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..