Hydrothermal Studies 



Introduction 



During 1987, Northeast Utilities Service Com- 

 pany (NUSCO), on behalf of Northeast Nuclear 

 Energy Company (NNECO), completed a survey 

 of the extent and configuration of the thermal 

 plume generated from the combined output of all 

 three nuclear power plants at the Millstone Nu- 

 clear Power Station (MNPS). This survey was 

 completed to meet two needs. The first was to 

 determine whether the NPDES Permit (CT 

 0003263) condition restricting the plume size had 

 been met. The Permit states that 



the permitee shall operate all facilities in such 

 a manner as not to raise the average temper- 

 ature of the receiving waters more than 4°F or 

 increase the normal temperature of the receiv- 

 ing waters above 83°F. For purposes of this 

 condition, cognizance will be given to reason- 

 able time and distance to allow mixing of 

 effluent and receiving waters, but the boundary 

 of the mixing zone shall not exceed a radius 

 of 8,000 feet from the discharge outlet at the 

 quarry cut. 



The second was a commitment made by NUSCO 

 on behalf of NNECO in a letter dated 23 July 

 1 986, to the Connecticut Department of Environ- 

 mental Protection (CT DEP) (NUSCO 1986). 

 In this letter NUSCO agreed 



to verify thermal plume predictions and to map 

 the extent of the 3-unit thermal plume during 

 varying tidal conditions, and to undertake ad- 

 ditional thermal plume studies in 1987. The 

 objective would be to implement the field stud- 

 ies when all three units were operating .umul- 

 taneously at or near 100% power. 



The work completed during 1987 included water 

 temperature and dye concentration surveys during 



the period 21-27 August 1987 to determine the 

 water temperature rise due to the plant discharge 

 and resulted in plots of isotherms of plume con- 

 figurations during four tidal regimes. 



The specific objectives of the field studies were 



1 . Provide dye concentration measurements that 

 would distinguish temperature rise in Long 

 Island Sound due to heated circulating water 

 discharge from heating due to solar radiation 

 and other natural sources. 



2. Provide information on the effect of three- 

 unit circulating water discharge on the 

 hydrographic and thermal characteristics of 

 the receiving waters off Millstone Point dur- 

 ing four tidal phases: max ebb, slack after 

 ebb, max flood and slack after flood 



The thermal plume survey was a cooperative 

 effort involving Ocean Surveys, Inc. (OSI), and 

 NUSCO staff. OSI provided instrumentation for 

 positioning, dye delivery, flourometry and tem- 

 perature recording and expertise for thermal con- 

 tour mapping. NUSCO provided necessary boat 

 and staff support. The survey data were supple- 

 mented by continuously recorded temperature 

 data collected during October 1987 at strategic 

 locations by NUSCO staff. This document sum- 

 marizes past hydrographic studies, presents the 

 methodology and results of those done during 

 1987 and compares the results of these studies to 

 three-unit thermal plume predictions. 



Review of past studies 



Since 1966, NUSCO has retained several differ- 

 ent investigators to perform hydrographic and 

 hydrothermal surveys in the vicinity of Millstone 

 Point (Table 1). Synopses of the results foUow. 



Hydrothermal Studies 323 



