GEOLOGICAL SURVEY CIRCULAR 745

Water Consumption by 

Nuclear Powerplants and 

Some Hydrological Implications
Water Consumption by 

Nuclear Powerplants and 

Some Hydrological Implications

By E. V. Giusti and E. L. Meyer

GEOLOGICAL SURVEY CIRCULAR 745

1977
United States Department of the Interior 

CECIL D. ANDRUS, Secretary

Geological Survey

V. E. McKelvey, Director

First printing 1977 

Second printing 1978

Library of Congress Cataloging in Publication Data

Giusti, Ennio V.

Water consumption by nuclear powerplants and some hydrological implications

(geological Survey Circular 745) 

Bibliography: p. 14. 

Supt. of Docs. No.: I 19.4/2:745

1. Atomic powerplants-United States-Water supply. 2. Water supply-United States. 3. Stream measurementsUnited States. I. Meyer, E. L., joint author. II. Title. HI. Series: United States. Geological Survey Circular 745. 

QE75.C5 no. 745 [TK1343] 557.3'08s [333.9'123J 77-608091

Free on application to Branch of Distribution, U.S. Geological Survey, 1200 South Eads Street, Arlington, VA 22202
CONTENTS

Abstract ............................................................................................. ..................... 1

Introduction ................................................................................................................ 1

Water use in nuclear powerplants ........................................................................ 1

Annual water consumption .................................................................................... 4

Streamflow constraints on water consumption for cooling ........................ 8

Selected low-flow data in the conterminous United States ...................... 13

Conclusions ................................................................................................................ 13

References .................................................................................................................. 13

ILLUSTRATIONS

Page

FIGURE 1. Sketch showing energy balance and cooling systems of nuclear powerplants .................................................. 2

2. Graph showing water consumption by nuclear powerplants.................................................................................... 4

3. Map showing location of nuclear powerplants in the conterminous United States ........................................ 7

4. Graph showing relationship between cooling pond surface area and plant's electrical capacity ............. 8

5. Map showing water consumption by nuclear powerplant in cubic feet per second ....................................... 9

6. Map showing stream reaches where the low flow is at least 300 cubic feet per second ............................... 12

TABLE

 Page

TABLE 1. Selected data on nuclear powerplants in the United States which are operating, in construction, or

proposed through about 1985 .............................................................................................................................................. 5

ill
Water Consumption by Nuclear Powerplants and 

Some Hydrological Implications

By E. V. Giusti and E. L. Meyer

ABSTRACT

About 63 percent of the energy input to a nuclear powerplant is discharged as heat to the cooling water system. 

Published data show that estimated water consumption 

varies with the cooling system adopted, being least in oncethrough cooling (about 18 cubic feet per second per 1,000 

megawatts electrical) and greatest in closed cooling with 

mechanical draft towers (about 30 cubic feet per second per 

1,000 megawatts electrical). When freshwater is used, this 

large variation carries important implications to the regional water resources economy in view of the increasingly keen 

competition for water. Thus it is felt that there is an urgent 

need to verify these findings with measurements.

The critical need for cooling water at all times by the 

nuclear powerplant industry, coupled with the knowledge 

that water withdrawal in the basin will generally increase 

with time and will be at a maximum during low-flow 

periods, indicates a need for reexamination of the design 

low flow currently adopted and the methods used to estimate it. In particular there is a need to develop models which 

account and evaluate low flows in terms of the ground-water 

inflow to the river basin network, the evapotranspiration 

from the basin, and the basinwide water consumption by 

man.

The amount of power generated, the name of the cooling 

water source, and the cooling method adopted for all 

nuclear powerplants projected to be in operation by 1985 

in the United States are tabulated and the estimated annual 

evaporation at each powerplant site is shown on a map of 

the conterminous United States. Another map is presented 

that shows all nuclear powerplants located on river sites as 

well as stream reaches in the United States where the 7-day, 

10-year low flow is at least 300 cubic feet per second or where 

this amount of flow can be developed with storage.

INTRODUCTION

The quest in the United States for selfsufficiency in energy, at some time in the future, 

is bound to elicit large increases in water 

demand. A good part of this demand for water 

will be directed to what is now the largest user of 

water in the Nation: the electric power industry. 

To the extent that nuclear powerplants will 

occupy a progressively larger share of the future

electric power generation, the demand for water 

for cooling by this industry will increase vastly. 

The purpose of this paper is to document the 

water consumption by all nuclear powerplants 

envisioned in operation in the United States by 

1985, to discuss some hydrological implications 

concerning freshwater consumption, and to 

provide some hydrologic information for 

planning the siting of powerplants along rivers.

WATER USE IN NUCLEAR POWERPLANTS

The production of electricity by nuclear 

powerplants involves, first, the transformation 

of the kinetic energy generated in the fission 

reaction into heat and, second, the conversion 

of this heat into stream and finally into electricity through turbine-driven generators. As 

shown by Hughes (1975), about one-third of the 

available thermal energy is converted into electricity; the remainder, as shown in figure 1, is 

expended as a direct loss to the atmosphere 

from the plant (5 percent) or is transmitted to 

the condenser cooling system (63 percent). The 

amount of water needed to cool the condenser 

varies with the amount of electricity generated 

as well as with the type of cooling system 

adopted. A theoretical estimate of the amount of 

water needed may be computed by equating the 

residual energy after electric generation, with 

the equivalent amount of energy that would be 

needed to evaporate water. The reasoning is as 

follows.

