Papers by Keyword: Radiation Damage

Abstract: Research and development of materials in the nuclear industry, including the assessment of irradiated components in nuclear power plants, rely heavily on research infrastructures that facilitate the preparation and analysis of radioactive samples. The metallographic preparation of samples must be carried out in shielded, hermetically sealed boxes and hot cells, as a large amount of dusty radioactive particles is released during the sample preparation process. Small samples, thin films for scanning and transmission electron microscopy (SEM, TEM), and powdered samples from biological shielding concretes are prepared in glove boxes, where a constant negative pressure is maintained. The enclosures of the boxes are constructed of thick-walled steel plates to shield against the ionizing radiation emitted by radioactive samples, protecting personnel. Subsequent microscopic analyses allow for the assessment of material degradation in operational nuclear power plant components caused by radiation-induced microstructure damage. These analyses focus both on existing materials, with the aim of extending the lifespan of nuclear power plants, and on newly tested materials irradiated as part of domestic and international programs, including those conducted at research reactors like LVR-15, operated by the Řež Research Center (CVŘ). Different analytical requirements call for the preparation of various types of samples. At CVŘ, X-ray diffraction analysis of powdered samples is primarily used to evaluate structural and phase changes in the cement and aggregate of concrete structures caused by radiation aging, which can impact the overall integrity of the structure. Monitoring these changes and predicting material behavior are essential for evaluating the safety, stability and durability of concrete used in biological shielding, containment structures, spent nuclear fuel storage pools and future deep radioactive waste repositories. Thin films for electron microscopy are prepared specifically to assess detailed changes, such as radiation-induced microstructure damage in reactor internals or fuel cladding, which result in dimensional changes and the degradation of mechanical properties due to neutron radiation.

Abstract: Lubricants and O-rings are necessarily used for the construction of many accelerator-driven facilities as spallation sources or facilities for the production of radioactive isotopes. During operation, such component will absorb high doses of mixed neutron and gamma radiation, that can degrade their mechanical and structural properties. Experimental radiation damage tests of these components are mandatory for the construction of the facility. Methodologies for irradiation in nuclear reactor mixed fields and post-irradiation examination of lubricating oils, greases and O-rings were developed and are here presented. Samples were characterized with standard mechanical and physical-chemical tests. Parametric studies on the dose rate effects have been performed on O-rings. A case studies for a specific O-ring application in a gate valve has been developed. Some of the tested samples showed a dramatic change of their properties with dose, while others remain stable. Results were collected on nine commercial greases, on one oil and on four commercial elastomeric O-rings. The most radiation resistant among the selected products are now considered for application in facilities under construction. The main mechanisms of neutron and gamma radiation damage on these polymers were investigated at the mechanical and structural level.

Abstract: The degradation of austenitic stainless steels under irradiation environment is a known problem for nuclear reactors, which starts from atoms displacement cascade. Here, molecular dynamics (MD) simulations have been used to investigate the formation of atomic displacement cascade in γ-iron for energies of the primary knock-on atom (PKA) up to 40 keV at 300 K. The number of Frenkel pairs increased sharply until a peak value was reached, which occurred at a time in the range of 0.1-2 ps. After that, a number of defects gradually decreased and became stabilized. Compared with α-iron, there was less defects in the stable stage, and more clustered defects were produced in γ-iron. Within the range of PKA energies, two regimes of power-law energy-dependence of the defect production were observed, which converge on 16.8 keV. The transition energy also marks the onset of the formation of large self-interstitial atom (SIA) clusters and vacancy clusters. Interstitial and vacancy clusters were in the form of Shockley, Frank dislocation loops and Stir-Rod dislocation loops.

Abstract: The oxide dispersed strengthened (ODS) ferritic-martensitic steel was irradiated by 100MeV iron ion whose energy was degraded by using a Ta foil of 4 μm thick, 100 keV Hydrogen and 200 keV Helium at 480, 515, 550 and 580 °C. The irradiation fluences were 1×10¹⁶, 1.1×10¹⁵ and 6.8×10¹³/cm², respectively for Fe, H and He. The techniques of positron annihilation lifetime and Doppler broadening of slow positron beam were utilized to examine the produced radiation damage. At 550 °C the maximal positron annihilation lifetime and S parameter of Doppler broadening were observed, implyin g tha t 550 °C is the pea k temperature of swelling. The S parameter and annihilation lifetime of the sample irradiated at 515 °C by the single Fe ion beam were smaller compared to the triple beam irradiation at the same temperature, implying that the triple beam irradiation caused more severe damage than the single beam irradiation.

