Download Principles of Technology

Principles of Technology
CH 16 NUCLEAR ENERGY 3
Name_______
KEY IDEAS
Nuclear reactions can by induced by bombarding nuclei with sub-atomic particles such as alpha particles. The
energies of the bombarding particles needed to produce a reaction are achieved by introducing the particles into
a particle accelerator such as a cyclotron. Only charged particles may be accelerated in this way.
When a neutron bombards certain heavy nuclei, a fission reaction results in which the nucleus splits into two
nuclei of similar size; this fission is accompanied by the release of large quantities of energy. The process,
known as nuclear fission, produces more neutrons, which can be used in turn to bombard other atoms, creating a
chain reaction. Fission reactors use this principle and harness the energy created to generate electricity.
Energy may also be produced by forcing light nuclei to combine, a process known as nuclear fusion. Stars use
fusion reactions to pro duce their immense energies. At present, physicists are experimenting with ways to
control fusion and to build fusion reactors as energy sources.
It is now known that the proton and the neutron are not fundamental particles. The existence of these nucleons
and other nuclear particles has been explained by the presence of six varieties of particles called quarks. The
electric charge on these particles is either —1/3 or +2/3 of the elementary charge.
KEY OBJECTIVES
At the conclusion of this chapter you will be able to:
• Explain how particle accelerators are used to induce nuclear reactions.
• Describe various types of induced nuclear reactions.
• Describe the process of nuclear fission and the reactions by which it produces vast quantities of energy.
• Describe the components of a fission reactor and the purposes for which each component is used.
• Describe the process of nuclear fusion, and compare it with nuclear fission.
• Describe how quarks are involved in nuclear structure.
16.10 INDUCED NUCLEAR REACTIONS
All of the nuclear reactions we have studied so far have been natural processes. We now turn our attention to
nuclear changes that have been produced artificially or induced. To induce a nuclear reaction, a target nucleus is
bombarded with a nuclear particle.
Particle Accelerators
A particle accelerator is a device that uses electric and magnetic fields to provide a charged bombarding nuclear
particle with sufficient kinetic energy to induce the desired nuclear reaction. As an analogy, consider a bullet
fired at a wall at a speed of 10 miles per hour. At this slow speed, the kinetic energy of the bullet would have
hardly any effect on the wall. If the bullet were fired at 600 miles per hour, however, its effect on the wall
would be devestating!
Examples of modem particle accelerators include the Van de Graaff accelerator, the linear accelerator, the
cyclotron, the synchrotron, and the large electron-positron (LEP) collider. These devices can supply
bombarding particles with kinetic energies ranging from 1 03 to 1012 electron-volts.
Artificial Transmutation
The first induced nuclear reactions used alpha particles (because of their large masses) as the bombarding
particles. In 1919, Rutherford bombarded nitrogen- 14 nuclei with alpha particles and noted that protons were
emitted. In this reaction:
nitrogen- 14 was artificially changed or transmuted into oxygen- 17. In 1932, English physicist Sir James
Chadwick bombarded beryllium-9 and identified a stream of uncharged particles that we now call neutrons. The
reaction is shown below:
In 1934, French physicists Frédéric Joliot-Curie and Irene Joliot-Curie bombarded aluminum-27 and produced
the first artificially radioactive isotope, phosphorus-30:
The phosphorus-30 undergoes positron decay (see pages 472—473) and forms silicon-30.
1. Which statement is false?
a. Nuclear reactions can be natural processes or produced artificially or induced. To induce a nuclear
reaction, a target nucleus is bombarded with a nuclear particle.
b. A particle accelerator is a device that uses electric and magnetic fields to provide a charged bombarding
nuclear particle with sufficient kinetic energy to induce the desired nuclear reaction.
c. Examples of modem particle accelerators include the Van de Graaff accelerator, the linear accelerator,
the cyclotron, the synchrotron, and the large electron-positron (LEP) collider. These devices can supply
bombarding particles with kinetic energies ranging from 1 03 to 1012 electron-volts.
d. Artificial Transmutation involves gamma particle drag.
Nuclear Fission and Fusion
In the bombardment reactions studied so far, only minor changes to the target nuclei occurred. We will now
discuss processes that cause major nuclear changes.
Referring to the graph of binding energy per nucleon on page 469, we see that the nuclides located in the left
and right portions of the graph have lower values than the nuclides located toward the center. The implication is
that very light and very heavy nuclei ought to be less stable than nuclei having moderate masses. We might
guess that the heavy elements could split, and the lighter elements could join, to form nuclei whose masses
would lie toward the middle of the binding-energy curve.
