Module 1: Introduction to Nuclear Physics
Prepared By:
Samir Hamasha
NUCLEAR ENERGY FUNDAMENTALS
5. Radioactive Decay
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The nuclei of some isotopes are unstable.
They emit particles or release energy to become stable.
This process is called radioactive decay.
After radioactive decay, the element changes into a
different isotope of the same element or into entirely
different element.
 The released energy and matter are collectively called
nuclear radiation.
5. Radioactive Decay
 Radiation can refer to light or energy transfer.
 Nuclear radiation will be used to describe
radiation associated with nuclear change.
 There are four different types of nuclear
radiation
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Alpha particles.
Beta particles
Gamma rays
Neutrons.
5. Radioactive Decay
Radiation Type
Symbol
Mass (kg)
Charge
Alpha particle
α, 42He
6.646 x 10-27
+2
9.109 x 10-31
–1, (+1*)
Beta particle
Gamma ray
γ
none
0
Neutron
n
1.675 x 10-27
0
* Beta particles are often fast electrons but may also be positively charged
particles called positrons which have the same mass as electrons.
5. Radioactive Decay
 The following example gives an idea about how a
nuclear radiation occurs.
 Example:
Uranium-230 nuclei emit alpha radiation and
become nuclei of thorium-226:
230
92U
226
90Th
→
4
2He
+
which can also be written as:
230
U
→
226
Th
+
α
 Remember that an alpha particle is identical to a
helium nucleus. Notice that the mass number goes
down by 4 and the atomic number goes down by 2.
5. Radioactive Decay
Fig. Types and penetration power of radiation.
6. Radiation Sources
The main natural radiation sources are
 Cosmic radiation from space,
 Radiation from the rocks and soils around us,
 Radon gas which comes from the natural decay
of uranium,
 The radioactive materials in our bodies, mainly
from foods we eat.
The sum of the exposures from these sources is
called background.
6. Radiation Sources
Moreover, human activity has added to
radiation by creating and using artificial sources
of radiation. These include :
 Radioactive waste from nuclear power
stations.
 Radioactive fallout from nuclear weapons
testing and ,
 Medical X-rays.
6. Radiation Sources
 Fig. 1.12 shows the contribution of different sources to the
background radiation.
 Fig. 1.12: Sources of radiation and their proportion.
 Artificial sources account for about 15 per cent of the average
background radiation dose.
 Nearly all artificial background radiation comes from medical
procedures such as receiving X-rays for X-ray photographs.
7. Radioactive Decay Rates
 If you were asked to determine the age of a rock,
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you would probably not be able to do so.
After all, old rocks do not look much different
from new ones.
One way to find the age of involves radioactive
decay.
It is impossible to predict the moment when any
particular nucleus will decay, but it is possible to
predict the time required for half of the nuclei in a
given radioactive sample to decay.
The time in which half of a radioactive substance
decays is called the substance’s half life.
7. Radioactive Decay Rates
 radioactive decay of carbon-14.
7. Radioactive Decay Rates
 Different substances have different half-lives
as indicated Table 1.3 below.
Isotope
Half-life
Nuclear radiation
emitted
Thorium-219
1.05 x 10-6 seconds
α
Radon-222
3.82 days
α, γ
Carbon-14
5730 years
β
Uranium-235
7.04 x 108 years
α, γ
Uranium-238
4.47 x 109 years
α, γ
8. Nuclear Fission
 Nuclear power stations use the heat released by
nuclear reactions to boil water to make steam.
 The type of nuclear reaction used is called nuclear
fission.
 In nuclear fission the uranium nucleus is
bombarded by a neutron that makes a large and
unstable atom.
 Due to that the uranium nucleus splits. Atoms of
two different elements are created along with more
neutrons. These neutrons can then collide with
more uranium nuclei.
These processes are repeated continuously, forming a chain
reaction.
8. Nuclear Fission
 In 1938 Hahn and Strassman
in their experiment
observed that when
bombarding uranium-235
with neutrons as shown in
Fig. 1.14, the set of products
includes two lighter nuclei,
barium-141 and krypton-92,
together with neutrons and
energy.
 This is nothing but one of
the examples of fission of
uranium-235.
