04/02323 Probabilistic safety analysis of a Greek Research Reactor: Aneziris, O. N. et al. Annals of Nuclear Energy, 2004, 31, (5), 481–516

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  • 04/02323 ProbabUistic safety analysis of a Greek Research Reactor Aneziris, O. N. et al. Annals of Nuclear Energy, 2004, 31, (5), 481-516. This paper presents the work and the results of a Level 1 Probabilistic Safety Assessment performed for the Greek Research Reactor of the National Centre for Scientific Research 'Demokritos'. Event trees have been used to study the response of the installation to various initiating events whereas fault trees have been used in the modelling of safety system failures. Using generic data, the probability of an accident leading to severe core damage (_>10% of core melted) from internal initiating events has been estimated to be 310 .6 per year of reactor operatiom The largest contributions to the probability of small damage are made by accidents initiated by loss of cooling and excess reactivity. Large releases of radioactivity are expected with a negligibly low frequency of about 1 xl0~ l~/year.

    04102324 Trends in computing systems for large fusion experiments How, J. A. et al. Fusion Engineering and Design, 2004, 70, (2), 115-122. The major European fusion facilities, JET, Tore Supra and Asdex Upgrade, have a 30-year history from conception to the present day. This period has seen a rapid evolution in computer techniques. Experience has shown that IT architectures and standards adopted early in the construction phase of a project are carried into the main programme and affect the project for its entire lifetime. This paper selects a few of the more important computer-related lessons learned in Europe, particularly with respect to the EFDA-JET facility, and makes recommendations towards the next generation of machines.

    06 ELECTRICAL POWER SUPPLY AND UTILIZATION

    Scientific, technical

    04/02325 A flexible approach for overcurrent relay characteristics simulation Karegar, H. K. et at. Electric Power Systems Research, 2003, 66, (3L 233-239. Accurate models of overcurrent (OC) with inverse time relay characteristics play an important role for the coordination of power system protection schemes. This paper proposes a new method for modelling OC relays curves. The model is based on fuzzy logic and artificial neural network. The feedforward multilayer perceptron neural network is used to calculate the operating times of OC relays for various time dial settings (TDS) or time multiplier settings (TMS). The new model is more accurate than traditional models. The model has been validated by comparing the results obtained from the new method with linear and non-linear analytical models.

    04102326 A global thermo-electrochemical model for SOFC systems design and engineering Petruzzi, L. et al. Journal of Power Sources, 2003, 118, (1-2), 96-107. At BMW AG in Munich high-temperature solid oxide fuel cells (SOFCs) are being developed as an auxiliary power unit (APU) for high-class car conveniences. Their design requires simulation of their thermo-electrochemical behaviour in all the conditions that may occur during operation (i.e. heat-up to about 600C, start-up to operating temperature, energy-delivering and cool-down). A global thermo- electrochemical model was developed for the whole system and a three- dimensional geometry code was performed using MATLAB program- ming language. The problems in developing SOFCs are now so many and so different that a very flexible code is necessary. Thus, the code was not only designed in order to simulate each of the operating conditions, but also to test different stack configurations, materials, etc. In every event, the code produces a time-dependent profile of temperatures, currents, electrical and thermal power density, gases concentrations for the whole system. The heat-up and start-up simulations allow: (1) to evaluate the time the cell stack needs to reach operating temperature from an initial temperature distribution, (2) to check the steepest temperature gradients occurring in the ceramic layers (which result in material stresses) and (3) to obtain important information about the pre-operating strategy. Simulation of energy-dellvering gives a detailed profile of the temperatures, currents, power density,, and allows to define the guidelines in system-

    06 Electrical power supply and utilization (scientific, technical)

    controlling. Simulation of cooling-down gives important advises about insulation designing. The aim of this work is to build up a tool to clearly individuate the best designing criteria and operating strategy during the development and the engineering of a SOFC system.