NUCLEAR FISSION
In the 1930s, Enrico Fermi, an Italian-American physicist, suggested that neutrons be used in bombardment
reactions because they are uncharged particles and therefore would not be repelled by the target nuclei. In 1938,
German physicists Otto Hahn and Fritz Strassman bombarded uranium atoms with neutrons and discovered that
some of the uranium atoms split into roughly two equal fragments. This reaction, known as nuclear fission, also
produced more neutrons and a large amount of energy.
One Type of Fission Reaction
There are many types of fission reactions; one type is shown below:
2. Which statement is false?
a. In the bombardment reactions studied so far, only minor changes to the target nuclei occurred. We will
now discuss processes that cause major nuclear changes.
b. Very light and very heavy nuclei ought to be less stable than nuclei having moderate masses. Heavy
elements could split, and the lighter elements could join, to form more stable nuclei.
c. Neutrons are always used in bombardment reactions because they are highly charged particles.
d. When uranium atoms are bombarded with neutrons some of the uranium atoms split into roughly two
equal fragments. This is nuclear fission and also produced more neutrons and a large amount of energy.
Fission Reactors
A number of physicists recognized that the neutrons released in nuclear fission could be used, in turn, to cause
other uranium-235 nuclei to fission, producing a chain reaction. If this chain could be controlled, the fission
reactions could provide a continuous source of energy. This controlled chain reaction is the principle behind the
fission reactor. (Uncontrolled chain reactions are produced in fission weapons such as the atomic bomb.)
Fission reactors are used in the production of electrical energy, as research tools and as a means of producing
other radioactive isotopes.
The primary system of a fission reactor has the following components:
• The containment vessel (concrete and steel) provides shielding for the reactor.
• The fuel rods, located in the core, serve as sources of energy. Uranium-233 and uranium-235 are used as fuels.
• The moderator, also located in the core, slows the neutrons so that they will be absorbed by the fuel nuclei.
These slow neutrons have kinetic energies that are close to the kinetic energies of air molecules at room
temperature. For this reason, they are known as thermal neutrons. Moderators are usually composed of water
(containing either hydrogen-1 or hydrogen-2), graphite or beryllium.
 The control rods in the core regulate the rate of fission by absorbing neutrons. Control rods are usually made
of cadmium or boron.
 The coolant (water or liquid sodium) removes thermal energy from the core.
 The heat exchanger receives the thermal energy and produces steam for the generation of electrical energy by
the secondary system (turbine) of the reactor.
The most common isotope of uranium, uranium-238, comprises about 9% of the naturally occurring element but
does not undergo fission. The isotope uranium-235 does undergo fission but it comprises less than 1% of
the naturally occurring element. For this reason, it is necessary to enrich uranium in order to use it as a
fissionable fuel.
When uranium-238 absorbs a neutron, it is converted into uranium-239, which then decays in two steps to form
plutonium-239:
The Pu-239 isotope of plutonium does fission, and it has been suggested that breeder reactors be constructed.
These reactors would be able to pro duce their own fuel. No breeder reactors have been operated in the United
States, however, because of the serious health and environmental hazards attributable to plutonium and also
because of the possibility that the plutonium might be used to produce nuclear weapons.
A number of problems are associated with any fission reactor. For example, the heat energy produced by the
reactor contributes to thermal pollution. There is also the serious problem of radioactive waste disposal. Solid
and liquid wastes are placed in corrosion-resistant containers and stored in isolated underground areas. Wastes
with low levels of radioactivity may be diluted until they are considered harmless and then released into the
environment. Gaseous wastes, such as krypton-85, nitrogen-16, and radon-222, are stored and allowed to decay
until it is considered safe to release them into the atmosphere.
3. Which statement is false?
a. The controlled fission chain reaction is the principle behind the fission reactor. Uncontrolled
fission chain reactions are produced in weapons such as the atomic bomb.
b. The primary system of a fission reactor has the following components: The containment
vessel, the fuel rods, the moderator, the control rods the coolant and the heat exchanger.
c. All uranium can be used as fissionable fuel.
d. Problems associated with fission reactor include pollution, radioactive waste and meltdown.
NUCLEAR FUSION
The joining of light nuclei to form heavier, more stable nuclei is known as nuclear fusion and is the process by
which stars, including our own Sun, produce their energy. In stars, fusion is a series of reactions that depend on
the temperature of the particular star. The net fusion reaction in our Sun is as follows:
where γ represents a gamma photon and v represents a neutrino.
For a fusion reaction to occur, very high temperatures are needed to give the positively charged nuclei the
kinetic energy they need to overcome their mutual repulsion. For this reason, all fusion devices are referred to
as thermonuclear devices.
