Papers by Keyword: Nuclear Materials

Abstract: The high sensitivity of positrons to directly probe atomic scale defects revealing their structure and characteristics makes it a unique tool in materials science research covering all types of materials from hard to soft matter. This review focuses on applications of positron annihilation spectroscopy (PAS) in hard materials. However, it is not intended as a comprehensive review of the foundations of positron annihilation spectroscopy and description of its techniques. These exist in numerous publications cited in this review. Instead, the aim here is to facilitate employing PAS and interpretation of its measurements by illustrating the advantages, limitations, and challenges and guiding the reader on how to overcome technical problems and how to interpret PAS results in meaningful ways. Applications of PAS in electronic and photonic materials, nuclear and irradiated materials, and engineering materials are discussed. Examples are given to guide the reader on how PAS can be combined with complementary methods to uncover the fundamentals of defect physics and reveal interesting new phenomena in condensed matter.

Abstract: Uranium silicides are envisaged as potential nuclear materials for the next generation. U₃Si is featured by the high actinide density and the better thermal conductivity relative to UO₂. To properly and safely utilize nuclear materials, it is crucial to understand their chemical and physical properties. First-principles in theory is mostly used to analyze the point defect structures for uranium silicides nuclear fuels. The lattice parameters of U₃Si and USi₂ are calculated and the stability of different types of point defects are predicted by their formation energies. The results show that silicon vacancies are more prone to be produced than uranium vacancies in β-USi₂ matrix. The most favorable sites of fission products are determined in this work as well. According to the current data, rare earth elements cerium and neodymium are found to be more stable than alkaline earth metals strontium and barium in a given nuclear matrix. It is also determined that in USi₂ crystal lattice fission products tend to be stabilized in uranium substitution sites, while they are likely to form precipitates from the U₃Si matrix. It is expected that this work may provide new insight into the mechanism for structural evolutions of silicide nuclear materials in a reactor as well as to provide valuable clues for fuel designers.

Abstract: Diffusion at infinite dilution of U in metals, with particular emphasis in those used in nuclear facilities, is revisited. Early works present some particularities such as activation enthalpies lower than the vacancy formation enthalpy in the matrix, large differences with self-diffusion in the base material, up to four orders of magnitude differences between measurements performed by different authors in similar temperature ranges, etc. In particular U self-diffusion was qualified as abnormal when compared with other metals. Recent studies by means of α-spectrometry reveal a normal behaviour: activation enthalpies and pre-exponential factors similar to the self-diffusion one and diffusion coefficient values in the same order of magnitude than self-diffusion. The possible influence of short circuits, impurities and/or uncertainties in the techniques used in the early works is discussed in order to explain the differences obtained.

Abstract: The paper presents an approach to evaluating the effectiveness of physical protection systems (PPS). Considering the fact that different processes are subject to universal physical laws and principles of development, a parallel was drawn between the concepts of conflict resolution field to description of interaction in the system “outsider – PPS”. The description of mathematical model in the form of dependence with respect to the real conflict in the system “outsider – PPS” is given. The result of the model analysis is determination of the parameter, allowing estimating the rate of threat increase in conflict situation. Also competence of PPS operator is considered in the framework of the work. Furthermore, actions of operator were taken as a key factor affecting the efficiency of the whole security system. To resolve identified issues ways to develop special qualities required for the operator and their professional skills through training were suggested. Also the issues of control of psychophysiological characteristics of PPS staff were addressed.

Abstract: Ceramic matrix composites (CMC’s), particularly silicon carbide (SiC) fiber-reinforced SiC-matrix (SiC/SiC) composites, have been studied for advanced nuclear energy applications for more than a decade. The perceived potentials for advanced SiC/SiC composites include the ability to operate at temperature regimes much higher than heat-resistant alloys, the inherent low inducedactivation nuclear properties, and the tolerance against neutron irradiation at high temperatures. This paper reviews the recent research and development of the advanced radiation-resistant SiC/SiC composites for nuclear applications. Additionally, remaining general and specific technical issues for SiC/SiC composites for nuclear applications are discussed.