Week
6-8: Conventional Technologies
Nuclear
Power Issues
How
Reactors Work
Siting Difficulties
Effects of Radiation
Why
Nuclear Reactors?
Good fuel/output ratio
High heat/unit of
fuel
Available sources
of uranium in U.S.
Endorsement of the
energy industry
Desire to cut back
on fossil fuels and reduce carbon dioxide releases
Why
Not?
Inherent dangers of
exposure to radiation
Miners, power plant
workers
Low-level releases
into air and water
Possible accidents
Difficulties of siting
facilities in the U.S.
Lengthy and costly
siting procedures (10 years)
Need for safety
redundancy
NIMBY attitudes
Evacuation plans are
difficult to implement
Principles
of Nuclear Fission
Utilization of U-235
as a fissionable material
Small proportion of
yellowcake ore
U-238 more abundant
(about 99.3% of ore)
Both are radioactive,
with long half-lives
4.5 billion years
for U-238
0.7 billion years
for U-235
U-235
� A Fissionable Material
In the U.S. most
uranium
deposits are found in sandstone (about 30% of the world�s deposits)
The tailings are
weakly
radioactive but must be carefully handled
Ore must be enriched
to 3-4% U-235 by weight (compared to >90% for weapons)
Pellets of enriched
uranium are fabricated into uranium oxide (UO2) fuel rods (packed in
zircaloy
tubing, called �cladding�)
How
Nuclear Reactors Work
Fuel rods are placed
in the reactor core
Fission process takes
place in a reactor
As fissions occur
in the reactor, more reactions are stimulated, to reach �criticality�
Fuel rods are
surrounded
by a �moderator� such as water
Heat is produced,
to make steam for electricity generation
Safety
Features of Reactors
Safety redundancy
guarantees that no emissions of radioactivity would be able to occur
during
an accident (�hopefully�)
Reactor core is placed
within a containment building
Cooling system removes
heat (helps to avoid exceeding a �critical� temperature)
�Control rods� control
the fission process
Types
of Reactors
Light Water Reactors
Contain ordinary water
as the moderator
Common reactors in
the United States
Graphite Cooled
Reactors
(Russian type)
Breeder Reactors
(LMFBRs)
Can be utilized for
reprocessing spent fuel rods
No currently operating
facilities in the U.S.
Can use U-238 as one
of the fuels
Cooling
Techniques
Nuclear reactions
must be controlled
Cannot allow reactor
contents to overheat or exceed a controlled level of fissions
Could result in a
runaway reaction
Reactors have coolants
to prevent overheating
Steam generated via
nuclear reactions must be condensed and returned to a waterway
Can lead to thermal
pollution of a lake or bay
Waste
Products from Reactors
Spent fuel rods (SURF)
Very radioactive
(after
about 3 years of use)
Usually stored in
water pools on reactor site
Could be �reprocessed�
into uranium and plutonium (done in France, Russia, Japan)
High-level radioactive
wastes (HLW)
Comes from
reprocessing
of the fuel rods
Generated from weapons
production in U.S.
Transuranic Wastes
(TRU)
Material with long
half-life, alpha-particle emitting radioactive isotopes (100 nCi/g)
Primarily defense
wastes
Mining or processing
wastes
U-238 that could be
used in breeder reactors
Mine tailings (contain
uranium, radium and radon gas)
Low-level radioactive
wastes (LLW)
Wastes not classified
as SURF, HLW, or TRU
Weakly radioactive
Associated with
medical
and educational-research institutions, commercial, and defense
facilities
Highly radioactive
wastes covered by the Nuclear Waste Policy Act of 1982
Yucca
Mountain Waste Facility
U.S. national
depository
for nuclear wastes
Located near Las
Vegas,
Nevada
Underground complex
(about 3 square miles in size) of interconnected tunnels
Located in dense
volcanic
rock 305 m beneath the mountain
Transport issues for
truck and rail routes
Life
Expectancy of a Reactor
Estimated 25-30 years
of active life
Decommissioning is
required after site becomes too radioactive to guarantee safety of
workers
and surrounding community
Most reactors
presently
operating in the U.S. are approaching their life expectancy
Mothballing the site,
or total disassembly
Adds to the cost of
nuclear-generated power
Decommissioning
Nuclear Plants
Storage of existing
plants (guarded by the utility company for 50-100 years)
Dismantling would
be safer at a later time because some radioactive materials would
decay,
but accidental leaks are possible
Entombment
(permanently
encasing the facility in concrete)
Would be intact for
about 1,000 years
Decommissioning
(dismantling
of plant immediately after closure)
Workers would wear
protective clothing
May possibly utilize
robotics
Transport to a
permanent
storage site
Most responsible
action
to take
Need permanent storage
sites
Shippingport
(Pennsylvania)
� first nuclear power plant in the U.S.
