Transcript aabvzvzcx - Harvard University

Energy, Environment, and
Industrial Development
Michael B. McElroy
Frederick H. Abernathy
Lecture 22
May 1, 2006
Elements of Nuclear Physics
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As seen earlier, the nucleus of an atom
consists of a specified number of protons and
neutrons. Collectively, we refer to the protons
and neutrons as nucleons. The number of
protons in the nucleus determines its electric
charge (positive). The sum of the number of
protons and neutrons determines its mass.
mass number
For example: 92U235
atomic number, number of protons
In this case, the nucleus contains 92 protons and
143 neutrons.
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Elements of Nuclear Physics
 Example of a fission process:
1 +
235 
90 +
143 + 3 n1
n
U
Sr
Xe
0
92
38
54
0
Neutron, zero charge, mass 1
Products in this case are strontium, xenon and 2
extra neutrons.
 Fission of U235 produces a variety of fission
products: there are almost 50 modes of fission.
The average neutron yield is about 2.5. Energy
released in a fission event is about 200 million
electron volts (MeV). To put this in context,
energy involved in a typical chemical reaction
is about a few eV  difference of more than a
factor of 106
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Elements of Nuclear Physics
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Energy released by fission of 1 mole of U235
(235g).
Number of atoms in mole = 6.02x1023
Energy per atom = 200 MeV = 2x108 eV
= (2x108)(1.602x10-19)J
= 3.204x10-11 J
Energy per mole = 6.02x1023 x 3.204x10-11 J
= 1.93x1013J
= 1.93x1013 / 4.184 cal
= 4.61x1012 cal = 4.61x109 kcal
1Btu = 2.52x102 cal
Energy per mole = 4.61x1012 / 2.52x102
= 1.83x1010 Btu
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Elements of Nuclear Physics
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Remember: total annual US energy use
is about 100 Quad
1 Quad = 1015 Btu
US annual use: 1017 Btu
Could be supplied by
1017/1.83x1010 moles of U235
= 5.5x106 moles
= 5.5x106 x 2.35 x102g
= 1.3x109 g = 1.3x103 tons
By comparison, we burn more than 109
tons coal per year
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Elements of Nuclear Physics
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Natural uranium consists mainly of U235
and U238, with U238 most abundant
U235 consists of about 7x10-3 of total U
(0.7%)
To obtain 1.3x103 tons of U235 we would
require 1.3x103/7x10-3 tons U
= 1.9x105 ton U
 190,000 metric tons U
 about 10,000 times less than
requirement for coal
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Contemporary Nuclear Power
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Most nuclear power plants today use enriched
uranium. Concentration of U235 is increased from
0.7% to between 4~5%.
Enrichment is accomplished either by centrifuge or
by differential diffusion. Bombs require much more
extensive enrichment – typically more than 90%
Key ingredients of the nuclear reactor in most
common use:
Fuel Rods
Control Rods – used to absorb neutrons and
cut off the chain reaction in event of an
emergency. Metal cadmium is a useful material
for the control rods
Water used as a moderator whose function is
to slow neutrons so that most of neutrons react
with U235 rather than U238
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Contemporary Nuclear Power
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The system of fuel rods, control rods and moderator
is contained in a pressure vessel composed of
thick steel. Second line of defense is a heavy
concrete containment building. The Chernobyl
reactor was not enclosed in a containment building.
The energy released is contained mainly in the
kinetic energy of the fission fragments – about 82%
of total energy released
Heat from the fuel rods is absorbed by the water in
the moderator and is transformed to steam in an
isolated system. The steam drives the turbines
producing electricity.
After about 2 years, sufficient quantities of U235 has
been converted to fission products and fission
products have accumulated to the point where the
fuel rods must be removed and replaced.
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Contemporary Nuclear Power
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Reactors are refueled about every 18 months
in the U.S. One third of the fuel rods are
replaced. The procedure takes between 2 and
3 weeks
When fuel rods are removed they contain
large quantities of highly radioactive material.
Major heat source. They are stored initially in
a large tank of water. When they cool down,
fuel cells can be removed and stored in
concrete cylinders filled with metal inner liners
or separate metal containers.
