An analytical approach for the sequential excavation of axisymmetric lined tunnels in viscoelastic rock

The main factors for the observed time dependency in tunnel construction are due to the sequence of excavation, the number of liners and their times of installation and the rheological properties of the host rock. A generalized derivation procedure for any viscoelastic model is presented accounting for the sequential excavation of a circular tunnel supported by any number of liners of different thicknesses and stiffnesses installed at different times in a viscoelastic surrounding rock under a hydrostatic stress field under plane strain axisymmetric conditions. Sequential excavation was accounted for assuming the radius of the tunnel growing from an initial value to a final one according to a time-dependent function to be prescribed by the designer. The effect of tunnel advancement was also considered. For the generalized Kelvin viscoelastic model, explicit analytical closed form solutions were presented, which can be reduced to the solutions for the Maxwell and Kelvin models

On the stability of fissured slopes subject to seismic action

A set of analytical solutions achieved by the upper bound theorem of limit analysis and the pseudo-static approach is presented for the assessment of the stability of homogeneous c, ϕ slopes manifesting vertical cracks and subject to seismic action. Rotational failure mechanisms are considered for slopes with cracks of either known or unknown depth and location. A validation exercise was carried out based on numerical limit analyses and displacement-based finite-element analyses with strength reduction technique. Charts providing the stability factor for fissured slopes subject to both horizontal and vertical accelerations for any combination of c, ϕ and slope inclination are provided. The effect of the direction of the vertical acceleration on slope stability is specifically analysed. Yield seismic coefficients are also provided. When the presence of cracks within the slope can be ascertained with reasonable confidence, maps showing the zones within the slope where they have no destabilising effect are provided. Finally, Newmark's method was employed to assess the effect of cracks on earthquake induced displacements. To this end, displacement coefficients are provided in chart form as a function of the slope characteristics. Two examples of slopes subjected to known earthquakes are illustrated

Literature review of lead-lithium thermophysical properties

Abstract In recent years, the scientific community of nuclear fusion raised the issue of thermophysical properties of lead lithium alloys. These alloys are foreseen to be used in several Breeding Blanket concepts in an almost eutectic composition, but only few data on the properties are available in literature and large differences on the same property exist between different authors. Moreover, apparently each organization used different available properties correlations, making practically pointless every comparison of results with the other organizations involved in the design of Breeding Blankets. The aim of this paper is to identify the properties to be used in the design of the Breeding Blankets, performing a literature review of the available data and suggesting a correlation for each of the main properties. These correlations were chosen based on the accurateness of the paper and on the similarities between different authors, where it was possible (e.g., density). The table with the correlations should represent a starting point for a discussion to reach a general consensus on the property database, which should be mandatory in order to allow a comparison of the results from different organizations. Very likely new experiments will be necessary to definitely measure at least the properties with the biggest scattering of the data (e.g., specific heat), encouraging a consensus and reducing the errors in the design activities

Validazione su base sperimentale del codice RELAP5 mediante campagna di prove sperimentali su impianto HEFUS-3

Presso il centro di ricerca ENEA Brasimone, nell'ambito dell' Accordo di Programma tra Ministero dello Sviluppo Economico ed ENEA, è stata svolta un'attività sperimentale sull'impianto ad elio Hefus3 ai fini di validare il codice di calcolo RELAP5. Le attività sperimentali sono state condotte con portate fino a 900 g/s a 70 bar, in regime di refrigerazione isoterma e a lungo termine, incidente di LOFA (Loss of Flow Accidents) ed incidente di LOCA (Loss of Coolant Accidents). I set di dati generati sono stati impiegati per integrare i dati precedentemente acquisiti con range di portate fino a 0.35 kg/s a 4 MPA per la validazione dei codici T/H per l'analisi di reattori a gas ad alta temperatura mediante codice RELAP5

Verification and validation of COMSOL magnetohydrody-namic models for liquid metal breeding blankets technologies

Liquid metal breeding blankets are extensively studied in nuclear fusion. In the main proposed systems, the Water Cooled Lithium Lead (WCLL) and the Dual Coolant Lithium Lead (DCLL), the liquid metal flows under an intense transverse magnetic field, for which a magnetohy-drodynamic (MHD) effect is produced. The result is the alteration of all the flow features and the increase in the pressure drops. Although the latter issue can be evaluated with system models, 3D MHD codes are of extreme importance both in the design phase and for safety analyses. To test the reliability of COMSOL Multiphysics for the development of MHD models, a method for verification and validation of magnetohydrodynamic codes is followed. The benchmark problems solved regard steady state, fully developed flows in rectangular ducts, non-isothermal flows, flow in a spatially varying transverse magnetic field and two different unsteady turbulent problems, quasi-two-di-mensional MHD turbulent flow and 3D turbulent MHD flow entering a magnetic obstacle. The computed results show good agreement with the reference solutions for all the addressed problems, suggesting that COMSOL can be used as software to study liquid metal MHD problems under the flow regimes typical of fusion power reactors

Cost-benefit methodology for road slope stabilisation

\ua9 2024 The Author(s). Cost-benefit analyses are conducted to evaluate the cost efficiency of road slope stabilisation measures to aid road planning, design, maintenance, and repair. Most cost analyses are based on a statistical framework that requires a database of slope failures. However, databases can be costly to compile, and they tend to compare options that satisfy the same global factor of safety or partial factors of safety (e.g. EC-7) neglecting the fact that each measure reduces the risk of slope failure by a different extent. Here, we present a novel methodology to evaluate the cost efficiency of different road slope stabilisation measures based on direct costs and a rigorous but parsimonious mechanistic and probabilistic geotechnical slope stability assessment. Unlike other cost analyses for slope stability, our methodology accounts for uncertainty in slope geomaterial characteristics, as well as for hillslope hydrology. Probabilistic slope stability analyses accounting for the effect of time-varying slope seepage are performed using the CUTSTAB-P methodology to estimate the frequency of slope failure. The methodology is demonstrated on a cut slope in Nepal, assessing four different road slope stabilisation measures that are implemented in Nepal: (1) the cut slope with no additional support; (2) reprofiling to a shallower inclination; (3) a mortared masonry wall; and (4) an anchoring system. We find that an anchoring system is the most cost-efficient road slope stabilisation measure for this cut slope, and that a mortared masonry wall is least cost-efficient. This is despite the mortared masonry wall having much lower initial investment costs than the anchoring system. Mortared masonry walls are hugely common along roads in Nepal. We also make an approximation of indirect costs. With this addition, we find that the anchoring system remains the most cost-efficient method

DEM of triaxial tests on crushable cemented sand

Using the discrete element method, triaxial simulations of cemented sand consisting of crushable particles are presented. The triaxial model used features a flexible membrane, allowing realistic deformation to occur, and cementation is modelled using inter-particle bonds. The effects of particle crushing are explored, as is the influence of cementation on the behaviour of the soil. An insight to the effects that cementation has on the degree of crushing is presented

Experimental and RELAP5-3D results on IELLLO (Integrated European Lead Lithium LOop) operation

The experimental facility IELLLO (Integrated European Lead Lithium LOop) was designed and installed at the ENEA Brasimone Research Centre to support the design of the HCLL TBM (Helium Cooled Lithium Lead Test Blanket Module).This work presents the results of the experimental campaign carried out within the framework of F4E-FPA-372 and which had three main objectives. First, to produce new experimental data for flowing LLE (Lead-Lithium Eutectic) for an analysis of the loop and the characterization of its main components. Then, to evaluate performances of commercial instrumentation as available instrumentation is not designed for use in LLE. Lastly, to use the data for validation of the model developed with the system code RELAP5-3D. The data collected could prove helpful to analyze the behavior of the LLE loop of ITER and DEMO in accidental conditions.The results show that the regenerative countercurrent heat exchanger has an efficiency ranging from 70 to 85%, mainly depending on the LLE mass flow rate. It was verified that the air cooler has the capability to keep the cold part of the loop at 623. K, even in the most demanding situation (700. rpm and maximum temperature of the hot part). The instrumentation tested showed good accuracy, with the exception of the turbine flow meter. Nevertheless, specific limitations in the upper operative temperatures were found for the LLE direct contact pressure transducer. RELAP5-3D simulations fit very well the associated experimental results achieved

Experiments on the MHD Effect on the Drainage of a LiPb Channel and Supporting Numerical Computations with the Level Set Method

To analyze the impact of the magnetohydrodynamics (MHD) effect on the fast draining of a LiPb channel (lithium-lead eutectic, 15.7 at. % Li) for a liquid metal fusion blanket such as the water-cooled lithium-lead test blanket system of ITER or DEMO, an experimental campaign was carried out with the support of the Integrated European Lead Lithium LOop experimental facility (IELLLO), installed at the ENEA Brasimone research center, Italy. The experiments were carried out by measuring the drainage time of the internal permanent magnet pump channel, normally used to circulate the LiPb in the loop, with and without the magnetic field. Moreover, this paper proposes a new numerical methodology to study the time delay induced by the MHD by using the commercial software COMSOL Multiphysics. In this way, it was possible to evaluate the LiPb fraction present at each time step in the computational domain and to estimate the time necessary for the complete drainage of the channel. The level set method was used to describe the transient behavior of the MHD flow under low-Rm approximation. The developed code was compared with the experimental results and showed good agreement, and it constitutes the first step in model validation as a possible application to ITER and DEMO. The experimental and numerical analyses performed in this work can be used as a benchmark case for MHD code development

Design of the Test Section for the Experimental Validation of Antipermeation and Corrosion Barriers for WCLL BB

Tritium permeation into the Primary Heat Transfer System (PHTS) of DEMO and ITER reactors is one of the challenging issues to be solved in order to demonstrate the feasibility of nuclear fusion power plants construction. Several technologies were investigated as antipermeation and corrosion barriers to reduce the tritium permeation flux from the breeder into the PHTS. Within this frame, alumina coating manufactured by Pulsed Laser Deposition (PLD) and Atomic Layer Deposition (ALD) are two of the main candidates for the Water Cooled Lithium Lead (WCLL) Breeder Blanket (BB). In order to validate the performance of the coatings on relevant WCLL BB geometries, a mock‐up was designed and will be characterized in an experimental facility operating with flowing lithium‐lead, called TRIEX‐II. The present work aims to illustrate the preliminary engineering design of a WCLL BB mock‐up in order to deeply investigate permeation of hydrogen isotopes through PHTS water pipes. The permeation tests are planned in the temperature range between 330 and 500 °C, with hydrogen and deuterium partial pressure in the range of 1–1000 Pa. The hydrogen isotopes transport analysis carried out for the design and integration of the mock‐up in TRIEX‐II facility is also shown