ML18283B707

Submits Natural Heating and Cooling in the Wheeler Reservoir Approach to the Browns Ferry Nuclear Plant, Advance Report No. 23, Report No. 63-50

ML18283B707
Person / Time
Site:	Browns Ferry
Issue date:	03/31/1975
From:	Waldrop W Tennessee Valley Authority
To:	Office of Nuclear Reactor Regulation
References
Advance Report No. 23, Report No. 63-50
Download: ML18283B707 (16)

Text

Tennessee Vali ey Authority Division of Water Control Plannin Engineering Laboratory Cj y1 Ej I

C NATURAL HEATING AND COOLING IN THE WHEELER RESERVOIR APPROACH TO THE BROWNS FERRY NUCLEAR PLANT g 5 f Browns Ferry Nuclear Plant Advance Report No. 23 i .1$

Prepared by William R. Waldrop Norris, Tennessee March 1975 Report No. 63-50

NATURAL HEATING AND COOLING IN THE WHEELER RESERVOIR APPROACH TO THE BROWNS FERRY NUCLEAR PLANT INTRODUCTION Tennessee River temperatures in the'icinity of the TVA Browns Ferry Nuclear Plant are monitored every quarter of an hour to assure com-pliance with the Alabama thermal water quality standards. These tempera-tures are recorded by five permanent floating monitors and telemetered to the plant for analysis. This report addresses the natural heating and cool-;

ing in the Wheeler Reservoir as the river'low approaches the plant.

PLANT FEATURES AND SITE DESCRIPTION Browns Ferry Nuclear Plant, now under construction, is located in north central Alabama on the north shore of Wheeler Reservoir at Tennessee River mile 294 as shown on Plate 1. When completed, the plant will contain three nu'clear-powered steam-electric generating units, each with a rated'.

capacity of 1152 megawatts.

During late 1973 unit 1 at the plant began operating with the con-denser cooling water system arranged in a once-through cooling mode. In this mode of operation heated water is passed directly from the steam condenser tnto Wheeler Reservoir through a 600-foot long diffuser pipe. The condenser flow is 1450 cubic feet per second, and the temperature rise across the condenser is 25'. The condenser cooling water pumping station is situated upstream from the powerhouse at the end of a short intake channel.

The cooling water diffuser system for the plant is constructed from perforated, corrugated, galvanized steel pipes laid side by side across the bottom of the main river channel. The diffuser location is shown by the map on Plate 2. The pipes are 17 feet, 19 feet, and 20 feet 6 inches in diameter and of different lengths as shown in detail on Plate 3. Each has the last 600 feet perforated on the downstream side with more than 7,000 holes two inches in diameter spaced six inches on centers in both directions.

The diffuser section for unit 1 occupies the center portion of the channel while unit 2 occupies the left-hand third, and unit 3 the right-hand third.

The topography of the reservoir at the site consists of a main navigation channel about 1,800 feet wide and 30 feet deep bordered by shallow overbank areas which vary from 2 to 10 feet in depth and 2,000 to 6,000 feet in width. A broad shallow area along the right bank (plant side) extends for several miles upstream.

HYDRO-THERMAL REGIME IN WHEELER RESERVOIR The hydraulic regime in the reservoir is controlled by the opera-tion of Guntersville (upstream) and Wheeler (downstr earn) Dams. These proj-'cts are operated primarily for hydro power production and navigation and at some periods of the year for flood control. Under present operating rules, the water level in Wheeler Reservoir does not vary more than six feet during the year. The mean annual flow in the Tennessee River at Browns Ferry is about 45,000 cubic feet per second; and the mean monthly average, estimated from the monthly averages at Guntersville Dam for the period 1959-1968, is presented in Plate 4.

Water temperatures in the reservoir have been monitored by perma-nent recording stations since the fall of 1968., Dates of initial operation of these monitors are listed in Table l. The natural water temperatures'n the reservoir range from about 40'. in the winter to a maximum (at the surface) of 85'-95'. in the summer with a maximum top to bottom vertical temperature difference of about 5'-8'.

At present, five temperature monitors are used to determine com-pliance of Bpowns Ferry Nuclear Plant with the Alabama State Water Tempera-ture Standards. The temperature of the heated discharge plume is determined by three stations (Nos. '9, 10, and 11) spaced across the reservoir at river mile 292.5. The plant receiving water temperature is determined by station (No. 6) at, river mile 309.7 or, if station 6 is inoperative, by station (No.

