Lecture 10 nuclear

Environmental Science
A Study of Interrelationships
Eleventh Edition

Enger & Smith

Chapter 10
Nuclear Energy

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Outline

 The Nature of Nuclear Energy
 The History of Nuclear Energy Development
 Nuclear Fission Reactors
 Investigating Nuclear Alternatives
 The Nuclear Fuel Cycle
 Nuclear Concerns
 The Future of Nuclear Power

The Nature of Nuclear Energy

 The nuclei of certain atoms are unstable and
spontaneously decompose. These isotopes are
radioactive.
 Neutrons, electrons, protons, and other larger
particles are released during nuclear
disintegration, along with a great deal of energy.
 Radioactive half-life is the time it takes for half
the radioactive material to spontaneously
decompose.


 Nuclear disintegration releases energy from the
nucleus as radiation, of which there are three
major types:
• Alpha radiation consists of moving particles composed
of two neutrons and two protons.
– It can be stopped by the outer layer of skin.
• Beta radiation consists of electrons from the nucleus.
– It can be stopped by a layer of clothing, glass, or
aluminum.


• Gamma radiation is a form of electromagnetic
radiation.
– It can pass through your body, several centimeters of lead,
or a meter of concrete.

 The absorbed dose is the amount of energy
absorbed by matter. It is measured in grays or
rads.
 The damage caused by alpha particles is 20 times
greater than that caused by beta particles or
gamma rays.


 The dose equivalent is the absorbed dose
times a quality factor.
 When alpha or beta particles or gamma
radiation interact with atoms, ions are formed.
Therefore, it is known as ionizing radiation.
 Ionizing radiation affects DNA and can cause
mutations.
• Mutations that occur in some tissues of the body may
manifest themselves as abnormal tissue growths
known as cancers.


 Large doses of radiation are clearly lethal.
 Demonstrating known harmful biological effects
from smaller doses is much more difficult.
 The more radiation a person receives, the more
likely it is that there will be biological
consequences.
 Time, distance, and shielding are the basic
principles of radiation protection.
• Water, lead, and concrete are common materials
used for shielding from gamma radiation.


Protective equipment


 Nuclear fission occurs when neutrons impact and
split the nuclei of certain other atoms.
 In a nuclear chain reaction, splitting nuclei
release neutrons, which themselves strike more
nuclei, in turn releasing even more neutrons.


Nuclear fission chain reaction


 Only certain kinds of atoms are suitable for
development of a nuclear chain reaction.
• The two most common are uranium-235 and plutonium-
239.
• There also must be a certain quantity of nuclear fuel
(critical mass) for the chain reaction to occur.

The History of Nuclear Energy
Development
 The first controlled nuclear chain reaction
occurred at Stag Field in Chicago in 1942.
 That event led directly to the development of the
atomic bombs dropped on the Japanese cities of
Hiroshima and Nagasaki.
 Following WW II, people began exploring other
potential uses of nuclear energy.
 The U.S. built the world’s first nuclear power
plant in 1951.

The History of Nuclear Energy Development

 In December 1953, President Dwight D. Eisenhower
made the following prediction in his “Atoms for
Peace” speech:
• “Nuclear reactors will produce electricity so cheaply that it
will not be necessary to meter it.”
 Today’s Reality:
• Accidents have caused worldwide concern about the
safety of nuclear power plants.
• As of 2006 there were 24 nuclear power plants under
construction in 11 countries. Many experts predict an
increase in building as energy demands increase.

Nuclear Fission Reactors

 A nuclear reactor is a device that permits a
controlled fission chain reaction.
• When the nucleus of a U-235 atom is struck by a
slowly moving neutron from another atom, the
nucleus splits into smaller particles.
• More rapidly-moving neutrons are released, which
strike more atoms.
• The chain reaction continues to release energy until
the fuel is spent or the neutrons are prevented from
striking other nuclei.


 Control rods made of a non-fissionable material
(boron, graphite) are lowered into the reactor to
absorb neutrons and control the rate of fission.
• When they are withdrawn, the rate of fission
increases.
 A moderator is a substance that absorbs
energy, which slows neutrons, enabling them to
split the nuclei of other atoms more effectively.


 In the production of electricity, a nuclear reactor
serves the same function as a fossil-fuel boiler: it
produces heat, which converts water to steam,
which turns a turbine, generating electricity.
• Lightwater reactors make up 90% of current reactors;
use ordinary water to cool reactor cores.
• The two types of lightwater reactors are:
– Boiling Water Reactors (BWR)
– Pressurized Water Reactors (PWR)
• Heavy water reactors and gas-cooled reactors are
two other kinds of nuclear reactors.


