Diss. (sammanfattning) Umeå : Sveriges lantbruksuniversitet, 2009. / Härtill 4 uppsatser.

The present work has been produced as part of an on-going collaboration between the University of Manchester and Amec, with the primary aim of furthering mechanistic understanding of corrosion processes in zirconium alloys out-of-reactor. Zirconium alloys are used as cladding material for nuclear fuel pellets, and correct understanding of the corrosion process in autoclave is essential to predicting material behaviour in-reactor. This EngD thesis is composed of five proposed papers that investigate observations and hypotheses under the theme of mechanical degradation in oxides formed on zirconium alloys in autoclave. First investigations concern observed stress relaxation in zirconium oxide. Finite element analysis is used to capture mechanical aspects of the corrosion process and apply this to stress behaviour determined previously using synchrotron x-ray diffraction. The results indicate that a mechanism other than creep or hydrogen induced lattice strain must be present to account for the observed stress relaxation. One such potential mechanism is crack formation; statistical analysis of scanning electron microscopy images has been used to identify a link between the development of roughness at the metal-oxide interface, crack formation in the oxide and transition points or acceleration in the corrosion kinetics. Parameters such as the median radius of curvature and profile slope (Rdq) have been applied, as these parameters do not require the definition of a periodic wavelength or amplitude. These and other parameters are related to information in literature to indicate that for samples of Zircaloy-4 and ZIRLOTM, which go through transition, the interface roughness changes in a way that would increase localised stress concentrations. The third material is an experimental low tin alloy, which under the same oxidation conditions, and during the same time period, does not appear to go through transition and does not develop an interface roughness in the same way. A critical assessment of finite element analysis applied to oxidising non-planar interfaces shows the significant limitations in the existing mechanism for representing oxidation expansion and stress formation. Autoclave oxidation experiments of artificially roughened samples of Zircaloy-4 were carried out to further understand the impact of out-of-plane stress generation. The results indicate a divergence based on surface roughness after ~86 days oxidation. SEM examination of images in cross section highlighted accelerated oxidation above surface roughness peaks, and an increased crack area in rougher samples. Finally, finite element analysis of the tetragonal to monoclinic phase transformation showed that biaxial compressive stress relaxation, or the tri-axial tensile stress associated with an advancing crack tip, could reduce the transformation strain energy and destabilise the tetragonal phase. The volumetric expansion and shear strain associated with the phase transformation produces stress in the surrounding oxide sufficient to generate nano-scale cracks perpendicular to the metal-oxide interface. This would allow fast ingress routes for oxygen containing species, and therefore acceleration in the corrosion kinetics.

The aim of this work is to study microbial production of polyhydroxyalkanoates (PHA). Theoretical part is focused on production of PHA using microoganisms and transgenic plants. Bacterial strain Cupriavidus necator H16 was used for laboratory production of PHA. Various waste oils were used as sole carbon and energy source. Salt of propionic adic and 1-propanol were used as intermediate for 3-hydroxyvalerate monomer (3HV) unit. Incorporation of 3HV to polymer can improve material features of PHA. The major part of experimental work was focused to study influence of aeration (concentration of dissolved oxygen) to bacterial growth, selected metabolic pathways and formation of PHA. Furthermore, influence of aeration to monomer composition of polymer was evaluated. According to experimental conclusion of this work it was approved dependance between aeration and monomer composition of PHA. Moreover, it was approved that higher concentration of oxygen supports bacterial growth and influences PHA content in cells. In addition, NADPH is one of the substrates influencing flux of acetyl-CoA throughout the metabolism; higher intracellular concentration of NADPH inhibits TCA cycle and enhances accumulation of PHA in cells. For this reason, specific enzymatic aktivity of several selected intracelular enzymes were measured, including those enzymes which can generate NADPH.