    04102327 A two-phase flow model for hydrogen evolution in an electrochemical cell Mat, M. D. M. D. el al. International Journal of Hydrogen Energy, 2004, 29, (~0), 1015-1023. Hydrogen evolution, flow field and current density distribution in an electrochemical cell are investigated with a two-phase flow model. The mathematical model involves solutions of transport equations for the variables of each phase with allowance for inter-phase transfer of mass and momentum. The buoyancy force generated due to density difference between two phases modifies flow profile and increases fluid velocity at the vicinity of the electrode. The current density decreases over the electrode mainly because of the decrease in effective conductivity of electrolyte. It is found that the hydrogen generation significantly increases at higher electrolyte flow by reducing the residence time of bubbles over the electrode. The predicted results satisfactorily agree with data available in the literature.

    04/02328 Bipolar plates for PEM fuel cells Middelman, E. et al. Journal of Power Sources, 2003, 118, (1 2), 44-46. The bipolar plates are in weight and volume the major part of the PEM fuel cell stack, and are also a significant contributor to the stack costs. The bipolar plate is therefore a key component if power density has to increase and costs must come down. Three cell plate technologies are expected to reach targeted cost price levels, all having specific advantages and drawbacks. NedStack has developed a conductive composite materials and a production process for fuel cell plates (bipolar and mono-polar). The material has a high electric and thermaI conductivity, and can be processed into bipolar plates by a proprietary molding process. Process cycle time has been reduced to less than 10 s, making the material and process suitable for economical mass production. Other development work to increase material efficiency resulted in thin bipolar plates with integrated cooling channels, and integrated seals, and in two-component bipolar plates. Total thickness of the bipolar plates is now less than 3 ram, and will be reduced to 2 mm in the near future. With these thin integrated plates it is possible to increase power density up to 2 kW/1 and 2 kW/kg, while at the same time reducing cost by integrating other functions and less material use.

    04/02329 Capacitance of the polypyrrole/polyimide composite by electrochemical impedance spectroscopy Iroh, J. O. and Levine, K. Journal o/'Poa,er Sources, 2003, 117, (1-2), 267-27Z Electrically conducting polymers and their composites attract a lot of attention because of their high charge storage ability. As was shown previously, the composite of polypyrrole (PPy) and polyimide (PI) possesses better charge storage properties then pure PPy. Probably the PI matrix protects PPy from oxidative destruction and serves as a high molecular weight dopant. In this paper, the PPy/PI composite was studied by as electrochemical impedance spectroscopy at different applied dc polarization potentials. The properties of the coatings were found to differ significantly between pure PI and the PPy/P] composite, depending on the applied potential and on the amount of the conducting polymer. An additional time constant related to the PPy/ PI interface appears when PPy is added to PI. The pseudocapacitance of the PPy/PI composite increased significantly with increased cathodic potential. This was explained by additional doping of PPy by the PI matrix.

    04/02330 Capacity fade study of lithium-ion batteries cycled at high discharge rates Ning, G. et al. Journal of Power Sources', 2003, 1 t7, (1-2), 160-169. Capacity fade of Sony US 18650 Li-ion batteries cycled using different discharge rates was studied at ambient temperature. The capacity losses were estimated after 300 cycles at 2C and 3C discharge rates and were found to be 13.2 and 16.9% of the initial capacity, respectively. At 1C discharge rate the capacity lost was only 9.5%. The cell cycled at high discharge rate (3C) showed the largest internal resistance increase of 27.7% relative to the resistance of the fresh cells. The rate capability losses were proportional with the increase of discharge rates. Half-cell study and material and charge balances were used to quantify the capacity fade due to the losses of primary active material (El +), the secondary active material (LiCoOz/C)) and rate capability losses. It was found that carbon with 10.6% capacity loss after 300 cycles dominates the capacity fade of the whole cell at high discharge rates (3C). A mechanism is proposed which explains the capacity fade at high discharge rates.

    Fuel and Energy Abstracts September 2004 329

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