• Nuclear Reactions
• Decisions about nuclear
energy require some
understanding of nuclear
reactions and the nature
of radioactivity. This is
one of the three units of
Palo Verde Nuclear
Generating Station in
Arizona. With all three
units running, enough
power is generated to
meet the electrical needs
of nearly 4 million
people.
• Natural Radioactivity
• Introduction
– Henri Becquerel discovered radioactivity in 1896
• Becquerel named the emission of invisible radiation
from uranium ore radioactivity.
• Radioactive materials was the name given to materials
that gave off this invisible radiation.
• Radioactivity was discovered by Henri Becquerel when he
exposed a light-tight photographic plate to a radioactive
mineral, then developed the plate. (A) A photographic film
is exposed to an uranite ore sample. (B) The film, developed
normally after a four-day exposure to uranite. Becquerel
found an image like this one and deduced that the mineral
gave off invisible radiation that he called radioactivity.
– Ernest Rutherford later discovered that there were three
kinds of radioactivity.
• Alpha particles () is a helium nucleus (2 protons
and 2 neutrons)
• A beta particle () is a high energy electron
• A gamma ray () is electromagnetic radiation with a
very short wavelength.
• Radiation passing through a magnetic field shows that
massive, positively charged alpha particles are deflected one
way, and less massive beta particles with their negative
charge are greatly deflected in the opposite direction.
Gamma rays, like light, are not deflected.
– Radioactivity is the spontaneous emission of particles or
energy from an atomic nucleus as it disintegrates.
– Radioactive decay is the spontaneous disintegration of
decomposition of a nucleus.
• Nuclear Equations
– The two subatomic particles that occur in the nucleus, the
proton and the neutron, are called nucleons.
• The number of protons is the atomic number which
determines the identity of the element.
• The number of protons and neutrons determines the
atomic mass of the element.
• Different isotopes of an element have the same atomic
number (same number of protons) but different atomic
masses (different number of neutrons)
• The three isotopes of hydrogen have the same number of
protons but different numbers of neutrons. Hydrogen-1 is
the most common isotope. Hydrogen-2, with an additional
neutron, is named deuterium, and hydrogen-3 is called
tritium. Neutrons and protons are called nucleons because
they are in the nucleus.
– Just like any other chemical reaction, we use
symbols to show a nuclear reaction
• As an example, when uranium 238 emits an
alpha particle, it loses 2 protons and 2
neutrons.
U   Th  He
238
92
234
90
4
2
– Nuclear reactions must balance just like any
other chemical reaction, but we must also be
aware of balancing protons and neutrons
• The Nature of the Nucleus.
– Protons and neutrons are held together by a nuclear force
when they are very close together.
– The shell model of the nucleus visualizes the protons and
neutrons moving on energy levels or shells, much like the
electrons move in shells.
– We can predict the stability of a nucleus by using some
simple rules
• All isotopes heavier than atomic number 83 have an
unstable nucleus
• Isotopes with 2, 8, 20, 28, 50, 82, or 126 protons or
neutrons in their nucleus occur in the most stable
isotopes.
• Nuclei are the most stable with pairs of protons and
neutrons, so those with all protons and all neutrons
paired up are the most stable.
• Isotopes with an atomic number less that 83 are most
stable when the ratio of protons to neutrons is 1:1.
• The dots indicate
stable nuclei, which
group in a band of
stability according
to their neutron-toproton ratio. As the
size of nuclei
increases, so does
the neutron-toproton ratio that
represents stability.
Nuclei outside this
band of stability are
radioactive.
• Type of Radioactive Decay
– Alpha emission
4
• This is the expulsion of an alpha particle 2
– Beta Emission
• Emission of a beta particle
• a beta particle is an electron that is ejected form the
nucleus
– Gamma emission
• This is a high energy burst of electromagnetic
radiation
– Emission occurs as nuclei try to obtain a balance between
nuclear attractions, electromagnetic repulsions, and a low
quantum of nuclear shell energy.
He
• Unstable nuclei
undergo different
types of radioactive
decay to obtain a
more stable nucleus.
The type of decay
depends, in general,
on the neutron-toproton ratio, as
shown.
• Radioactive Decay Series
– Radioactive decay produces a simpler and more stable
nucleus.
– A radioactive decay series occurs as a nucleus
disintegrates and achieves a more stable nuclei
– There are 3 naturally occurring radioactive decay series.
