ML13289A158
| ML13289A158 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 06/30/2003 |
| From: | Harper W Tennessee Valley Authority |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML13289A109 | List:
|
| References | |
| WR2003-1-45-149 | |
| Download: ML13289A158 (20) | |
Text
DRAFT TENNESSEE VALLEY AUTHORITY River System Operations & Environment River Operations Study to Confirm the Calibration of the Numerical Model for the Thermal Discharge from Sequoyah Nuclear Plant as Required by NPDES Permit No. TN0026450 of August 2001 WR2003-1-45-149 Prepared by Walter L. Harper Norris, Tennessee June 2003
DRAFT i
EXECUTIVE
SUMMARY
A system of real-time measurements and computer models is used to verify compliance of Sequoyah Nuclear Plant (SQN) operations with the instream thermal limits contained in the National Pollutant Discharge Elimination System (NPDES) permit. The NPDES permit of August 2001 also requires that a calibration study of the numerical model be used to compute downstream temperatures once per permit cycle.
Temperature data collected at the downstream end of the SQN mixing zone in 11 field surveys performed at SQN between 1982 and 2003, along with measured data from the SQN instream temperature monitoring system were compared with computed downstream temperatures for the same time periods. Sensitivity tests were performed for the two calibration parameters for the diffuser mixing model, effective diffuser slot width, and plume entrainment coefficient. The results showed acceptable agreement between computed and measured temperatures, particularly at river temperatures greater than 75ºF. The accuracy at lower river temperatures, although not as good, is still considered acceptable, since instream temperatures resulting from SQN operations do not approach the NPDES limits as closely at lower river temperatures.
DRAFT ii CONTENTS Page Executive Summary....................................................................................................................... i List of Figures................................................................................................................................ iii List of Tables................................................................................................................................. iii Introduction................................................................................................................................... 1 Background................................................................................................................................... 4 Model Summary............................................................................................................................ 7 Calibration..................................................................................................................................... 9 Previous Calibrations and Field Surveys............................................................................. 9 New Calibration Data.......................................................................................................... 9 Diffuser Slot Width.............................................................................................................. 9 Plume Entrainment Coefficient.......................................................................................... 11 Results of Updated Calibration.......................................................................................... 12 Conclusions................................................................................................................................. 15 References................................................................................................................................... 16
DRAFT iii CONTENTS (continued)
Page LIST OF FIGURES
- 1.
Location of the Sequoyah Nuclear Plant............................................................................. 2
- 2.
Chickamauga Reservoir in the Vicinity of Sequoyah Nuclear Plant................................... 3
- 3.
Locations of Instream Temperature Monitors at Sequoyah Nuclear Plant.......................... 5
- 4.
Sequoyah Nuclear Plant Diffuser Design Details................................................................ 8
- 5.
Sensitivity of Computed Downstream Temperature to Diffuser Effective Slot Width.......................................................................................................... 11
- 6.
Sensitivity of Computed Downstream Temperature to Plume Entrainment Coefficient.. 13
- 7.
Comparison of Computed and Measured Downstream Temperatures for Field Studies from April 1982 through April 2003.................................................................... 13
- 8.
Comparison of Computed and Measured 24-Hour Averaged Downstream Temperatures from Compliance Monitoring Records for 8/09/2001 through 10/15/2002............................................................................................................ 14
- 9.
Comparison of Computed and Measured Hourly Averaged Downstream Temperatures from Compliance Monitoring Records for 8/09/2001 through 10/15/2002............................................................................................................ 15 LIST OF TABLES
- 1.
Summary of Instream Thermal Limits for SQN Outfall 101............................................... 6
- 2.
Thermal Surveys at Sequoyah Nuclear Plant from April 1982 through March 1983....... 10
- 3.
Thermal Surveys at Sequoyah Nuclear Plant from March 1996 through April 2003....... 10
DRAFT INTRODUCTION The Sequoyah Nuclear Plant (SQN) was built by the Tennessee Valley Authority (TVA) on the right bank of Chickamauga Reservoir, at Tennessee River Mile (TRM) 484.5. The plant is about 18 miles (29 kilometers) northeast of Chattanooga, Tennessee, and about 13 miles (21 kilometers) upstream of Chickamauga Dam (Figure 1). The reservoir in the vicinity of SQN can be characterized as having a roughly rectangular main channel approximately 900 feet (274 meters) wide and 50 to 60 feet (15 to 18 meters) deep, depending on the pool elevation, with extensive and highly irregular overbank areas which are usually less than 20 feet (6.0 meters) deep (Figure 2).
