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.
Theoretical investigation of the MTC noise estimate in 1-D homogeneous systems
In this paper, the accuracy of the noise-based determination of the moderator temperature coefficient (MTC) is investigated theoretically and quantitatively. It is known from earlier work that the noise method systematically underestimates the MTC. In this paper, it is found that the main reason for the underestimation lies with the radial incoherence of the temperature fluctuations. The deviation of the reactor response from point-kinetics is another possible reason, but it was found to play a quite insignificant role. The theory of neutron noise, induced by spatially random perturbations is elaborated and by its help the inaccuracy (bias) of the noise based MTC estimation was quantitatively investigated. It was found that a relatively short correlation length of the temperature fluctuations, which is in agreement with experimental evidence, can explain the observed underestimation of the MTC by the noise method.
Study of the effect of substrate roughness on adhesive joints by SEM image analysis
The effect of adherend surface roughness on epoxy bonded aluminium joints has been studied. Several epoxy adhesives were tested to evaluate the influence of adhesive nature on the roughness effect. The aluminium pre-treatments applied were abrasion and impression processes, generating different texture levels. The abrasion with grinding papers of different grain sizes provides surfaces with high density of low summits. In contrast, the surfaces subjected to impression process present low density of very deep valleys. The roughness measurements were made by image analysis of micrographs obtained by Environmental Scanning Electron Microscopy (ESEM). In addition to the average roughness, other surface descriptors were determined to characterise completely the surfaces, allowing to evaluate the influence on the adhesive strength of different texture variables, such as the height, shape and density of the peaks and valleys which constitute the roughness profile. An optimum value of surface roughness was found for the joint strength measured by the lap shear tensile test. An increase of adherend roughness causes an increase of effective area but, at the same time, decreases the ability for adhesive penetration. It has also been shown that the joint strength depends on the main characteristics of the adherend surface, such as the density and depth of the protuberances. Finally, it has been found that the roughness effect seems be influenced by the adhesive nature. Different epoxy adhesives with similar mechanical properties present different joint strengths due to their different penetration abilities.