The amount of heat needed to evaporate 

water, its latent heat of vaporization, varies 

with temperature; it may be expressed as 1,040 

Btu per pound of water at a temperature of 

about 70°F. Thus the amount needed to evaporate 1 cubic foot of water (62.3 pounds) is 65,000 

Btu. Given that 1 Btu per second equals 1,055
Input 

100%

Thermal energy from 

nuclear fuel

ENERGY BALANCE

Output

5% + 32% + 

Direct loss Electricity

63

Rejected 

cooling 

to

Closed-cycle cooling .

FIGURE 1. Sketch showing energy balance and cooling systems of nuclear powerplants.
watts, it is concluded that about 69 MW (megawatts) are needed to evaporate a water flow of 1 

ftVs (cubic foot per second). The energy balance 

of figure 1 shows that only 32 percent of the 

input energy is converted into electricity, and 

thus the production of 1,000 MW of electric 

energy requires an input of 3,120 MW of thermal 

energy. Also from figure 1 it is seen that 63 

percent of this thermal energy (about 2,000 MW) 

is transmitted to the water cooling system. In 

terms of the equivalent evaporative energy 

previously computed (that is, 69 MW=1 ftVs 

evaporated water), the amount of cooling water 

required for a 1,000 MWe nuclear powerplant is 

about 29 ftVs.

The above calculations assume that all the 

rejected heat is dissipated by evaporation, 

whereas in practice some heat is dissipated by 

conduction and convection. For example, it is 

generally assumed (for example, Hughes, 1975) 

that cooling towers dissipate about 80 percent of 

the heat by evaporation and about 20 percent by 

conduction and convection through the airflowing through the towers. In this sense, less water 

would be consumed in the cooling process. 

However, with wet cooling towers, some water 

is lost as drift (that is, water drops carried by the 

rising air column), and some is lost in leaks, and 

so forth. In the end it is estimated in round 

numbers that about 25 ftVs would be consumed 

by a condenser cooling system with wet towers 

serving a 1,000 MWe plant.

The water cooling systems commonly used 

are shown in figure 1. In once-through cooling, 

water is circulated through the steam condenser 

once only and the heated water is discharged 

directly to the water body from which it was 

taken. In closed cooling systems, water is continuously recirculated through the condensers, 

and the cooling of the water is effected mainly 

through evaporation by means of towers, 

ponds, spray canals, and similar means. The 

water consumed is replaced with water taken 

from a water body (lake, river, or other water 

source). An additional small amount is taken 

and an equivalent small amount is continuously discharged to the water body to prevent 

an excessive buildup in the concentration of 

minerals in the circulating cooling water and to 

maintain steady-state conditions in the quantity and quality of water used in cooling. The

water discharged into the water body is called 

blowdown. The water taken to replace that lost 

by evaporation and drift plus an amount equivalent to the blowdown is called makeup.

An analysis of cooling water consumption of 

nuclear powerplants has been made by the 

Directorate of Regulatory Standards of the U. S. 

Atomic Energy Commission (now the Nuclear 

Regulatory Commission) on the basis of data 

presented in the environmental impact reports 

prepared by the electric companies in connection with their applications for constructing 

and operating specific nuclear plants at specific 

sites (U. S. Atomic Energy Commission, 1974). 

The data cover most of the cooling systems in 

present use, and the evaporation for each cooling system was estimated for each specific site 

where a nuclear powerplant was proposed. Also, 

a series of plots was shown in which the estimated evaporation rates were correlated with 

the net electric power production for various 

cooling systems. A similar plot of these data is 

shown in figure 2, and a line is drawn for each 

identified cooling system. The slopes of the lines 

are as follows:

Mechanical draft

wet towers ................ b = 0.031 or 31 ftVs/1,000 MWe

Natural draft

wet towers ................ b = 0.029 or 29 ftVs/1,000 MWe

Cooling ponds

(not including

natural

evaporation)............ b = 0.021 or 21 ftVs/1,000 MWe

Once through .............. b = 0.018 or 18 ftVs/1,000 MWe

In comparison with theoretical computations 

based on latent heat of vaporization of water, 

the average slope for mechanical draft wet 

towers, that is 31 ftVs/1,000 MWe, slightly 

exceeds the 29 ftVs of evaporative energy previously computed. Furthermore, it should be 

pointed out that the 29 ftVs of evaporative 

energy computed from the latent heat of vaporization represents a maximum which assumes 

that all rejected heat is converted into evaporation, whereas only about 80 percent of it, or 

about 23 ftVs, is estimated to do so. It would 

appear thus that the data presented by the U. S. 