4) at mile 297.8. Plate 5 shows the location of these monitors. At 15-minute intervals a data logging system scans the stations and displays tem-perature data from the five-foot depth in the Browns Ferry control room to

ensure compliance with the state thermal standards. The monitoring system has been described in more detail in Reference l.

NATURAL HEATING AND COOLING UPSTREAt1 OF THE PLANT The river upstream of the monitor at Station 6, the primary monitor for determining the upstream temperature, may be described as riverine and generally confined to the natural river banks; consequently, velocities, are relatively h'igh between Guntersville Dam and Station 6. Slightly downstream of Station 6, the reservoir spreads over wide and shallow overbank regions.

Because of the large surface area along this stretch of the .reservoir, the heat flux to or from the atmosphere may be significant. During the spring and summer, the reservoir may warm considerably during daylight. hours. In the fall, the opposite is true and appreciable cooling of the reservoir may occu} along this overbank region. A qualitative description of this warming and cooling in the overbank region is shown in the aerial infrared imagery of Plates @ and 7N Plate 6r presents about a seven-mile stretch of the reservoir irrmediately downstream of Station 6. The lighter shades of gray indicate warmer surface water. Notice how the river tends to flow into the overbank area upstream and either warm or cool, depending upon the season.

, Plate 7; presents aerial infrared imagery of the right overbank region for about an eight-mile stretch of the reservoir immediately upstream .

of the plant. The same trend of warming or cooling continues until the flow along the overbank is mixed with the flow of the main channel in the vicinity of the plant.

Because of the large cross-sectional areas in the reservoir between

'tation 6 and Browns Ferry, the mean travel time is quite long. For instance, assuming a constant river flow of 35,000 cfs, 42 hours4.861111e-4 days <br />0.0117 hours <br />6.944444e-5 weeks <br />1.5981e-5 months <br /> are required for a slug Reference 1: "tlonitoring of Mater Temperatures in Wheeler Reservoir to Determine Compliance of Browns Ferry Nuclear Plant with Water Temperature Standards." Advance Report No. 21. Tennessee Valley Authority, Division of Mater Control Planning, Engi-neering Laboratory.

of water to travel from Station 6 to Browns Ferry. This, of course, is based upon mean cross-sectional velocities. Considerable natural heating or cool-ing, possibly 4 F. or more, can occur during this time as is shown from the hourly recorded temperatures at Station 1 presented in Plates 8 and 9. This monitor is located nearest the plant and probably best reflects the natural river temperatures in the vicinity of the diffuser before operation of'he plant.

'A statistical analysis of the natural heating and cooling along this stretch of the reservoir has also been performed. The hourly differ-s ences between temperatures at the five-foot depth at Stations 1, 4, and 6 were analyzed on a monthly basis. Results of preplant operation are pre-sented in Plate 10 as'monthly mean values of hourly temperature differences and standard deviations about the mean. Recall that Station 1 is located furthest downstream and Station 6 is furthest upstream. This clearly indi-cates that during Hay 1973 the river warmed an average of 2'. between Station 6 and the plant and a natural warming of 3'. was not uncomnon.

The autumn cooling trend is also apparent in these results.

A similar analysis of the mean temperature differences between data from the five-foot depth of Stations 4 and 6 was performed. This com-parison covers a longer time span because both are upstream .of the plant and hence not influenced by the thermal discharge as was Station 1. The yearly cycle of heating and cooling is apparent in the results shown in Plate 11.

The spring heating during 1974 was apparently not as rapid as during 1973, but differences of 1.5'. between these two stations were not uncommon.

CONCLUSIONS TVA has positioned permanent floating monitors upstream and down-stream of the thermal di'scharge of the Browns Ferry Nuclear Plant to assess the incremental heating which the plant adds to the reservoir. The upstream monitor located 15 miles above the plant should determine the ambient temper-ature of the river before the thermal discharge. For several miles upstream of the plant, the 1/heeler Reservoir is very broad and often quite shallow.

As a result, mean velocities are low and overbank natural heating and cooling

is substantial during much of the year. After analyzing the available data which now spans several years, it is concluded that the upstream monitor at Station 6 is located too far upstream of the plant to provide reliable temperatures of the ambient river for assessing the incremental heating input of the plant.

TABLE 1 OPERATIONAL DATES OF BFNP RIVER TEMPERATURE MONITORS

. Monitor River flile . Ince tion Date 293.55 R Sept 1968 3 275. 0 L Dec 1968 297.78 L Sept 1969 285.25 R Sept 1969 309. 7 R Apr 1973

'7 295.7 R Sept 1973 294.5 L Sept 1973 9 292.5 R July 1973 10 ~

292. 5 L July 1973 292. 5 H July 1973

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