Dynamic Behavior of Thermal Plume

The thermal front phenomenon was observed during field surveys at the Ginna Nuclear Power Plant. Temperature oscillations were observed with amplitudes of over 3 °C measured from the mean with periods ranging from approximately 100–700 sec. A physical scaled model of the Ginna discharge was also used to study the thermal fronts and plume characteristics. The model was 31 ft (9.4 m) long and 16 ft (4.9 m) wide and geometric scale ratios of 1–50 and 1–100 were used to model the plume. Temperature versus time for several positions inside the model thermal plume exhibit a clear oscillatory pattern with amplitudes varying from approximately 1–5 °C (average differences between the discharge and ambient were 11 °C). The discharge and ambient temperature traces revealed no significant temperature fluctuations. Predominant periods ranged from 10–70 sec in the model and approximately 100–700 sec in the prototype. The power of the dominant oscillations tended to increase with depth at the same position inside the plume as did the magnitude of the temperature variations. The mechanism responsible for the thermal fronts is believed to be due to a Kelvin‐Helmholtz instability. This instability results in vortex formation at a shear layer due to the movement of the heated surface plume across the cooler ambient lake water. The Richardson number (J) computed from the measured temperature and velocity profiles, for locations in the near field plume with temperature oscillations, fell well within the unstable range when compared to J for the neutral curve developed for the Kelvin‐Helmholz instability.