Donato Aquaro

Preliminary experimental evaluation of thermal conductivity of ceramic pebble beds

This paper illustrates the preliminary experimental tests for determining the effective thermal conductivity of ceramic pebble beds versus temperature and compression strains. Ceramic pebble beds are promising candidates to be used in breeding blankets for nuclear fusion reactor as breeder and neutron multiplier. The tests were performed with an experimental rig, built at the DICI-University of Pisa, which permits to determine the thermal conductivity of pebble beds in steady state conditions, at several temperatures and compression forces. The values of thermal conductivity of pebble beds are obtained as function of a known conductivity of an alumina disc. The assessment of the method has been performed determining the effective thermal conductivity of alumina pebbles beds of different diameters. Void fraction and compression strains are the parameters that mainly influence the variability of the thermal conductivity of the beds.

THERMAL MECHANICAL ANALYSIS OF A SOLID BREEDING BLANKET

Abstract This paper deals with a theoretical model of thermal mechanical behaviour of pebble beds, used as neutron multiplier or tritium breeder in the breeding blanket of a fusion nuclear reactor. The model tries to sum up the advantages of the two approaches (‘discrete’ method and macroscopic method), presently used for analysing the pebble bed behaviour, without their intrinsic disadvantages. The developed method has the capability to describe the microscopic behaviour of the single sphere (as the discrete approach does), and the capability to model complex structures under variable loads, typical of the macroscopic approach, without doing the unrealistic assumption of continuum homogeneous and isotropic material. The model describes the thermal mechanical behaviour of a single sphere compressed in elastic plastic conditions. The obtained relations have been extrapolated to regular lattices of spheres and subsequently to pebble beds (characterised by a macroscopic parameter called ‘packing factor’) of simple geometric shapes using statistical considerations. The results of the model have been assessed by comparison with results obtained by means of numerical simulations and experimental tests. The ongoing activity is the implementation in a FEM code of a new finite element, which represents one or several regular lattices of spheres, the non linear stiffness of which is obtained from the mono dimensional compression model of one sphere. The results of the numerical simulation permits to construct and display the strain and stress distribution of the single spheres by means of an implemented graphical interface.

Numerical-experimental analyses by Hot-Wire method of an alumina cylinder for future studies on thermal conductivity of the fusion breeder materials

The determination of the thermal conductivity of breeder materials is one of the main goal in order to find the best candidate material for the fusion reactor technology. Experimental tests have been and will be carried out with a dedicated experimental devices, built at the Department of Civil and Industrial Engineering of the University of Pisa. The methodological approach used in doing that is characterized by two main phases strictly interrelated each other: the first one focused on the experimental evaluation of thermal conductivity of a ceramic material, by means of hot wire method, to be subsequently used in the second phase, based on the test rig method, to determine the thermal conductivity of pebble bed material. To the purpose, two different experimental devices have been designed and built. This paper deals with the first phase of the methodology. In this framework, the equipment set up and built to perform Hot wire tests, the ceramic material (a cylinder of alumina), the experimental procedure and the measured results obtained varying the temperature, are presented and discussed. The experimental campaign has been lead from 50°C up to 400°C. The thermal conductivity of the ceramic material at different bulk temperatures has been obtained in stationary conditions (detected on the basis of the temperature values measured during the experiment). Numerical analyses have been also performed by means of FEM code Ansys©. The numerical results were in quite good agreement with the experimental one, confirming also the reliability of code in reproducing heat transfer phenomena.

Numerical and Analytical Analyses of a High Temperature Heat Exchanger

In this paper some analytical and numeric analyses of a high temperature heat exchanger are performed. This heat exchanger should be employed in a test loop of a EFCC (Externally Fired Combined Cycle), placed in a experimental facility owned by the Italian electric utility, ENEL. The heat exchanger is the crucial element in this cycle, as it undergoes temperatures above 1000°C and pressures of about 7 bars. The enthalpy of the combustion products of low cost fuels, such as coal, bottom tar, residuals from refineries, is used to heat a clean working fluid, in this case pressurized air. There are some outstanding benefits for the turbine, in regard to the manufacturing and maintenance costs, and also for its life. The heat transfer components are some bayonet tubes, assembled in 4 modules. A half of them is made of ceramic materials, the others of an advanced metallic material (ODS), due to the burdensome operating conditions. First of all, the heat exchanges are evaluated by means of a simplified analytical model. The radiant contribution also has been taken into account, due to the presence of non-transparent gases. Subsequently, the in-tube fluid temperature increase is calculated for all the heat exchanger modules, through an enthalpy balance and with some simplifying assumptions. Moreover, a comparison is made between the analytical solution and the results of a numerical model implemented in a CFD code. A good agreement is found, which indicates that the analytical model is reasonably valid. In fact, the whole heat exchanger temperature change is determined by means of the two methods with a difference of about 7% for both the streams. Finally, these results are to be compared with the experimental data which should be available in the near future, when the facility will begin working. Also, by this way, the developed calculation model would get a validation.Copyright

Experimental thermal characterization of concrete to be used in CP5.2 packaging system

This work deals with the experimental evaluation of the thermal conductivity of a concrete matrix to be used for embedding LILW bituminised Wastes into the packaging system. Such a type of packaging, identified with the acronym CP5.2, has been also qualified by executing at the Lab. Scalbatraio of Dep. of Civil and Industrial Engineering of the University of Pisa, an experimental test campaign accordingly to the IAEA regulations. In particular, the knowledge of the thermal conductivity is important because of the fire or furnace test to carry out in oven at 800 °C for 30 minutes. These data allowed to simulate pre-test conditions and to set up safety and operational fire test procedures. The concrete thermal conductivity was obtained by performing hot wire tests on cylindrical concrete samples for temperatures ranging from 100° to about 800°C. Thermal conductivity is determined at steady state condition. Results indicate a monotonically reduction of the thermal conductivity as the temperature increases. The comparison with concrete thermal conductivity data available in literature indicates a quite good agreement. Finally, visual and X-ray inspection of sample did not highlight the presence of micro/macro damages that would have affected the thermal performance of the concrete under study.