The two-unit Sequoyah Nuclear Plant has a net generating capacity of 2440 MWe and an associated waste heat load of 4800 MWe, or 16.4 x 109 Btu/hr. The heat transferred from the steam condensers to the cooling water is dissipated to the atmosphere by way of two natural draft cooling towers, to the river through a two-leg submerged multiport diffuser, or by a combination of both.
Compliance of SQN operation with the instream temperature limits specified in the National Pollutant Discharge Elimination System (NPDES) Permit is based on downstream temperatures which are calculated on a real-time basis by a numerical computer model. A calibration study of that model is required by NPDES Permit TN0026450 of August 2001. In particular, Part II, Section G states:
The numerical model used to determine compliance with the temperature requirements for Outfall 101 shall be subject of a calibration study once during the permit cycle. The study should be accomplished in time for data to be available for the next permit application for re-issuance of the permit. A report of the study will be presented to the division of Water Pollution Control. Any adjustments to the numerical model to improve its accuracy will not need separate approval from the Division of Water Pollution Control; however, the Division will be notified when such adjustments are made.
This report documents the findings of the required calibration study.
DRAFT Figure 1. Location of the Sequoyah Nuclear Plant
\\
SEQUOYAH NUCLEAR PLANT TRM 4B4.5 WATTS BAR STEAM PLANT WATTS BAR NUCLEAR PLANT CLEVELAND
~
(3ATHENS CHICKAMAUGA DAM I
'/'
/
, HIWASSEE DAM CHATTA~:~G:71.0 ___ I~ NNESSEE _____ 1 t~~J~N~ __
.F---
DRAFT 3
Denotes Reservoir areas of water depth less than 20 feet Station 13 Station 8 Mixing Zone SQN Figure 2. Chickamauga Reservoir in the Vicinity of Sequoyah Nuclear Plant
DRAFT 4
BACKGROUND In August 1983, the Tennessee Valley Authority (TVA) reported the results of six field studies of the Sequoyah Nuclear Plant (SQN) diffuser performance under various environmental and plant operating conditions (TVA, 1983a). One purpose of that report was to support the use of computerized compliance as a method for verifying compliance with the thermal limitations in the NPDES Permit for the plant. The field validations from the report showed that the temperature variation that occurs in space and time at the downstream edge of the mixing zone made the use of the two downstream temperature monitors, then in place, inadequate for obtaining a representative cross-sectional average temperature. Because of the necessity to maintain an unobstructed navigation channel, a temperature monitoring station could not be positioned downstream of the mid-point of the diffuser. Instead, two monitors were placed at the edges of the navigation channel, one downstream of either end of the diffuser (Figure 3). It was found that Station 11, at that time located on the right side of the river (looking downstream),
was normally not in the main flow path of the discharge plume and did not always show elevated temperatures. Station 11 has subsequently been removed. The remaining downstream monitor, Station 8, may not be in an ideal location because it must also be positioned outside the navigation channel.
In the report, TVA described the use of a dedicated microcomputer to provide real-time assessment of compliance with thermal discharge limitations in Chickamauga Reservoir. The condenser waste heat is discharged to the reservoir through two multiport diffusers located on the riverbed. The procedure for evaluating the effects of the thermal discharge upon the river temperatures requires data gathered at the intake and discharge of the plant and from upstream and downstream dams. A microcomputer, located in the Environmental Data Station (EDS), is used to compute the plant-induced effects upon river temperatures (TVA, 1983b).
The August 1983 report was sent to the Environmental Protection Agency (EPA) and the State of Tennessee (TN) requesting approval to use the computerized compliance as the first method of verifying thermal compliance. The advantages of the method include an improved representation of downstream temperatures (cross-sectional averages) that is at least as good as the instream temperature measurements. The computerized compliance method also provides consistency in the modeling that is used for scheduling upstream and downstream dam releases and SQN operation to meet the thermal limits. The consistency in modeling allows TVA to minimize the number and duration of thermal noncompliances. The operators and schedulers can anticipate undesirable thermal conditions and implement changes that will ensure compliance with the thermal limits in a timely fashion.