Dismantled in 1989;
moved to Hanford Nuclear Reservation in Washington state
Many nuclear plants
ready for retirement
87 nuclear plants
permanently retired � 1999
93 plants over 25
or more years old in 1999
Costly process ($370
million for Yankee Rowe)
Types
of Nuclear Accidents
LOCA (Loss of Coolant
Accident)
Can permit the reactor
to overheat
May lead to other
consequences; other backup systems may fail
Example: Three Mile
Island, Harrisburg, PA (March 28, 1979)
Partial meltdown
occurred
(50%), but held within the containment building
Core Meltdowns
Caused by excessive
heat whereby the contents of the reactor can melt down
Can lead to
groundwater
contamination and offsite effects
Explosion and Fire
Can release large
amounts of radioactivity
Example: Chernobyl,
Ukraine (April 26, 1986)
More
About Chernobyl
Radiation spread
quickly
into Belarus and throughout northern Europe (Sweden, Norway, France,
Switzerland)
116,000 people
evacuated
(within a 30-km radius); eventually 170,000 people had to abandon their
homes
Damaged reactor was
encased in concrete (sarcophagus)
Long-term health and
economic effects
Reactor design was
flawed (RBMK)
Graphite moderator
(reacts to dramatic temperature changes
Reactor was not
encased
in a containment building
Reactor was extremely
unstable at low power
Human error
contributed
to accident
Radiation is presently
escaping from the site
Effects
of Radiation Exposure
Somatic Effects
Can include burns,
organ damage, reduced immunity, or even be fatal
Genetic Effects
Alters sperm or egg
cells, thereby creating mutations that can be passed on to offspring
Acute vs. Chronic
Exposure
Varies with dosage
and duration
Measures
of Radiation Exposure
Curie � 3.7 million
disintegrations/second
Picocurie �
1/trillionth
(10-12) curie (pCi)
Roentgen � basic unit
of radiation energy
Measures of Radiation
Exposure
Rads (amount of
radiation
exposure)
Rems (human exposure
equivalency)
(1 rem = 100 ergs
of absorbed radiation/gram of matter, multiplied by a factor for each
type
of radiation, e.g. 20 for alpha radiation)
Background radiation
is about 100 mrem
European term: Sieverts
(1 sievert=100 rads)
More
About Radon
Radon-222 is a
byproduct
of the breakdown of uranium
Flux of radon
everywhere,
but more concentrated in some areas (e.g., with granite rock formations)
Half-life of 4 days
Decays into
radioactive
lead, polonium, bismuth (radon daughters)
Radon daughters are
extremely carcinogenic
Particularly a problem
in �tight� houses that are located over high-uranium rocks
Common situation in
New England and in many Western states
5,000 to 30,000 lung
cancer deaths/year are due to radon daughters formed indoors from
radon-222
Houses need to be
tested for radon
EPA programs for this
purpose
4 pCi/L (4 picocuries
per liter of air) is the standard used by the EPA
Equivalent to smoking
4 cigarettes/day
Outside air is usually
0.8 to 1.5 pCi/l and indoor air averages 1.0 to 2.0 pCi/l
Principles
of Nuclear Fusion
Fusion process also
takes place in a reactor
Called a tokamak
(Russian
for �fusion reactor�)
Requires excessively
high temperatures and pressures for reactions to occur
Based on fusing of
different atoms, with a heat release
Not yet commercially
available ($10 billion already invested by U.S.)
Strong interest in
future applications
Fusion process could
produce considerable amounts of energy
Isotopes of hydrogen
are fuel (e.g., nuclei of deuterium and tritium unite to form helium)
Deuterium, called
�heavy hydrogen� is found in water and easy to separate
Tritium is radioactive
and does not occur in nature (formed from lithium found in seawater)
Illustrations
of Reactors
U.S. Department of
Energy Web Site
http://www.energy.gov
Overview
of a Nuclear Facility
Student Team Web Site
http://web.bryant.edu/~langlois/ems/nuclear.htm
copyright
Gaytha
A. Langlois, Ph.D., 1999
Bryant
University, Smithfield, RI 02917
e-mail:
langlois@bryant.edu
Last
Updated: August 2006