Boiling water reactor


Pressurized-water reactor


Advanced gas-cooled reactor

Investigating Nuclear Alternatives

 Breeder reactors are nuclear fission reactors that
produce heat to be converted to steam to generate
electricity.
 They also form a new supply of radioactive
isotopes during operation. (i.e., U238 turns into Pu239)
 The liquid metal fast-breeder (LMFBR) appears to
be the most promising model.
• Because Pu-239 is very hazardous to humans, and can
be made into nuclear weapons, development has
slowed significantly in most regions of the world.


Formation of Pu-239 in a breeder reactor


 Nuclear fusion is a process in which two
lightweight atomic nuclei combine to form a
heavier nucleus, resulting in the release of a
large amount of energy.
 The energy produced by the sun is the result of
fusion.
 Nuclear fusion has a huge potential for energy,
but technical difficulties of attaining necessary
conditions make this an unlikely fuel candidate
for the immediate future.


Nuclear fusion

The Nuclear Fuel Cycle

 The nuclear fuel cycle begins with the mining of
low-grade uranium ore.
 It is milled, and crushed and treated with a solvent
to concentrate the uranium.
 Milling produces yellow-cake, a material
containing 70-90% uranium oxide.


 Naturally occurring uranium contains about
99.3% nonfissionable U238, and .7% fissionable
U235.
• It must be enriched to 3% U235 to be concentrated
enough for most nuclear reactors.
• Material is fabricated into a powder and then into
pellets.
• The pellets are sealed into metal rods (fuel rods) and
lowered into the reactor.


 As fission occurs, U-235 concentration decreases.

 After about three years of operation, fuel rods
don’t have enough radioactive material remaining
to sustain a chain reaction, thus spent fuel rods
are replaced by new ones.

 Spent rods are still very radioactive, containing
about 1% U-235 and 1% plutonium.


 Initially, scientists proposed spent fuel rods could
be reprocessed and used to manufacture new fuel
rods.
 This process would reduce the amount of nuclear
waste.
 At present, India, Japan, Russia, France and the
United Kingdom operate reprocessing plants as an
alternative to storing rods as waste.


Steps in the nuclear fuel cycle


 All of the processes involved in the nuclear fuel
cycle have the potential to generate waste.
 Each step in the nuclear fuel cycle involves the
transport of radioactive materials.
 Each of these links in the fuel cycle presents the
possibility of an accident or mishandling that
could release radioactive material.

Nuclear Concerns

 Nuclear power provides a significant amount of
electricity for the people of the world.
 However, many factors have caused some
countries to reevaluate their commitment to
nuclear power:
• Accidents have raised questions about safety.
• Use of nuclear material by terrorists.
• Concern about worker and public exposure to radiation.
• Weapons production.
• Contamination and disposal problems.

Nuclear Concerns

 Reactor Safety
• The Three Mile Island nuclear plant in Pennsylvania
experienced a partial core meltdown on March 28,
1979.
– It began with pump and valve malfunction, but operator
error compounded the problem.
– The containment structure prevented the release of
radioactive materials from the core, but radioactive
steam was vented into the atmosphere.
– The crippled reactor was defueled in 1990 at a cost of
about $1 billion.
– Placed in monitored storage until its companion reactor
reaches the end of its useful life.

Nuclear Concerns

 Reactor Safety
• Chernobyl is a small city in Ukraine, north of Kiev.
• It is the site of the world’s largest nuclear accident,
which occurred April 26, 1986.
– Experiments were being conducted on reactor.
– Operators violated six important safety rules.
– They shut off all automatic warning systems, automatic
shutdown systems, and the emergency core cooling system
for the reactor.

Nuclear Concerns

Chernobyl

Nuclear Concerns

 Reactor Safety
• In 4.5 seconds, the energy level of the reactor
increased 2000 times.
• The cooling water converted to steam and blew the
1102-ton concrete roof from the reactor.
• The reactor core caught fire.
• It took 10 days to bring the burning reactor under
control.

Nuclear Concerns

 Reactor Safety
• There were 31 deaths; 500 people hospitalized (237
with acute radiation sickness); 116,000 people
evacuated.
– 24,000 evacuees received high doses of radiation.
– Children or fetuses exposed to fallout are showing
increased frequency of thyroid cancer.
 Some scientists fear the worst is yet to come.
Whether people living in the shadow of
Chernobyl remain at risk for health problems is a
question under close scrutiny.