The current instream temperature limits for the SQN diffuser (Outfall 101) are summarized in Table 1. Compliance with instream temperature limits are based on computed downstream temperatures at a depth of 5.0 feet. Upstream temperatures are measured at the 5.0-foot depth at monitoring Station 13, located on the SQN intake skimmer wall. A spatial average of measurements at depths of 3.0, 5.0, and 7.0 feet is used as the primary backup downstream temperature measurement in the event of computer failure or invalid or missing input data for the computed downstream temperature.
DRAFT 5
The model is calibrated by adjusting the plume entrainment coefficient and effective diffuser slot width to achieve the best match of the computed downstream temperatures with field survey measurements and recorded instream monitor data. Higher priority is given to matching data from field surveys, since those measurements are made across the entire width of the plume mixing zone and, therefore, are believed to be more representative of the plume than data from the backup compliance monitor (Station 8), which is limited to the left edge of the mixing zone.
`
Td dTd/dt T=Td-Tu Tu Station 13 Station 12 Station 8 Mixing zone Station 11 (removed)
Figure 3. Locations of Instream Temperature Monitors at Sequoyah Nuclear Plant
DRAFT 6
Table 1. Summary of Instream Thermal Limits for SQN Outfall 101 Type of Limit Averaging Period (hours)
NPDES Limit2 Tmax 1
243 86.9°F (30.5°C)4 Tmax 1
1 93.0°F (33.9°C)
T1 243 5.4 / 9.0 F° (3.0/5.0Cº)5 dT/dt Mixed6
+/- 3.6 F°/hr (2.0 Cº/hr)
Notes:
- 1.
Compliance with the river limitations (river temperature, temperature rise, and rate of temperature change) shall be monitored by means of a numerical model that solves the thermohydrodynamic equations governing the flow and thermal conditions in the reservoir. This numerical model will utilize measured values of the upstream temperature profile, flow, and temperature of the diffuser discharge, releases at Watts Bar and Chickamauga Dams, and the diffuser performance characteristics. In the event that the modeling system described here is out of service, an alternate method will be employed to measure water temperatures at least one time per day. Depth average measurements can be taken at a downstream backup temperature monitor (left bank TRM 483.4) or by grab sampling from boats. Boat sampling will include average 5-foot depth measurements (average of 3-, 5-, and 7-foot depths).
Sampling from a boat shall be made outside the skimmer wall and at quarter points and mid-channel at downstream TRM 483.4. Sampling from boats will be used to verify compliance with temperature rise and maximum river temperature limits. The downstream reported value will be a depth (3-, 5-, and 7-foot) and lateral (quarter points and midpoint) average of instream measurements. Monitoring in the alternative mode using boat sampling shall not be required when unsafe boating conditions occur.
- 2.
Compliance with river temperature, temperature rise, and rate of temperature change limitations shall be applicable at the edge of a mixing zone which shall not exceed the following dimensions: (1) a maximum length of 1500 feet downstream of the diffusers, (2) a maximum width of 750 feet, and (3) a maximum length of 275 feet upstream of the diffusers. The depth of the mixing zone measured from the surface varies linearly from the surface 275 feet upstream of the diffusers to the top of the diffuser pipes and extends to the bottom downstream of the diffusers. When the plant is operated in closed mode, the mixing zone shall also include the area of the intake forebay.
- 3.
Daily maximum temperatures for the ambient temperature upstream of the discharge, the downstream river temperature at the edge of the mixing zone, and temperature rise shall be determined from 24-hour average values.
The average temperature shall be calculated every 15 minutes using the previous 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> of data, thus creating a rolling average. The maximum of the ninety-six averaged observations generated by this procedure shall be reported as the daily maximum value.
- 4.
The maximum 24-hour average river temperature is limited to 30.5°C as a daily maximum. Since the states criteria makes exception for exceeding the value as a result of natural conditions, where the 24-hour average ambient temperature exceeds 29.4°C and the plant is operated in helper mode (full operation of one cooling tower, at least three lift pumps, per operating unit), the maximum temperature may exceed 30.5°C. In no case shall the plant discharge cause the 1-hour average downstream river temperature to exceed the temperature of 33.9°C without the consent of the permitting authority.
- 5.
