Reactor Produced Radionuclides

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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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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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..

Reactor Produced Radionuclides

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