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.
Dynamic subcriticality measurements using the {sup 252}CF-source-driven noise analysis method
This paper reports on dynamic measurements of the subcritical neutron multiplication factor K{sub eff} using the {sup 252}Cf-source-driven neutron noise analysis method performed for an unreflected 25.1-cm-i.d. cylindrical tank containing aqueous uranyl nitrate as the solution height changed at rates of 1 to 23 cm/min, with corresponding changes in k{sub eff} from 4 {times} 10 {sub {minus} 4} to 0.01/s. These experiments, which were the first test of the method to measure k{sub eff} while it is changing, showed the following: this method has the capability to measure subcriticality for a multiplying system to a k{sub eff} as low as 0.30; experimental k{sub eff} values can be obtained from the ratio of spectral densities with as little as 6 s of data accumulation and a small fraction of a second analysis time while the solution tank is drained from a height of 29.5 to 6.5 cm in {approx} 60 s, with corresponding changes in k{sub eff} from 0.95 to 0.30; the measured k{sub eff} values obtained do not depend on the speed at which the solution height is changed or whether it is filling or draining; the results of the dynamic measurements agreed with the static measurements; where static measurementsmore » were practical (limited to k{sub eff} down to {approx} 0.5 by detection efficiency) with {sup 3} He proportional counters sensitive to leakage neutrons only, the results agreed with those from measurements with scintillation detectors sensitive to gamma rays and neutrons escaping from the system; as in previous experiments the ratios of spectral densities at low frequency were used successfully to obtain k{sub eff} values using a modified point kinetics interpretation of the data; the neutron multiplication factors from independent measurements using the break frequency noise analysis method agree with the value of k{sub eff} from the measured ratios of spectral densities of k{sub eff} values of 0.65.« less
Shape and surface texture of molten droplets deposited on cold surfaces
When a molten droplet impacts on a cold surface, the interaction between solidification and fluid dynamics can lead to different shapes and surface textures of deposited droplets. This article presents an experimental study on deposition of small droplets (39 μm diameter) of molten wax ink on solid surfaces. Our results show that both the final shape and the surface texture of deposited droplets are extremely sensitive to deposition conditions, which include initial temperature of droplets, type of substrate surface and distance between substrate and printhead. Different surface textures are observed and shown to depend on the thermal and flow histories of ink droplets during the process of impact. A test using crystalline wax reveals crystallization as one of the major factors in determining the final texture.