FAMILIARISATION WITH NUCLEAR TECHNOLOGY

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FAMILIARISATION WITH NUCLEAR TECHNOLOGY

• RADIOACTIVITY AND FISSION
• Peter D. Wilson

DURATION ABOUT 40 MINUTES
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COMPONENTS OF AN ATOM
Atom of lithium-7 (conventional representation)
Not to scale - the electrons would be better
regarded as a cloud of negative charge occupying
a volume around 1/100,000,000 cm across, i.e.
some 10,000 times the diameter of the nucleus
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ENERGY RELEASED BY FISSION
Packing protons and neutrons into a nucleus
involves some gain or loss in mass per nucleon, a
quantity represented by the packing fraction. For
fission products it is lower than in the parent
nucleus. Fission of heavy elements thus releases
some surplus mass as energy.
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MORE ABOUT ATOMIC COMPONENTS
The central nucleus contains practically all the
mass Electrons occupy practically all the
volume The simplest nucleus (of hydrogen)
consists of a single proton Other nuclei have
more protons held together by a similar or larger
number of neutrons In a neutral atom the positive
protons are matched by negative electrons, each
with unit charge Chemistry depends on the
behaviour of electrons The number of protons (the
atomic number) therefore determines the chemical
identity of the atom
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PERIODIC TABLE
SHOWING CHEMICAL ELEMENTS BY ATOMIC NUMBER AND
CHEMICAL SYMBOL
Elements in vertical columns have more or less
similar chemistry
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ISOTOPES
The number of protons determines chemical
identity Neutrons provide the remaining nuclear
mass but may vary somewhat in number without
other effect on chemistry Forms of an element
with different numbers of neutrons are called
isotopes, distinguished by mass number (sum of
protons neutrons) e.g uranium-235,
uranium-238 Isotopes have identical chemistry
(except for usually trivial effects of mass) but
different nuclear properties Not all nuclear
combinations are stable - many decay
spontaneously and are radioactive Specific
combinations of protons and neutrons are
generically called nuclides, and if unstable
radionuclides
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COMMON DECAY REACTIONS
UNSTABLE NUCLEI
Alpha (?) decay and fission are confined to the
heaviest elements beta (?) and gamma (?) decay
may occur in any element.
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INTERNAL EFFECTS
COMMON NUCLEAR REACTIONS
Thus all but simple ? -emission result in a
change of chemical identity
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NUCLEAR RADIATION TYPES
EXTERNAL EFFECTS
A high dose of radiation to the body over a short
time is likely to cause illness or death A low
dose (comparable with natural levels) may or may
not have adverse effects they cannot be
identified against the natural background and the
likelihood is subject to dispute
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LOW-LEVEL RADIATIONRISKS OF HARMFUL EFFECTS
(illustrative, not to scale)
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SOURCES OF RADIATION EXPOSURE (UK PROPORTIONS,
2005)
Radon, internal (e.g. K-40), terrestrial and
cosmic contributions are natural
Average proportions levels vary widely from
place to place
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NUCLEAR DECAY CHARACTERISTICS
Decay of radioactive nuclei is a matter of chance
and can be predicted only statistically A
characteristic proportion of those present decays
in any unit of time The time taken for half to
decay is the half-life (unalterable for any given
radionuclide) The longer the half-life, the
feebler the radioactivity The pattern of decay is
identical for all pure radionuclides but with
different time scales, so for a mixture can be
complex
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NEUTRON ABSORPTION EFFECTS
Any nucleus can absorb a free neutron -
likelihood varies enormously Likelihood and
consequence varies widely according to neutron
energy and nuclear composition exchanging a
neutron in the nucleus for a proton can make an
enormous difference Absorption can have one of 4
possible consequences (not necessarily
immediate)-

• Excitation
• Emission
• Conversion
• Fission

then loss of energy as gamma-rays but with no
further change of one or more neutrons of a
neutron to a proton with emission of a
beta-particle - transmutation to next higher
element in the Periodic Table (may be
repeated) splitting into two major parts plus
some free neutrons.
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NEUTRON ABSORPTION TERMS
The probability that a nucleus will interact with
unit flux of neutrons (1 neutron per sq. cm. per
second) in one second has the dimensions of
area. It may be imagined as the area presented by
the nucleus to the neutron flow and is known as
the cross-section. The unit of cross-section is
the barn (from the expression, as easy as
hitting a barn door)1 barn
1/1,000,000,000,000,000,000,000,000 sq. cm (10-24
cm2). Neutron density in a given space is
inversely proportional to speed. Absorption is in
general therefore likelier with slow than fast
neutrons. After absorption, fission is likelier
the higher the energy. Absorption is especially
likely at resonance energies.
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RESONANCE SPECTRUM
Fission probability rises above 1/V trend with
increasing energy
1/V trend
Fission cross-section of uranium-233 (typical -
no significance in choice)
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ACTIVATION AND TRANSMUTATION
Activation n ?-?
e.g. Co-59 ? Co-60 ? Ni-60 5.3
years Transmutation n ?-?
?-? e.g. U-238 ? U-239 ? Np-239 ?
Pu-239 24 min 2.4
days Activation and transmutation involve the
same processes the difference lies in the
time-scale and point of interest. In activation a
stable nucleus is made radioactive, usually with
a fairly long half-life (days to years) In
transmutation a new element is formed more or
less quickly (seconds to days)
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EXAMPLES OF TRANSMUTATION
Thus fissile Pu-239 is generated from
non-fissile U-238
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PROPERTIES RELATED TO FISSION
Once a neutron is absorbed, the likelihood of
fission rather than other effects increases with
neutron energy. Neutrons with energy matching
their surroundings are thermal. Neutrons as
released by fission are fast. Neutrons rather
faster than thermal are epithermal. Nuclei that
can undergo fission with thermal neutrons (e.g.
uranium-235) are fissile. Nuclei that undergo
fission only with fast neutrons (e.g.
uranium-238) are fissionable. U-238, not itself
fissile, is converted by neutron absorption to
fissile Pu-239 and so is fertile.
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CRITICALITY
Fission in a heavy nucleus may occur
spontaneously but is more readily caused by
absorbing a neutron. Each fission releases
several initially fast free neutrons that in
principle could cause a further fission, and so
on in a chain reaction. If on average exactly one
neutron from each fission goes on to cause
another, the chain reaction continues
indefinitely at a constant rate - criticality -
the condition required in a power reactor. If
less than one causes further fission, the chain
dies away more or less rapidly. If more than one
causes further fission, the reaction accelerates
until controlled naturally or artificially.
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CONDITIONS FAVOURING CRITICALITY
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FISSION PRODUCT DISTRIBUTION
Fission usually yields products differing
considerably in mass. Symmetric fission is much
less common, as shown here for U-235 in a thermal
neutron flux. Very fast neutrons lead to a
distribution with a shallower minimum
In terms of elements, fission peaks are roughly
from krypton to palladium and iodine to europium
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INSTABILITY OF FISSION PRODUCTS
The proportion of neutrons to protons needed for
stability rises with atomic number. Thus the
primary fission products have too many. They
therefore convert some of the excess to protons
by emitting energetic electrons (?-particles),
and usually ?-radiation Accordingly they rise
by one atomic number unit at each step but keep
unchanged mass number.
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A FEW THOUGHTS
Heavy elements are remnants from stars that
exploded before the Earth was formed. Only the
heaviest are subject to fission. All beyond lead
are more or less unstable. The heaviest to have
survived is uranium. The only natural fissile
nuclide on Earth is U-235. So the very
possibility of nuclear energy depended upon the
last element available to us ... or did it?