Atomic Energy Commission (1974) regarding 

water consumption by nuclear powerplants are 

somewhat conservative in that they overestimate the water consumption of the cooling
100

EXPLANATION

- o Mechanical draf t towers 

 Natural draft towers

 A Cooling ponds 

x Once through

50

i I I I I I I

0 1000 2000 3000 4000 

ELECTRICAL CAPACITY, IN MEGAWATTS ELECTRICAL

FIGURE 2. Graph showing water consumption by nuclear 

powerplants.

process. In recognition of this, the electrical 

capacity data and the slope of the lines of figure 

2 are used to estimate the total water consumption (rather than the cooling water consumption only) of all nuclear powerplants operating, 

in construction, or proposed in the United 

States through about 1985.

It is recognized, of course, that the data are 

derived from theoretical considerations and on 

the basis of unverified mathematical models of 

evaporation for the various cooling methods. 

Because of this it is felt that there is an urgent 

need to calibrate and verify the mathematical 

models in use with field measurements made at 

operating powerplants.

ANNUAL WATER CONSUMPTION

Data on the projected buildup of the nuclear 

powerplant industry through 1985 are available 

from various sources. The data considered in 

this paper consist of those published by the 

NUS Corporation (1974) plus data obtained 

directly from the Nuclear Regulatory Commission. The data given in table 1 consist of the 

names of all nuclear powerplants operating,

under construction, and proposed for completion through 1985. The name of the water source 

for the plant, the cooling method employed, and 

the net electrical power generated are also 

given. While the location of the plants can be 

obtained roughly from table 1 the State and 

the water body concerned a more accurate 

location of each plant was obtained and plotted 

on a map by West (1975). Figure 3, after West, 

has been modified to show the location of all the 

powerplants of table 1 as well as the urban 

centers of power consumption.

In order to estimate the annual water consumption for each plant, the relationships defined in figure 2 for the various cooling systems 

were used. The appropriate slope values are 

multiplied by the given net electrical power 

shown in table 1 to obtain the total water consumption for cooling. It should be noted that 

these data represent the net evaporation arising 

from cooling only and do not include water 

amounts used in plants, for other operations 

such as boiler feed, domestic use, and irrigation 

of plant grounds as well as water losses from 

leaks, seepage, drift, and so forth. However, the 

amounts of water involved for these purposes 

are small and are ignored here especially since 

the amounts of cooling water computed from 

figure 2 are thought to be somewhat high.

The evaporation rates of figure 2 must be 

increased if cooling ponds are used. In cooling 

ponds the natural evaporation from the water 

surface must be assessed. An estimate of this 

evaporation can be computed from data given 

by the U. S. Atomic Energy Commission (1974). 

The area of cooling ponds of selected powerplants is plotted in figure 4 versus the electrical capacity of the plants. The ordinate on the 

right side of this figure shows an estimate of the 

natural pond evaporation. It was scaled by 

assuming various net annual evaporation rates 

taken from maps prepared by Kohler, Nordenson, and Baker (1959) and is considered applicable for those regions where it is proposed to use 

artificial ponds for cooling nuclear powerplants. This graph was used together with that 

of figure 2 to estimate the total water consumption of those nuclear powerplants which adopt 

closed cycle cooling systems utilizing ponds or 

lakes built for this purpose.

A map of annual water consumption by nuclear powerplants is shown in figure 5. The data
TABLE 1. Selected data on nuclear powerplants in the tfnited States which are operating, in construction, or proposed

through about 1985

Units and status: Number of letters equals number of units in given status: 

Ooperating, C=cona traction, P=proposed

Cooling (method): CT=cooling towers (natural draft), Once=once-through cooling, RS=cooling pond, MCT=mechanical cooling towers, SP=spray canal or 

pond, Mixed=combination of cooling methods, usually once through and 

wet cooling towers

Item Name

Units and 

status

Cooling Power

State Source Method (net £ MWe) Remarks

1 Alien's Creek.................. PP

2 Arkansas ........................ O,C

3 Atlantic............................ PPPP

4 Bailly................................C

5 Barton ..............................PPPP

6 Beaver Valley................CC

7 Bellefonte........................ PP

8 Big Rock Point..............O

9 Black Fox........................CC

10 Blue Hills........................ PP

11 Braidwood ...................... PP

12 Browns Ferry ................ OO,C

13 Brunswick ...................... CC

14 Byron................................PP

15 Callaway ........................ PP

16 Calvert Cliffs ................ OO

17 Catawba.......................... PP

18 Cherokee..........................PPP

19 Clinch River .................. P

2Q Clinton ............................ PP

21 ComanchePeak ........ PP

22 Cook..................................O,C

23 Cooper.............................. O

24 Crystal River ................ O

25 Davis Besse.................... C.PP

26 Diablo Canyon ..............CC

27 Douglas Point................ PP

28 Dresden............................ OOO

29 Duane Arnold................ O

30 Parley, J. ........................ CC

31 Fermi................................ C,P

32 Fitzpatrick...................... O

33 Forked River.................. C

34 Fort Calhoun ................ O

35 Fort St. Vrain................ O

36 Fulton .............................. PP

37 Ginna................................ O

38 Grand Gulf .................... PP

39 Greenwood...................... PP

40 Haddam Neck................ O

41 Hanford .......................... O

42 Hartsville........................ PPPP

43 Hatch Edwin.................. O,C

44 Hope Creek .................... PP

45 Humboldt Bay............... O

46 Indian Point.................. OO,C

47 Jamesport ...................... PP

48 Kewaunee........................ O

49 Koshkonong .................. PP

50 LaCrosse.......................... O

51 LaSalle ............................ CC

52 Limerick.......................... CC

53 Maine Yankee................O

54 McGuire .......................... CC

55 Midland .......................... CC

56 Millstone.......................... O,C,P

Tex............ Brazos River ............................................ RS 2,300

