perm filename NUCLEA[L,VDS] blob sn#250518
filedate 1976-11-27 generic text, type C, neo UTF8
COMMENT ⊗ VALID 00002 PAGES
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Americans use an incredible amount of energy each year. In fact,
with only about 6% of the total world's population, the U.S. uses 35% of
the world's energy. In l970, the U.S. consumption of total energy per capita
was about 250 KWh per day, and of that, the electrical consumption per day was
about 21 KWh per person. Last year, in l975, the FEA's estimate of the year's
electrical production alone was 1.9 x l0 KWh per year. (That is l.9 TRILLION
KWh per year!) And, more importantly, the present doubling time of electrical
generation in the U.S. is l0 years, its present growth rate is 2%, which itself
is accelerating. If Americans continue their present energy utilization trends,
they can expect an astronomical amount of over three and a half trillion KWH of
electricity needed in the year l985!
Unfortunately, as America's electrical energy consumption steadily soars,
the amount of fossil fuel resources steadily declines. Now, as the fossil fuel
resources continue to become more and more scarce, the price correspondingly becomes
more and more expensive. (The beginnings of which were demonstrated in the "energy
crisis" of l973.) Americans are now being forced to seriously seek other energy
alternatives which can competitively compete with the rising fossil fuel costs.
And, the people must find another means for supplying these enormous amounts of
energy within the next two decades if they are planning to keep up their present
patterns of energy consumption. Although coal, solar, geothermal, and hydro power
are being considered as major contributors to the future energy needs, it looks as
if nuclear power will, by necessity, become the dominant source of electricity
generation before the year 2000.
In l975, nuclear power supplied only 11% of the total electrical energy
needs, but the FEA predicts a 27% share in l985, while the Exxon Company forecasts
a possible 50% share in l980. Even though nuclear power is presently being hotly
debated by a substantial number of concerned Americans, the fact remains that
there are 58 nuclear power plants on line today, with a projected number of 170 in
the year l985. With energy utilization rates increasing now, we can expect that
nuclear power use will most likely increase greatly within the next decade. This
new power source will bring along with it not only the problems such as those
involved with waste disposal, fuel processing, and plant safety, but also the
possible problems arising from thermal pollution.
Thermal pollution is caused when the heat that is wasted in industrial
processes is transferred to a body of cooling water, raising its temperature 10 to
20 C., and then dumped back into the waterways. This temeperature rise can cause
a great deal of harm to the water ecology and its dependent life, thus making this
a potentially serious pollution problem.
It must be stressed that thermal pollution is not a new problem which has
been caused only by nuclear power plants. ALL power plants reject this unusable heat,
which is known as waste heat, whether they are coal, oil, nuclear, solar, or any
type of heat source. The reason for this is summed up in the second law of thermo-
dynamics which says that all processes cannot be l00% efficient. There is simply a
thermodynamical limit on the conversion of random heat energy to organized energy
(work). This is a limit imposed upon us from nature, and cannot be argued with.
The reason that people are becoming very concerned about it now though, is because a
nuclear power plant gives off about l5% more waste heat than a fossil fuel plant,
which gives off some of its heat directly into the atmosphere through the stacks.
The efficiency of a power plant can easily be calculated by employing the
Carnot equation, since the majority of plants today operate by using the steam cycle.
In this cycle, pressurized hot steam drives a turbine, which turns fuel energy into
mechanical energy in the turbine. This energy is converted into electrical energy,
in the form of electricity, and is then distributed to the consumers. The Carnot
equation allows us to calculate the actual amounts of waste heat which is produced
in this cyclic process. Pictured on the next page is a typical plower plant using
the Rankine Cycle (a steam cycle.) 1→2 is the conversion of thermal energy to
mechanical energy. 2→3 is the heat rejection due to condensing. 3→4 is the only
work input into the cycle, the work needed to run the water pump. Finally, 4→1 is
the heat input provided by the boiler.
The maximum efficiency of the power plant's heat engine is determined
by the Carnot efficiency, which is simply: MAXIMUM EFFICIENCY = 1-T(2)/T(1).
By looking at this equation, we can see that we want a very high entering temper-
ature (T(1)), and a very low exit temperature (T(2)), in order to obtain a high
engine (cycle) efficiency. Thus, the efficiencies depend on the temperature and
pressure of tte steam generated, which is limited by the temperature capacity of the
metal which must enclose this heat, and also, by the temperature of the environmental
Let us look at some typical power plant efficiencies. A modern fossil fuel
plant has a thermal efficiency of 40%, if it uses a steam cycle with superheat.
