1,721,047 research outputs found
Preliminary investigation of Li4SiO4 pebbles structural performance
No full textOne 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
Aircraft impact effects on an aged NPP
No full textThe impact of an aircraft is widely known to be one of the worst events that can occur during the operation of a plant (classified for this reason as beyond design). This can become much more catastrophic and lead to the loss of strength of/collapse of the structures when it occurs in the presence of ageing (degradation and alteration) materials. Therefore, since the performance of all plant components may be affected by ageing, there is a need to evaluate the effect that aged components have on system performance and plant safety. This study addresses the numerical simulation of an aged Nuclear Power Plant (NPP) subjected to a military aircraft impact. The effects of impact velocity, direction, and location were investigated together with the more unfavorable conditions to be expected for the plant. The modelling method was also validated based on the results obtained from the experiments of Sugano et al., 1993. Non-linear analyses by means of finite element (FE) MARC code allowed us to simulate the performance of the reinforced concrete containment build-ing and its impact on plant availability and reliability. The results showed that ageing increases a plant’s propensity to suffer damage. The damage at the impact area was confirmed to be dependent on the type of aircraft involved and the target wall thickness. The greater the degradation of the materials, the lower the residual resistance capacity, and the greater the risk of wall perforation
Inverse Heat Conduction Problem in Estimating Nuclear Power Plant Piping Performance
No full textMost of today’s operating nuclear plants that were originally designed for 30 or 40-year life are facing the long-term operation issues. Therefore, it is of meaningful importance to assess the time-dependent degradation and the aging of the relevant nuclear systems, structures, and components because of resulting loss of structural capacity. In this framework, the inverse method is implemented starting from temperatures at an accessible boundary, which are measured through a monitoring system. The reconstruction technique uses the elaborated signal provided by the monitoring system to determine temperature at inaccessible surface: this is the so-called inverse heat transfer problem. The inverse space marching method is applied. Analytical and numerical studies are performed taking into account thermal transient conditions in order to determine thermal loads. In particular, the developed code demonstrates to be able to reconstruct temperature and stress profiles in any section of the pipe with a good accuracy. In addition, the thermal loads obtained suggest that the investigated transient condition is not able to jeopardize the integrity of nuclear power plant, confirming the possibility of the plant extension of life
Preliminary study of the effects of ageing on the long-term performance of NPP pipe
Full text linkMost of today's operating nuclear plants are facing long-term operation (LTO) issues caused by the time degradation and/or deterioration suffered by the system, structure, and components (SSCs). These phenomena are known as ageing and are responsible for the change of material properties and, in turn may affect the structural integrity of plant SSCs. The paper deals with the analysis of the performance of a primary pipe of a typical PWR subjected to ageing mechanisms. To the aim an inverse space marching method is applied. From reconstructed temperature it is possible to determine e.g. temperature values at surfaces that are difficult to reach and inspect. Accordingly, based on the thermal gradient across the pipe wall, the residual thickness of the pipe may be determined and used for structural capacity verification. Analytical and numerical (thermo-mechanical) analyses are performed considering several thinning rates. The effects of both homogeneous and heterogeneous thinning are also investigated. The results suggest that an excessive (general or local) thinning may affect the strength capacity of pipe. The performance of the pipeline confirms the possibility of the life extension if the thinning rate is kept below 0.5 mm/year, even when the plant operating conditions are outside the prescribed operating limits
NUMERICAL INVESTIGATION OF CEMENTED-BASED MATERIAL TO BE USED FOR PACKAGING SYSTEM
No full textThe selection of immobilization waste technologies based on cementation should demonstrate that the obtained matrices are reliable, durable, and stable. This study deals with the numerical simulation of cemented package. Both the normal and accidental storage condition are considered. The FE numerical model is adopted to benchmark several RLOW simulants composition and correlate/compare the structural properties. A 3D cylindrical shaped model was implemented. Homogeneous mechanical material properties of defined stiffness were assumed. The thermal properties have been obtained experimentally by performing hot wire test on several samples of the cement inert matrix. These data were used as input for the numerical simulation. Moreover, pseudo-dynamic analysis was carried out in order to determine the damage level in the case of an accidental drop. Ageing effects have been also investigated. Results indicate that the thermal conductivity monotonically decreases of 20% as the temperature increases up to 800 °C. The compressive strength, even reduced mainly as consequence of increased material porosity, confirmed to be dependent on w/c ratio. The results obtained were consistent with literature values, indicating a strong correlation between the simulation results and the established knowledge in the field