Ark............ Arkansas River .................................. Once 1,762

N.J. .......... Atlantic Ocean.................................... Once 4,600

Ind ............ Lake Michigan........................................ CT 660

Ala............ Coosa River.......................................... MCT 4,636

Pa.............. Ohio River ............................................ MCT 1,704

Ala............ Reservoir on Tennessee River............ CT 1,800

Mich ........ Lake Michigan.................................... Once 75

Okla.......... Verdigris River.................................... MCT 1,900

Tex............ Sabine River ........................................ MCT 1,836

111 .............. Kankakee River...................................... RS 2,240

Ala............ Lake Wheeler........................................ Once 3,195

N.C. Cape Fear Estuary ............................ Once 1,642

111 .............. Rock River ................................................ CT 2,240

Mo ............ Missouri River ........................................ CT 2,300

Md ............ Chesapeake Bay.................................. Once 1,690

S.C. .......... Lake Wylie............................................ Once 2,306

S.C. .......... Broad River.......................................... MCT 3,840

Tenn ........ Clinch River ........................................ MCT 350

111 .............. Salt Creek.................................................. RS 1,910

Tex............ Squaw Creek Reservoir........................ RS 2,300

Mich ........ Lake Michigan.................................... Once 2,120

Nebr.......... Missouri River .................................... Once 778

Fla ............ Gulf of Mexico .................................... Once 825

Ohio.......... Lake Erie .................................................. CT 2,718

Calif.......... Pacific Ocean ..................................... Once 2,190

Md ............ Potomac River ........................................ CT 2,356

111 .............. Illinois River............................................ RS 1,818

Iowa.......... Cedar River .............................................. CT 569

Ala............ Woodruff Reservoir............................ MCT 1,658

Mich ........ Lake Erie ............................................ Mixed 2,303

N.Y........... Lake Ontario........................................ Once 821

N.J. .......... Barnegat Bay .......................................... CT 1,070

Nebr.......... Missouri River .................................... Once 457

Colo .......... South Platte River.............................. MCT 330

Pa.............. Susquehanna River................................ CT 2,280

N.Y........... Lake Ontario........................................ Once 490

Miss.......... Mississippi River.................................... CT 2,580

Mich ........ Lake Huron .............................................. SP 2,400

Conn ........ Connecticut River .............................. Once 575

Wash........ Columbia River ...................................... CT 850

Tenn ........ Cumberland River.................................. CT 4,820

Ga.............. Altamaha River.................................. MCT 1,581

N.Y........... Delaware Estuary .................................. CT 2,134

Calif.......... Humboldt Bay..................................... Once 63

N.Y........... Hudson Estuary.................................. Once 2,103

N.Y........... Long Island Sound ............................ Once 2,300

Wis............ Lake Michigan.................................... Once 541

Wis............ Lake Koshkonong.................................. CT 1,800

Wis............ Mississippi River................................ Once 50

111 .............. Kankakee River ...................................... RS 2,156

Pa ............ Schuykill River........................................ CT 2,130

Maine ...... Back River Estuary .......................... Once 790

N.C........... Lake Norman ...................................... Once 2,360

Mich ........ Saginaw River ........................................ RS 1,310

Conn ........ Niantic Bay.......................................... Once 2,639

Mixed cooling 

method in future. 

Do.

Breeder Reactor 

Plant.
TABLE 1. Selected data on nuclear powerplants in the United States which are operating, in construction, or proposed