• Thorium 232 ending in lead 208
• Uranium 235 ending in lead 207
• Uranium 238 ending in lead 206
• The radioactive decay series for uranium-238. This
is one of three naturally occurring series.
– When there are a large number of nuclei the ration of the
rate of nuclear decay per unit time to the total number of
radioactive nuclei will be a constant
• k=rate
•
n
• The radioactive decay constant is specific for each
isotope
– The rate of radioactive decay is expressed in terms of
half-life
• The half-life of an element is the time required from
one-half of its unstable nuclei to decay
• The half-life of an element is related to the ration of
0.693 to its radioactive decay constant
– t ½ = 0.693/k
• The decay constant for U238 is 4.87 X 10-18/s
• The half life is therefore
– t ½ = 0.693/4.87 X 10-18/s = 1.42 X 1017s = 4.5 X
109 years
• The half-life of U238 is 4.5 billion years.
• Radioactive decay of a hypothetical isotope with a half-life
of one day. The sample decays each day by one-half to some
other element. Actual half-lives may be in seconds, minutes,
or any time unit up to billions of years.
• The half-life of each step in the uranium-238
radioactive decay series.
• Measurement of Radiation
• Measurement Methods
– Film badges
• Workers who are exposed to radioactivity carry film
badges
• The film is exposed and the optical density of the film
shows the workers exposure levels during the time the
film badge was worn.
– Ionization counter.
• Measure ions that are produced by radiation
– Scintillation counter.
• Measures the flashes of light that occur when radiation
strikes a phosphor.
– Geiger counter
• Measures pulses of electrons released from the
ionization of gas molecules in a metal cylinder
• Each pulse of electrons is heard as a pop or click
• This is a beta-gamma probe, which can measure beta
and gamma radiation in millirems per unit of time.
• The working parts of a Geiger counter.
• Radiation Units
– Curie (Ci)
• Measurement of the activity of a radioactive source.
• Measured as the number of nuclear disintegrations per
unit of time
• A curie is 3.70 X 1010 nuclear disintegrations per
second.
– Rad
• Measures the amount of energy released by radiation
striking living tissue
• Short for radiation absorbed dose
• One rad releases 1 X 10-2 J/kg
– Rem
• Short for roetgen equivalent man
• This takes into account the possible biological damage
to humans of certain types of radiation.
• Radiation Exposure
– Background radiation is constantly present in our
environment.
• Most people are exposed to between 100 to 500
millirems per year.
• This background radiation comes from many natural
source.
– The harm that radiation does to living organisms is due to
the fact that it produces ionization which can:
• Disrupt chemical bonds in biological macromolecules
such as DNA
• Produce molecular fragments which can interfere with
enzyme action and essential cell functions.
– The linear model of exposure proposes that any
exposure above zero is damaging and can produce cancer
and genetic damage, mostly through its effect on DNA
– The threshold model proposes that there is a threshold
limit of exposure up to which the human body can repair
damage caused by the exposure
• It is not until we reach and exceed this threshold that
we begin to see irreversible damage.
• Graphic representation of the (A) threshold model and (B)
linear model of low-level radiation exposure. The threshold
model proposes that the human body can repair damage up
to a threshold. The linear model proposes that any radiation
exposure is damaging.
• Nuclear Energy
• Introduction.
– Albert Einstein showed us that energy and matter are the
same thing, both are inter-convertible.
• E=mc2
– Using mass losses during nuclear reactions, one can
calculate the energy change of a system.
• E=mc2
– There is a difference between the mass of the individual
nucleons that make up a nucleus and the actual mass of
the nucleus.
• This is called the mass defect of the nucleus.
• The mass defect occurs as energy is released when nucleons
join to form a nucleus.
– The energy that is released is called the binding energy.
• This is also the energy that is required to break the
nucleus into its individual protons and neutrons.
• The ratio of the binding energy to the nucleon number
is a measure of a nucleus’ stability
• Massive nuclei can gain stability by breaking into
smaller nuclei with a release of energy.
• Smaller nuclei can gain stability by joining together
with the release of energy.
• The maximum binding energy per nucleon occurs around
mass number 56, then decreases in both directions. As one
result, fission of massive nuclei and fusion of less massive
nuclei both release energy.
– Splitting massive nuclei apart with the release of energy
is called nuclear fission.
