weather and x/q 1 impact of weather changes on tva nuclear plant chi/q ( /q) kenneth g. wastrack...
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Weather and X/QWeather and X/Q 11
Impact Of Weather Impact Of Weather Changes On TVA Nuclear Changes On TVA Nuclear
Plant Chi/Q (Plant Chi/Q (/Q)/Q)
Kenneth G. WastrackKenneth G. WastrackDoyle E. PittmanDoyle E. PittmanJennifer M. Call Jennifer M. Call
Tennessee Valley AuthorityTennessee Valley Authority
Presented at the 12Presented at the 12thth NUMUG Meeting NUMUG MeetingCharlotte, NC / 25-27 June 2008Charlotte, NC / 25-27 June 2008
Weather and X/QWeather and X/Q 22
BackgroundBackground
The Tennessee Valley Authority (TVA) The Tennessee Valley Authority (TVA) operates three nuclear power plants along operates three nuclear power plants along the Tennessee River in north Alabama and the Tennessee River in north Alabama and southeast Tennesseesoutheast Tennessee
Browns FerryBrowns Ferry SequoyahSequoyah Watts BarWatts Bar
Weather and X/QWeather and X/Q 33
+
+
Browns Ferry
+
WattsBar
Sequoyah
Weather and X/QWeather and X/Q 44
BackgroundBackground
• Wind direction/speed and air temperature are collected at Wind direction/speed and air temperature are collected at the 91-, 46-, and 10-meter levels.the 91-, 46-, and 10-meter levels.
• Dewpoint temperature is collected at 10 meters.Dewpoint temperature is collected at 10 meters.• Solar radiation and rainfall are collected at the surface.Solar radiation and rainfall are collected at the surface.
• All data are reported hourly.All data are reported hourly.• Wind direction/speed, air temperature, and rainfall are Wind direction/speed, air temperature, and rainfall are
also reported every 15 minutes for emergency also reported every 15 minutes for emergency preparedness. preparedness.
At each plant, meteorological data are collected on At each plant, meteorological data are collected on a 91-meter tower.a 91-meter tower.
Weather and X/QWeather and X/Q 55
Initial Plant Design AssumptionsInitial Plant Design Assumptions
The Nuclear Regulatory Commission (NRC) The Nuclear Regulatory Commission (NRC) specifies dose limits that apply to emissions from specifies dose limits that apply to emissions from nuclear power plants.nuclear power plants.
The “dose limit margin” (the The “dose limit margin” (the difference between dose from difference between dose from plant emissions and the plant emissions and the regulatory limit) ensures regulatory limit) ensures compliance with regulatory compliance with regulatory requirements and permits requirements and permits flexibility of operations for the flexibility of operations for the power plant.power plant.
No emissions
Regulatory Limit
Dose from plant operations
DoseLimit
Margin
Weather and X/QWeather and X/Q 66
Initial Plant Design AssumptionsInitial Plant Design Assumptions
• Meeting dose limits involves more that just Meeting dose limits involves more that just controlling emissions.controlling emissions.
• Weather conditions are also a major factor.Weather conditions are also a major factor.• The impact of meteorological conditions is The impact of meteorological conditions is
estimated by calculating the relative air estimated by calculating the relative air concentration, or Chi/Q (concentration, or Chi/Q (/Q), values for specific /Q), values for specific points.points.
• These These /Q values are multiplied by the emission /Q values are multiplied by the emission rate (Q), to determine the concentrations (rate (Q), to determine the concentrations () at ) at those points. those points.
Weather and X/QWeather and X/Q 77
Chi/Q (Chi/Q (/Q) Calculations/Q) Calculations
2z
2
zy 2
Hexp
u
1Q
= Ambient Concentration
Q = Emission Rate
H = Effective Stack Height (m) u = wind speed (m/s)
y
z
= Horizontal diffusion coefficient (m)
= Vertical diffusion coefficient (m)= 3.14 . . .