The 24-hour average temperature rise shall be computed ninety-six times at 15-minute intervals. The temperature rise is the difference between the upstream ambient 24-hour average river temperature and the 24-hour average downstream temperature at the edge of the mixing zone for that 15-minute interval. The 24-hour average temperature rise shall be limited to 3.0 C° during the months of April through October. The 24-hour average temperature rise shall be limited to 5.0 C° during winter operation months of November through March.
- 6.
The rate of temperature change instream shall be computed at 15-minute intervals based on the current 24-hour average ambient temperature, current 24-hour-hour average river flow, and current values of flow and temperature of water discharging through the diffuser pipes (recorded every 15 minutes). The 1-hour average rate of temperature change shall be calculated for each 15-minute interval by averaging that 15-minute value with the previous four 15-minute values. The 1-hour average rate of temperature change shall be limited to 2 C° per hour.
DRAFT 7
MODEL
SUMMARY
The diffusers at SQN lie submerged at the bottom of the navigation channel of Chickamauga Reservoir. Each diffuser leg is 350 feet long, and contains seventeen 2-inch ports per linear foot of pipe arranged in seventeen rows over an approximately 18 degree arc of the pipe (Figure 4).
The two diffuser legs rest on an elevated pad approximately 10 feet above the channel bottom, occupying the 700 feet of navigation channel nearest the plant (left side of the channel, looking downstream). The flow field in the immediate vicinity of the holes is far too complex to be analyzed on a real-time basis with current computer technology. Therefore, a simplifying assumption must be made that the diffuser can be treated as a slot jet with a length equal to that of the perforated sections of the pipe. The width of this assumed slot then becomes one of two empirical constants or relationships which are used to calibrate the model; the other is the coefficient used to compute the entrainment of ambient water at the turbulent boundaries of the plume.
The development of the current diffuser model is described in detail by Benton (2003), and will only be summarized here. The model is a two-dimensional numerical solution of the equations conservation of momentum, mass, and energy for a submerged diffuser in a thermally stratified cross-flow. The governing differential equations are solved by a fourth-order Runge-Kutta scheme, using measured diffuser discharge flow and temperature, upstream temperature profile and river surface elevation, and river flow and velocity computed by a one-dimensional unsteady flow model as boundary conditions. The inputs to the river flow model are measured releases at the upstream and downstream dams (Watts Bar and Chickamauga Hydro Plants) and the measured river surface elevation at SQN.
The downstream temperature and instream temperature rise are computed at 15-minute intervals, using instantaneous values of measured diffuser discharge flow and temperature, upstream temperature profile, and river elevation, and computed river flow, which is, in turn based on measured upstream and downstream hydro releases. All computations are performed once every 15 minutes. One-hour and 24-hour averages are computed from the 15-minute solution results.
The one-hour averages are based on the last four 15-minute values, while the 24-hour averages are based on the last ninety-six 15-minute values.
The rate of instream temperature change is computed differently, being computed at 15-minute intervals based on the current 24-hour average ambient temperature, current 24-hour average river flow, and current values of flow and temperature of water discharging through the diffuser pipes. This method was adopted in August 2001, in order to distinguish between rate of instream temperature change due to changes in SQN operations (i.e., changes in discharge flow and/or temperature) and non-SQN induced temperature changes such as passage past the plant of packets of water which have been raised to higher than normal ambient surface temperatures in upstream shallow embayments and then flushed downstream by unsteady river flows. Prior to this change, SQN was held accountable for numerous temperature spikes over which it had no control and very little influence.
DRAFT Figure 4. Sequoyah Nuclear Plant Diffuser Design Details EL 680 II II II II II II II 00 a cDlO.,.
"'cD cD RIVER FLOW" I I I I I I I I
I I I I
-I-I I J '
I-350ft.
350 ft.
..l..1 600ft.
~
I II 1
I I
I I A.J a,..,
cD 17 fI OIAMETER UPSTREAM DIFFUSER LEG EL 626.~.
RIVER 16fl DIAMETER DOWNSTREAM DIFFUSER LEG
... DISCHARGE PORTS EL635 SECTION A-A I I I
I II I I I. ~
I I I I I III I I 000 a a It).,. 10 cD...
cDlDlD cD cD NORMAL MAXIMUM POOL EL 683.."
