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7/31/2019 Reactor Produced Radionuclides
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REACTORPRODUCED
RADIONUCLIDES
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REACTORPRODUCED RADIONUCLIDES
A variety of radio nuclidesare produced in nuclearreactors.
A nuclear reactor is
constructed with fuel rodsmade of fissile materialssuch ad U and Pu.
The fuel rods may beindividually loaded into the
reactor (into tubes locatedin the reactor) or may beorganized into fuelassemblies.
The fuel nuclei undergospontaneous fission .
Fission is defined as
breakup of a heavy nucleusinto two fragments ofapproximately equal massaccompanied by theemission of two or three
neutrons with mean energy1.5Mev.
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The atomic number for
Ba is 56 and Kr is
36.
Uranium -235 absorbs aneuron and becomes
uranium 236 the
fission reaction.
And 3 neutrons
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In each fission there is aenergy release of 200Mev that appears as heatand is usually removed byheat exchangers toproduce electricity in thenuclear plant.
Coolant - Used to removethe heat from the fuelrods directly if themoderator and coolantare the same material.
In cases where aseparate moderator isused, coolant tubes arerouted through the
moderator, removing heatfrom the moderatordirectly and fuel rodsdirectly or indirectly
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Water is often used as
a coolant.
However, sometimes
heavy water, liquidmetals (sodium,
potassium), or even
gases (carbon dioxide)
may be used.
Moderator - Used to
slow down the neutrons
to thermal energies.
Typical materials thatcan be used as
moderator include
water, heavy water, and
graphite.
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Neutrons emitted in
each fission can cause
further fission of other
fissionable nuclei in thefuel rod, provided the
right conditions exist.
This will initiate a chain
reaction and ultimately
leading to the melt
down of the reactorcore.
This chain reaction
must be controlled
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A possible nuclear fission chain
reaction
1. A uranium-235 atom
absorbs a neutron, and
fissions into two new
atoms (fissionfragments), releasing
three new neutrons and
a large amount of
binding energy.
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Nuclear fission chain reaction
2. One of those neutronsis absorbed by an atom ofuranium-238, and doesnot continue the reaction.Another neutron leavesthe system without beingabsorbed.
However, one neutrondoes collide with an atom
of uranium-235, whichthen fissions and releasestwo neutrons and morebinding energy.
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Nuclear fission chain reaction
3. Both of those
neutrons collide with
uranium-235 atoms,
each of which fissionsand releases a few
neutrons, which can
then continue the
reaction.
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Control rods
Control Rods - Used to
regulate the distribution
of power in the reactor
while the reactor inoperating.
The most important
function is to insert to
shutdown or stop thenuclear fission process
when required.
To maintain a self
sustained chain reaction
, only one fission
neutron is needed andexcess neutrons ( more
than one) are removed
by positioning cadmium
rods in the reactor core.
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The control rods are
made of materials that
quickly stop the nuclear
reaction by absorbingthe neutrons
Cadmium has a high
probability of absorbing
a thermal neutron.
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Neutron moderator
Neutrons emitted with amean energy of 1.5 Mevfrom the surface of the fuelrod have a low probability
of interacting with othernuclei and therefore do notserve any useful purpose.
The neutrons with thermalenergy (0.025 eV)interact
with many other stablenuclei efficiently , producingvarious radio nuclides.
To make use of highenergy neurons or fastneutrons more useful theyare slowed down by
interaction with lowmolecular weight materialssuch as heavy water , water, beryllium and graphite.
These are called
moderators.
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Neutron moderator
Neutron moderator areused to reduces thespeed of fast neutrons,thereby turning them intothermal neutrons capableof sustaining a nuclearchain reaction involvinguranium-235.
Commonly usedmoderators includeregular (light) water (75%of the world's reactors),solid graphite (20% ofreactors) and heavy water(5% of reactors).
Beryllium has also beenused in some
experimental types, andhydrocarbons have beensuggested as anotherpossibility.
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The flux or intensity of
the thermal neutrons so
obtained ranges from
10 to 10neutrons/cm .sec
And they are useful in
production of many
radio nuclides.
When a target element
is inserted in the
reactor core , a thermal
neutron will interactwith the target nucleus
with a definite
probability of
producing anothernuclei.
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In the reactor two types
of interaction with
thermal neutrons occur
to produce variousradio nuclides
Fission of heavy
elements
Neutron capture
Fission of heavy
elements :Fission or (n ,
f) Reaction
When a target heavyelements is inserted in
the reactor core, heavy
nuclei absorb thermal
neutrons and undergofission
Fi i ( f) R ti
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Fission or (n , f) Reaction
Fissionable heavy
elements are U , Pu
,Np , U and Th
and many others havingatomic numbers greater
than 92.
Nuclides produced by
fission may range inatomic number from 28
to 65.
These isotopes ofdifferent elements areseparated by appropriatechemical procedures that
involve1. Precipitation
2. Solvent extraction
3. Ion exchange
4. Chromatography
5. and Distillation
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Fission or (n , f) Reaction
The fission radionuclides are normallycarrier-free andtherefore radio nuclides
of high specific activityare available fromfission.
The fission products areusually neutron rich anddecay by emission
Many clinically useful
radio nuclides such as
I , Mo , Xe and
Cs are produced byfission ofU
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Fission or (n , f) Reaction
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Neutron capture or ( n, ) Reaction
In neutron capture reaction the target nucleuscaptures one thermal neutron and emits raysto produce an isotope of the same element.
The radio nuclide so produced are not carrierfree , and its specific activity is very low.
Some examples of neutron capture reactions areMo (n, )Mo , Hg (n, ) Hg and Cr (n, )
Cr. Molybdenum -99 so produced is called the
irradiated molybdenum.
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Characteristics of commonly used
Radio Nuclides
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CONTD..
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CONTD..