WATTS BAR. NUCLEAR PLANT SUPPLEMENTAL CONDENSER COOLING WATER PROJECT Thermal Plume Modeling Prepared by WalterHarper Tennessee Valley Authority Engineering Laboratory Norris, Tennessee December 5, 1997 Index No: 444 Title: Watts Bar Nuclear Plant Supplemental Condenser Cooling Water Project EA -August 1998- Thermal Plume Modeling - December 5,1997 WBN Supplemental Condenser Cooling Water Project Thermal Plume Modeling Executive Summary Project statement This project would construct and operate a Supplemental Condenser Cooling Water (SCCW) system for the Watts Bar Nuclear Plant. The system would connect existing water intake and discharge piping originally operated as a part of the Watts Bar Fossil Plant to the cooling towers at the Nuclear Plant. This system would increase power production by the Watts Bar Nuclear Plant by reducing main turbine condenser temperature. The gravity flow SCCW system would operate continuously except during times of chemical water treatment for the Nuclear Plant. Operation of the existing cooling tower was optimized previously at 105% of design capacity, however, warm weather power losses continue due to undersizing of cooling tower capacity to support maximum main turbine generator power and condenser cooling capacity. Without installation of the SCCW system, no increase in generation would be realized. CORMIX was used to model the mixing of the discharge from the WBF discharge structure into the river. CORMIX is a steady state computer model which only accepts input through keyboard entry. Therefore, it was not feasible to run CORMIX for all possible combinations of river flows and elevations, meteorological conditions and plant operations. Instead, CORMIX was run for a range of river flows, river elevations, ambient river temperatures, discharge flow rates, and plant temperature rises (difference between discharge and intake temperature) which encompassed the maximum and minimum expected values for each parameter. The results of these CORMIX runs were used as the basis for an interpolation scheme by which the instantaneous and average instream river temperature rise due to discharge from the WBF discharge structure was computed. Conclusions The proposed SCCW system can be operated without significant risk of non-compliance of the instream thermal limits of 30.5 0C (86.9 TF) maximum discharge temperature, 3 °C (5.4 oF) maximum delta temperature, and +2 TC (3.6 'F) rate of rise. It would be necessary to operate the bypass system during the months of December, February, March, and April. The number of predicted non-compliances of the instream temperature rise limit during the months of November and January are relatively small. This indicates that some operation of the bypass would be needed in January and possibly in November, but not on a continuous basis. The conditions under which the bypass would be needed are a combination of low river temperature and higher than normal air temperature. Given reasonably accurate weather forecasts, these conditions can be predicted sufficiently far in advance to turn the bypass on if needed during these months. There is no significant likelihood of the 85 'F (29.4 °C) WBN intake temperature safety limit being exceeded due to operation of the SCCW system. However, should the intake temperature approach the limit, special WBH operations could be used to reduce the temperature at the WBN intake. If such WBH operations are not feasible, the SCCW system could be shut down, reducing the WBN intake temperature to that of the WBH discharge temperature. No adverse influence on niver temperatures at Sequoyah Nuclear Plant is expected from operation of the SCCW system. 2 Table of Contents Executive Summary 2 1.0 Introduction 4 2.0 Plant Operating Characteristics 4 2.1 Watts Bar Hydro Plant 4 2.2 Watts Bar Fossil Plant 4 2.3 Watts Bar Nuclear Plant 4 2.3.1 Current Design 4 2.3.2 Proposed Design Modifications 5 3.0 Hydrothermal Analyses 5 3.1 Discharge Through WBF Discharge Structure 5 3.2 Discharge Through WBN Diffuser 6 3.3 Near-Field Effects on River Temperature of Combined Discharges Through WBN Diffuser and WBF Discharge Structure 7 3.3.1 Computer Model Inputs 7 3.3.2 Model Results 9 3.4 Effect of SCCW System Operation on WBN Intake Temperatures 11 3.5 Effect of Watts Bar Nuclear and Fossil Plant Discharges on Sequoyah Nuclear Plant 11 4.0 Conclusions 12 5.0 Tables 13 6.0 Figure;s 31 7.0 References 43 3 1.0 Introduction This evaluation provides details of the computer modeling of the effects of Watts Bar Nuclear Plant (WBN) thermal discharge on Chickamauga Reservoir. The operating characteristics of WBN and Watts Bar Hydro Plant are described. Results of computer simulations of plant operations using dam releases, reservoir elevations, and ambient water and air temperatures for 1976 through 1993 are presented. 