NORMAL MINIMUM POOL. EL 675;\\
EL650.5 o
0 o 0 0
I.,
ARRANGEMENT OF PORTS tr:
~
DIFFUSER POND DIKE PORT DIAMETER 2" ALTERNATING COUJMNS OF 8 AND' PORTS
DRAFT 9
CALIBRATION Previous Calibrations and Field Surveys Calibrations and/or field surveys were performed in 1981, 1982, 1983, 1987, 1996, 1997, 1999, 2000, 2002, and 2003. On July 24, 1981, TVA conducted a test under one unit SQN operation (TVA, 1982). Ambient temperatures ranged from 82.4°F (28°C) at the surface to 78.8°F (26°C) in the bottom layers. The diffuser discharge was 1240 cfs at a temperature of about 102.2°F (39°C). Adequate agreement was achieved with measured data and model projections. In cases where there were discrepancies, the model under-predicted the observed dilutions (over-predicted temperatures).
Between April 1982 and May 1983, six field surveys were performed to acquire data for validation of the computed compliance technique (TVA, 1983a). Only one SQN unit was operating during the March 1983 test; the other five tests were for two-unit operation. The results of the simulation program compared favorably with the field-measured downstream temperatures. On average, the discrepancy between the measured and computed downstream temperatures was 0.4 F° (0.22 C°). Considering that the accuracy of the temperature sensors is 0.25 F° (+/-0.14 C°), the agreement between the field measurements and the computer model is quite close. A similar comparison of the monitored temperatures with the measured lateral averages revealed that the discrepancy for Station 8 was 0.79 F° (0.44 C°) and for Station 11 was 0.65 F° (0.36 C°). Consequently, it was concluded that the computerized compliance method not only provides an accurate representation of the downstream temperature, but also is superior to the monitored approach. The results of these surveys are shown in Table 2.
In 1987, TVA released a report describing the field surveys in support of SQN computed compliance validation and calibration which had been performed up to that date (TVA, 1987).
In addition, a table was introduced which described the ambient and operational conditions for which field surveys had been performed or were still needed. This table indicated combinations of river flow, time of year, and number of operating units, showing which tests had been performed, and assigning relative priorities for tests which were still needed.
New Calibration Data Since March 1996, six field surveys have been performed to measure downstream temperatures with various river flows and at different times of year. The results of these surveys are shown in Table 3.
Diffuser Slot Width The effective slot width for a multiport diffuser of the type at SQN can logically be assumed to fall somewhere between the width of a rectangle with the same length as the perforated diffuser section and area equal to the total area of the ports, and that a rectangle of the same length, but
DRAFT 10 Table 2. Thermal Surveys at Sequoyah Nuclear Plant from April 1982 through March 1983 DATE TIME River (All temperature data in these columns are hourly averages.)
T-Upstream T-Downstream Temp Rise Flow Elev Measured Measured Computed Measured Computed cfs ft
°F
°F
°F F° F° 04/04/1982 0900 CST 19900 676.46 56.8 61.9 61.0 5.1 4.2 04/04/1982 1000 CST 19800 676.46 56.7 60.1 61.1 3.4 4.4 04/04/1982 1100 CST 19600 676.47 56.7 61.2 61.0 4.5 4.3 04/04/1982 1200 CST 19700 676.50 57.2 61.9 61.4 4.7 4.2 04/04/1982 1300 CST 19700 676.45 57.4 62.2 61.5 4.8 4.1 05/14/1982 0900 CDT 7200 682.43 74.5 71.8 74.5
-2.7 0.0 05/14/1982 1100 CDT 9100 682.40 73.4 71.8 73.4
-1.6 0.0 05/14/1982 1300 CDT 6300 682.42 72.1 73.6 72.1 1.5 0.0 09/02/1982 1400 CDT 38500 680.30 78.1 80.1 79.8 2.0 1.7 11/10/1982 1300 CST 36200 677.57 59.0 60.1 60.3 1.1 1.3 11/10/1982 1400 CST 31600 677.59 59.0 60.6 60.6 1.6 1.6 11/10/1982 1500 CST 32300 677.58 59.0 60.4 60.5 1.4 1.5 03/31/1983 1100 CST 9800 676.34 51.4 54.3 54.3 2.9 2.9 03/31/1983 1200 CST 9400 676.34 50.4 54.7 54.2 4.3 3.8 03/31/1983 1300 CST 9300 676.34 52.5 54.5 54.4 2.0 1.9 03/31/1983 1400 CST 9500 676.34 51.4 54.9 54.4 3.5 3.0 03/31/1983 1500 CST 9400 676.36 51.4 54.9 54.4 3.5 3.0 Table 3. Thermal Surveys at Sequoyah Nuclear Plant from March 1996 through April 2003 DATE TIME River (All temperature data in these columns are hourly averages.)