Abstract: Small prismatic dislocation loops in BCC metals have Burgers vectors either ½<111> or <100> and are usually close to circular shape. In atomistic simulations constructing prismatic dislocation loops of different shapes is straightforward, however, it is difficult to compare their formation energies, since loops of different shapes or different Burgers vectors do not necessarily have exactly the same size. Here we develop a general method to correctly compare loops of similar size but different shapes and the Burgers vectors. This method is combined with molecular statics simulations to identify the most energetically favorable shapes of prismatic dislocation loops in elastically isotropic tungsten and anisotropic α-iron.

Abstract: Neutron bombardment on semiconductor material causes defects, one such primary physical effect is the formation of displacement defects within the crystal lattice structure, and such defects effectively decrease the mean free path and thus shorten the recombination time. Ionizing radiation causes creation of electron-hole pair in the gate oxide and in parasitic insulating layers of the MOS devices. Calculations show increase of the dark current in depletion region caused by a single neutron. Determination of energy and angular distribution of primary knock on atoms, with 14 MeV neutron irradiation in silicon are presented.

Abstract: The interaction of atom-atom collisions cascades with Ni-Al interphase boundary was studied by the method of molecular dynamics. It was shown that the interphase boundary partially absorbs the cascade energy. The degree of energy absorption of the cascade by the interphase boundary increases with the growth of the structural imperfection of the boundary and density of the misfit dislocations, and also with increase of distance between the boundary and the place of the cascade initiation.

Abstract: 4H-SiC Schottky barrier diodes (SBDs) were irradiated to neutron fluence of 3.55 x10¹⁶ cm^-2 and 6.6 x 10¹⁵ cm^-2 (15,000 kGy) electrons respectively. In general, characterization of the irradiated samples show that the current characteristics of the diodes decreased. The performance of Schottky gate contact is less for electron irradiated sample compared to neutron irradiated sample. The d-spacing, crystallite sizes and lattice strains were calculated from X-ray diffraction (XRD) measurements. SiC Schottky interface damage and radiation defects, as observed in atomic force microscopy (AFM) topography and scanning electron microscope (SEM) morphology images is possibly the main reason for this reduction in performance.

Abstract: Passage of heavy ions produces radiation-damage trails known as latent tracks in a variety of solid-state nuclear-track detectors (SSNTDs). These tracks are made visible in an optical microscope by a simple process known as chemical etching. It is a well-known fact that latent tracks are radiation damage trails in SSNTDs, which can be annealed by thermal heating. Modgil-Virk formulation of single-activation-energy model of radiation damage annealing was proposed as an empirical approach for explaining the thermal fading of nuclear tracks in SSNTDs. The empirical formulation of this model is based on track annealing data collected from both isothermal and isochronal experiments performed on different types of SSNTDs using a variety of heavy ion beams and fission fragments. The main objective of this empirical model was to resolve some contradictions of variable activation energy derived by using Arrhenius plots to study annealing in mineral SSNTDs. Some equivalent versions of the Modgil-Virk model have been proposed but the concept of single activation energy is vindicated in all empirical formulations. The model always yields a unique value of activation energy independent of the nature of the ion beam used and the degree of annealing. The anisotropy of the mineral SSNTDs is revealed by variation in activation energy along different crystal planes and even with different orientations of the ion beam on the same plane. Some recent experiments are a pointer to the successful exploitation of this model for future cosmic-rays studies using SSNTDs.

Abstract: Long-term effects of time-dependent solarization behavior on optical density and optical band gap of the gamma and beta irradiated Ta₂O₅ thin films were investigated to examine the optical aging of films at different absorbed dose levels. Irradiation process caused to induce defect centres. The population inversion of the colour centres was increased by controlling the absorbed dose at certain radiation dose levels. The efficient absorption of photons in tantalum reduced transmittance. The grain boundaries of the Ta₂O₅ thin film decreased at the end of irradiation process. The average grain boundaries changed after the irradiation process at 4.5 kGy and average grain boundaries decreased from ~ 11.5 to 5.5 μm. The optical fading behavior of induced color centers addressed to the potential opportunities of Ta₂O₅ thin film by monitoring absorption of photons in optoelectronic technology. Optical fading of the Ta₂O₅ thin film presented an importance to investigate the material stored ionizing radiation dose in it with the increase of elapsed time after the irradiation treatment.