Nuclear Concerns

The accident at Chernobyl

Nuclear Concerns

 A consequence of the Chernobyl accident in
particular has been a deepened public concern
over nuclear reactor safety.
 Increased public opposition after Chernobyl:
• United Kingdom: from 65% to 83% opposed.
• Germany: from 46% to 83%
• United States: from 67% to 78%

 The number of new nuclear plants being
constructed has been reduced, and many have
been shut down prematurely.

Nuclear Concerns

 After Sept. 11, 2001, fear arose regarding
nuclear plants as potential targets for terrorist
attacks.
 The consensus is that there is not great cause
for concern of an airliner attack on a reactor or
containment building.
 Probably the greatest terrorism-related threat is
from radiological dispersal devices (RDDs), or
dirty bombs.
• The point of a dirty bomb is to cause disruption and
panic, rather than kill large numbers of people.

Nuclear Concerns

 Each step in the nuclear fuel cycle poses a
radiation exposure problem.
 Prolonged exposure to low-level radiation
increases workers’ rates of certain cancers.
 Enrichment, fabrication, and transport of nuclear
materials offer potential exposure hazards and
risks of accidental release of radioactive
materials into the environment.

Nuclear Concerns

 Producing nuclear materials for weapons and
other military uses involves many of the same
steps used to produce nuclear fuel for power
reactors.
 The U.S. Department of Energy (DOE) is
responsible for nuclear research for both weapons
and peaceful uses.
 Historically, research and production dealt with
hazardous chemicals and minor radioactive
wastes by burying them, storing them in ponds, or
releasing them into rivers.

Nuclear Concerns
 Despite environmental regulations that prohibited
such behaviors, the DOE (formerly the Atomic
Energy Commission) maintained they were
exempt from such laws.
 As a result, DOE has become steward of:
• 3,700 contaminated sites.
• 330 underground storage tanks with high-level
radioactive waste.
• More than one million 55-gallon drums of waste.
• 5,700 sites with wastes moving in soil.
• Millions of cubic meters of radioactive wastes.

Nuclear Concerns

 Clean-up will take decades and cost billions of
dollars.
• Local residents and host states are not completely trustful.
• Clean-up is a new mission for DOE.
• Additional problem of disposing of dismantled nuclear
weapons.

Nuclear Concerns

 There are four general categories of nuclear
wastes:
• Transuranic wastes are highly radioactive waste that
contain large numbers of atoms that are larger than
uranium with half-lives greater than 20 years.
• Uranium mining and milling wastes have low levels
of radioactivity but are above background levels.
• High-level radioactive wastes are spent fuel rods
and highly radioactive materials from the reprocessing
of fuel rods.
• Low-level radioactive wastes have low levels of
radioactivity and are not classified into one of the
other categories.

Nuclear Concerns

 Transuranic waste from former nuclear weapons
sites is transported to the Waste Isolation Pilot
Plant (WIPP) near Carlsbad, NM, which began
accepting waste in March, 1999.
 Waste management activities of mining and
milling waste include:
• Building fences.
• Putting up warning signs.
• Establishing land use restrictions.
• Covering the wastes with a thick layer of soil and rock
to prevent erosion, windblown particles, and
groundwater contamination.

Nuclear Concerns

DOE high-level radioactive transuranic waste sites

Nuclear Concerns

 Disposal of high-level radioactive waste is a
major problem for the nuclear power industry.
• In the U.S., about 30,000 metric tons of highly
radioactive spent fuel rods are stored in special
storage ponds at nuclear reactor sites.
• There is no permanent storage facility, and many
plants are running out of temporary storage.
• Most experts feel the best solution is to bury waste in
a stable geologic formation.

Nuclear Concerns

 In 1982, Congress called for a high-level
radioactive disposal site to be selected by March
1987, and to be completed by 1998.
 In 2002, the Secretary of Energy indicated the
choice of a site at Yucca Mountain in Nevada was
based on “scientifically sound and suitable
science.”
 Current work is primarily exploratory and is
seeking to characterize the likelihood of
earthquake damage and water movement.

Nuclear Concerns

 If completed, the facility would hold about 70,000
metric tons of spent fuel rods and other highly
radioactive material.
• Not to be completed before 2015.
• By that time, waste produced by nuclear power plants
will exceed the storage capacity of the site.

