Nuclear energy:

1
Nuclear energy In the transformation Thermal
energy -gt mechanical-gt electrical energy Which
dominates our energy economy, the sources of
thermal energy are currently fossil fuel burning
and nuclear fission of uranium or plutonium. In
order to discuss the latter we need to introduce
some elementary facts about atomic and nuclear
structure.
2
Structure of matter All matter normally
encountered consists of atoms. Atoms are about 1
hundred millionth of a centimeter in size. They
consist of a positively charged nucleus which is
about 20,000 times smaller in diameter than the
entire atom and which carries almost all
the mass of the atom. The rest of the atom is
electrons which are negatively charged and orbit
around the nucleus. All chemistry involves the
rearrangement of the electrons around the atoms,
causing binding or dissociation of atoms in
chemical reactions.
3
For example, fossil fuel burning involves This
electronic rearrangement during the reaction C
O2 -gt C O2 However in nuclear power plants,
the heat is being generated, not by
rearrangement of the electrons but by
rearrangement of the structure of the nuclei of
the atoms in the fuel. To understand this you
need to understand a Little about the structure
of nuclei in atoms.
4
Since I have told you that I want to understand
some of the simple chemistry, for example in fuel
cells, we will digress to review the rules
which chemical reactions must obey Conservation
of charge The total charge on each side of each
reaction must be the same. Often this total
charge is zero. For example in the fuel cell
reaction H2 -gt 2H 2e- The total charge on
the left is zero and the total charge on the
right is also 2e-2e0 because the
charge of the hydrogen ion is e
5
Chemical reactions, continued This conservation
of charge law is also obeyed by nuclear
reactions, which we will discuss later. Chemical
reactions also must have the same number of each
chemical species on each side of the reaction,
though the charge associated with each of those
species can change (but not the total
charge.) For example in the reaction H2 -gt 2H
2e- the number of Hs on each side is the
same. (The subscript 2 on H2 means that the
two hydrogen atoms are combined into a molecule.)
6

• Which of the following chemical reactions is NOT
• possible?
• C O2 -gt CO2
• O2 4e- 4H -gt 2H2O
• Cu HCl -gt Cu H e-Cl-
• PbO2 4H 2e- SO42- -gt Pb SO4 2H2O

7
Nuclei in atoms have two constituents, called
protons and neutrons. Neutrons and protons have
approximately the same mass. Protons have
positive electrical charge, exactly equal in
magnitude to the negative charge of
electrons. Neutrons have zero electrical charge.
(Neutrons by themselves are not stable. An
isolated neutron decays into a proton and an
electron and an almost massless particle called
an antineutrinon a little less than15 minutes. )
8
The number of protons in a nucleus,
usually denoted by the letter Z is called the
atomic number. It determines the number of
electrons in the corresponding neutral atom as
well, Because the electrons determine the
chemical behavior, Z also determines how the
atom will behave chemically. Atoms also have
names, which are the names of the chemical
elements. There are known atoms for Z between 1
(hydrogen) and 113 (ununtrium, discovered 2004,
only lives ½ second) In addition to Z and a name,
there is also a chemical symbol For each
element. In all cases except hydrogen, there are
also neutrons in the nucleus. The total number
of neutrons plus protons is called A, the atomic
mass number, so the number of neutrons is A-Z.
9
Notation for nuclei

Mass number
A
X
Chemical symbol
Z
Atomic number
The atomic number Z and the chemical symbol X
contain the same information. Sometimes Z is
omitted in the symbol.
10
Examples
12
C
Atoms with these 2 nuclei behave
identically chemically but have different
numbers of neutrons and therefore different
masses. We say they are different Isotopes of
carbon (C).
6
14
C
6
11
12
14
C
C
6
6

• How many neutrons are in these two nuclei
• 12 in the first and 14 in the second.
• 6 in the first and 8 in the second
• 6 in both
• 8 in both