Analysis of feasibility of a new core catcher for the in-vessel core melt retention strategy
Full text linkThis study deals with the feasibility study of a new in-vessel core melt retention (IVCMR) strategy capable to extend the coping period in the event of adverse situations, involving the melting of the core. Since Fukushima accident, many studies have been carried out to resolve the severe accident mitigation issues related to the corium stabilization inside and outside the reactor vessel. This is in fact one of the most relevant safety issues to secure LWRs from the point of view of severe accident mitigation and containment integrity. As for the corium stabilization inside the reactor vessel, in this study it is proposed a new IVCMR concept, developed at the University of Pisa, based on the adoption of an original core catcher design made of batches of ceramic material. By profiting of its low thermal conductivity, this core catcher is capable to retard the heat-up of the lower head of the vessel during the phase of relocation of the corium. To support the feasibility of its design analytical and numerical analyses have been performed assuming homogeneous pool condition. Results show that the adoption of the proposed core catcher solution extends the severe accident coping period: after 1 h from the initiating event, the maximum temperature of the vessel wall is below the limit for which localized failure may appear
Anomalies Detection in Structures, System and Components for Supporting Nuclear Long Term Operation Program
No full textLong Term Operation (LTO) of nuclear power plants (NPPs) will play a key role to reach net zero target. Monitoring and predictive approach detecting in advance faulty SCCs conditions may provide a further key tool in LTO framework. Detecting anomalies may allow the transition from time-based to condition-based predictive maintenance of the NNPs. Predictive algorithms could reduce the number of unplanned outages caused by reactor system failures (one-day outage of a 1000-MW NPPs causes losses of about 500 k$), improving the capacity factor, and keeping high safety margin level of NPPs. To this end, innovative approach by unsupervised machine learning technique (ML) is proposed to detect anomalies of SSCs. Based on principal component analysis and mahalanobis distance is possible to detect in advance the failure of the components. To the purpose a 2D digital twin of primary nuclear pipe under nominal conditions (inner temperature of 300° and an internal pressure of 15.5 MPa) is implemented in finite element code to provide a dataset for unsupervised ML code. The algorithm is then tested under anomaly pattern that deviate from nominal conditions. The results show good code prediction capabilities anticipating the pipe failure. Traditional monitoring combined with ML technique may support LTO program increasing the safety and competitiveness of NPPs
Feasibility of Li4SiO4 pebbles by drip casting and preliminary characterization
No full textOpen issues for fusion reactor technology are related to the breeder material to be used for the nuclear fusion reaction. Despite in the last decades several studies were carried out, the selection, characterization and reliability of breeder material are still open issues. In this study, the attention is focused on the production of lithium orthosilicate (Li4SiO4) by means of drip casting method. The feasibility of this method was verified using an experimental rig designed and constructed purposely at the Pisa University, Department of Civil and Industrial Engineering (DICI). The green pebbles were produced starting from a suspension of Li4SiO4 precursors (i.e. LiOH and Si(OCH2CH3)4) prepared by sol-gel method. These pebbles were dried at 40 °C for 4 h, and then sintered in air at 900 °C for 10 h. The morphological and physical properties of the obtained pebbles were analyzed by scanning electron microscope (SEM), X-ray and compression tests. The X-ray results showed the main characteristic peaks of Li4SiO4 crystalline phase and small peaks attributable to calcium metasilicate, while the SEM analysis showed a homogeneous and uniform structure of the pebbles. The obtained preliminary results showed the feasibility of dripping at room temperature and provided further useful information for the optimization of the process
Pipe performance in long term operation framework: Ageing issues
No full textThe Long-Term Operation (LTO) process involves a full screening of structures, systems, and components (SSCs), for ageing assessment in order to verify their residual safety margin is still acceptable. At today, 46% of the operating Nuclear Power Plant (NPP) has lifetime between 31 and 40 years, while 19% is in operation since more than 40 years. LTO currently represents the highest priority for all the nuclear industry. To propose and plan suitable management strategies, first step is to intensify the efforts for studying phenomena that influence the performance of SSCs and, in turn, may threat the plant safe operation. This study deals with the investigation of the performance of a primary piping (Class 1 component of a 2nd Generation PWR) subjected to the effects of alteration of material properties as caused by ageing. In this study it is proposed a numerical investigation of a piping characterized by a complex geometry. Numerical analyses were performed by means of MSC©MARC FE code. The (quantitative) influence of ageing and corrosion processes onto bent pipe are so studied. Particularly, corrosion effects generated from an operation of beyond 35 years are considered. The methodology and results may have an influence on future issues about LTO of NPPs
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