Weather and X/QWeather and X/Q 88
Chi/Q (Chi/Q (/Q) Calculations/Q) Calculations
• Wind speed is directly measured.Wind speed is directly measured.• The two diffusion coefficients are derived from The two diffusion coefficients are derived from meteorological measurements. They can be meteorological measurements. They can be based on stability class as determined by the based on stability class as determined by the vertical temperature difference.vertical temperature difference.
• The remaining variables are constant for a The remaining variables are constant for a release type.release type.
It is important to have accurate It is important to have accurate meteorological measurements.meteorological measurements.
Weather and X/QWeather and X/Q 99
Chi/Q (Chi/Q (/Q) Calculations/Q) Calculations
• Wind speed (u) is in the denominator. Wind speed (u) is in the denominator. Therefore, as wind speed increases, Therefore, as wind speed increases, /Q /Q decreases; as wind speed decreases, decreases; as wind speed decreases, /Q /Q increases. increases.
• The diffusion coefficients (The diffusion coefficients (yy) and () and (zz) are in the ) are in the denominator. Therefore, as diffusion coefficients denominator. Therefore, as diffusion coefficients increase, increase, /Q decreases; as diffusion /Q decreases; as diffusion coefficients decrease, coefficients decrease, /Q increases. /Q increases.
Weather and X/QWeather and X/Q 1010
The diffusion coefficients are largest with the most The diffusion coefficients are largest with the most unstable class (stability class A) and decrease with unstable class (stability class A) and decrease with distance from the source.distance from the source.
Chi/Q (Chi/Q (/Q) Calculations/Q) Calculations
yy zz
Distance (meters)Distance (meters) StabilityStability Distance (meters)Distance (meters)
500500 10001000 15001500 20002000 ClassClass 500500 10001000 15001500 20002000
113113 209209 298298 384384 AA 105105 454454 10711071 19681968
8383 154154 221221 286286 BB 5151 109109 171171 234234
5555 103103 149149 193193 CC 3232 6161 8989 115115
3636 6868 9999 128128 DD 1818 3232 4242 5050
2727 5151 7474 9696 EE 1313 2222 2828 3333
1818 3434 4949 6464 FF 88 1414 1818 2222
1212 2323 3333 4242 GG 66 99 1212 1414
Weather and X/QWeather and X/Q 1111
Trends and ChangesTrends and Changes
• There has been an overall upward trend in There has been an overall upward trend in TVA TVA /Q values since the 1970’s./Q values since the 1970’s.
• In more recent years (1990’s and 2000’s), In more recent years (1990’s and 2000’s), the highest average the highest average /Q values are as /Q values are as much as 2.6 times the 1970’s values. much as 2.6 times the 1970’s values.
• Therefore, plant operations must be altered Therefore, plant operations must be altered to maintain the desired dose limit margin to maintain the desired dose limit margin
Weather and X/QWeather and X/Q 1212
Trends and ChangesTrends and Changes Browns Ferry (Ground-Level)
2.0
3.0
4.0
5.0
19
76
19
78
19
80
19
82
19
84
19
86
19
88
19
90
19
92
19
94
19
96
19
98
20
00
20
02
20
04
20
06
Ma
x X
/Q (
10
-6)
Sequoyah (Ground-Level)
3.0
4.5
6.0
7.5
19
76
19
78
19
80
19
82
19
84
19
86
19
88
19
90
19
92
19
94
19
96
19
98
20
00
20
02
20
04
20
06
Ma
x X
/Q (
10
-6)
Watts Bar (Ground-Level)
8.0
10.0
12.0
14.0
16.0
18.0
20.0
19
76
19
79
19
82
19
85
19
88
19
91
19
94
19
97
20
00
20
03
20
06
Ma
x X
/Q (
10
-6)
Weather and X/QWeather and X/Q 1313
Trends and ChangesTrends and Changes
• This upward trend is not unique to the TVA This upward trend is not unique to the TVA plants, but has been observed elsewhere. plants, but has been observed elsewhere.
• Two meteorological conditions must be Two meteorological conditions must be examined to determine the reasons for the examined to determine the reasons for the increase in increase in /Q values./Q values.
o Atmospheric stability patterns.Atmospheric stability patterns.o Wind speed. Wind speed.
