Download nuclear fission student handout ppf200.03.ho.01

NUCLEAR FISSION
STUDENT HANDOUT
PPF200.03.HO.01
PPF200.03.HO.01 Rev. 1
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INTRODUCTION
The fission process is the foundation upon which the entire nuclear power industry is based. It is
a wonder of nature when certain nuclei absorb neutrons and then split releasing large quantities
of energy. Also, because fission itself yields additional neutrons to carry on the process with
other nuclei, this permits the design of a self-sustaining nuclear Reactor. In this section, we will
describe some of the basic properties of an atom, the forces that hold an atom together, and the
fission process and resulting products.
ATOMIC STRUCTURE
To provide a foundation for understanding the fission process we must first discuss the atom and
its structure. Atom comes from the Greek word meaning indivisible. An atom is the smallest
particle of any element that can be identified as an element. Atoms are made up of negativelycharged electrons and a nucleus which contains protons with a positive charge and neutrons
with no charge. However, the atom is almost completely defined by its protons and neutrons
because of their mass. The masses of a neutron and a proton are about the same (≈1.67 E-27
Kg). The mass of an electron is about 2000 times less.
ATOMIC NOTATION
A notation format is used to easily express identifying information about an atom. It is seen in
one of the following formats:
A
Z
XN , or
A
Z
X , or
Z
XA
WHERE:
X = Chemical Symbol for an element
Z = Atomic Number (Number of Protons)
A = Atomic Mass Number (# of Protons plus Neutrons)
N = Number of Neutrons (not often used)
TERMS
Described below are several terms you will encounter in your study of nuclear and reactor theory.
•
Nuclide =
A species of atom characterized by the constitution of its nucleus. For
example, Uranium-235, can be written as:
235
92
U143
or
235
92
U
or
92
U 235 or
U-235
This indicates that it contains 92 protons, 143 neutrons, and 235 total
protons plus neutrons.
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•
Nucleon = a constituent particle of an atomic nucleus, that is, a proton or a neutron.
For example, referring to 235
92 U , it is correct to state that U-235 contains a
total of 235 nucleons. The number of nucleons is the same as the atomic
mass number.
•
Isotope =
one of two or more nuclides with the same atomic number, Z, but with
different atomic mass numbers (A). A chemical element may have several
isotopes but will still be the same chemical element. An equivalent
statement is that the nuclei of isotopes have the same number of protons, but
different number of neutrons. For example,
92
U 233 ,
92
U 235 ,
92
U 238
are all isotopes of the element Uranium. The left subscripts indicate the
same atomic number, 92. However, the nucleus of these isotopes contains
different neutron numbers, 141, 143, and 146, respectively. This makes
their atomic mass numbers, A, different. Isotopes of the same element have
identical chemical properties but have different nuclear properties.
FORCES AT WORK IN THE ATOM
The atom is so small it cannot be seen even with very powerful microscopes. As small as a
single atom is, it consists of even smaller particles which give rise to the nuclear properties of the
atom. In the case of the atom, several forces are at work – Gravitational, Electrostatic, and
Nuclear forces.
Similar to the gravitational forces acting between the sun and the planets in our solar system, the
electrons and proton/neutrons exhibit gravitational attraction. Newton postulated that the
gravitational force is directly proportional to the product of the mass of the bodies and indirectly
proportional to the square of the distance between the bodies.
F=G
m1m 2
, G is the universal gravitational constant
d2
The mass of a proton or neutron is approximately 1.67 E-27 kg and an electrons mass is
approximately 9.1 E-31 kg. The mass of the electrons and the nucleus are very much smaller
than the overall distance of the electron's orbit around the nucleus. Thus, there is another
similarity between the small world of the atom and our solar system. If an atom were enlarged
such that the nucleus would be about the size of a softball, the outer electrons would be about the
size of a marble and would be in orbit about one-half mile or 880 yards away. As you can see,
the atom is almost all empty space. The size and weight of an atom are very small so that a large
number of atoms exist in small quantities of material. One cubic inch of water, weighing a little
more than one-half ounce, contains about 5.5 x 1023 atoms.