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.

Mitigation of a Core Meltdown Scenario by Means of a Core Catcher Located Inside the Reactor Pressure Vessel

This paper describes an original solution of core catcher to managing the in vessel retention of the Corium in the accidental event of the core meltdown. The solution envisaged intends to verify the possibility of managing the accidental event within the pressure vessel, ensuring that the CORIUM is confined and cooled. The core catcher, elaborated at the DIMNP, is made of a ceramic pebbel bed (Alumina Al2 O3 ) contained in a metallic or Ceramic Matrix Composite (CMC) structure. The paper illustrates a theoretical model to simulate the thermal-mechanical behaviour of the pebble beds under extremely high loads, developed by the authors. This model has been used to design the core catcher and to determine the effective conductivity and the effective stiffness of the pebble bed. These values have been used in order to implement a numerical model of the core catcher. The results of the thermal and mechanical coupled simulation have permitted to determine the maximum time that the core catcher could resist and the mechanical resistance of the core catcher in the case of RPV external or internal cooling. The preliminary analyses performed have emphasised the good performance of pebble bed core catcher in order to mitigate the envisaged severe accident.Copyright

Application of PHADEC Method for the Decontamination of Radioactive Steam Piping Components

The dismantling of nuclear plants is a complex activity that originates often a large quantity of radioactive contaminated residue.In this paper the attention was focused on the PHADEC (PHosphoric Acid DEContamination) plant adopted for the clearance of Caorso NPP (in Italy) metallic systems and components contaminated by Co60 (produced by the neutron capture in the iron materials), like the main steam lines, moisture separator of the turbine buildings, etc.The PHADEC plant consists in a chemical off line treatment: the crud, deposited along the steam piping during life plant as an example, is removed by means of acid attacks in ponds coupled to a high pressure water washing.Due to the fact that the removed contaminated layers, essentially, iron oxides of various chemical composition, depend on components geometry, type of contamination and time of treatment in the PHADEC plant, it becomes of meaningful importance to suggest a procedure capable to improve the control of the PHADEC process parameters.This study aimed thus at the prediction and optimization of the mentioned treatment time in order to improve the efficiency of the plant itself and to achieve, in turn, the minimization of produced wastes. To the purpose an experimental campaign was carried out by analysing several samples, i.e. taken along the main steam piping line.Smear tests as well as metallographic analyses were carried out in order to determine respectively the radioactivity distribution and the crud composition on the inner surface of the components. Moreover the radioactivity in the crud thickness was measured. These values allowed finally to correlate the residence time in the acid attack ponds to the level of the achieved decontamination.Copyright

Seismic response of reactor vessel internals in the IRIS reactor

The intent of this paper is the presentation and discussion of a methodology for the evaluation and analysis of seismic loads effects on a nuclear power plant. To help in focussing the presented methodology, a preliminary simplified analysis of an integral, medium size next generation PWR reactor structure (IRIS project, an integral configuration PWR under study by an international group) was considered as an application example also for models/codes evaluation. The performed preliminary seismic analysis, even though by no means complete, is intended to evaluate the method of calculating the effects of dynamic loads propagation to the reactor internals for structural design as well as geometrical and functional optimisation purposes. To this goal, finite element method and separated (sub) structures approaches were employed for studying the overall dynamic behaviour of the nuclear reactor vessel. The analysis was set up by means of numerical models, implemented on the MARC FEM code, on the basis of Design Response Spectra as indicated on the relevant rules for Nuclear Power Plants (NRC 1.60) design. The seismic analysis is indented to evaluate the dynamic loads propagated from the ground through the Containment System and Vessel to the Steam Generator’s tubes.© 2006 ASME

Damage Analysis of Gas Turbine Vanes Using a Thermal Fluid Dynamic and Mechanical Approach

Gas turbine combined–cycle systems work with high inlet temperatures, requiring the use of components made of advanced high temperature resistant materials and coatings. These components must be controlled to avoid serious damage to the plants. The durability of these materials and coatings is of great concern to equipment users. This paper deals with a procedure based on thermal fluid dynamic and mechanical integrated analyses of high temperature loaded components. The methodology is applied to uncooled last stator stages vanes of an industrial 165 Mw gas turbine. Several cracks were revealed on these vanes during periodical inspection and mechanical and metallographic investigations were performed. These analyses were used to identify the critical areas of the vanes, from which the component residual life depends on. The procedure was applied to study the damage undergone by gas turbine vanes to discover the causes of crack nucleation and the nucleation mechanism connected to load histories. It has a diagnostic scope, not a predictive one, but it can be considered as the first step of a residual life evaluation and, consequently, of a load cycle optimization: by modifying the future load histories, it could be possible evaluate the best operating conditions to extend component life. The numerical results of these analyses were compared with the damage to vane rows determined during periodical inspections. A good agreement between the analyses results and the inspection data was obtained in terms of critical points and crack locations. The implemented methodology seems to be a powerful tool for increasing the reliability of critical components of gas turbine combined–cycle systems.Copyright