people, are compiled in various countries. The united list of significant radionuclides suitable for  the  whole variety of nuclear facilities does not exist. Therefore, it is relevant to assess the nuclide composition of nuclear fuel and activated materials, considering materials of nuclear fuel and reactor structures and characteristics of neutron flux, and to define safety-relevant radionuclides from the radiation safety viewpoint. It is particularly relevant to nuclear power plants with RBMK-1500 reactors, as the spent nuclear fuel operational and other radioactive waste treatment, storage and disposal technologies during decommissioning of the plant are implemented for the first time. Accumulated scientific knowledge on the theoretical and experimental evaluation of the nuclide composition of operational and decommissioning radioactive waste would be  useful  when  selecting  optimal technologies, assessing possible scenarios of radionuclide migration from repositories, predictions and outcomes of unwanted radioactive pollution. The increasing demand for nuclear energy worldwide stimulates the relevance  of these problems and  solution of arisen tasks requires new scientific knowledge and its creative practical implementation.
Justina Šapolaitė
Author: Justina Šapolaitė
Dissertation title: Dynamics of Radioactive Nuclides in Heterogeneous Dispersive Systems
Fields of science: Physical sciences, Physics (02P)
Scientific supervisor: Dr. Galina Lujanienė
Defence of the dissertation: 2009-11-18
The aim of the work is to evaluate equilibrium and kinetic sorption parameters in heterogeneous systems as weH as to investigate sorption mechanisms and processes responsible for radionuclide removal from liquid medium and retention by artificial and natural barriers.
Andrej Rožkov
Author: Andrej Rožkov
Dissertation title: Artificial Radionuclides and their Interaction with Aerosol Particles in the Ignalina NPP and its Environment
Fields of science: Physical sciences, Physics (02P)
Scientific supervisor: Dr. Rimvydas Jasiulionis
Defence of the dissertation: 2009-03-05
137Cs is one of the most important artificial radionuclides released into the environment during nuclear weapon tests and the Chemobyl nuclear power plant (NPP) accident that detennines ionizing radiation doses to the population up till now.  Long­ term measurements of radionuclide activity concentrations in the ground-level air and deposition enable evaluating geophysical  factors that determine the change of radionuclide activity concentrations over a time [I]. These measurements carried out at the geophysical station of the Institute of Physics (Phi station), located  in the  lgnalina NPP impact zone, enable the investigation  of the impact of the lgnalina NPP and other sources of artificial radionuclide emission in the air [2].
The Ignalina NPP RBMK  type reactor contains one cooling loop with the branch for the overheated steam, in which the steam condensation takes place in the presence of high ionizing radiation doses. Conditions of radionuclide interaction  with  aerosol particles (temperature, water steam pressure, chemical composition) on the way from the radionuclide formation place to the ground-level air change in a broad range, which can result in the formation of radioactive aerosol particles due to the absorption of water molecules.
The classical method  of the radionuclide activity determination  in aerosol particle samples, collected on filters, does not provide information whether aerosol particles are fanned due to the absorption of water molecules. The leaching of aerosol  particle samples using the distilled water and measuring of radionuclide activities in  samples before and after the experiment was studied in order to investigate the presence of radionuclides  in soluble and insoluble fractions of aerosol particles.
The  objects  of this  work  were  artificial  radionuclides  and  their  interaction  with aerosol particles in the system "Ignalina NPP- environment".
Aurimas Čerškus
Author: Aurimas Čerškus
Dissertation title: Effect of Beryllium and Silicon Impurities on Radiation from GaAs/AlAs Quantum Wells
Fields of science: Physical sciences, Physics (02P)
Scientific supervisor: Prof. Habil. Dr. Algis Jurgis Kundrotas
Defence of the dissertation: 2009-10-23
The semiconductor nanostructures are grown  using  modem  technology  methods. Such structures are grouped into two-dimensional (2D), named quantum wells (QWs), one-dimensional (lD), named  quantum  wires,  and  zero-dimensional  (OD),  named quantum dots. The investigation and application of quantum  wells  and  superlattices  are one of the most developing areas of solid-state physics. This thesis focus on the physical properties of GaAs/AlAs quantum wells. They differ  from  GaAs/Al,Ga 1 ._,As  quantum wells because they achieve a higher barrier height and thereby obtain maximum energy levels splitting in these quantum wells.
