<p> Formation of a high density of Mn-Ni-Si nanoscale precipitates in irradiated reactor pressure vessel steels could lead to severe, unexpected embrittlement, which may limit the lifetimes of our nation&rsquo;s light water reactors. While the existence of these precipitates was hypothesized over 20 years ago, they are currently not included in embrittlement prediction models used by the Nuclear Regulatory Commission. This work aims to investigate the mechanisms and variables that control Mn-Ni-Si precipitate (MNSP) formation as well as correlate their formation with hardening and embrittlement. </p><p> A series of RPV model steels with systematic variations in Cu and Ni contents, two variables that have been shown to have a dominant effect on hardening, were irradiated in a series of test reactor and power reactor surveillance irradiations. Atom probe tomography (APT) measurements show that large volume fractions (f_v) of MNSPs form in all the steels irradiated at high fluence, even those containing no added Cu, which were previously believed to have low sensitivity to embrittlement. It is demonstrated that while Cu enhances the rate of MNSP formation, it does not appear to significantly alter their saturation f_v or composition. The high fluence MNSPs have compositions consistent with known intermetallic phases in the Mn-Ni-Si system and have f_v very near those predicted by equilibrium thermodynamic models. In addition, X-ray diffraction experiments by collaborators shows that these precipitates also have the expected crystal structure of the predicted Mn-Ni-Si phases. </p><p> Post irradiation annealing experiments are used to measure the hardness recovery at various temperatures as well as to determine if the large f_v of MNSPs that form under high fluence neutron irradiation are thermodynamically stable phases or non-equilibrium solute clusters, enhanced or induced by irradiation, respectively. Notably, while post irradiation annealing of a Cu-free, high Ni steel at 425&deg;C results in dissolution of most precipitates, a few larger MNSPs appear to remain stable and may begin to coarsen after long times. A cluster dynamics model rationalizes the dissolution and reduction in precipitate number density, since most are less than the critical radius at the annealing temperature and decomposed matrix composition. The stability of larger precipitates suggests that they are an equilibrium phase, consistent with thermodynamic models. </p><p> Charged particle irradiations using Fe³⁺ ions are also used to investigate the precipitates which form under irradiation. Two steels irradiated to a dose of 0.2 dpa using both neutrons and ions show precipitates with very similar compositions. The ion irradiation shows a smaller f_v, likely due to the much higher dose rate, which has been previously shown to delay precipitation to higher fluences. While the precipitates in the ion irradiated condition are slightly deficient in Mn and enriched in Ni and Si compared to neutron irradiated condition, the overall similarities between the two conditions suggest that ion irradiations can be a very useful tool to study the susceptibility of a given steel to irradiation embrittlement. </p><p> Finally, the large f_v of MNSPs that are shown to form in all steels, including those low in Cu, at high fluence, even those without added Cu, result in large amounts of hardening and embrittlement. A preliminary embrittlement prediction model, which incorporates MNSPs at high fluence, is presented, along with results from a recent test reactor irradiation to fluences representative of extended lifetimes. This model shows very good agreement with the data.</p>

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007. / Source: Dissertation Abstracts International, Volume: 68-07, Section: B, page: 4797. Adviser: James F. Stubbins. Includes bibliographical references (leaves 164-171) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-05, Section: B, page: 3243. Adviser: James F. Stubbins. Includes bibliographical references (leaves 230-240) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-11, Section: B, page: 7075. Adviser: Robert S. Averback. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

<p> Developing new multifunctional materials in recent years for nuclear systems has become increasingly critical owing to the high demand on better shielding in extreme environments. The purpose of this research was to design, manufacture, and evaluate the feasibility of utilizing novel light weight close-cell composite metallic foam (CMF) and open-cell Al foam with fillers as radiation shields at nuclear facilities to attenuate the background of ionization radiations to a minimum level for creating a safer workplace, meeting regulatory requirements and maintaining high quality performance. </p><p> Steel-steel composite metal foams (S-S CMFs) and Aluminum-steel composite metal foams (Al-S CMFs) with various sphere sizes and matrix materials were manufactured and investigated for nuclear and radiation environments applications. 316L stainless steel, highspeed T15 steel and aluminum materials were used as the matrix material together with 2, 4 and 5.2 mm steel hollow spheres to manufacture various types of composite metal foams (CMFs). High-speed T15 steel is selected due to its high tungsten and vanadium concentration (both high-Z elements) to further improve the shielding efficiency of CMFs. This new type of S-S CMF is called High-Z steel-steel composite metal foam (HZ S-S CMF). Open-cell Al foams with fillers were obtained by infiltrating original empty pores with variety of hydrogen-rich compounds: petroleum wax, borated polyethylene, water, and borated water. </p><p> All the foams were investigated for their radiation shielding efficiency in terms of X ray, gamma ray and neutron. X-ray transmission measurements were carried out on a highresolution microcomputed tomography (microCT) system. Gamma-emitting sources: 3.0mCi ⁶⁰Co, 1.8mCi ¹³⁷Cs , 13.5mCi ¹²⁴Am, and 5.0mCi ¹³³Ba were used for gamma-ray attenuation analysis. The evaluations of neutron transmission measurements were conducted at the Neutron Powder Diffractometer beam facility at North Carolina State University. The experimental results were verified theoretically through XCOM and Monte Carlo Z-particle Transport Code (MCNP). </p><p> A mechanical investigation was performed by the means of quasi-static compressive testing. Thermal characterizations were carried out through effective thermal conductivity and thermal expansion analyses in terms of high temperature guarded-comparativelongitudinal heat flow technique and thermomechanical analyzer (TMA), respectively. The experimental results were compared with analytical results obtained from respectively Brailsford and Major&rsquo;s model and modified Turner&rsquo;s model for verification. Flame test was performed in accordance with United States Nuclear Regulatory Commission (USNRC) standard. CMF sample and a 304L stainless steel control sample were subjected to a fully engulfing fire with an average flame temperature of 800&deg;C for a period of 30 minutes. Finite Element Analysis was conducted to secure the credibility of the experimental results. </p><p> This research indicates the potential of utilizing the lightweight close-cell CMFs and open-cell Al foam with fillers as shielding material replacing current heavy structures with additional advantage of high-energy absorption and excellent thermal characteristics.</p>