This is quite a high temperature, most plants have efficiencies around 35%. The
efficiencies of nuclear plants are varied, it depends on the type of reactor. A
Light Water Reactor (LWR), with a T(1) pf 277 C and a T(2) of l7 C, has an efficiency
33%. A High Temperature Reactor, (HTR) is 40% efficient, because T(1) can be
increased to 530 C. Generally speaking, nuclear power plants using the steam cycle
have low efficiencies and reject low-grade heat (usually about 30 C), while gas
turbines can produce high quality heat with efficiencies on the order of 40%. High
grade heat can be obtained from the steam engine, by extracting some of the higher
temperature steam from the turbine, but only at the expense of reducing electricity
production. High grade heat can be much more easily used than the low-grade heat,
as other none-electrical applications of this heat need higher temps. Now, the major
emphasis by the power stations is on producing maximum ELECTRICAL efficiency. Should
this emphasis be shifted towards an emphasis on maximum ENERGY efficiency ? (That is,
obtaining some of the high grade heat from the steam engine in order to effectively use
it. Would the benefits derived from the possible uses of the high grade waste heat
outweigh the disadvantage of decreasing the electrical capacity? I think so, and
have included at the end of this paper some uses of waste heat which I think can become
greatly beneficial. ENERGY efficiencies of 80% have been agreed upon to be possible
if this waste heat is used effectively.
Present research in areas such as fluid mechanics, combustion, heat transfer
and lubrication is striving to improve the Carnot efficiencies. Research is still
being devoted towards finding the "miracle metal" which will be able to extend
the present metallurgical limit of 1100 F. in the modern steam cycle power plants.
Some research is now directed towards finding alternatives to the low efficient
Rankine cycle. Those being explored are magnetohydrodynamics (MHD) and fuel cells.
No major breakthroughs on them have been discovered as of now to make the replacement.
With an efficiency of 33%, a 3,000 MW LWR would dump 2,000 MW of heat into
the environment. With hundreds of nuclear power plants each dumping out amounts
such as these, we can see where the temperature of the receiving water could become
quite high if the waste heat was continually dumped into the rivers, lakes, and
estuaries. This is where the problem of thermal pollution comes in.
By a rough estimate, it is assumed that 1 GW of electrical capacity causes
a body of water to be raised 10 C, (Assuming an effiency of 33%), and requires 50
cubic meters per second of cooling water. The Evironmental Protection Agency (EPA)
has limited the maximum temperature increase in receiving waters to be 5 C. Thus,
we can see that l GW will require 100 cubic meters per second of cooling water to
meet these limits! These huge water withdrawals can cause water shortages in areas
where water supplies are already deficient.
The U.S. has a run-off of 53,000 cubic meters per second, making a limit
of only 530 GW of power then to be obtained from the nuclear power plants. With
our high energy consumption rates, this figure should be reached by around l980.
Therefore, it is obvious that this heat will have to be dissipated elsewhere
besides in the runoff water. This is why I feel that immediate emphasis should
be placed on obtaining the high-grade heat and effectively using it, instead of
just throwing it away into the rivers.
But, WHAT IF the waste heat is continued to be dumped into the rivers?
What are the effects of adding this heat to the water ecosystem and life?
Probably the most important effect in raising the water temperature of
river water is the decrease of oxygen solubility; the oxygen capacity of the
water is reduced, which always spells out trouble. Thus, the river re-aeration
rate is decreased, due to the reduced oxygen saturation deficit. Along with all
of this is, of course, and increase in Biological Oxygen Demand. This is why dumping
sewage into warm water can be very serious--the wastes are oxidized at a much
faster rate, which requires a very high BOD. If there is not enough oxygen in the
water, the decomposition of the wastes will be anaerobic, which is undesirable
because it leads to such end products as methane, ammonia, hydrogen sulfide, and
CO . Thus, there is a lower waste assimilation capacity.
Other effects of the temperature rise include increase in evaporation
rates (which causes increased consumption rates of water), a reduction in ice
formation in winter, and an increase in chemical reactions. The viscosity of the
water can also be decreased, which can result in increased sedimentation, leading to
possible sludge problems. And, of course, raising the temperature also leads to a
qualitiative and quantitative change in the aquatic population, and an increase in
the undesirable aquatic flora.