2.0 Plant Operating Characteristics 2.1 Watts Bar Hydro Plant Watts Bar Hydro Plant (WBH) has five units with a total generating capacity of 166.5 MWe. The turbine discharge is approximately 9000 cfs per unit. WBH is normally used for peaking operation which entails storing water during periods of low electrical demand and releasing it during periods of high electrical demand. This usual pattern for peaking operation is low or zero turbine discharge at night and higher discharges during the day. Hourly WBH discharges have been archived since January 1, 1976. The average annual discharge at the hydro plant for the modeled period is approximately 26,300 cfs. Seasonal and monthly average discharges are shown in Table 1. The normal discharge through each of the five turbines ranges from 7500 to 10,000 cfs. The minimum flow through a single turbine is 3500 cfs, however, for maximum efficiency, the single unit flow is seldom less than 8000 cfs. The water surface elevation downstream of WBH in the vicinity of WBN is determined by the headwater elevation at Chickamauga Dam and the discharges from WBH and Chickamauga Hydro Plant (CHR). Normal Chickamauga headwater elevations vary from a minimum pool elevation of 675.0 feet in the winter to a maximum of 683.0 feet in the summer. 2.2 Watts Bar Fossil Plant Watts Bar Fossil Plant (WBF) is currently mothballed with no scheduled start-up date. Implementation of the proposed plan for supplemental condenser cooling water for WBN would prevent future WBF operation. When operated, the WBF condensers are cooled by a gravity feed cooling water system which withdraws water from Watts Bar Reservoir, through an intake structure located on the upstream face of Watts Bar Dam. The nominal flow rate of the WBF condenser cooling water (CCW) system is 626 cfs (280,968 gpm), with a temperature rise across the condensers of 10 F*(5.56 C0). After passing through the condensers' the cooling water was discharged into the Tennessee River below Watts Bar Dam. This discharge structure is a rectangular culvert seven feet wide by 10 feet high, which expands to a width of 15 feet at the discharge point. The elevation of the top of the culvert outlet is 675 feet, which coincides with the normal minimum pool elevation of Chickamauga Reservoir. At winter reservoir elevations, the culvert acts as an open channel discharge. At higher reservoir elevations, the top of the culvert opening is submerged to a maximum depth of 8 feet. 2.3 Watts Bar Nuclear Plant 2.3.1 Current Design Watts Bar Nuclear Plant is located on the right bank of the Tennessee River at Tennessee River Mile (TRM) 528. WBN is a two unit plant, however only Unit 1 is currently operational. Unit 1 is rated to produce 1270 MWe of electricity at full load. The plant utilizes a closed cycle heat dissipation system, consisting of one natural draft cooling tower per unit and a blowdown system. Water losses due to blowdown and evaporation are replenished with makeup water supplied via an intake channel and pumping stations at TRM 528.0. The average and maximum makeup rates are 64 cfs (28,725 gpm) and 71.5 cfs (33,348 gpm), respectively. 4 To maintain the concentration of dissolved solids in the cooling tower basin at approximately twice that found in the Tennessee River, blowdown discharge from the cooling tower is between 21.1 cfs (9470 gpm) and 31.4 cfs (14,093 gpm). All blowdown is either discharged into the Tennessee River through a submerged multiport diffuser system (TRM 527.8) or held for discharge at a later time. The diffuser system consists of two pipes branching from a central conduit at the right bank of Chickamauga Reservoir and extending in a direction perpendicular to Tennessee River flow. Each pipe is controlled by a 54-inch diameter butterfly valve located a short distance downstream of the central conduit wye. A physical description of the diffusers is given in Table 2. The National Pollutant Discharge Elimination System (NPDES) Permit for WBN stipulates that the diffusers should be operated only when the discharge from WBH is greater than 3500 cfs. To meet this requirement, blowdown from the cooling towers is diverted to a holding pond when WBH is not discharging. When WBH discharge resumes, the stored blowdown water is released through the diffusers. The holding pond discharge rate ranges from 0 to approximately 110 cfs (49,370 gpm), depending on the surface elevation of water in the pond and in the Tennessee River. The combined discharge from the holding pond and cooling towers ranges from 21.1 cfs (9470 gpm) to 138 cfs (61,940 gpm). An overflow weir (elevation 707 fL) on the south side of the holding pond allows discharge though a surface channel into the Tennessee River (TRM 527.2) should the pond capacity of 190 acre-feet be exceeded due to an extended emergency curtailment of WBH discharge.