Photoelectron Spectroscopy of High Temperature Superconductors and Related Compounds

In this thesis, the high temperature superconductors (HTSC) Ba0.6K0.4BiO3, La2CuO4+x, Bi2Sr2Ca1-xYxCu2O 8+d, Bi2Sr2CuO6+d as well as semiconducting Ba0.9K0.1BiO3 and the strongly magnetoresistant material La1-xSrxMnO3 have been studied by photoelectron spectroscopy. By using the tunability of synchrotron radiation, techniques such as resonant photoemission and x-ray absorption spectroscopy have been applied. The aim of the studies has been to investigate the electronic structure of these compounds with an emphasis on strong electron-electron correlation, the distribution of valence states, band structure effects, the importance of oxygen nonstoichiometry, and the influence of metal overlayers. Experimental results from the high quality single crystals are compared to band structure calculations and atomic absorption calculations. Resonant photoemission was performed on Ba1-xKxBiO3 at photon energies in the vicinity of the Ba 4d, Ba 5p and O 1s core levels. The enhancements seen in the valence band at photon energies corresponding to the Ba 5p-5d excitation are assigned to Ba states. From the resonance at the Ba 4d threshold, Ba states were also identified close to the Fermi level whereas no resonant enhancement was seen for photon energies close to the O K absorption threshold. In addition, angle resolved measurements were used to identify the G and X high symmetry points in Ba0.6K0.4BiO3, and laterally resolved photoelectron microscopy showed a strongly varying Fermi level intensity on a micron length scale. For Bi2Sr2CaCu2O8+d and Bi2Sr2CuO6+d, a satellite enhancement was seen for photon energies close to the main oxygen K-edge absorption threshold. From the data, the values of the electron-electron interaction energies Udd and Upp, as well as the charge transfer energy D, can be found. The core level shifts and the valence-band shape in cuprate HTSC were also investigated by means of yttrium substitution and surface derived electronic bands. For La1-xSrxMnO3, core-level spectroscopy and x-ray absorption measurements indicated that x and temperature dependent changes in the electronic structure can be connected to the various magnetic phases. Temperature dependent resonant photoemission at the O K and Mn L3 edges showed enhanced spectral intensity close to the Fermi level and for photon energies around the Mn L edge, strong enhancements were seen in the valence band. The importance of small deviations in oxygen content is common for all compounds investigated in this thesis, although x-ray absorption and resonant photoemission measurements showed that O 2p holes play different roles in the doping mechanisms of cuprate and bismuthate high temperature superconductors as well as of magnetoresistant manganites

Surveillance and diagnostics of the beam mode vibrations of the ringhals pwrs

Surveillance of core barrel vibrations has been performed in the Swedish Ringhals PWRs for several years. This surveillance is focused mainly on the pendular motion of the core barrel, which is known as the beam mode. The monitoring of the beam mode has suggested that its amplitude increases along the cycle and decreases after refuelling. In the last 5 years several measurements have been taken in order to understand this behaviour. Besides, a non-linear fitting procedure has been implemented in order to better distinguish the different components of vibration. By using this fitting procedure, two modes of vibration have been identified in the frequency range of the beam mode. Several results coming from the trend analysis performed during these years indicate that one of the modes is due to the core barrel motion itself and the other is due to the individual flow induced vibrations of the fuel elements. In this work, the latest results of this monitoring are presented

Investigation of the ex-core noise induced by fuel assembly vibrations in the Ringhals-3 PWR

© 2015 Elsevier Ltd. The effect of cycle burnup on the ex-core detector noise at the frequency of the pendular core barrel vibrations in the Ringhals-3 PWR core was investigated using a neutron noise simulator. The purpose of the investigations was to confirm or disprove a hypothesis raised by Sweeney et al. (1985) that fuel assembly vibrations could affect the ex-core detector noise and cause the corresponding peak in the auto power spectral density (APSD) to increase during the cycle due to the effects of fuel burnup, the change of boron concentration, flux redistribution etc. Numerical calculations were performed by modelling the vibrations of fuel assemblies at different locations in the core and calculating the induced neutron noise at three burnup steps. The APSD of the ex-core detector noise was evaluated with the assumption of vibrations either along a straight-line or along a random two-dimensional trajectory, with two different representations of the cross section perturbations caused by the vibrations. The results show the obvious effect of in-core fuel vibrations on the ex-core detector noise, but the monotonic increase of the APSD does not occur for all fuel elements, vibration types and cross section perturbation models. Such an increase of the of APSD occurs predominantly for peripheral assemblies with one of the perturbation models. However, assuming simultaneous vibrations of a number of fuel assemblies uniformly distributed over the core with random vibrations and the more realistic perturbation model, the effect of the peripheral assemblies will dominate and hence the surmised monotonic increase of the amplitude of the ex-core neutron noise during the cycle can be confirmed

Surveillance and diagnostics of the beam mode vibrations of the ringhlas PWRs

Surveillance of core barrel vibrations has been performed in the Swedish Ringhals PWRs for several years. This surveillance is focused mainly on the pendular motion of the core barrel, which is known as the beam mode. The monitoring of the beam mode has suggested that its amplitude increases along the cycle and decreases after refuelling. In the last 5 years several measurements have been taken in order to understand this behaviour. Besides, a non-linear fitting procedure has been implemented in order to better distinguish the different components of vibration. By using this fitting procedure, two modes of vibration have been identified in the frequency range of the beam mode. Several results coming from the trend analysis performed during these years indicate that one of the modes is due to the core barrel motion itself and the other is due to the individual flow induced vibrations of the fuel elements. In this work, the latest results of this monitoring are presented

Influence of Local Spectral Variations on Control-Rod Homogenization in Fast Reactor Environments