– The joining together of less massive nuclei with the
release of energy is called nuclear fusion
• Nuclear Fission
– As a nuclear reaction occurs, it has the ability to produce
a chain reaction
• A chain reaction is a reaction where the products are
able to produce more products in a self-sustaining
reaction series.
– In order to achieve a chain reaction there must be:
• A sufficient mass.
• A large concentration of fissionable nuclei
– The critical mass is when the mass and concentration are
high enough to sustain a chain reaction.
– A sub-critical mass is one that is too small to achieve a
chain reaction.
• The fission reaction occurring when a neutron is absorbed
by a uranium-235 nucleus. The deformed nucleus splits any
number of ways into lighter nuclei, releasing neutrons in the
process.
• A schematic
representation of a
chain reaction. Each
fissioned nucleus
releases neutrons,
which move out to
fission other nuclei.
The number of
neutrons can
increase quickly
with each series.
• Nuclear Power Plants
– The nuclear reactor contains the material and is the vessel
for the controlled chain reaction of fissionable materials
that will release the energy.
– Usually there is a fissionable enriched material made of
3% U235 and 97% U238 the is fabricated into small
beads.
– The beads are enclosed in a fuel rod.
– The fuel rods are locked in a fuel rod assembly by
locking collars and arranged so that pressurized water
can flow around the rods.
– Control rods are made of material that can absorb
neutrons and are inserted between the fuel rods.
– The rate of the chain reaction is controlled by raising or
lowering the control rod.
– In a pressurized water reactor the energy is carried way
from the reactor by water in a closed pipe called a
primary loop.
• This water acts as a coolant and also as a moderator to
slow neutrons so they can more readily be absorbed by
the U235 nuclei.
– The water from the primary loop is circulated to a heat
exchanger called a steam generator.
• The water in the heat exchanger moves through
hundreds of small loops and heats the feedwater in the
steam generator.
• The water then returns to the core to be heated again.
• A schematic representation of the basic parts of a nuclear
reactor. The largest commercial nuclear power plant reactors
are nine- to eleven-inch-thick steel vessels with a stainless
steel liner, standing about 40 feet high with a diameter of 16
feet. Such a reactor has four pumps, which move 440,000
gallons of water per minute through the primary loop.
• (A)These are uranium oxide fuel pellets that are stacked
inside fuel rods, which are then locked together in a fuel rod
assembly. (B) A fuel rod assembly.
• A schematic general system diagram of a
pressurized water nuclear power plant, not to scale.
The containment building is designed to withstand
an internal temperature of 300OF at a pressure of 60
lbs/in2 and still maintain its leak-tight integrity.
• Spent fuel rod assemblies are removed and new ones are
added to a reactor head during refueling. This shows an
initial fuel load to a reactor, which has the upper part
removed and set aside for the loading.
• The turbine deck of a nuclear generating station. There is
one large generator in line with four steam turbines in this
non-nuclear part of the plant. The large silver tanks are
separators that remove water from the steam after it has left
the high-pressure turbine and before it is recycled back into
the low-pressure turbines.
• The composition
of the nuclear fuel
in a fuel rod (A)
before and (B)
after use over a
three-year period
in a nuclear
reactor.
• Nuclear Fusion
– Nuclear fusion is the source of the energy from the Sun
and other stars.
– Fusion is a very desirable energy source as:
• Two isotopes of hydrogen (deuterium and tritium)
undergo fusion at a relatively low temperature.
• The supply of deuterium is unlimited with seawater
being a very large source
• Enormous amounts of energy are released with no
radioactive byproducts.
– The problems with utilizing fusion as an energy source
are:
• Temperature.
– The amount of energy required to bring two nuclei
together is enormous.
• Density
– The density of the reacting hydrogen nuclei must
be significantly high so that there are enough
reactions occurring in a short period of time.
• time
– These nuclei need to be confined to up to a second
or more at 10 atmospheres of pressure in order for
enough reactions to take place.
• A fusion reaction between a tritium nucleus and a
deuterium nucleus requires a certain temperature,
density, and time of containment to take place.
– Plasma.
• A very hot gas consisting of atoms that have been
stripped of their electrons and utilized as a confining
mechanism
– Inertial confinement
• An attempt to heat and compress small frozen pellets
of deuterium and tritium with energetic laser beams or
particle beams, producing fusion.
• The Source of Nuclear Energy
– When elements undergo the natural radioactive decay
process, energy is released and the decay products have
less energy than the reacting nucleus.
• When massive nuclei undergo fission, a great deal of
energy is Released