When we look at the effects of thermal pollution, we must be sure to
not just look at the direct effects that it has on the fish, which has been done so
often in the past. We must remember that the surface waters which can be penetrated
by sunlight and that can receive nutrients can support an active ecosystem, so the
added heat will effect the decomposers and producers, as well as the consumers. Because
all three groups are so tightly interdependent upon eachother for survival, it is of
utmost importance to weigh the effects on every living creature in the ecosystem--
the zooplankton, the phytoplankton, the microorganisms, the fish, and all the rest.
This over-all view, which deals with communities and the interrelationships of popu-
lations within them, and determines the over-all effect of any given factor on all the
species, is known as synecology.
Each species has its own tolerance range for temperature; one that has a wide
range is called Eurythermal,whereas those with a narrow range is called Stenothermal.
Some organisms can live in near freezing water, whereas some can survive at temperatures
at 88 C.(some bacteria). Thus, we can see that by raising the temp. of water 10 C.,
some may not be affected at all, others might be beneficially affected, and others
just might be completely wiped out. Physiologically, the upper lethal temperature
is determined by the degree of enzyme inactivation in a particular species. With
increased temperatures, respiration is increased, which means an increased rate of
chemical reactions within the body. These reactions must be catalyzed by temp.-dependent
enzymes. Above an optimum temp., these enzymes become inactivated and can't keep up
with the reactions. With higher temps., and more and more inactivation of the enzymes,
the organism eventually dies. Inefficient oxygen transport to cells, central nervous
system malfunctioning, or changes in the lipid structure of cell membranes might be
other reasons for deaths caused at elevated temperatures.
Autecology is the ecology of individual species, involving studies of factors
influencing their distribution in nature. Because fish are economically important,
let us look at some actual effects of thermal pollution upon them. Because temperature
is probably the single most important factor governing life, the possible consequences
of thermal pollution are very serious. Fish are very sensitive to temperature changes
in their waters, and they are unable to quickly regulate their body temperature.
An 18 F. increase causes the basic metabolic rate of fish to double, which
is in accordance with the Van't Hoff Principle, which states that the rate of chemical
reactions increase wih increased temperature. This principle is very important, because
this increase of metabolic rate includes the increased rate of respiration, which of
course increases the need for oxygen. At the same time, an increase in water temp.
directly correlates with a decrease in the amount of DO in the water because of a
decrease of oxygen solubility, a decrease in the river re-aeration rate, and an
increase in the Biological Oxygen Demand (BOD). All of this spells disaster for the
fish. And, to make matters even worse, the respiration difficulties are coupled with
the problem of the reduced hemoglobin affinity for oxygen, which means that there will
be a reduced efficiency of carrying oxygen to the tissues. These two effects alone
put the fish under severe stress.
Another serious problem occurs in he reproduction cycle of the fish. Usually,
fish spawn in the Fall or Spring. The temperature of the waters induce the seasonal
development of the gonads. At the critical temperature, the female deposits her eggs.
For instance, estuarine shellfish such as oysters and clams spawn within hours of the
critical temperature. With the temp. being increased artificially through the waste
heat, the hatching time of fish are greatly effected. Atlantic Salmon have a normal
hatch time of 114 days at 36 F., but only a 90 day period at 45 F. This then prevents
normal development of the eggs. The tiny crustacean, Gammarus, reacts to this in a
strange way...at temperatures greater than 46 F., the species lays only female offspring!
D'Arcy W. Thompson has also established the fact that accelerated stage development
by warmth shortens the duration of the life. Thus, the longevity of the fish are
threatened by the increased temperature.
Fish are fortunately able to acclimate to temperature shifts, providing that
they are not too sudden. An example of this is shown by the reaction by the largemouth
bass. When they are transferred suddenly to 85 F. temperature, from 65 to 70 F.
temperatures, 90% of their eggs will perish. However, if one gives them enough time
to adjust to the change, in this case, 40 hours, only 20% will die. Fish, as a general
rule, acclimate to elevated temperatures much more quickly than they acclimate to
Again, though, it is important to emphasize that the fish are only a part of
a very delicately balanced ecosystem. The food chains in the oceans consist of long
food chains, as opposed to relatively short one on land. Man must be very careful to
avoid destroying any link of this tightly woven, interdependent food chain. Ecosystem
stability is weakened by an increased water temperature environment, because the warmer
water usually causes a decline in species diversity.