through about 1985 Continued

Item Name

57 Montague ...................

58 Monticello .................

59 Nine Mile Pt .............

60 Norco ...........................

61 North Anna. ..............

62 Oconee.........................

63 Oyster Creek .............

64 Palisades ...................

65 Palo Verde .................

66 Peach Bottom ...........

67 Pebble Springs .........

68 Perkins .......................

69 Perry ...........................

70 Pilgrim .......................

71 Point Beach...............

72 Prairie Island ...........

73 Quad Cities ...............

74 Rancho Seco .............

75 River Bend. ................

76 Robinson ...................

77 Salem...........................

78 San Joaquin .............

79 San Onofre ...............

80 Seabrook.....................

81 Sequoyah ...................

82 Shearon Harris .......

83 Shoreham ...................

84 Skagit .........................

85 Somerset ....................

86 South Texas ............

87 St. Rosalie ................

88 Summer......................

89 Summit ......................

90 Surry ..........................

91 Susquehanna ...........

92 Sterling ......................

93 St. Lucie ....................

94 Three Mile Is. ..........

95 Trojan ........................

96 Turkey Point ............

97 Tyrone ........................

98 Vermont Yankee ....

99 Vidal ..........................

100 Vogtle A.W. ..............

101 Waterford ..................

102 Watts Bar..................

103 Wolf Creek ................

104 WPPSS 1 & 4 ..........

105 WPPSS 3 & 5 ..........

106 Yankee-Rowe ..........

107 Zimmer ......................

108 Zion ........................

Units and

status

.....PP

.....o

.....o,c

.....p

.....cccc

.....ooo

.....o

.....o

.....ppp

.....ooo .....PP

.....ppp

.....PP

.....o.c

.....oo

.....oo

.....oo

.....o

.....PP

.....o

.....cc

....pppp

.....o.cc .....PP

.....cc

.....pppp

.....c .....PP .....PP

.....PP .....PP

.....c .....PP

....OO.PP

....cc

...... p

.....cc

.....o,c

.....c

.....CO .....PP

...... o

...... PP

....cccc

...... p

.....cc

.....p

...... PP .....PP

.... o

...... C,P

oo

State

Mass ..

Minn ..

N.Y. ....

P.R. ....

Va. ......

S.C. ....

N.J. ....

Mich ..

Ariz ....

Pa .....

Oreg....

N.C. ....

Ohio....

Mass .. Wis ......

Minn ..

Ill ........

Calif....

La .....

N.C. ....

N.J. ....

Calif....

Calif....

N.H.....

Tenn. .

N.C. ....

N.Y. ....

Wash .

N.Y. ...

Tex.....

La.......

S.C. ...

Del .....

Va.......

Tenn .

N.Y. ...

Fla .....

Pa .....

Oreg...

Fla .....

Wis.....

Vt .......

Calif .

Ga.......

La.......

Tenn .

Kans . Wash . Wash .

Mass .

Ohio...

Ill

Cooling

Source

...... Connecticut River .........................

...... Mississippi River ...........................

...... Lake Ontario ...................................

...... Atlantic Ocean ...............................

...... North Anna Reservoir .................

...... Lake Keowee ...................................

...... Bamegat Bay .................................

...... Lake Michigan ...............................

...... Phoenix Sewage .............................

...... Lake Conowingo ...........................

...... Columbia River .............................

...... Yadkin River ...................................

...... Lake Erie .........................................

...... Cape Cod Bay.......................... .... ...

...... Lake Michigan ...............................

...... Mississippi River ...........................

...... Mississippi River ...........................

...... American River .............................

...... Mississippi River ..........................

...... Lake Robinson ..............................

...... Delaware Estuary ........................

...... Calif. Aqueduct...............................

...... Pacific Ocean ................................

...... Atlantic Ocean ..............................

....... Lake Chickamauga......... .............

...... Cape Fear River ............................

...... Long Island Sound ......................

....... Skagit River....................................

....... Lake Ontario ..................................

....... Colorado River ..............................

....... Mississippi River ..........................

....... Lake Monticello ............................

....... Chesapeake-Delaware Canal....

....... James River Estuary ..................

....... Susquehanna River......................

....... Lake Ontario ..................................

....... Atlantic Ocean ..............................

....... Susquehanna River......................

....... Columbia River ............................

....... Biscayne Bay ................................

....... Chippewa River ............................

....... Connecticut River ........................

...... Colorado River ..............................

....... Savannah River ............................

....... Mississippi River ..........................

....... Chickamauga Lake..... .................

....... Neosho River.................. ................

....... Columbia River ............................

.... . jChehalis River .............................

....... Lake Sherman .............................

....... Ohio River .-. ...............................

....... Lake Michigan ..............................

Method (i

......... CT

... Mixed

..... Once

..... Once

......... RS

..... Once

..... Once

... Mixed

..... MCT

..... MCT

......... RS

..... MCT

......... CT

..... Once

...... Once

..... MCT

..... Once

......... CT

..... MCT

...... Once

...... Once

......... CT

...... Once

...... Once

...... Once

... Mixed

...... Once

.......... CT

.......... CT

.......... RS

.......... RS

...... MCT

.... Mixed

.......... CT

...... Once

...... Once

.......... CT

.......... CT

.... Mixed

...... MCT

.... Mixed

.......... CT

.......... CT

...... Once

.......... CT

.......... RS

...... MCT

......... CT

..... Once

.......... CT

Once

Power

net 3i MWfe)

2,300

545

1,705

583

3,610

2,658

640

700

3,810

2,170

2,520

3,840

2,410

1,844

994

1,060

1,600

804

1,868

700

2,205

5,200

2,710

2,400

2,240

3,660

819

2,554

2,440

2,500

2,320

900

1,540

3,294

2,100

1,150

1,610

1,724

1,130

1,386

2,300

514

1,540

4,452

1,113

2,338

1,150

2,412

2,484

175

1,980

2,100

i Remarks

Irrigation Canal.

Possibly MCT.

Also CT in future.