Weather and X/QWeather and X/Q 1414
Trends and ChangesTrends and Changes
Annual average atmospheric stability patterns Annual average atmospheric stability patterns have been relatively unchanged, which indicates have been relatively unchanged, which indicates the increases in the increases in /Q values are not due to changes /Q values are not due to changes in atmospheric stability.in atmospheric stability.
• Browns Ferry and Watts Bar display relatively flat trends Browns Ferry and Watts Bar display relatively flat trends for the stability occurrences. for the stability occurrences.
• Sequoyah has a slight downward trend for stable cases Sequoyah has a slight downward trend for stable cases and an upward trend for neutral cases. However, this is and an upward trend for neutral cases. However, this is primarily due to observations in the 1970’s. Since 1980, primarily due to observations in the 1970’s. Since 1980, the Sequoyah trends have also been relatively flat.the Sequoyah trends have also been relatively flat.
Weather and X/QWeather and X/Q 1515
Trends and ChangesTrends and Changes
Average annual wind Average annual wind speed has shown a speed has shown a definite decrease with definite decrease with time for all plants. time for all plants.
Browns Ferry (Ground-Level)
4.0
5.0
6.0
19
76
19
78
19
80
19
82
19
84
19
86
19
88
19
90
19
92
19
94
19
96
19
98
20
00
20
02
20
04
20
06
Win
d S
pe
ed
(m
ph
)
Sequoyah (Ground-Level)
3.0
4.0
5.0
19
76
19
78
19
80
19
82
19
84
19
86
19
88
19
90
19
92
19
94
19
96
19
98
20
00
20
02
20
04
20
06
Win
d S
pe
ed
(m
ph
)
Watts Bar (Ground-Level)
3.0
4.0
5.0
19
76
19
78
19
80
19
82
19
84
19
86
19
88
19
90
19
92
19
94
19
96
19
98
20
00
20
02
20
04
20
06
Win
d S
pe
ed
(m
ph
)
Weather and X/QWeather and X/Q 1616
Trends and ChangesTrends and ChangesThe relationship between wind speed decreasing and The relationship between wind speed decreasing and /Q increasing is confirmed when both are plotted on /Q increasing is confirmed when both are plotted on the same graph for each plant.the same graph for each plant.
Note that wind speeds are plotted in reverse.Note that wind speeds are plotted in reverse.
Browns Ferry (Ground-Level)
2.0
3.0
4.0
5.0
6.0
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
Win
d S
pee
d (
mp
h)
0.00
1.50
3.00
4.50
Ma
x. X
/Q (
10
-6)
WS (mph) XoQ (10-6) Linear (WS (mph)) Linear (XoQ (10-6))
Weather and X/QWeather and X/Q 1717
Trends and ChangesTrends and ChangesSequoyah (Ground-Level)
2.0
3.0
4.0
5.0
19
76
19
78
19
80
19
82
19
84
19
86
19
88
19
90
19
92
19
94
19
96
19
98
20
00
20
02
20
04
20
06
Win
d S
pee
d (
mp
h)
0.00
1.50
3.00
4.50
6.00
7.50
Max
. X/Q
(10
-6)
WS (mph) XoQ (10-6) Linear (WS (mph)) Linear (XoQ (10-6))
Watts Bar (Ground-Level)
2.0
3.0
4.0
5.0
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
Win
d S
pee
d (
mp
h)
0.00
5.00
10.00
15.00
20.00
Max
. X/Q
(10
-6)
WS (mph) XoQ (10-6) Linear (WS (mph)) Linear (XoQ (10-6))
Weather and X/QWeather and X/Q 1818
AnalysisAnalysis
• What is the nature of the wind speed decrease?What is the nature of the wind speed decrease?• Is it due to local influences, or is it part of a Is it due to local influences, or is it part of a
wider regional pattern?wider regional pattern?