Dainius Šalucha
Author: Dainius Šalucha
Dissertation title: Passivation of the p-n Junction Edge in High-Power Semiconductor Silicon Devices
Fields of science: Technological sciences, Material Engineering (08P)
Scientific supervisor: Dr. Irena Šimkienė
Defence of the dissertation: 2009-05-19
In this work, passivation of moat surface by means of electrochemical etching, formation of hydrogen-rich porous silicon layers and glass in-melting steps has been investigated. Passivation quality was controlled by the measurements of surface recombination characteristics after each technological step using a non-invasive technique, which employed microwave probed photoconductivity transients (MW­ PCT). It has been shown that electrochemical etching - glass melting steps involved in passivation technological procedures resulted in a decrease of surface recombination velocity from 3·103 cm/s to 10 cm/s. Thus, the passivation quality approached to that of suppressed surface recombination velocity obtained  for silicon wafers exploiting the iodine ethanol solutions.
Jelena Devenson
Author: Jelena Devenson
Dissertation title: Fabrication and Investigation of Heterostructures Based on Lanthanum Manganites
Fields of science: Technological sciences, Material Engineering (08P)
Scientific supervisor: Doc. Dr. Bonifacas Vengalis
Defence of the dissertation: 2009-08-18
During the last few decades, great progress in electronics has been achieved mainly due to the developing of modern technologies and complex research of electronic transport phenomena in thin semiconductor films and related device structures. Up to now, the silicon still remains the most important material of modern electronics. However, increasing attention is paid also to a number of new materials exhibiting specific electrical and magnetic properties which are not typical for Si.
New  ferromagnetic  materials  and  particularly  ferromagnetic  oxides  such  as  mixedvalence manganites referred to by the general formula R 1.x.AxMn03 (here = La, Nd, and A= Ca, Sr, Ba, Pb, Ce) and exhibiting spin-polarized carriers are of key importance for the development of spin-dependent electronics (spintronics). To meet increasing needs of spintronics there were attempts to prepare various multilayered device structures composed of ferromagnetic oxides, semiconducting and dielectric thin films. In addition, attention was paid also to various hybrid device structures possessing integration of the ferromagnetic oxides into Si-based electronic circuits. A number of experiments have been carried out to elaborate high crystalline quality manganite films and related heterostructures containing traditional semiconductors. Use of the manganite-based  heterostructures has been demonstrated recently for the development of magnetic field sensors, magnetic memory elements and other devices.
New promising possibilities for future spintronics could be expected due to the integration of the so-called multiferroic oxides into various multilayer device structures. This kind of materials exhibit simultaneously ferroelectric and ferromagnetic (antiferromagnetic) properties and unusually strong correlation between polarization vector and internal magnetic moment. Bismuth  ferrite (BiFe03) with perovskite-Iike  structure demonstrating  multiferroic properties at room temperature is one of the most promising materials belonging to this group of compounds, however electrical and magnetic properties of the compound has not yet been fully understood and technology of thin BiFe03 films and related heterostructures with other magnetic oxides such as manganites has not been elaborated.
Up to now, major attention of researchers was focussed mainly on magnetic and electrical properties of thin oxide films. Individual manganite  films and their properties have been studied relatively well meanwhile their heterostructures continue to surprise us with new phenomena. One can conclude that following needs of rapidly developing spintronics more attention needs to be focused on both preparation  and complex investigation of multilayered structures and heterostructures based on magnetic oxides as manganites and multiferroic compounds.