Not yet decided.

represent the plants listed in table 1 and located 

in the map of figure 3. The water consumption is 

expressed as an average rate which would be 

applicable for those parts of the year when the 

plant is operating. It would be possible, of 

course, to reduce the amounts shown by some

percentage according to the past experience 

regarding the percentage of time that nuclear 

powerplants have been in operation. In design, 

it is often assumed that plants operate 80 

percent of the time, but in practice the percentages have so far been lower. It was felt, how-
EXPLANATION

 4

Location of nuclear powerplant 

Number is keyed to table 1

o 

Urban centers

 

Metropolitan areas

0 100 200 300 400 500 MILES

I III I I

I I I I I I

0 100 200 300 400 500 KILOMETERS

FIGURE 3. Map showing location of nuclear powerplants in the conterminous United St
15,000

10,000

5000

EVAPORATION RATE, IN 

CUBIC FEET PER SECOND 

 20 . 40 , 60

o

O

O

o

o

o

 10

- 5

 1

- 20

-10

! 1

o

ce. U_ 

O

30 I Q.

g

S

liUJ 0

|8 

18

UJ UJ z£

02 

co 5

h- 20

-10

L- 1

< CO

z o

uj E 

z $

1000 2000 3000 

ELECTRICAL CAPACITY, IN MEGAWATTS ELECTRICAL

4000

FIGURE 4. Graph showing relationship between cooling pond surface area and plant's electrical capacity.

ever, that to take this into account would have 

introduced an additional variable into the picture, and thus figure 5 should be viewed as 

showing the maximum average rates of water 

consumption.

It is believed that the data of figure 5 provide 

basic information and guidance for waterresource managers as well as nuclear powerplant planners, regarding the siting of additional water consuming industry, particularly 

on rivers. It is understood, of course, that the 

data presented concern only nuclear powerplants and thus represent only part of the waterconsumption picture of each river basin.

As stated previously, estimates given are 

based on theoretical computations of evaporation rather than on field data. This should not 

detract from the usefulness of the data shown in 

figure 5 if it is considered that they represent a 

first approximation subject to some error, possibly of overestimation. However, because the 

siting of nuclear powerplants on inland stream 

sites, and particularly those plants using good 

quality freshwater for cooling, is inherently 

related to competition for available water, planners should have at hand realistic data regarding evaporation rates from the various cooling 

systems in use. Such data can only be obtained 

by field measurements.

STREAMFLOW CONSTRAINTS

ON WATER CONSUMPTION

FOR COOLING

An inspection of the data listed in table 1 or 

plotted on figure 3 shows that many plants are 

located on sites next to streams. These plants 

draw water directly from the streams without 

provision of significant storage. A consideration related to plant siting along rivers concerns 

periods of low streamflow when the impact on 

water resources of total water consumed in the 

cooling process is at a maximum. This consideration is particularly relevant for those plants 

which do not use cooling ponds with a sufficiently large storage capacity to allow withinpond recirculation of water for several days 

without intake from or discharge to the stream. 

Estimates of low flow of streams for plants 

withdrawing water for cooling continuously are 

particularly important and require careful definition for the following reason: 

1. Safety. The regulatory staff of the U.S.
EXPLANATION

Location of nuclear powerplant 

Number is water use in cubic feet per second

Urban areas

Metropolitan areas

0 100 200 300 400 500 MILES

I I I.I I____I

I I I I \ I

0 100 200 300 400 500 KILOMETERS

FIGURE 5. Map showing water consumption by nuclear powerplant in cubic feet per se
Atomic Energy Commission (1972) in reference to safety analysis reports for nuclear 

power plants states: "estimate the probable 

minimum flow rate*** resulting from the 

most severe drought considered reasonably 

possible in the region as such conditions 

may affect*** the ability of the ultimate 

heat sink to perform adequately***."

2. Standards. Most States have issued standards regarding the maximum permissible 

mineral concentration in surface waters to 

be used for cooling. As is well known, this 

concentration is at a maximum during lowflow periods because then the flow consists 

of ground-water discharge which is normally more concentrated mineralogically than 

surface water. Additional concentration of 

the streamflow mineral content is brought 

about by basin evapotranspiration which 

is also at a maximum during low-flow 

periods.

3. Ecology. Maximum ecological impact on 

freshwater biota can occur on some streams 

during low-flow periods if the mineral concentration exceeds certain limits or if the 

flow is abruptly reduced by withdrawal at 

powerplants. Furthermore, the withdrawal 

itself entails loss of biota by physical entrainment on the intake screens or by physical injury on passage through the water 

pumps.

4. Plant operation. The conditions described 

above may be such as to force the shutdown 

of the plant, with contingent costs and loss 

of revenue to the plant operators and loss of 

service to consumers. While this may be 

considered an acceptable operational rule 

under exceptional circumstances, say once 

in 10 years, it becomes a serious problem of 

misdesign when recurring more often, say 

once every year.

Because the streamflow constraints imposed 

on the siting of a nuclear powerplant can be 

severe under some circumstances, the estimation of low flow is an extremely important 

aspect of site planning. At the same time, it 

is also an aspect of hydrologic analysis which 

is pervaded with uncertainty and complexity. 

This uncertainty arises from several considerations as brought out in the following 

discussion.

The low streamflow observed at any river site 

represents ground-water discharge from upstream save for losses due to:

1. Evapotranspiration from ground-water reservoirs upstream from the observation point.

2. Ground water originating upstream from the 

observation point but emerging downstream from it (underflow), or outside the 

basin.

3. Upstream water withdrawal returned to the 

stream below the observation point.

