CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)
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Research activities in the Czech Republic focusing on materials and technologies for advanced helium-cooled nuclear reactors
Full text linkOrganisations in the Czech Republic are involved in international research and development of advanced helium-cooled nuclear reactors, including both the very-high-temperature reactors (VHTRs) and the gas-cooled fast reactors (GFRs). To support this effort, a dedicated research infrastructure has been developed and constructed, incorporating large-scale facilities, such as the High-Temperature Helium Loop (HTHL), the S-Allegro helium loop, and other specialised equipment. Current studies are investigating the resistance of structural materials in high-temperature helium environments. Various metallic alloys and ceramic materials intended for high-temperature applications are tested at 750–900 °C. Additional activities focus on helium-coolant technologies – particularly purification, purity monitoring, recovery, and primary-circuit sealing – and on reactor safety and system behaviour under off-normal and emergency conditions
Testing the debris fretting phenomenon of new perspective materials under LWR conditions
Full text linkDespite decades of advancements and research aimed at improving fuel’s physical properties, fretting damage to fuel cladding remains a significant challenge. This study outlines the findings of experiments conducted on Zr-1%Nb alloy coated with protective layers composed of chromium-aluminium (CrAl) compounds and other new perspective materials, such as FeCrAl and CrNi alloys, at the Research Centre Řež. The experiments included debris-fretting tests performed under LWR conditions (320 °C and 15 MPa) and microscopic evaluations of groove depths. The primary goals were: (1) to evaluate the debris-fretting resistance of uncoated Zr-1%Nb samples compared to Zr-1%Nb with coatings and new alloys, and (2) to investigate the potential effects of coatings and new materials on the durability of fuel cladding. The conducted experiments and studies confirmed the benefits of the wear resistance of the ATF types of claddings in comparison to the standard Zr-1%Nb cladding material. Various parameters, such as wire wear and the influence of mutual position of the samples, were considered during the testing to provide the most precise insight into the wear resistance of the tested samples
Influence of thermal and moisture ageing on the mechanical behaviour of conventional and polymer-modified asphalt mixtures
Full text linkThis study aims to improve our understanding of how ageing affects the performance of asphalt mixtures used in pavements, focusing on two commonly applied binders: conventional Petroleum Asphalt Cement (PAC 50/70) and Polymer Modified Asphalt (PMA 55/75). Considering the tropical climate in Brazil, where temperature and moisture both play a crucial role in pavement durability, four ageing conditions were simulated: no ageing, short-term thermal ageing, long-term thermal ageing, and long-term thermal ageing combined with moisture exposure. The results indicate that ageing increases binder stiffness and improves resistance to permanent deformation, especially in polymer-modified mixtures. However, exposure to moisture reduces this resistance, primarily affecting mixtures with conventional binders as a result of adhesive failure. Statistical analysis confirms that there are significant differences in susceptibility to ageing between the two binder types. Overall, the polymer modification enhances the mixture is resilience against combined thermal and moisture ageing. These findings highlight the importance of incorporating realistic ageing scenarios and including moisture effects in laboratory evaluations to better predict the performance of pavements in tropical regions
Influence of MR thermometry on predicted temperature during regional hyperthermia treatment
Full text linkHyperthermia treatment involves heating tissues to 40–44 °C in order to enhance the efficacy of radiotherapy and/or chemotherapy. Temperature increase is usually induced by the constructive interference of several electromagnetic waves radiating from external sources and can be monitored using magnetic resonance (MR) thermometry. This study aimed to predict temperature increases from non-invasive MR thermometry electromagnetic exposure during deep regional hyperthermia treatment. A 1.5T MR birdcage coil was tuned to produce a homogeneous B1+ field and simulated together with the Sigma Eye hyperthermia applicator and three available patient models. Results show that electromagnetic exposure during MR measurements increases temperature by 0.027 °C, which is, in the hyperthermia temperature range, insignificant. In addition, our results show that the predicted B1+ field homogeneity was influenced by object size and material properties, especially by the water bolus filling the inner part of the applicator
Coordinated operation of battery swapping and charging stations for electric vehicles: a review
Full text linkOver the past decades, the number of electric vehicles (EVs) has grown rapidly due to advances in EV technologies, charging infrastructure, public charging services, and improvements in battery materials. The governments in many countries have implemented policies to prohibit fossil fuels and reduce greenhouse gas emissions. Nevertheless, EVs require longer charging times than refueling at gasoline stations because of charger limitations and battery characteristics, which hampers the promotion of EVs. Consequently, batteries can be swapped with fully charged ones within a few minutes at Battery Swapping Stations (BSS) similar to existing gasoline stations. However, Charging Stations (CS) are required to charge the batteries in EVs due to a lack of swapping facilities and individual constraints. Recently, researchers have examined the Battery Swapping and Charging Stations (BSCS) approach by proposing optimization methods, operational models for BSS and CS service operators, and implementations of BSS and CS at private and commercial locations. This study provides a comprehensive review of the key challenges associated with BSS and CS, including optimal location selection, coordination, charging infrastructure, charging strategies, feasibility, battery ownership models, grid integration, and battery management. Also, this article identifies key research gaps and future directions, emphasizing the need for technological innovations. It also provides integrated solutions to overcome these challenges and enhance the overall efficiency of EV infrastructure
Robust implementation of elastoplastic constitutive models using automatic differentiation in PyTorch