Advanced fast reactor concepts, such as the CFV core (French acronym of "Coeur a Foible effet de Vide Sodium," meaning "low sodium void effect core"), are characterized by a heterogeneous axial core arrangement, with an inner fertile zone and a sodium plenum above the fuel. Such concepts represent a strong challenge for accurate predictions of the control-rod antireactivity effects, and the surrounding local fuel pin power. Classical equivalence procedures, which were developed for axially homogeneous cores, are put to the test when applied to such axially heterogeneous cores. In this work, we investigate the influence of variations in the local neutron spectra, for different control-rod environments, with the objective of understanding the impact of spectral variations in control-rod homogenization. This was conducted by considering a simple one-dimensional model of the equivalence procedure in which a transition zone between the fuel and control rod was introduced to represent different control-rod environments. Two types of situations were studied, one corresponding to softened neutron spectrum environments, for which the impact in the homogenized control-rod cross section was found to be smaller than 5%. The second situation was with wide elastic scattering resonances in the control-rod environment, which could locally lead to differences of up to 15% in the resulting equivalent cross sections. The reactivity effect of these changes was calculated to be less than 2%. In some cases, the numerical stability of the equivalence procedure was adversely affected, mainly in high-energy groups, due to the softening of the neutron spectra

RECONSTRUCTING THE AXIAL VOID VELOCITY PROFILE IN BWRs FROM MEASUREMENTS OF THE IN-CORE NEUTRON NOISE

The problem of determining the axial velocity profile from the in-core neutron noise measurements is revisited, with the purpose of developing an objective method for the determination of the void fraction. Until now it was assumed that in order to determine a realistic velocity profile which shows an inflection point and hence has to be at least a third order polynomial, one needs four transit times and hence five in-core detectors at various axial elevations. However, attempts to determine a fourth transit time by adding a TIP detector to the existing four LPRMs and cross-correlate it with any of the LPRMs were unsuccessful so far. In this paper we thus propose another approach, where the TIP detector is only used for the determination of the axial position of the onset of boiling. By this approach it is sufficient to use only three transit times. Moreover, with another parametrisation of the velocity profile, it is possible to reconstruct the velocity profile even without knowing the onset point of boiling, in which case the TIP is not needed. In the paper the principles are explained and the strategy is demonstrated by concrete examples

RECONSTRUCTING THE AXIAL VOID VELOCITY PROFILE IN BWRs FROM MEASUREMENTS OF THE IN-CORE NEUTRON NOISE

The problem of determining the axial velocity profile from the in-core neutron noise measurements is revisited, with the purpose of developing an objective method for the determination of the void fraction. Until now it was assumed that in order to determine a realistic velocity profile which shows an inflection point and hence has to be at least a third order polynomial, one needs four transit times and hence five in-core detectors at various axial elevations. However, attempts to determine a fourth transit time by adding a TIP detector to the existing four LPRMs and cross-correlate it with any of the LPRMs were unsuccessful so far. In this paper we thus propose another approach, where the TIP detector is only used for the determination of the axial position of the onset of boiling. By this approach it is sufficient to use only three transit times. Moreover, with another parametrisation of the velocity profile, it is possible to reconstruct the velocity profile even without knowing the onset point of boiling, in which case the TIP is not needed. In the paper the principles are explained and the strategy is demonstrated by concrete examples

Investigation of the Equilibrium Core Characteristics for the Ringhals-3 PWR with Improved Thermal Margins Using Uranium-thorium Fuel

Nowadays, PWR core thermal margins have decreased because of higher enrichment of U-235 in the fuel assemblies to achieve higher discharge burnup, higher core power to improve economical competiveness, and reduction of fast neutron leakage to prevent rapid ageing of the pressure vessel. Thus, a new uranium-thorium-based fuel assembly is proposed for improving the thermal margins. The proposed fuel is simulated for the Swedish Ringhals-3 PWR equilibrium core in a realistic demonstration. The difference between the uranium-thorium and traditional fuel assemblies is the fuel pellets composition. In the uranium-thorium fuel assembly, 7 weight percentage of thorium oxide is added to some of the fuel pellets. In those fuel pellets, the enrichment is kept below 5 % enrichment of U-235 in order to avoid extra certification on transport, management and storage. The two most significant results showed in this paper are the improved pin peak power, and the axial offset in the core. The pin peak power margin could be improved by 75 % by using the uranium-thorium-based fuel compared with the reference core containing traditional fuel assemblies. The maximum axial offset was decreased by 33 % using the uranium-thorium-based fuel compared with the reference core. The improved thermal margins in the core would allow more flexible core designs with less neutron leakage, or could be used for power uprates while guaranteeing sufficient safety margins

Reducing axial offset and improving stability in PWRs by using uranium-thorium fuel

The axial offset (AO) is a parameter used to monitor the core axial power distribution, and is important for safe reactor operation. Previous works have showed that a new type uranium-thorium (U-Th) based fuel assembly could be used to achieve a more homogenous radial power distribution, and thereby improve the thermal margins in the core. In this paper, it is shown that a core containing U-Th fuel assemblies also reduces the AO and improves the core stability. For an equilibrium U-Th core, the maximum AO could be reduced by 33%, and the minimum AO could be reduced by 8% compared with a standard uranium core. In the U-Th core, the maximum AO was influenced by the more negative moderator temperature coefficient of reactivity, while the minimum AO is mainly affected by the more homogenous radial power distribution. These findings conclude that the U-Th core displays a more homogenous power distribution both axially and radially, thus offering better thermal margins for power uprates and flexible power operation. Moreover, xenon oscillations in the core due to perturbations induced by the control rods are 30% more effectively damped for the U-Th core compared with the standard core, improving the stability in AO. © 2014 Elsevier Ltd. All rights reserved