It appears that the increasing trend in It appears that the increasing trend in /Q values is /Q values is primarily due to decreases in wind speed.primarily due to decreases in wind speed.
Weather and X/QWeather and X/Q 1919
AnalysisAnalysis Local influences include construction of structures or other Local influences include construction of structures or other facilities that might obstruct wind flow, tree growth near the facilities that might obstruct wind flow, tree growth near the towers, or deterioration of wind speed instruments due to towers, or deterioration of wind speed instruments due to age. age.
None of these explanations appears to be plausible.None of these explanations appears to be plausible.
• TVA meteorologists have routinely inspected the TVA meteorologists have routinely inspected the meteorological sites since the early 1980’s, and noted meteorological sites since the early 1980’s, and noted no significant changes that would affect wind speed. no significant changes that would affect wind speed.
• As part of these inspections, tree growth was monitored. As part of these inspections, tree growth was monitored. Trees were cut when they become too tall. Trees were cut when they become too tall.
• Finally, TVA changed from cup anemometers to sonic Finally, TVA changed from cup anemometers to sonic wind sensors during 2000-2001, with no apparent wind sensors during 2000-2001, with no apparent change in the trend. change in the trend.
Weather and X/QWeather and X/Q 2020
AnalysisAnalysisReduced wind speeds appear to be a regional Reduced wind speeds appear to be a regional phenomenon. phenomenon.
Note that a different scale is used for NWS data.Note that a different scale is used for NWS data.
Observed Annual Average Wind Speed
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
1955
1958
1961
1964
1967
1970
1973
1976
1979
1982
1985
1988
1991
1994
1997
2000
2003
2006
TVA
Win
d S
peed
(mph
)
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
NW
S W
ind
Spe
ed (m
ph)
BFN - 10-m SQN - 10-m WBN - 10-m Huntsville NWS Chattanooga NWS
Weather and X/QWeather and X/Q 2121
A Look to the FutureA Look to the Future • The decrease in wind speed appears to be the The decrease in wind speed appears to be the
cause for increasing cause for increasing /Q values, but what does /Q values, but what does this mean?this mean?
• Data from the NWS stations indicates that the Data from the NWS stations indicates that the higher wind speeds was the normal condition higher wind speeds was the normal condition from the mid-1950’s until the 1970’s.from the mid-1950’s until the 1970’s.
• In the 1970’s, a noticeable drop in the regional In the 1970’s, a noticeable drop in the regional wind speeds occurred.wind speeds occurred.
• Since 1980 (and especially the past decade), the Since 1980 (and especially the past decade), the wind speed trend has been flat.wind speed trend has been flat.
• This indicates that recent years are more likely a This indicates that recent years are more likely a new “normal” period and that the base data from new “normal” period and that the base data from the 1970’s may no longer be applicable.the 1970’s may no longer be applicable.
Weather and X/QWeather and X/Q 2222
A Look to the FutureA Look to the Future • The relatively flat trend in the past decade The relatively flat trend in the past decade
indicates that a further significant decrease in indicates that a further significant decrease in wind speeds is unlikely. However, this is not wind speeds is unlikely. However, this is not certain.certain.
• In any case, planning based on the In any case, planning based on the /Q values /Q values should reflect the more recent data based on low should reflect the more recent data based on low wind speeds. While higher wind speeds are wind speeds. While higher wind speeds are possible, they should not be considered as possible, they should not be considered as “normal”.“normal”.
Weather and X/QWeather and X/Q 2323
A Look to the FutureA Look to the Future
LessonsLessons
• Meteorological conditions vary over time. Plant Meteorological conditions vary over time. Plant design and operation plans should assume design and operation plans should assume variability in variability in /Q values. /Q values.
• The initial data used for plant design may not be The initial data used for plant design may not be "normal" for the entire history of plant operations. "normal" for the entire history of plant operations.
• The dose limit margin should be adequate to The dose limit margin should be adequate to handle not only the variability of meteorological handle not only the variability of meteorological conditions from year-to-year, but must also conditions from year-to-year, but must also consider the possibility that the “base” data is not consider the possibility that the “base” data is not the “normal” data for the plant. the “normal” data for the plant.