T-Upstream T-Downstream Temp Rise Flow Elev Measured Measured Computed Measured Computed cfs ft
°F
°F
°F F° F° 3/1/1996 0929 -
1059 CST 42456 676.96 45.9 48.8 47.0 2.9 1.1 1344 -
1444 CST 28136 677.04 46.2 50.2 49.0 4.0 2.8 1444 -
1559 CST 21962 677.00 46.1 51.4 50.6 5.3 4.5 1629 -
1659 CST 20280 677.00 46.0 51.5 50.9 5.5 4.9 7/24/1997 1356 -
1548 CDT 40441 682.57 83.5 84.7 84.2 1.2 0.6 3/24/1999 1137 -
1251 CST 35731 677.46 51.9 54.5 53.2 2.7 1.3 8/2/2000 0744 -
0959 CDT 12472 682.20 82.1 85.1 85.2 3.0 3.2 0959 -
1103 CDT 8624 682.20 82.1 85.3 85.3 3.1 3.2 7/27/2002 1133 -
1251 CDT 17231 682.37 84.0 86.6 86.1 2.6 2.1 4/23/2003 1328 -
1444 CDT 34178 682.53 63.7 64.2 64.2 0.5 0.5
DRAFT 11 with area equal to the arc length of the perforated section of the diffuser. For the SQN diffuser, this slot width would be between 0.37 and 2.67 feet. Five slot widths ranging in equal increments from 0.37 foot to 3.437 feet were evaluated and compared with 27 measured data points from field surveys. The results, shown in Figure 5, were relatively insensitive to the assumed initial slot width, with only slightly better agreement with measured data being attained for the larger widths. The nominal arc length of the perforated section of the diffuser (2.67 feet) is therefore used as the diffuser slot width in the model.
Slot Width Sensitivity Test on Computed Downstream Temperature Field Data - 1982 - 2003 45 50 55 60 65 70 75 80 85 90 45 50 55 60 65 70 75 80 85 90 Measured (oF)
Computed (oF)
Line of perfect agreement B0 = 0.37 ft B0 =1.137 ft B0 = 1.903 ft B0 = 2.67 ft B0 = 3.437 ft Figure 5. Sensitivity of Computed Downstream Temperature to Diffuser Effective Slot Width Plume Entrainment Coefficient Two empirical relationships for the plume entrainment coefficient were evaluated. The first, proposed by McIntosh, was inferred for a slot diffuser from a relationship for the entrainment coefficient from the data reported by TVA (1983a).
=
00 55 0
00 27 0
27 0
.1
.1 0.75
/
0.75 2.5 r
r r
r F
F F
F (1)
The densimetric Froude number, Fr, is determined by
DRAFT 12 0
0 p
/
=
0 gB
/
w F
0 r
(2) where w0 is the initial discharge mass flux; g is the gravitational constant; B0 is the diffuser slot width; p is the density of the diffuser discharge, and 0 is the density of the ambient river water at the discharge depth.
The second entrainment coefficient, based on laboratory data, was developed by Benton in 1986 and is given by
(
)/2 2.0584)
(6.5430 1
1.69 0.31
x
+
+
=
rmf tanh
(3) where b
U rmf
/
3
=
(4) and
)
)/
((
0 p
0
x x
=
(g/l)
Q b
0 (5) where U is the ambient river velocity; Q0 is the diffuser discharge flow rate, and l is the length of the ported section of the diffuser.
Figure 6 shows the comparison with measured data of downstream temperatures computed with the McIntosh and Benton entrainment coefficients (i.e., Equation 1 and Equation 3, respectively).
Both entrainment coefficients result in relatively close matches with the measured data.
Although the McIntosh coefficient seems to perform better at low ambient river temperatures, temperatures computed using the Benton coefficient more closely match measured downstream temperatures at higher river temperatures. Since the accuracy of the computation is more critical at temperatures approaching the NPDES limit, the Benton coefficient is currently used in the compliance monitoring program.