Full text linkThis paper explores the use of automatic differentiation (AD) for implementing complex elastoplastic constitutive models in finite element analysis. Traditional approaches require manually deriving and coding the derivatives of residual functions governing implicit stress return mapping, a process that becomes cumbersome for advanced models. We demonstrate how AD can simplify the implementation of consistent material stiffness operators and improve code maintainability by using PyTorch and its autograd functionality. A drained triaxial shear test is used to compare AD with manually coded and finite difference derivatives, highlighting the efficiency of the proposed approach. The example shows that AD simplifies code development and reduces the required source code by over 50 %. These findings support the use of AD as a practical approach for implementing and testing constitutive models, especially in the early development stages
The impact of fire flames on the mechanical characteristics of basalt fiber-reinforced geopolymer concrete composed of sustainable components
Full text linkThe use of geopolymer concrete (GPC) has been proposed to reduce carbon dioxide (CO2) emissions linked to the cement production. Fire poses a significant risk to concrete structures, as it causes mechanical degradation of the concrete. This research used 70% Granulated Ground Blast Furnace Slag (GGBFS) and 30 % Fly Ash (FA) to synthesis Geopolymer Concrete (GPC). The alkaline activation solution was created by mixing sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) at a concentration of 12 molarity. The ratio of the solution to the cementitious material was 0.4. The weight ratio of sodium silicate to sodium hydroxide was 2.5:1. Basalt fibres were used for reinforcement at volume fractions of 0.5 %, 0.75 %, and 1 %. The geopolymer concrete specimens were subjected to an elevated temperature of 70 °C in an oven for 4 hours, which is similar to the curing time of 2 days. After 56 days, the specimens were burned at three different temperatures of 300 °C, 500 °C, and 700 °C for one hour. The required tests (compressive strength, flexural strength, splitting tensile strength, and mass loss percentage) were conducted before and after the burning procedure. The residual compressive strength percentages obtained were 90 %, 73 %, and 61 %, the residual flexural strength percentages were 91 %, 70 %, and 51 %, the residual splitting tensile strength percentages were 89 %, 68 %, and 50 %, and the mass loss percentages were 1.271 %, 1.557 %, and 2.035% for a 1% ratio of basalt fibre at 300 °C, 500 °C, and 700 °C, respectively. Geopolymer concrete is highly resistant to flames, even at temperatures of up to 700 °C
Experimental investigation of the effect of test speed on rupture performance of synthetic yarns
Full text linkThis study experimentally investigated the effect of testing speed on the mechanical behaviour of five types of synthetic fibres – aramid, high-modulus polyethylene (HMPE), liquid crystal polymer (LCP), polyamide, and polyester – through yarn break load (YBL) tests, ISO 2062. Twelve different speeds were evaluated, ranging from 50 to 1 000 mm min−1. The discrete rupture force and rupture strain results were stable and showed no statistically significant variations as a function of speed, which was reflected in the fitted mathematical models exhibiting stabilised curves across the studied range. The stress-strain curves also showed a high degree of similarity, with a particular emphasis on LCP, which demonstrated extremely consistent behaviour across all tested speeds. For HMPE, a slight trend of increased strain at lower speeds was observed, attributed to its viscoelastic nature. The tangent modulus curves revealed more noticeable variations in the initial tangent modulus response for each fibre, where lower speeds resulted in steeper slopes, indicating a greater instantaneous stiffness. It is concluded that, although the testing speed does not affect the overall rupture parameters (rupture force and strain), it can influence the behaviour in the initial regions, particularly regarding the determination of modulus and stiffness
Comparison of field pile load tests with the theoretical static equations using soil parameters
Full text linkPile bearing capacity is simply the maximum load that a pile can withstand before excessive settlement. Its estimation is usually complex due to various factors, such as installation techniques, construction methods, and ground conditions. Validating pile designs through testing is important in order to ensure reliability and address any uncertainties that may arise during the design and construction phases. This research paper presents a comprehensive comparison of field pile load tests with theoretical static equations for determining the bearing capacity of pile foundations. The study focuses on the accuracy of static equations in estimating the bearing capacity of piles in cohesive soils. The research analysed twenty different cases involving bored piles in cohesive soils and compared the results with actual field pile load tests. The findings of the study indicate that this research is able to predict the bearing capacity of a cohesive soil that will be in close agreement with the results of field pile load test results for cohesive soils. Static equations are generally accurate in estimating the ultimate bearing capacity of piles but can be improved by conducting thorough soil investigations and lab testing for a more accurate representation of the soil parameters. The study recommends the use of static equations as an initial assessment tool, but also emphasises the importance of conducting field load tests to confirm their accuracy. The Tangent method is also recommended for adoption as a standard practice in pile design alongside the static equations, as both had correlation coefficients of 0.95 with the pile loading tests. The results of this study provide valuable insights for engineers and researchers involved in the design and construction of deep foundations, highlighting the accuracy and usefulness of static equations
Frequencies of letters in infinite k-balanced sequences
Full text linkThe frequency of letters in a symbolic sequence u over a finite alphabet is one of the basic characteristics of u. The notion of k-balancedness captures the property that the number of any letter occurring in two arbitrary factors of u of equal length differs at most by k. For a fixed integer k and alphabet size d ∈ N, we discuss possible frequencies of letters in k-balanced d-ary sequences. For the size d of the alphabet, we introduce the notion of balancedness threshold BT(d) and provide an upper bound on it, where BT(d) is the minimum k such that there exists a k-balanced sequence over a d-letter alphabet for all possible letter frequencies