4. Out-of-basin water transfer.

5. Within-basin consumptive use of water.

Although low-flow analysis is a multidisciplinary problem, involving in part surfacewater hydrology, hydrogeology, and climatology, it has been dealt with, traditionally, as a 

surface-water problem through hydrograph 

analysis. From the practical point of view of 

engineering design, the frequency analysis of 

drought flow allows the analyst to choose that 

discharge and associated probability of occurrence that he feels to be appropriate to the 

problem at hand. Thus, for example, the socalled 7-day, 10-year low flow is used in evaluating environmental impacts to stream biota in 

the environmental impact statements of nuclear powerplants. It is interesting to note that 

hydrologists have resorted to the 7-day average 

low flow to smooth out or filter fluctuating 

hydrologic data such as that originating from 

rivers on which water-consuming activities 

might be shut down on weekends. In this respect it may also be noted that while the 7-day 

average low-flow index was devised for purposes of hydraulic design it has been retained 

also for assessing impacts on river biota. Clearly, however, for some rivers in which there is 

both a large fluctuation of instantaneous flow 

from day to day and a critical pollution level, 

environmental assessment based on the 7-day 

average flow may be misleading. In these rivers 

serious biological impacts may occur if the 

instantaneous flow during a 7-day low-flow 

period falls below a critical threshold even 

though the average during the 7-day period may 

not be particularly worrisome. The same kind of 

reasoning can be followed with respect to the 

chosen probability of occurrence (or return period) if the low-flow frequency curve is steep, that 

is if low flows decrease rapidly with decreasing 

return period. As an example one can postulate 

a situation in which a stream may have a low 

flow with a 10-year frequency which is acceptable for a given problem while the low flow with 

a 15-year frequency is not.

10
The choice of an appropriate low-flow index 

becomes particularly significant in the design 

of nuclear powerplants which require that a 

continuous source or reservoir of emergency 

cooling water be available to prevent major 

accidents even after shutdown. This is well 

recognized in the licensing regulations which 

stipulate that the probable minimum flow of 

streams providing cooling water be estimated 

and alternative sources provided for the time 

when the instantaneous flow could fall below 

the amount required for emergency cooling.

By desire or coincidence the probable minimum flow (pmf?) is analogous to the probable 

maximum flood (PMF), at least euphoniously. 

The latter has become well entrenched in usage, 

notwithstanding the existing disagreement between those who propound its use (determinists) 

and those who prefer a probabilistic approach 

to flood-level assessment. The former, the probable minimum flow, which is requested in the 

preparation of Preliminary Safety Analysis 

Reports for nuclear powerplants, is at present 

no more than a term, and there are no widely 

accepted standards for defining or calculating 

it.

Conceptually the probable maximum flood 

derives from meteorological arguments and is 

defined as follows by the World Meterological 

Organization (1974): "The probable maximum 

flood is that flood estimated to result if the most 

critical combination of severe meteorological 

and hydrological conditions considered reasonably possible in the region were to occur." 

Although the probable minimum flow would 

derive from hydrogeological and climatological 

arguments, H. C. Riggs (written commun., 1975) 

pointed out that the probable minimum flow 

would be a viable concept only for natural 

streams free of man's effects, and in times of 

severe droughts even these streams would not 

be free from emergency diversions. Taking this 

into consideration, the probable minimum flow 

may be defined in a manner similar to that of 

the probable maximum flood:

The probable minimum flow is that flow estimated to result 

if the most critical combination of severe climatological and 

hydrogeological conditions considered possible in the region were to occur. For natural streams the probable 

minimum flow may be defined as the ground-water discharge to the network of stream channels, assuming water 

depletion in the soil and weathered rock, minus evapotranspiration from the basin. For stream basins subject to 

significant regulations or diversions, the probable minimum flow includes abstractions due to water consumption 

or interbasin water transfer taking place upstream from the 

river reach under study.

While it is recognized that the hydrogeologic 

limiting conditions given above for natural 

streams are somewhat arbitrary, they were 

derived from a study by Kilpatrick (1963) in 

which direct field observations of sources of 

base flow to a stream were made in a trench dug 

across the flood plain of a small stream in the 

Piedmont Province in Georgia. In this study it 

was observed that low flow in the dry season 

was derived entirely from the underlying rock 

aquifer as ground water in the soil and that the 

colluvium was exhausted.

Given this definition of the probable minimum flow, even if subject to modifications, 

there remains the task of computing it. Because 

no operational methods for computing the probable minimum flow exist, the matter is still at 

the research stage. Progress in deriving an 

acceptable computational method can be expected to emerge from mathematical modeling 

of the hydrogeologic, climatologic, and waterresources components which control the magnitude of the low flow. Of the above components 

the hydrogeologic one may be treated by classical deterministic methods used in groundwater-flow analysis, and the water-resources 

component involves accounting of consumptive 

uses by man. Finally the climatologic component involves consideration of the rates of 

evapotranspiration from the basin as well as 

determination of the most severe regional 

drought considered possible in the region. In the 

end the model should be able to match the dry 

season portion of streamflow hydrographs with 

sufficient reliability to serve as a predictor of 

the probable minimum flow. It would be expected that the calculated value be checked with 

regional values of extreme low flows determined by frequency analysis.