 It does not always split into
Barium and Krypton but
usually into two fragments
with almost equal masses.
8.1 The strong nuclear force
 The nuclear force is the force between two or
more nucleons (both proton and neutron).
 It is responsible for binding of protons and
neutrons into atomic nuclei.
 The energy released causes the masses of
nuclei to be less than the total mass of the
protons and neutrons which form them.
 The difference in mass is converted to
energy.
8.2 Rate of energy released
 Due to the nuclear force, the energy released in
nuclear fission is far greater than the energy
released in a chemical reaction, such as burning fuel.
 This means that the power output of a nuclear
power station is large.
 The reaction also releases large amounts of energy
 Each dividing nucleus releases about 3.2 x 10-11 J of
energy
 The chemical reaction of one molecule of the
explosive (TNT) releases 4.8 x 10-18 J.
 In other words one nucleus undergoing fission
releases approximately the same amount of energy
as 6.7 millions TNT molecules do when they explode.
8.2 Rate of energy released
 Theory of relativity:(Albert Einstein presented in 1905 ).
 Energy = mass x (speed of light)2
E = mc2
 This equivalence means that matter can be converted
into energy, and energy into matter
 The constant, c, is equal to 3 x 108 m/s.
 The mass equivalent energy of 1 kg of matter is 9 x 1016 J,
which is more than the chemical energy of 22 million
tons of TNT.
 The huge difference in the amount of energy release is
due to the fact that in fission the mass is converted to
energy.
9. The Nuclear Reactor
9. The Nuclear Reactor
 The nuclear fuel is held in
metal containers called fuel
rods. These are lowered into
the reactor core.
 A coolant, usually water or
carbon dioxide, is circulated
through the reactor core to
remove the heat.
 Control rods are also
lowered into the core. These
absorb neutrons and control
the rate of the chain
reaction. They are raised to
speed it up, or lowered to
slow it down..
9. The Nuclear Reactor
 Uranium or plutonium
isotopes are normally
used as the fuel in
nuclear reactors,
because their atoms
have relatively large
nuclei that are easy to
split, especially when
hit by neutrons.
 Fig. 1.15 shows an
outline of a nuclear
reactor.
10. Nuclear Fission - Advantages and
Disadvantages
Advantages:
• No carbon dioxide is produced when the station is
operating, as stated earlier.
• There is a high power output.
• A small amount of fuel is needed, when compared with
coal or gas.
Disadvantages:
• Hazardous radioactive waste is produced.
• Building the power stations is quite expensive.
• Taking apart the power stations at the end of their
lifetime is very costly.
11. Nuclear Fusion
 Nuclear fusion involves two
atomic nuclei joining to make a
large nucleus.
 Energy is released when this
happens.
 The Sun and other stars use
nuclear fusion to release
energy.
Fig. 1.16: Nuclear fusion.
 The sequence of nuclear fusion
reactions in a star is complex,
but overall hydrogen nuclei join
to form helium nuclei
11. Nuclear Fusion
 Some scientists estimate that 1 kg of hydrogen in a fusion reactor
could release as much energy as 16 million kg of burning coal.
 The fusion reaction itself releases very little waste or pollution.
 Scientists are conducting many experiments in the United States,
Japan and Europe to learn how people can exploit fusion to create a
clean source of power that uses fuels extracted from ordinary water.
 ITER ( International Thermonuclear Experimental Reactor )
experimental nuclear fusion research reactor.
Practical fusion-based power
illustrated by the concept
drawing in Fig. 1.17 is far from
being a reality.
12. Nuclear Fusion – Advantages and Disadvantages
Advantage s:
 The fuel used for fusion (hydrogen) is very abundant .
 Earth’s oceans could provide enough hydrogen to meet current world
energy demands for millions of years.
Disadvantages:
 Fusion reactions can produce fast neutrons, a highly energetic and
dangerous form of nuclear reaction.
 This requires replacing the shielding material periodically which makes
the operation of the fusion power plant expensive and impractical.
 Lithium can be used to slowdown these neutrons, but lithium is
chemically reactive and rare so its use is not practical.
Research on nuclear fusion is still developing and successful experiments are
just beginning. The world is still waiting for the perfect fuel to come.