Modeling of the Thermal Discharge from a Power Generating Facility into a Tidal-Like Hydropower Controlled Pool on the Connecticut River

The Vermont Yankee Nuclear Power Station (Vermont Yankee), an electric-generating station, is located on the Connecticut River in Vermont above the Vernon Dam. Vermont Yankee uses a sophisticated cooling system that varies from an open cycle, once-through cooling water configuration to a closed cycle, cooling tower configuration that recirculates the cooling water. Its cooling system operation is based on river flow and temperature. In support of a request to increase discharge temperature limits during the summer period pursuant to a 316(a) of the Clean Water Act, this study was conducted to assess the thermal discharge from the plant and its effects on the River using a state-of-the-art, general curvilinear boundary fitted hydrothermal computer simulation model. The unique aspect of this model application was the time varying pool flow of the receiving water caused by hydropower generating activity. The flow varied on 1- to 4-day time scales with the diurnal being the most predominant. The pool response did vary widely during different measurement periods, however, sometimes with tidal (diurnal) regularity and other times with the diurnal component being only a small portion of larger multi day variations. A field program collected data from a set of continuous monitoring thermistors from May through October 2002 in the river with primary focus on the plant thermal plume. The data also included river flows and water temperatures obtained from permanently deployed instruments located both upstream and downstream of the plant and used in Vermont Yankee's operations. The model calibration and verification exercises used data sets acquired in August and June – July 2002, respectively. The August calibration period was chosen to be representative of warm river temperatures, low river flows and relatively regular water level variations. The June – July verification period was chosen to include warmer river temperatures occurring during fishway operations at a downriver dam. Results of the calibration and verification indicated that the model predicted both river flows and temperatures well. The flow predictions for the calibration and verification periods were very successful, exhibiting RME and ECV values less than 3% and 4%, respectively, which were smaller (better) than U. S. EPA guidance levels (30% and 10%, respectively). Correlations between the flow predictions and observations were excellent, with r² values of 0.92 and 0.98 for the calibration and verification periods, respectively, which were larger (better) than the guidance level of 0.88. The calibration results indicated that temperature predictions were also good. Best results were obtained at the upper layer and were somewhat reduced with depth. Squared correlation coefficients at the surface were usually larger (better) than the U. S. EPA guidance level of 0.71 but the correlations became smaller with depth. The average surface r² value was 0.74 while the overall average was 0.61. However, average RME and ECV values were 1.5% and 2.7%, respectively, smaller (better) than the EPA guidance levels (25% and 45%, respectively) and suggesting that the predictions throughout the water column were acceptable. The validation results indicated that the model performed excellently in simulating the thermal conditions of June – July 2002. The temperature predictions showed an average RME value of 2.1% compared to the guidance level of 25%, an average ECV value of 3.7% compared to the guidance level of 45%, and average r² of 0.91 compared to the guidance level of 0.71. Based on the successful calibration and verification, the model was then used for predictions under selected plant and river conditions. Two sets of scenario simulations were performed for the August and June – July periods to investigate the effect of a proposed new 0.6 °C (1 °F) increase in temperature limits within the portion of the lower Vernon Pool affected by the Vermont Yankee discharge during the summer permit compliance period and during the period of fishway operation, respectively. For the August period, a 0.6°C (1°F) increase from 1.1°C (2°F) to 1.7°C (3°F) resulted in a marginal increase in the temperature of the water column. For worst conditions occurring only 1% of the time, 30% of the volume increased by 0.3°C (0.5°F) and 20% increased by 0.4°C (0.7°F). For typical conditions occurring 50% of the time, 30% of the volume increased by 0.2°C (0.4°F) and 20% increased by 0.3°C (0.5°F). Similar results were obtained for changes in bottom area and for the June – July period. Based on the hydrothermal modeling results, a 0.6°C (1°F) increase in the permit limit from 1.1°C (2°F) to 1.7°C (3°F) resulted in de minimus changes in the thermal structure of the lower Vernon Pool.