Weather and X/QWeather and X/Q 2424
Case StudyCase StudyDelta-T Layers Used to Determine Stability Delta-T Layers Used to Determine Stability
Class for Elevated ReleasesClass for Elevated Releases • In Revision 1 of Regulatory Guide 1.23, NRC In Revision 1 of Regulatory Guide 1.23, NRC specifies that the temperature difference for specifies that the temperature difference for determining stability class applicable to elevated determining stability class applicable to elevated releases should be releases should be “. . . between the 10-meter “. . . between the 10-meter (33-foot) level and a higher level that is (33-foot) level and a higher level that is representative of diffusion conditions from release representative of diffusion conditions from release points . . .”points . . .”
Weather and X/QWeather and X/Q 2525
Case StudyCase StudyDelta-T Layers Used to Determine Stability Delta-T Layers Used to Determine Stability
Class for Elevated ReleasesClass for Elevated Releases • For TVA, this affects the For TVA, this affects the /Q estimates for /Q estimates for elevated releases from the stack at Browns Ferry elevated releases from the stack at Browns Ferry Nuclear Plant.Nuclear Plant.
• Previously, stability class for these releases was Previously, stability class for these releases was based on the temperature difference between based on the temperature difference between 46 46 meters and 91 metersmeters and 91 meters (old method). (old method).
• The new Regulatory Guide revision specifies that The new Regulatory Guide revision specifies that the temperature difference between the temperature difference between 10 meters 10 meters and 91 metersand 91 meters (new method) be used instead. (new method) be used instead.
Weather and X/QWeather and X/Q 2626
Case StudyCase StudyThe frequency of neutral (D) and near neutral (E) cases The frequency of neutral (D) and near neutral (E) cases decreased with the new method, while the frequency of stable decreased with the new method, while the frequency of stable (F, G) and unstable (A, B, C) cases increased. The increase in (F, G) and unstable (A, B, C) cases increased. The increase in unstable cases (+7.17) was essentially equal to the increase in unstable cases (+7.17) was essentially equal to the increase in stable cases (+7.18).stable cases (+7.18).
15-Year Average Annual Frequency of Occurrence (percent)15-Year Average Annual Frequency of Occurrence (percent)
StabilityStability Old methodOld method New methodNew method Difference (New - Old)Difference (New - Old)
AA 0.010.01 1.001.00 +0.98+0.98
BB 0.070.07 2.172.17 +2.10+2.10
CC 0.630.63 4.724.72 +4.09+4.09
DD 54.8954.89 43.4043.40 -11.49-11.49
EE 34.1834.18 31.4431.44 -2.74-2.74
FF 8.028.02 13.5713.57 +5.55+5.55
GG 2.062.06 3.693.69 +1.63+1.63
Weather and X/QWeather and X/Q 2727
Case StudyCase StudyChanges in stability class affect Changes in stability class affect /Q values in /Q values in three ways. three ways.
1.1. The diffusion coefficients change based on the The diffusion coefficients change based on the stability class. stability class.
2.2. The height of the plume rise changes. The height of the plume rise changes. 3.3. The distance of the point of maximum plume The distance of the point of maximum plume
impact changes. impact changes.
Weather and X/QWeather and X/Q 2828
Case StudyCase Study• It was not possible to isolate the effects of It was not possible to isolate the effects of
specific variables in the specific variables in the /Q equation. /Q equation. • Annual average X/Q values were calculated for Annual average X/Q values were calculated for
15 years using the old method, re-calculated for 15 years using the old method, re-calculated for the same 15 years using the new method, and the same 15 years using the new method, and results were compared.results were compared.
• The annual average The annual average /Q values /Q values dropped dropped approximatelyapproximately 12 percent12 percent (individual annual (individual annual averages ranged from a 7 percent increase to a averages ranged from a 7 percent increase to a 37 percent decrease).37 percent decrease).
• Net result was beneficial to Browns Ferry.Net result was beneficial to Browns Ferry.