12
Ways to display the atoms and nuclei For
chemical purposes The periodic table. This
lists the elements in order of their Z
values. It does not list the A values, but
shows the average A value appearing in nature
for that Z. For example, in nature one finds
that 76 of nuclei with Z17 (Cl or chlorine)
the A35 and for 24 of the atoms with Z17,
the A37 so the listed value of atomic mass
in the periodic table is .76 x 35 .24 x 37
35.5 For understanding nuclear energy Chart
of the nuclei
13
Fig. 13-14a, p. 449
14
http//133.53.31.105/CN04/index.html
15
Nuclear forces In atoms and the nuclei at their
centers, electromagnetic forces are still
operating (as are gravitational forces, but on
atomic and nuclear scales they have a small
effect). You can see that electrical forces can
hold the electrons near the nuclei because the
electrons are negatively charged and the nuclei
are positively charged and opposite
charges charges attract . However for these
electromagnetic forces to hold an atom together,
the positively charged protons would have to be
interspersed between the electrons in the atom.
16
A model like this was proposed in the early
20th century but was shown to be incompatible
with experiment by experiments by Geiger,
Marsden and Rutherford which established that the
nucleus is very small compared to the size of the
atom. TO HOLD THIS SMALL NUCLEUS TOGETHER SOME
OTHER FORCE MUST EXIST WHICH IS NOT GRAVITATIONAL
OR ELECTROMAGNETIC. This is the nuclear force.
It turns out that there are actually two kinds of
nuclear force, called strong and weak. The
strong force is the ultimate source of the energy
which powers nuclear weapons and nuclear power
plants.
17
Some properties of the nuclear force. It only
acts over extremely small distances (roughly
10-15 of a meter) It is attractive so it holds
the nucleus together. It acts on protons and
neutrons approximately equally. The strong
component of the force does not act on
electrons. Where it acts (at short distances) it
can be much larger than electrical forces.
18
Fig. 13-3, p. 430
19

• Which of the following would NOT be true if only
• electromagnetic forces were acting between
• the protons and neutrons of the nucleus.
• The nucleus could not stay together unless it
• were nearly as big as the whole atom.
• b. The neutrons could not be held in the atom.
• The number of protons would have to be less than
• the number of electrons.
• Chemistry would be different because the
  electrons
• would be arranged differently in the atoms.

20
Nuclear reactions In most chemistry, the nuclei
in the atoms are stable during the reactions.
That is the nuclei dont change. However many
nuclei are unstable which usually means that
over time, they lose some of their neutrons or
protons (though neutrons and protons can be
gained as well). Instability of nuclei was first
discovered in the late nineteenth century. Many
of the unstable nuclei are nuclei of atoms high
in the periodic table. The products of the
disintegration of the nucleus have very large
kinetic energy, This is basically because the
potential energy associated with the strong
nuclear forces is very large.
21
Modes of nuclear decay Alpha (a ) particle
decay An alpha particle Is the nucleus of a
helium atom, denoted 4He

2
having charge 2e . Many nuclei decay by
emitting an alpha particle , for example
238 234 4

U -gt Th He 92
90 2
22
Modes of nuclear decay (II) Beta (ß) decay A
beta particle is an electron. For example the
thorium isotope of the last example decays by
emission of an Electron 234 Th -gt
234 Pa ß- antineutrino 90
91 -1 The antineutrino
is not mentioned in your book And is not of much
practical importance. Antineutrinos Have very
little mass but can carry a lot of energy.