When removed, the fuel rods contain not only
fission products but also plutonium-239
produced by capture of a fast neutron by U238.
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Contemporary Nuclear Power
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Pu239 can also serve as a fissionable fuel,
like U235. But it is also the ingredient for
nuclear bombs. Normally when the fuel
rods have been in the reactor for as much
as 18 months, the fuel rods contain a mix of
Pu isotopes. Not very useful for potential
bomb makers.
But, if the fuel rods are withdrawn much
earlier, the Pu may be much higher in
Pu239. Proliferation risk.
A large reactor produces about 1.5 tonnes
of fission products a year.
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The Nuclear Waste Problem
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Many of the radioactive products decay
rapidly. But some are long lived and can pose
a problem for hundreds or even thousands of
years
Current US strategy is to store wastes on site.
Eventually, wastes should be transferred to a
permanent geological reservoir.
Problems with identifying a suitable long-term
depository
Is reprocessing a solution?
Economic issues. Arguments for temporary
storage.
Is the breeder reaction the solution?
Story of the Integrated Fast Reactor (IFR).
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The Story of Chernobyl
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Used a pipe of graphite to slow neutrons.
Pipes inside carried the fuel rods, control rods,
and cooling water. No concrete containment
building.
April 25, 1986. Power level surges. Reaction
vessel burst. Hot steam + graphite +
(combustible) zirconium metal in fuel rods
reacted to produce an intense fire
Fire plume lifted to high altitude spreading
radioactive debris over large area
Remains of reactor later enclosed in a layer of
thick concrete, entombing radioactive waste
level inside
Plant reached 150 times normal power before
high pressure steam caused the disaster.
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The Story of Chernobyl
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Immediate deaths small – 31. Twenty
square miles permanently uninhabitable.
The number dying of wider spread fall-out
is difficult to estimate.
Conservatives would argue that the
Chernobyl accident was no more serious
than the Bhopal chemical plant or the
Texas City explosion of a shipload of
ammonium nitrate.
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The Story of Three Mile Island
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Accident took place on 28 March 1979.
Location near Harrisburg, Pa.
Minor malfunction led to series of errors,
shutting down for a while main and emergency
cooling systems
Heat melted part of core creating free
hydrogen.
Reactor vessel was not breached.
Containment vessel survived intact.
Minimal radiation released. Death toll: zero,
although some radioactive gases were vented
to the outside.
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Historical Milestones
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October 12, 1939, President Roosevelt
authorized government funding of atomic
(nuclear) research responding to letter from
Albert Einstein
February 1940, Uranium Committee granted
Enrico Fermi and Leo Szilard contract to build
a reactor at Columbia University
Arthur Compton, Nobel prize winner and Dean
of Physical Sciences at University of Chicago,
established laboratory to promote nuclear
research. Samuel K. Allison selected to direct
project
Graphite/uranium pile built under west stands
of Stagy Field Stadium at University of
Chicago.
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Historical Milestones
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First self-sustained reactor began 9.45am,
Dec. 2, 1942, under direction of Fermi.
George Weil pulled the last control Rod.
Experiment ended, 3.45pm
August 1, 1946. President Truman signed
the Atomic Energy Act. Federal
Government gives long-term responsibility
for nation’s nuclear laboratories.
Professor Abernathy invented the nuclear
fly-wheel
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Historical Milestones
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Atomic Energy Commission (AEC) established a
nuclear test side in Arco, Idaho, on April 4, 1949,
became location for development of a number of
experimental reactors including naval submarine
reactor, Argonne fast-breeder reactor, and others
April 1986. Argonne engineers demonstrated passive
cooling capability of integral nuclear power plant EBR11 (Experimental Breeder Reactor)
Research on EBR-11 led to development of the integral
fast reactor (IFR)
Spent fuel recycled in situ
IFR can “burn” plutonium from nuclear weapons and
actinides (including plutonium) from commercial light
water reactors
1994. President Clinton and Congress terminate funding
for advanced reactor technology at Argonne National
Laboratory. EBR-11 placed in a radiologically and
industrially safe condition.
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