The motion of a heavy tethered sphere and its wake were measured in a closed loop water channel using a time resolved, high-speed particle image velocimetry technique in a horizontal plane. Measurements were performed for nondimensional reduced velocities ranging from 2.8 to 31.1 that include three bifurcation regions. In order to analyze the vortex shedding characteristics, the directional swirling strength parameter was computed in addition to the vorticity as the former enables vortex identification. In the first bifurcation region, the sphere remained stationary and the wake was characterized by a train of hairpin vortices exhibiting symmetry in the vertical plane similar to visualization results obtained for stationary spheres. The second bifurcation region was characterized by large amplitude periodic oscillations transverse to the flow. Phase-averaged results for the swirling strength showed that although the shedding mechanism was identical for several reduced velocities, the phase at which vortices were shed increased with V R . Spatiotemporal swirling strength characteristics revealed counter-rotating vortex pairs in the far wake of the sphere. In addition to primary vortex pairs, secondary weaker vortical structures were also observed. In the third bifurcation region, nonstationary vortex shedding was characterized by high frequencies associated with shear layer instabilities causing pinch-off of small scale vortices. In addition, large scale undulations of the wake associated with the sphere motion were observed.

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The main purpose of this benchmark paper is to study and compare point and spatial neutronic approaches used to calculate ULOF and UTOP transients in sodium cooled fast reactors. A second objective is to compare deterministic and Monte Carlo calculations with two different calculation codes. The first one is based on a deterministic (discrete ordinate S N) approach, using tabulated self-shielded cross sections, where the core reactivity and the power shape distribution are evaluated at each time step of the transient calculation. The second model relies on the Transient Fission Matrix (TFM) approach, condensing the response of a Monte Carlo neutronic code in time dependent Green functions characterizing the local transport in the reactor. This second approach allows a fast estimation of the reactivity and of the flux redistribution in the system during the transient with a precision closed to that of the Monte Carlo code. Both models have been coupled to the thermalhydraulics and applied on an ASTRID representative assembly. This application case is supposed to be sensitive to power redistributions. A second comparison between spatial kinetics and point kinetics calculations has been led to study this point. Finally we obtain a good agreement between spatial and point kinetics on ULOF and UTOP calculations, while some discrepancies are observed between the TFM and the S N approaches on the power level stabilization, due to difference on the feedback estimation in both models.

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Variation of temperature in time and space was recorded at multiple vertical locations in the course of initiation of a heated water layer in an open-pool research reactor of the Soreq Nuclear Research Center. The pool was initially filled with warm water, and heavier cooler water was then injected at the bottom of the facility. Different modes of coolant injection were employed in two different experiments. In both cases, a finite width thermocline that separated cool water at the lower part of the pool from the warm water in its upper part was observed. The thermocline gradually moved up eventually attaining a constant raise velocity. In both experiments, the thermocline characteristics were different, but wave-trains with notable temperature fluctuations were observed within the thermocline. The characteristic frequencies of oscillations were below the Brunt–Väisälä frequencies that characterize the density gradient within the thermoclines. The finite dimensions of the tank impose conditions in which standing internal waves with the length commensurate with tank size can be expected. The oscillations were thus associated with resonant internal waves excited by disturbances introduced by the coolant flow at the lower part of the pool. In both experiments, the measured wave spectra agree with the results of linear analysis of two-layer and three-layer stratification models.

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Optical fibers and gratings were exposed to intense ionizing radiation in a research-grade nuclear reactor. Radiation induced attenuation, and changes in Rayleigh scattering (fibers) and Bragg wavelengths (gratings) were monitored in real time.