We have seen then that thermal pollution can cause serious effects to the water
ecosystem if action is not done. The good thing now is that most of the nuclear power
plants have not yet been built, and so plans can be made to incorporate methods into
the plants for properly disposing of this waste heat and controlling thermal pollution
from the very start. For instance, proper siting of the plant can reduce the harmful
impact of the heat. Many factors must be considered, such as the atmosphere. How
much wind, rain, humidity is there? What is the average temperature? The receiving
water must also be evaluated...what are the flow characteristics, the turbulence, the
stratification, and the volume? One must look at the biological factors...what types of
aquatic flora and fauna is there? What are the ecosystems? What is the DO and the
BOD levels? How much waste and pollution is already present in the water? And finally,
one must ask what the other uses of the receiving water are...recreation, irrigation,
water supplies? Everything must be taken into account and carefully evaluated.
By using cooling towers or cooling ponds, the problem of thermal pollution
may have been taken care of (depending upon the validity of assumptions that thermal
air pollution will not cause detrimental effects to the enivronment). However, it is
obvious that this energy in the form of waste heat is simply being wasted. With energy
costs and possible future environmental problems, serious attempts should be made to
effectively utilize this heat energy.
Realizing the importance of utilizing as much of this waste heat as we can,
research is advancing rapidly in this field. Provisions must be made now for waste
heat use in the original designs of the nuclear reactors. The two main considerations
that must be taken into account when developing uses for the heat is reducing enivron-
mental effects and in creating an overall economic gain. The most important difficulty
which limits the development of the heat uses is the economic factor. Much progress
has been made in developing uses for the waste heat in areas such as aquaculture, mari-
culture, agriculture, airport deicing, space heating, airconditioning, and uses in
many industrial processes. However, much more research is needed to actually turn these
experimental programs into working realities.
One promising use of the waste heat could be in using it for heating the
sewage in a secondary sewage treatment plant, which uses the activated sludge process.
This process involves running the sewage effluent into an aeration tank, where the
bacteria feed upon the organic matter and converts it into the end products of H O,
CO , sulfates, nitrates, and new bacterial cells. This process can be greatly acceler-
ated by increasing the temperature, because a 10 C. rise can double the biochemical
reaction rate. Also, the decreased water viscosity can cause a faster sedimentation
of the activated sludge particles. An estimate is that if the sewage temperature
coul be raised from 70 to 100 F., by passsing the sewage effluent through the power
plant condenser, the sewage could be treated at twice the normal rate of a 70 F.
facility. This means that the sewage plant will be able to save a lot because of lower
capital costs. The power plant will be able to save money by having less capital costs
in cooling towers, as well as their operating expenses.
It would be highly desirable to locate the waste plant close to the nuclear
power plant, or vice versa, so that the two could take advantage of each other's
contributions and requirements. This is known as the group utility concept. This
could be easily done by combining a power generation plant with a sweawater desalting
plant, or perhaps a huge greenhouse or animal shelter complex. Grouping them together
is important because of the costs that would be involved in otherwise transporting the
heat through a system of pipes to the desired place. For instance, if a 10 MGD treatment
plant was situated one mile from the power plant, it would cost from 1.5 to 3.9 cents
per 1,000 gallons. (Depending if the sewage plant is located downhill from the power
plant, which would not require the high pumping costs.) Located five miles away, it
would cost from 2.9 to 5.4 cents per 1,000 gallons. Thus, we can see that locating the
two together is very important in the economic analysis.
Of course, there are many problems which must be solved before programs can
be started for using this waste heat. A major problem is the unreliability of the
nuclear reactor. A nuclear reactor can shutdown unpredictably, for various reasons,
such as for maintenance or malfunctions. This means that in almost all of the uses,
back up structures would be necessary. Would the added extra cost of buying a structure
for the utilization of the waste heat be worth it economically? Another factor is the
variation in the availability and temperature of the thermal effluent, due to the
load variation of the plant. Another problem is the consumptive use of the water in
applications where the water is directly used (in warm water irrigation, for instance).
Of course, the major problem is the distance between the plant and the point of applica-
tion...people are very reluctant now to site the plant within 15 miles of a major city.
The problems are there. But, it is important to continue to try to find the
solutions to them. We simply cannot continue to waste two-thirds of our energy now.
Economic and environmental considerations will someday, soon, force this utilization.
Therefore, we must plan now, before the reactors are built, so that a possible tran-
sition later will not be so difficult and costly.