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2009. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3760. Adviser: Roy Axford. Includes supplementary digital materials. Includes bibliographical references (leaves 260-263) Available on microfilm from Pro Quest Information and Learning.

Nuclear fuel performance is highly affected by the behaviour of fission gases, particularly<p>at elevated burnups, where large amounts of gas are produced and can<p>potentially be released. The importance of fission gas release was the motivation<p>for large efforts, both experimentally and theoretically, in order to increase our<p>understanding of the different steps of the process, and to continuously improve<p>our models.<p>Extensions to higher burnups, together with the growing interest in novel types<p>of fuels such as inert matrix fuels envisaged for the transmutation of minor actinides,<p>make that one is still looking for a permanently better modelling, based<p>on a physical understanding and description of all stages of the release mechanism.<p>Computer simulations are nowadays envisaged in order to provide a better<p>description and understanding of atomic-scale processes such as diffusion, but even<p>in order to gain insight on specific processes that are inaccessible by experimental<p>means, such as the fuel behaviour during thermal spikes.<p>In the present work simulation techniques based on empirical potentials have<p>been used, focusing in a first stage on pure uranium dioxide. The behaviour of<p>point defects was at the core of this part, but also the estimation of elastic and<p>melting properties.<p>Then, in a second stage, the study has been extended to the behaviour of helium<p>and xenon. For helium, the diffusion in different domains of stoichiometry<p>was considered. The simulations enabled to determine the diffusion coefficient and<p>the migration mechanism, using both molecular dynamics and static calculation<p>techniques. Xenon behaviour has been investigated with the additional intention<p>to model the behaviour of small intragranular bubbles, particularly their interaction<p>with thermal spikes accompanying the recoil of fission fragments. For that<p>purpose, a simplified description of these events has been proposed, which opens<p>perspectives for further work.<p>/<p>Les performances du combustible nucléaire sont fortement affectées par le comportement<p>des gaz de fission, et ce particulièrement lorsqu’un taux d’épuisement<p>élevé est atteint, puisque d’importantes quantités de gaz sont alors produites<p>et peuvent potentiellement être relâchées. Les enjeux, entre autre économiques,<p>liés au relâchement de gaz de fission ont donné lieu à d’importants efforts, tant<p>sur le plan expérimental que théorique, afin d’accroître notre compréhension des<p>différentes étapes du processus, et d’améliorer sans cesse les mod`eles. Les extensions<p>à des taux d’épuisements encore plus élevés ainsi que l’intérêt croissant pour<p>de nouveaux types de combustible tels que les matrices inertes, envisages en vue<p>de la transmutation des actinides mineures, font qu’à l’heure actuelle, le besoin<p>permanent d’une meilleure modélisation, basée sur une compréhension et une description<p>physique des différentes étapes du processus de relâchement de gaz de<p>fission, est toujours de mise.<p>Les simulations par ordinateur ont ainsi été considérée comme un nouvel angle<p>de recherche sur les processus élémentaires se produisant à l’échelle atomique, à la<p>fois afin d’obtenir une meilleure compréhension de processus tels que la diffusion<p>atomique ;mais aussi afin d’avoir accès à certains processus qui ne sont pas observables<p>par des voies expérimentales, tels que la le comportement du combustible<p>lors de pointes thermiques.<p>Dans ce travail, deux techniques, basées sur l’utilisation de potentiels interatomiques<p>empiriques, ont permis d’étudier le dioxyde d’uranium, dans un premier<p>temps en l’absence d’impuretés. Cette partie était principalement centrée sur le<p>comportement des défauts ponctuels, mais a aussi concerné différentes propriétés<p>élastiques, ainsi que le processus de fusion du composé.<p>Ensuite l’étude a été étendue aux comportements de l’hélium de du xénon. Pour<p>ce qui a trait à l’hélium, la diffusion dans différents domaines de stoechiométrie<p>a été considérée. Les simulations ont permis de déterminer le coefficient de diffusion<p>ainsi que le mécanisme de migration lui-même. Quant au xénon, outre les<p>propriétés de diffusion, l’intention fut de se diriger vers la modélisation des petites<p>bulles intragranulaires, et plus précisément vers leur interaction avec les pointes<p>thermiques, créées lors du recul des fragments de fission. Une description simplifiée de ce processus a été proposée, qui offre de nouvelles perspectives dans ce<p>domaine.<p><p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished