benefits of seismic isolation for nuclear structures subjected to severe earthquake

The Fukushima accident has reiterated that the seismic safety is a clear necessity in the design of nuclear power plants. To overcome the weaknesses of the plant design, appropriate measures or interventions have thus to be put in place to improve the nuclear safety. In this study, seismic isolation, widely adopted for conventional constructions, is considered as retrofit measure to provide superior performance of plant itself, even when exceptional events occur. In this paper, we numerically investigate the dynamic behaviour of a Small Modular Reactor (SMR) plant subjected to 0.6g PGA; in doing that time-history analysis has been performed assuming the reactor building with and without isolators. For that purpose, a suitable FEM model has been implemented to provide in-structure response spectra at safety relevant locations and subsystem supports. Adequate steel and concrete properties as well as isolators properties, experimentally determined, have been assumed. Results have shown the benefits of seismic isolation for NPP that can so sustain levels of loading beyond the design input and demonstrated that failure of an isolation system cannot occur before failure of the isolated structure. However, the large horizontal displacements of the structure require appropriate considerations in the layout and interfaces for interconnected systems

Demonstration of structural performance of IP-2 package by simulation and full-scale horizontal free drop test

Packaging systems for the transportation of radioactive wastes have to be designed according to rigorous acceptance criteria and requirements in order to protect people and environment against radiation exposure and contamination risk. The IAEA requirements for type IP2 (Industrial Package Type 2) packages include to carry out free drop tests that represent normal conditions of transport. In such conditions, obviously, the required containment capability of the package has to be ensured. In this study the mechanical performances of a new Italian packaging system for the transportation of low and intermediate level wastes (LILW) undergoing horizontal free drop test are investigated. Especially, deformations caused in the sealing area of the package, which can affect the capability of the containment system, are evaluated. The carried out numerical analyses and experimental tests, at the lab. Scalbatraio of the DICI- University of Pisa, are presented and discussed. Numerical analyses (by qualified MARC® code) have been performed to investigate the stress histories in the bolts, lid, and package body as well as the deformations in the sealing area and the compression conditions of the gasket. Localised stress appeared at the flange and at the bottom of packaging system. The maximum stresses resulted lower than the stress limits, so the structural integrity of the package was maintained and confirming its tightness. As a consequence of the primary impact a local deformation appeared at the primary lid, no cliff edge or loss of the safety features resulted

Preliminary investigation of Li4SiO4 pebbles structural performance

Abstract One of the main purposes of a breeder blanket is to remove the heat produced in the blanket by the fusion reaction neutrons, and to breed the tritium required to sustain it. To achieve these requirements, several breeder materials (solid or liquid lithium-bearing ones) have been investigated in the past decades. To date it has not yet been possible to identify a stable material, with high thermal conductivity and melting point. This paper deals with the mechanical characterization of the lithium orthosilicate (Li4SiO4) in form of pebbles, produced at the University of Pisa at room temperature by a drip casting forming technique, starting from an aqueous suspension of Li4SiO4 precursor prepared by a sol-gel synthesis method. To investigate also numerically, by means of FE code, the breeder blanket behaviour, it is of meaningful importance the mechanical characterization of such pebbles. To the purpose, either static or cyclic uniaxial compression tests, without radial constraints, have been performed on several produced pebbles of about 1.5 mm diameter in order to determine the collapse and crushing loads and the stiffness. Moreover, the carried-out post-test SEM examination allowed to evaluate the failure mode and the crack shapes on the contact surface. Results show the influence of the elastic properties and matrix flaw population on the crushing load. The pebbles produced by the sol-gel method showed also a high strength, the value of which is comparable to that of the pebbles obtained by melting process

Analysis of feasibility of a new core catcher for the in-vessel core melt retention strategy

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Structural performance of an IP2 package in free drop test conditions: numerical and experimental evaluations

The casks or packaging systems used for the transportation of nuclear materials, especially spent fuel elements, have to be designed according to rigorous acceptance requirements, like the IAEA ones, in order to provide protection to human beings and environment against radiation exposure and contamination. This study deals with the free drop test of an Italian design packaging system to be used for the transportation of low and intermediate level radioactive wastes. Impact drop experiments were performed in the Lab. Scalbatraio of the DICI - University of Pisa. Dynamic analyses too have been carried out, by refined models of both the cask and target surface to predict the effects of the impact shock (vertical drop) on the package. The experimental tests and numerical analyses are thoroughly compared, presented and discussed. The numerical approach shows to be suitable to reproduce with good reliability the test situations and results

Characterization of the thermal conductivity for ceramic pebble beds

The evaluation of the thermal conductivity of breeder materials is one of the main goals to find the best candidate material for the fusion reactor technology. The aim of this paper is to evaluate experimentally the thermal conductivity of a ceramic material by applying the hot wire method at different temperatures, ranging from 50 to about 800°C. The updated experimental facility, available at the Department of Civil and Industrial Engineering (DICI) of the University of Pisa, used to determine the thermal conductivity of a ceramic material (alumina), will be described along with the measurement acquisition system. Moreover it will be also provided an overview of the current state of art of the ceramic pebble bed breeder thermos-mechanics R&D (e.g. Lithium Orthosilicate (Li4SiO4) and Lithium Metatitanate (Li2TiO3)) focusing on the up-to-date analysis. The methodological approach adopted is articulated in two phase: the first one aimed at the experimental evaluation of thermal conductivity of a ceramic material by means of hot wire method, to be subsequently used in the second phase that is based on the test rig method, through which is measured the thermal conductivity of pebble bed material. In this framework, the experimental procedure and the measured results obtained varying the temperature, are presented and discussed

em zooming procedure in ansys maxwell 3d

Abstract The severity of electromagnetic (EM) loads produced by plasma disruptions is one of the most concerning issues for the ITER in-vessel components design. To investigate the effects of fast EM transients on plasma surrounding structures during a disruption the Secondary Excitations (SE) method is used. This is an interface procedure to couple 2D plasma equilibrium codes with Finite Elements (FE) software. The Zooming Approach (ZA) used for the analyses presented here is a particular implementation of the SE method. The aim of this work is the demonstration that the ZA can be effectively applied in case of ANSYS Maxwell 3D analyses combining the ease of use of the Maxwell code with the computational efficiency of the ZA. The work has been carried out evaluating the EM loads acting on the ITER Diagnostic Equatorial Port Plug (EPP) during major disruptions scenario and comparing these loads with those obtained in previous analyses. Additional analyses have been performed to study the effect of ferromagnetic materials on EM loads in order to investigate ANSYS Maxwell capabilities in simulating non-linear magnetic properties

Thermal tests of a scaled down mock-up of CP5.2 packaging system: Post-test analysis

Abstract In this study, the thermal performances of an Italian CP5.2 packaging system aimed at the transportation of bituminised wastes (i.e. engulfing fire of 800 °C for 30 min according to the IAEA regulation) are presented. Due to the high risk of auto-ignition of the bituminised wastes, that are stowed in the drums, in turn, immersed in the cement matrix of the CP 5.2, it was decided to test firstly a small scale mock-up. The mock up, containing only one drum with bituminised waste, was designed and built at the Department of Civil and Industrial Engineering (DICI) of the University of Pisa. The experimental test was carried out at Lab. Guerrini of the University of Pisa. Results demonstrated that after half an hour of fire exposure at 800 °C, the temperature in the bituminised waste package is below that of auto-ignition of the bitumen. The obtained results allowed in addition to set up the test procedure to adopt for fire test of a full scale CP5.2 system. Post test analysis, which was carried out by performing FEM analysis, is also presented and results compared to the experimental ones

An improved substructuring approach for dynamic modelling – The example of the ITER vacuum vessel

Abstract This study deals with the substructuring method that is adopted for treating complex structures, such as the ITER Tokamak, as an assembly of different components or substructures. This method uses basic mass and stiffness matrices combined with conditions representative of the geometrical compatibility along the substructure boundaries. This approach makes it possible to simulate the dynamic behaviour of the (complex) ITER Vacuum Vessel (VV), subjected to dynamic loading (e.g. seismic), by means of a simplified model (finite element discretization) with little loss of accuracy. In this framework, a simplified model of the ITER VV for use in global seismic analyses has been developed and implemented. In order to minimize the wavefront (and hence computational time), the simplified model is made of multiple superelements, each representing a part of the VV. Moreover, in order to reduce the number of Degrees of Freedom (DoFs) needed along the boundaries of the different substructures, a new approach has also been developed for the breakdown of the benchmark model