For a given mass of fuel, nuclear fusion yields more energy than nuclear fission. For this reason, work is being
done to develop fusion reactors as a means of producing power. The three fusion reactions most likely to
succeed in a reactor are as follows:
At this time, no successful fusion reactor has been constructed. The problems to be overcome include the
production of a high ignition temperature, the packing of nuclei into a space small enough to allow a sufficient
number of collisions, the control of the fusion reaction once it has begun, and the development of materials that
can withstand the high operating temperatures and radiation levels of the reactor.
4. Which statement is false?
a. The joining of light nuclei to form heavier, more stable nuclei is known as nuclear fusion and is the
process by which stars, including our own Sun, produce their energy. In stars, fusion is a series of
reactions that depend on the temperature of the particular star.
b. For a fusion reaction to occur, very high temperatures are needed to give the positively charged nuclei
the kinetic energy to overcome their mutual repulsion (fusion devices are thermonuclear devices)
c. For a given mass of fuel, nuclear fusion yields more energy than nuclear fission. For this reason, work is
being done to develop fusion reactors as a means of producing power.
d. At this time, no successful fusion reactor has been constructed. The problems to be overcome include
the production of a high ignition temperature, the packing of nuclei into a space small enough to allow a
sufficient number of collisions, the control of the fusion reaction once it has begun, and the development
of materials that can withstand the high operating temperatures and radiation levels of the reactor.
e. Cold fusion, fusion without heat, is possible and is what powers the Sun and other cold, dark stars.
16.11 FUNDAMENTAL PARTICLES AND INTERACTIONS
The model of nuclear structure continues to evolve. Over the years, as physicists probed the nucleus, a host of
particles were discovered whose functions were largely unknown. In an attempt to explain the existence of these
particles, a number of theories were developed.
One of the most successful models is known as the standard model, which assumes that four fundamental
interactions (also known as fundamental forces) operate in the universe: electromagnetic, weak, strong, and
gravitational. The standard model describes the behavior and relationships among the first three of these
interactions. At present, attempts to incorporate gravitational force within the standard model have been
unsuccessful. (At this writing, experiments on a nuclear particle known as the muon suggest that the standard
model might need to be reformulated in order to explain the results of the experiments.)
An important feature of the standard model is the recognition that there are fundamental particles, known as
quarks, which have fractional elementary charges and are the building blocks of protons, neutrons, and other
nuclear particles. There are six varieties of quarks (and their antimatter counterparts), whimsically named up,
down, charm, strange, top, and bottom. The property with which the names of the quarks are associated is
known as flavor. (Who says scientists don’t have a sense of humor?) The fractional elementary charges of the
six quarks are shown in the table:
According to the standard model scheme, the proton has the structure uud and the neutron has the structure udd.
Using the table, we can calculate the charges on the proton and neutron:
Proton = uud = [ (+2/3e) + (+2/3 e) + (-1/3 e)] = e
Neutron = udd = [+2/3 e) + (-1/3 e) + (-1/3 e)] = 0
These results are in complete agreement with well-known scientific observations. The energies needed to
reduce protons, neutrons, and other nuclear particles into their constituent quarks are so high that these quarks
cannot be isolated as separate particles. Therefore, their existence has been demonstrated only by indirect
means. Whether quarks represent the ultimate structure of matter or whether there are even smaller subunits
continues to be the subject of research.
5. Which statement is false?
a. The standard model states that there are fundamental particles, known as quarks, which have fractional
elementary charges and are the building blocks of protons, neutrons, and other nuclear particles.
b. There are six varieties of quarks (and their antimatter counterparts) named up, down, charm, strange,
top, and bottom. The property with which the names of the quarks are associated is known as flavor.
c. The energies needed to reduce protons, neutrons, and other nuclear particles into their constituent quarks
are so high that these quarks cannot be isolated as separate particles. The existence of quarks has been
demonstrated only by indirect means.
d. Quarks were the ultimate structure of matter until smaller subunits called Force Meticlorians were discovered.
Elementary Particles:
The search for the origin of matter means the understanding of elementary particles. And with the advent of
holism, the understanding of elementary particles requires an understanding of not only their characteristics,
but how they interact and relate to other particles and forces of Nature, the field of physics called particle
physics. The study of particles is also a story of advanced technology begins with the search for the primary
constituent. More than 200 subatomic particles have been discovered so far, all detected in sophisticated
particle accelerators. However, most are not fundamental, most are composed of other, simpler particles. For
example, Rutherford showed that the atom was composed of a nucleus and orbiting electrons. Later physicists
showed that the nucleus was composed of neutrons and protons. More recent work has shown that protons and
neutrons are composed of quarks.