Nuclear Concerns

 In February 2002, President George W. Bush
notified Congress that he considered the Yucca
Mountain site qualified to receive a construction
permit, and in July 2002, he signed a joint
resolution allowing construction to proceed.
 Members of Congress have vowed to continue
to fight this designation, and the state of Nevada
has several lawsuits pending in federal court.

Nuclear Concerns

High-level nuclear waste disposal

Nuclear Concerns

 Currently, the U.S. produces about 800,000
cubic meters of low-level radioactive waste
annually.
• Disposal of this type of radioactive waste is very
difficult to control, and estimates indicate that much of
it is not disposed of properly.
• It is presently buried in various scattered disposal
sites.
• Most states still do not have a permanently available
place to deposit low-level radioactive wastes.

Nuclear Concerns

 Thermal pollution is the addition of waste heat to
the environment.
• It is especially dangerous in aquatic systems.
– In a nuclear power plant, 1/3 of heat is used to generate
electricity while the other 2/3 is waste.
– Fossil fuel plants are 50:50.
• To reduce the effects of waste heat, utilities build
cooling facilities.

Nuclear Concerns

 The life expectancy of most electrical generating
plants (fossil fuel or nuclear) is 30-40 years.
 Unlike other plants, nuclear plants are
decommissioned, not demolished.
• Decommissioning involves removing the fuel,
cleaning surfaces, and permanently barring access.
• Over 80 nuclear power plants in the world have been
decommissioned.

Nuclear Concerns

 The decommissioning of a plant is a two-step
process:
• Stage 1: The plant is shut down, all fuel rods are
removed; all of the water used as a reactor-core
coolant, as a moderator, or to produce steam is
drained; and reactor and generator pipes are cleaned
and flushed.
– Everything must be safely stored or disposed of.
– This removes 99% of radioactivity.
• Step 2: Dismantle all parts of the plant except the
reactor, and safely contain the reactor.

Nuclear Concerns

 Utilities have three options in decommissioning a
plant:
• Decontaminate and dismantle plant as soon as it is shut
down.
• Shut down plant for 20-100 years, allowing radiation to
dissipate, then dismantle.
• Entomb plant by covering the reactor with reinforced
concrete, and placing a barrier around the plant.
 Recent experience indicates decommissioning a
large plant will cost between $200 and $400 million.

The Future of Nuclear Power

 Environmental and economic issues have
transformed into political stands where nuclear
power is concerned.
 The potential for climate change has had major
implications for nuclear power.
• Because nuclear power plants do not produce carbon
dioxide, many people have reevaluated the value of
nuclear power as a continuing part of the energy
equation.


 Many governments, setting their policy based on
how their citizens vote, have declared
themselves to be nonnuclear countries.
• Germany and Sweden have set goals to
decommission all of their nuclear power plants.


 Currently, there are 439 nuclear power reactors in
31 countries.
• They have a combined capacity of 354 gigawatts.
• In 2004, they provided 16% of world’s electricity.
 Currently, 24 reactors are under construction in 11
countries.
• Most of the planned reactors are in Asia, where China,
India, Japan, and South Korea plan to add 40 plants
over the next 10-20 years.


 Plant Life Extension
• Most nuclear power plants originally had a normal
design lifetime up to 40 years.
• Engineering assessments have established many
plants can operate much longer.
• Extending the operating life of a plant reduces the
cost of producing power because the
decommissioning is delayed and no new power plant
needs to be built to replace the lost capacity.
 New reactor designs are improving safety.

Summary

 Nuclear fission is the splitting of the nucleus of
an atom.
 Various kinds of nuclear reactors have been
constructed, and scientists are conducting
research on the possibilities of using nuclear
fusion to generate electricity.
 All reactors contain a core with fuel, a moderator
to control the rate of the reaction, and a cooling
mechanism to prevent the reactor from
overheating.

Summary

 The nuclear fuel cycle involves mining and
enriching the original uranium ore, fabricating it
into fuel rods, using the fuel in reactors, and
reprocessing or storing the spent fuel rods.
 Fuel and wastes must also be transported.
 Each step in the process presents a danger of
exposure.

Summary

 Public acceptance of nuclear power plants has
been declining. Accidents at Three Mile Island
and Chernobyl have increased concerns about
the safety of nuclear power plants.
 Other concerns are the unknown dangers
associated with exposure to low-level radiation,
the difficulty of agreeing to proper long-term
storage of high-level and low-level radioactive
waste, and thermal pollution.

Lecture 10 nuclear

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