Results of Updated Calibration Computed and measured downstream temperatures for the 27 downstream temperature data points collected in SQN field surveys since March 1982 are shown in Figure 7. The average discrepancy between the measured and computed downstream temperatures was 0.68 Fº (0.38 Cº). For downstream temperatures above 75ºF, the average discrepancy improved to 0.40 Fº (0.22 Cº).
DRAFT 13 Entrainment Coefficient Test on Computed Downstream Temperature Field Data - 1982-2003 45 50 55 60 65 70 75 80 85 90 45 50 55 60 65 70 75 80 85 90 Measured (oF)
Computed (oF)
Line of perfect agreement Benton Entrainment Coefficient McIntosh Entrainment Coefficient Figure 6. Sensitivity of Computed Downstream Temperature to Plume Entrainment Coefficient Field Survey Data - Downstream Temperature @ 5 ft Depth 45 50 55 60 65 70 75 80 85 90 45 50 55 60 65 70 75 80 85 90 Measured (oF)
Computed (oF)
Line of perfect agreement Field Data 1982 - 2003 Figure 7. Comparison of Computed and Measured Downstream Temperatures for Field Studies from April 1982 through April 2003
DRAFT 14 Measured downstream 24-hour averaged temperatures from the downstream temperature monitor (Station 8) are compared to computed values in Figure 8. The figure shows data collected since the adoption of the 24-hour averaging period in August 2001 through mid-October 2002. The data includes a period in December 2001, when one of the temperature probes began to fail, resulting in erroneously high measured temperatures. The overall average discrepancy between the measured and computed 24-hour averaged downstream temperatures was 0.63 Fº (0.35 Cº), and was 0.31 Fº (0.17 Cº) for downstream temperatures above 75ºF.
Measured downstream hourly averaged temperatures from the same time period are compared to computed values in Figure 9. As should be expected, the temperature data are much more widely scattered for the hourly averaged temperatures. The average discrepancy between the measured and computed hourly averaged downstream temperatures was 0.94 Fº (0.52 Cº) for the full range of river temperatures, decreasing to 0.55 Fº (0.31 Cº) for downstream temperatures above 75ºF.
Compliance Data hr Avg. Downstream Temperature @ 5 ft Depth 45 50 55 60 65 70 75 80 85 45 50 55 60 65 70 75 80 85 Measured (oF)
Computed (oF)
Line of perfect agreement Measured 2001 Measured 2002 Note - defective probe in December 2001.
Figure 8. Comparison of Computed and Measured 24-Hour Averaged Downstream Temperatures from Compliance Monitoring Records for 8/09/2001 through 10/15/2002
DRAFT 15 Compliance Data hr Avg. Downstream Temperature @ 5 ft Depth 45 50 55 60 65 70 75 80 85 45 50 55 60 65 70 75 80 85 Measured (oF)
Computed (oF)
Line of perfect agreement Measured 2001 Measured 2002 Note - defective probe in December 2001.
Figure 9. Comparison of Computed and Measured Hourly Averaged Downstream Temperatures from Compliance Monitoring Records for 8/09/2001 through 10/15/2002 CONCLUSIONS The model appears to compute the temperature at the downstream end of the mixing zone with sufficient accuracy for use as the primary method of verifying thermal compliance. The current calibration appears to be more accurate at higher river temperatures than at lower temperatures.
This is considered acceptable, since the accuracy is more critical as the downstream temperatures approach the NPDES instream limit(s). Any future calibration efforts intended to improve accuracy at lower river temperatures should place a high priority on maintaining accuracy at the more critical higher river temperatures.
DRAFT 16 REFERENCES
- 1. TVA (1982), A Field Verification of Sequoyah Nuclear Plant Diffuser Performance Model One-Unit Operation, Report No. WR28-1-45-110, TVA Division of Air and Water Resources, Water Systems Development Branch, October 1982.
- 2. TVA (1983a), Validation of Computerized Thermal Compliance and Plume Development at Sequoyah Nuclear Plant, Report No. WR28-l-45-115, Tennessee Valley Authority, Division of Air and Water Resources, Water Systems Development Branch, August 1983.
- 3. TVA (1983b), Real-Time Computation of Compliance with Thermal Water Quality Standards, Proceedings of Microcomputers in Civil Engineering, University of Central Florida, Orlando, Florida, November 1983.