The view held here, which is implicitly embodied in the definition given above of the 

minimum probable flow, is that ground water is 

a conservative element of the total water cycle 

of natural stream basins and that its seasonal 

fluctuations within the year are small when 

compared to those of streamflow. In addition it 

is believed that the large variations of low flow

11
EXPLANATION

Seven-day, ten-year low flow is 

at least 300 cubic feet per second

Streamflow sufficient to yield 300 cubic feet 

per second with storage

Nuclear power plant site

0 100 200 300 400 500 MILES

I I .1.1. I____I

I \ \ I I I

0 100 200 300 400 500 KILOMETERS

FIGURE 6. Map showing stream reaches where the low flow is at least 300 cubic feet per s
from year to year arise mainly from climatological variability. In the end, the assessment of 

the probable minimum flow in natural streams 

will rest on the assessment of "the most*** 

severe climatological*** conditions considered 

possible in the region."

SELECTED LOW-FLOW DATA

IN THE CONTERMINOUS

UNITED STATES

Stankowski, Limerinos, and Buell (1976) 

have examined the low flow in the conterminous United States for the purpose of providing 

a general appraisal of potential sites for 

cooling water. A map prepared by them identifies those streams for which the average 7-day 

low flow with a recurrence interval of 10 years is 

at least 300 ftVs. Their map also portrays those 

stream reaches which could sustain a flow of 

300 ftVs if adequate storage were provided.

An adaptation of this map is shown in figure 

6 together with the location of those powerplants which use streamflow as sources of 

cooling water. Although the choice of 300 ftVs 

as a flow index was arbitrary, it provides 

preliminary information for investigating the 

potential siting of powerplants. For 1,000 MWe 

plants, the low-flow index is conservative since 

water consumption in these plants would be at 

most (with wet mechanical draft cooling 

towers) about 30 ftVs, or 10 percent of the flow. 

For larger powerplants the data of figure 6 

would have to be used with some judgment and 

interpretation, especially in view of the discussion of the previous section.

The information given in figure 6, the location of all powerplants using rivers as sources of 

cooling water (table 1), and also the water 

consumption data shown in figure 5 can serve 

as a first point of reference for plant siting. The 

next step, from a hydrological point of view, and 

in line with the previous discussion concerning 

low flow, should be the computation of the 

minimum streamflow to be expected in the 

given stream basin, taking into account present 

and future water withdrawals throughout the 

basin and making some allowance for possible 

emergency withdrawals during severe 

droughts.

CONCLUSIONS

Theoretical computations of water consumption by nuclear powerplants for cooling indicate 

that it is very large and varies greatly among 

the various cooling systems, being least in oncethrough cooling (18 ftVs per 1,000 MWe) and 

greatest in closed cooling via mechanical draft 

cooling towers (30 ftVs per 1,000 MWe).

Plants utilizing nonestuary seawater for cooling are not expected to have problems with the 

availability of water. Also, no hydrological 

problems regarding water quantity are foreseen 

for plants utilizing freshwater in large rivers 

and estuaries, even though environmental considerations have already brought about the 

preferential use of closed-cycle cooling which 

consumes more water than once-through cooling.

Problems of water scarcity are expected to 

arise, however, during low-flow periods in some 

small- or medium-sized basins (say, where low 

flows are less than 300 ftVs), especially where 

competing water demands exist now or are 

likely to emerge in the future. It appears that in 

the siting of nuclear powerplants in mediumand small-sized basins the relationship between water consumption and cooling method 

should be thoroughly evaluated with respect to 

the minimum streamflow expected to occur.

Because the data available on evaporation by 

various cooling methods are derived theoretically, there is an urgent need to verify these 

data with field measurements at operating 

plants. An equally urgent need exists to derive 

an operational model for the computation of 

the probable minimum flow.

REFERENCES

Hughes, G. H., 1975, Perspective on use of fresh water for 

cooling systems of thermoelectric power plants in Florida: U.S. Geol. Survey Water-Resources Inv. 43-75,30 p.

Kilpatrick, F. A., 1963, Source of base flow of streams, Internat. Assoc. Sci. Hydrology Pub. 63.

Kohler, M. A., Nordenson, T. J., and Baker, D. R., 1959, 

Evaporation maps for the United States: U.S. Weather 

Bur. Technical Paper 37.

NUS Corporation, 1974, Commercial nuclear power plants: 

Southern Nuclear Dept. Edition no. 7, 64 p.

Stankowski, S. J., Limerinos, J. T., and Buell, S. E., 1977, 

Stream reaches in the conterminous United States 

where dependable flows of 300 cubic feet per second are 

available or could be provided: U.S. Geol Survey Natl. 

Atlas (in press).

U.S. Atomic Energy Commission, 1972, Standard format 

and content of safety analysis reports for nuclear power 

plants: Revision 1 (manual).

 1974, Nuclear power facility performance character13
istics for making environmental impact statements: 

WASH-1355. 

West, M., 1975, Nuclear power reactor sites in the United

States: U.S. Geol. Survey open-file map 75-523. 

World Meteorological Organization, 1974, Guide to hydrological practices: World Meteorol. Organization, No. 

168, p. A. 13.

14