23
Nuclear decay (III) Gamma (?) decay Gamma
rays are very high frequency electromagnetic
waves. Many nuclear decays are accompanied
by gamma rays. They do not change the A or Z of
a nucleus but they do carry away energy.
24
Fission High Z nuclei can fall into two
roughly equal pieces if struck by a neutron as
in n 235U -gt 93Kr 141Ba 2n
92 36 56
In this example, the extra neutrons coming out
lead to the possibility of a CHAIN REACTION in
which the neutrons coming out cause more decays
if more of the uranium is around. This is what
happens in a nuclear reactor (and in one type of
nuclear bomb.)
25
Rates of nuclear reactions. One could try to
characterize the rate of a nuclear decay by
dividing the amount of product by the time (which
would give a rate). However, as time goes on the
reaction goes slower and slower if measured this
way. A better way is to measure the time it
takes for ½ of the original nucleus to decay
away. This is called the ½ life and characterizes
the rate of the nuclear reaction. Any particular
nucleus can decay at any time, but if, as usual,
there are a lot of them, then ½ of them will be
gone after one ½ life. The other half will
continue decaying until, after another ½ life, ¾
are gone and so forth.
26
In the reaction 27Al 4 He -gt 30 P
n
13 2 15
What symbols should be added, for completeness ,
to the n ? a. 0n b. 1n c. 0n d. 1n
0
0
1
1
27
Quantities which are balanced in nuclear
reactions Charge (as in chemical
reactions) Total mass number A (but not total
mass) Notice that, unlike chemical reactions,
the numbers of nuclei of a given chemical
species are not the same on the two sides of a
nuclear reaction, though they are the same in a
chemical reaction.
28
Energy conservation in nuclear reactions Energy
is conserved in nuclear reactions The energy is
stored in the nucleus in the form of potential
energy associated with the nuclear forces. When
a nucleus decays, the amount of this nuclear
potential energy is reduced and the energy shows
up as the kinetic energy of the products of the
reaction. This kinetic energy is the origin of
the heat which makes steam in a nuclear power
plant and of the destructive power of nuclear
weapons.
29

• A new feature is that the MASS of a nucleus
• is a measure of the amount of potential energy
• stored in it. (Actually this is true of other
  systems
• we have studied also, but the effect is larger
  for
• nuclei because the nuclear forces are so big.)
• The relation is the famous one due to Einstein
• Change in kinetic and electromagnetic energy
• between the right and left sides of a nuclear
  reaction
• (Mass of the left hand reactants
• Mass of the right hand products) x c2

30
Units for designating nuclear masses One atomic
mass unit (Mass of 12C) /12 1.66053886
10-27 kilograms
6
Because of the equivalence of mass and
energy mentioned above, the masses of nuclei
are only approximately A amu. The difference Is
due to the potential energy stored in the nuclear
forces between the neutrons and protons in the
nucleus.
31
In the reaction 1H 7Li -gt 4He 4 He
1 3 2 2

• The masses on the left(reactants) add up to
  8.02387051207 amu
• The masses on the right(products) add up to
  8.0052065083amu
• Which statement is true of the kinetic energies
  (KE)?
• The KE of products will be greater than that of
  reactants.
• b. The KE of reactants will be greater than that
  of products
• c. The KE of products and reactants will be the
  same.
• d. It depends on the initial KE of the reactants.

32
Fusion and the origin of the suns energy There
is very strong evidence that the origin of the
electromagnetic energy emitted by the Sun is a
series of nuclear reactions taking place In the
suns interior. These are typically
fusion reactions such as 1H 1H -gt 2H
ß neutrino
1 1 1
1H 2H -gt 3He ?
1 1 2
3He 3He -gt 4He 2 1H
2 2 2 1
33
Regrettably, similar fusion reactions have been
caused to occur in so called thermonuclear
weapons, which are the most destructive kind of
nuclear bombs. The US and Russia are the only
nations known to have fusion nuclear weapons.
About 20,000 such bombs are believed to
exist, each with destructive power much greater
than that of the bombs used by the US on
Hiroshima and Nagasaki in World War II. Despite
50 years of effort, it has not been possible
to produce controlled fusion reactions for use
in a power plant, though such efforts continue.
34
Energy units for nuclear reactions. The
energies associated with atomic and nuclear (per
nucleus of atom) are typically very Small and
become significant because one is Dealing with a
lot of atoms or nuclei. For this reason the
electron volt is often used To describe the
energies. One electron volt Is the energy
gained by an electron if it passes Through a
source of emf of 1 volt. Because the Electrons
charge is 1.6 x 10-19 coulombs we Have 1
electron-volt 1.6 x 10-19 joules
35
In chemical reactions, the energies exchanged Are
typically a few electron volts In nuclear
reactions, the energies exchanged Are typically a
few Million electron volts.