6. Which statement is false?
a. The origin of matter is unrelated to atomic or elementary particles.
b. More than 200 subatomic particles have been discovered so far, all detected in complex particle
accelerators. Most are not fundamental, but composed of other, simpler particles.
c. Rutherford showed that the atom was composed of a nucleus and orbiting electrons. Later physicists
showed that the nucleus was composed of neutrons and protons.
d. More recent work has shown that protons and neutrons are composed of quarks.
Quarks and Leptons:
The two most fundamental types of particles are quarks and leptons. The quarks and leptons are divided into 6
flavors corresponding to three generations of matter. Quarks (and antiquarks) have electric charges in units of
1/3 or 2/3's. Leptons have charges in units of 1 or 0.
Normal, everyday matter is of the first generation, so we can concentrate our investigation to up and down quarks,
the electron neutrino (often just called the neutrino) and electrons.
7. Which statement is false?
a. The two most fundamental types of particles are quarks and leptons. The quarks and leptons are divided
into 6 flavors corresponding to 3 generations of matter.
b. Every moment you come into contact with each type of quark, lepton, antiquark and antilepton
c. Quarks (and antiquarks) have electric charges in units of 1/3 or 2/3's. Leptons have charges in units of 1 or 0.
d. Normal matter is of the first generation, such as up and down quarks, the neutrino and electrons.
Anti-Matter and Anti-Particles
Note that for every quark or lepton there is a corresponding antiparticle. For example, there is an up antiquark,
an anti-electron (called a positron) and an anti-neutrino. Bosons do not have antiparticles since they are force
carriers (see fundamental forces).
8. Which statement is false?
a. Note that for every quark or lepton there is a corresponding antiparticle.
b. There is an up antiquark, an anti-electron (called a positron) and an anti-neutrino.
c. Bosons do not have antiparticles since they are force carriers (see fundamental forces).
d. Gravity has an anti force known as anti-gravity.
Baryons and Mesons:
Quarks combine to form the basic building blocks of matter, baryons and mesons. Baryons are made of three
quarks to form the protons and neutrons of atomic nuclei (and also anti-protons and anti-neutrons). Mesons,
made of quark pairs, are usually found in cosmic rays. Notice that the quarks all combine to make charges of 1, 0, or +1.
Thus, our current understanding of the structure of the atom is shown below, the atom contains a nucleus
surrounded by a cloud of negatively charged electrons. The nucleus is composed of neutral neutrons and
positively charged protons. The opposite charge of the electron and proton binds the atom together with
electromagnetic forces.
The protons and neutrons are composed of up and down quarks whose fractional charges (2/3 and -1/3)
combine to produce the 0 or +1 charge of the proton and neutron. The nucleus is bound together by the nuclear
strong force (that overcomes the electromagnetic repulsion of like-charged protons)
9. Which statement is false?
a. Quarks combine to form the basic building blocks of matter, baryons and mesons.
b. Baryons are made of three quarks to form the protons and neutrons of atomic nuclei (and also anti-protons and antineutrons). Mesons, made of quark pairs, are usually found in cosmic rays; quarks all combine to make charges of 1, 0, or +1.
c. The like charge of the electron and proton binds the atom together with gravitational forces.
d. The protons and neutrons are composed of up and down quarks. The nucleus is bound together by the nuclear
strong force (that overcomes the electromagnetic repulsion of like-charged protons)
Quark Confinement:
There can exist no free quarks, i.e. quarks by themselves. All quarks must be bound to another quark or
antiquark by the exchange of gluons. This is called quark confinement. The exchange of gluons produces a
color force field, referring to the assignment of color charge to quarks, similar to electric charge.
The color force field is unusual in that separating the quarks makes the force field stronger (unlike
electromagnetic or gravity forces which weaken with distance). Energy is needed to overcome the color force
field. That energy increases until a new quark or antiquark is formed (energy equals mass, E=mc2).
Two new quarks form and bind to the old quarks to make two new mesons. Thus, none of the quarks were at
anytime in isolation. Quarks always travel in pairs or triplets.
10. Which statement is false?
a. There can exist no free quarks, i.e. quarks by themselves. All quarks must be bound to another quark or
antiquark by the exchange of gluons. This is called quark confinement.
b. The exchange of gluons produces a color force field, referring to the assignment of color charge to
quarks, similar to electric charge. The color force field is unusual in that separating the quarks makes
the force field stronger (unlike electromagnetic or gravity forces which weaken with distance).
c. Energy is needed to form lone quarks which exist freely and do not interact with each other. That
energy increases until a new quark or antiquark is formed (energy equals mass, E=mc2).
d. Two new quarks form and bind to the old quarks to make two new mesons. Thus, none of the quarks
were at anytime in isolation. Quarks always travel in pairs or triplets.