Investigation of the validity of the point-kinetic approximation and of the break-frequency method in 2-D subcritical systems
The accuracy and applicability of the break-frequency method for estimating the reactivity in a subcritical 2-D, 2-group reflected system with a distributed source is investigated for two different system sizes. The source is distributed according to the static thermal flux. This method could be useful in power reactor cores during loading conditions for determining the subcriticality. The validity of the point-kinetic approximation in the same systems is also tested. A neutron noise simulator is used to calculate the actual neutron noise induced by fluctuations of the source which is compared with the analytically calculated point-kinetic term. The break-frequency method is used to estimate the reactivity from the neutron noise calculated by the simulator. The conclusion of this work is that the point-kinetic approximation in general is not valid in 2-D, 2-group reflected systems and hence the break-frequency method is not valid either.
Investigation of the validity of the point-kinetic approximation for subcritical heterogeneous systems in 2-group diffusion theory for measurement of the reactivity in ADS
In this paper, calculations are performed on a 2-D heterogeneous reflected subcricital system to determine the spatial distribution of the neutron noise induced by fluctuations of a neutron source located in the middle of the core. Likewise, the point-kinetic component of the neutron noise is determined and compared to the actual neutron noise. It is found that close to the source, the amplitude of the actual neutron noise is larger than its point-kinetic component, while smaller close to the core periphery. For increasing core subcriticalities and/or decreasing frequencies, the deviation of the reactor response from point-kinetics becomes smaller. Based on these observations, the source modulation method is considered for the evaluation of the core subcriticality, since this method is based on point-kinetics. Although the reactor response only deviates slightly from point-kinetics in practical cases such as for an ADS (i.e. a deeply subcritical medium), the reactivity estimated from the source modulation technique significantly deviates from its actual value. Small deviations of the reactor response from point-kinetics lead to large inaccuracies in the estimation of the reactivity, whereas perfect agreement with point-kinetics leads to a pathological case where the formula for estimating the reactivity is ill-posed. Therefore, the source modulation technique seems to be of limited applicability.
Stability of planar PEMFC in Printed Circuit Board technology
Abstract The use of planar PEMFCs in printed circuit board (PCB) technology with a thickness of less than 3.5 mm are presented. This planar design consists of an open cathode side which allows a completely passive, self-breathing operation of the fuel cell. Power densities of 100 mW/cm 2 at 500 mV with hydrogen as a fuel were achieved. A steady operation with this type of fuel cell was demonstrated over a week without any cell flooding. Since the electrical conducting elements of PCBs usually are made of copper, corrosion in the wet environment of a PEMFC is expected. Thus, a significant degradation in performance of fuel cells made of plain copper PCBs was seen in long-term operation. In order to avoid corrosion, the copper layer has to be coated. Fuel cells in PCB design with diverse coatings were tested in long-term operations up to 1000 h under load. Moreover, corrosion currents have been determined by the use of potentiodynamic scans. Promising coatings are electroplated Cr and Ni and a combination of Ni/Au.