The gravitational force between a nucleus and an electron is relatively weak because of the small
masses and relatively large distance between them.
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Another force existing between the atomic particles is the Electrostatic force of charged
particles, commonly called Coulombic forces. The orbiting electron has a negative charge and
the proton, in the nucleus, has an equal positive charge. Since "like" charges repel and "unlike"
charges attract, the electrons want to be drawn in to the proton. However, the electrons are held
in discrete orbital positions by a balance between the inward electrostatic attraction and the
outward centrifugal forces. The Coulombic forces in an atom are much greater than the
gravitational forces.
The force holding the nucleus of an atom together is called the strong nuclear force. This force
acts equally between a neutron to neutron, neutron to proton, or proton to proton. The strong
nuclear force is greater than the coulombic repulsion force between the protons. The nuclear
force, however, has a very short range (≈ 2.0 E-13 cm). The diameter of a nucleus is ≈1 E-12 cm.
Above mass numbers of ≈ 65, the average nuclear force begins to weaken and above mass
numbers of 250 is not strong enough to overcome the coulombic forces. Thus, the absence of
extremely massive nuclides. Fission occurs when the coulombic repulsion force in the nucleus
overcomes the short range nuclear force.
FISSION PROCESS
In explaining the fission process, it is convenient to compare it to the splitting of a liquid drop,
such as a drop of water. A drop of water is held together by surface tension. This is comparable
to the nuclear forces that hold together the protons and neutrons in the nucleus. If the drop of
water is struck by some particle, such as another drop of water, it will begin to vibrate. If enough
energy has been absorbed by the drop of water, the vibration will cause the drop to neck down
into a dumbbell shape and split into two parts.
In the Reactor, there are two types of fission events occurring:
•
Spontaneous fissions
•
Neutron induced fissions
Spontaneous fission occurs when a nuclide splits without any outside help. Although some
nuclides or atoms will spontaneously fission, the majority of the fission events occurring in the
reactor are neutron induced fissions.
Neutron induced fission will occur when the absorption of a neutron results in a new excited
nucleus and the coulombic forces overcome the nuclear force. Therefore, the fission is induced
by the absorbed neutron. The minimum amount of energy required to cause a nuclide to fission
is defined as the critical or threshold energy.
When a U-235 nucleus absorbs a neutron, the resulting U-236 nucleus is excited and begins to
vibrate. The excited U-236 nucleus could rid itself of this extra energy by emitting a gamma ray,
but, in most cases, it will eventually neck down into a dumbbell shape. Since both ends of the
dumbbell-shaped nucleus contain protons (positive charges), the two ends begin to repel each
other.
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As the dumbbell shape is formed, the nuclear forces holding the nucleus together weaken due to
their short range. At some point, the coulombic repulsion forces overcome the nuclear forces and
the nucleus of the atom splits or fissions into two parts called fission fragments. Since these two
parts are both positively charged, they will be pushed apart.
Each fission of a Uranium-235 atom typically produces two fission fragments, 2.5 neutrons on
average, and radiation energy.
FISSION ENERGY RELEASE AND DISTRIBUTION
The purpose of the Reactor in the power plant is simply to heat primary water. The energy
released in the fission process is the energy used to heat the water. The heat from the primary
water is transferred to the secondary water to produce steam. This steam turns a turbine which is
connected to an electrical generator which produces electricity.
The products of the fission process all possess energy. The products lose their energy as they
interact with matter. As an example, a neutron collides with an atom imparting some of its
kinetic energy to the atom. This causes the atom to vibrate faster (get hotter). The energy
released in the fission process is transferred to the reactor coolant (typically water) that flows past
the fuel rods.
SUMMARY
In this lesson, we have discussed the basic planetary model of the atom, each of its main parts
and their strengths, and the fission process with its resulting products. The next section will
discuss the neutron cycle and the factors required to maintain a constant neutron population.