Three stage Indian Nuclear program

The three stages of Indian Nuclear Program

STAGE-1:

Construction of pressurised heavy water reactors. These reactors use natural Uranium. Spent fuel from these reactors is reprocessed to obtain Plutonium.

STAGE-II:

Construction of fast breeder reactors fuelled by Plutonium produced in stage-I. These reactors would also breed U-233 from Thorium.

STAGE-III:

Construction of power reactors using U-233/Thorium as fuel.

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What are Isomers?

In earlier days, if two half lives were observed in a given sample, it was assumed that two different isotopes were present, each decaying with particular half life.

One interesting example is isotope of Protactinium, Z=91, A=234 which is formed in beta decay of its parent, Thorium Z = 90, A=234[UX1]. The isotope of Protactinium was found to decay by the emission of beta particles of two distinct half lives, one of 1.18 min, the other of 6.6hr. It was assumed that these two half lives were due to two different isotopes and these were given separate names UX2 and UZ respectively.

In 1921, "Hahn" showed that these  two substances form a pair of Nuclear Isomers; i.e they are different energy states of same nucleus.

"Feather" and "Bretscher" later showed that these nuclear isomers are genetically realted; i.e. one type of nucleus is formed from other.


The nucleus called UX2 is an isomeric state of Pa-234 at an energy of 0.394MeV above ground state called UZ.

The nucleus may decay by beta particle emission directly from isomeric state of higher energy with a half life of 1.18 min or it may first emit a gamma ray photon of 0.394MeV, going to ground state of Pa, and then undergo beta decay to U-234 with a half life of 6.66 hours.

An isomeric state differs from ordinary excited state of a nucleus in that it lasts for measurable time.

Thus,

" Isomers are atoms which have same atomic number and mass number but differ from one another in their nuclear energy states and exhibit differences in their internal structure. These nuclei are distinguished by their different life times". 

 

Determination of Avogadro's number

Quantity of any substance whose mass, in grams, is numerically equal to its molecular weight is called a mole.

The volume occupied by a mole of any gas is called gram molecular volume. At 0oC and 76 cm pressure the gram molecular volume of any gas is 22.4 liters.

On the basis of Avogadro's hypothesis, every mole of a substance contains the same number of molecules. This number is referred to as Avogadro number.

Determination of Avogadro's number:

The behavior of electrolytic cells can be summarized in terms of two laws formulated by "Faraday".

First law:

It states that the quantity of any substance liberated from the solution depends only on the total charge passing through the circuit,

M = KQ; --------------------------------(1)

where 'M' is mass of material liberated at one electrode 
         'Q' is quantity of charge transferred
         'K' is factor of proportionality called electrochemical equivalent of the substance. It is mass   liberated per unit charge transferred, usually expressed in grams per coulomb.

Second law:

For any substance, the mass liberated by the transfer of quantity of electric charge 'Q' is proportional to chemical equivalent of substance,

M = (A/V) *(1/F)* Q  ------------------------(2)

where (A/V)  is the ratio of atomic mass to the valence of element, is the chemical equivalent of the element and 'F' is a constant of proportionality known as Faraday's constant.

From equations (1) & (2) it could be noted that

F = A/KV -------------------(3)

The value of 'F' can be determined from the results of experiments on electrolysis.

For case of silver, where K=0.0011180 grams/coulomb, A = 107.88 gms/gram atomic mass and 'V' is unity; we get

F = 96,500 Coulombs.gram atomic mass.

Thus the transfer of 96,500 coulomb of charge will deposit a gram atomic mass of a monovalent element. Since the valency of silver is unity, for every atom of silver deposited on the cathode, a charge equivalent to one electron has been transferred through the solution.

If 'e' is charge of one electron, then N*e is the total charge transferred when one gram atomic mass of silver is deposited on cathode.

F = N*e = 96,500 Coulombs/gram-atomic mass

hence N = 6.022 x 10^23 gms/gram atomic mass.

The first direct determination of Avogadro number was made by "Perrin" in 1908 in an investigation of motion and distribution of very small particles suspended in a fluid. 

 


  

What are PROMPT and DELAYED Neutrons in Nuclear Physics?

In the process of fission, the capture of a Neutron leads to formation of excited compound nucleus and thus breaks into two nuclear fragments having excess neutrons and energy of about 8 MeV which is sufficient to expel neutrons. Such neutrons which gets emitted in time scale of order 10^-14 sec are called prompt neutrons. They have energies of order of few MeV generally 1-2MeV.

     On the other hand, some fission fragments decay in various modes to become stable. In this process, fragments which undergo beta decay, some times forms product nucleus left in excited state with an energy more than average binding energy which then emits neutron to reach stable state. Aas this neutron emission follows beta decay of preceding nucleus and so neutron activity of that element will have some apparent half life due to beta activity of parent nuclide, usually of order of mSec to few Sec. This type of emission is called Delayed Neutron Emission and neutrons emitted are called delayed neutrons.     

You can see the decay scheme for two well known fission fragments Br-87 & I-137 exhibiting delayed neutron activity.

DELAYED NEUTRON EMISSION

 

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ELECTROMAGNETIC SPECTRUM

Gamma Rays:

Wavelength Range:  from 0.0001 to 1Angstroms.

Production: Nuclear origin. Emitted on disintegration of nuclei of atoms.

Properties: Highly penetrating and uncharged. Exhibits fluoroscence, Phosphoroscence, ionization and chemical reaction on photographic plates.

Application: Gives information about structure of nuclei.

X-Rays:

Wavelength Range:  from 1 to 100 Angstroms.

Production: by striking high speed electrons on heavy target.


Properties: All properties of gamma rays holds good for these rays also, but less penetrating.


Application: helpful in medical diagnosis, study of crystal structure. 

Ultraviolet Rays:

Wavelength Range:  from 100 to 4000 Angstroms.

Production: by sun, arc, spark and ionized gases.

Properties:  All properties of X-rays but less penetrating. They produce photoelectric effect.

Application: used in medical applications. Detection of finger prints, forged documents. 


Visible Region:

Wavelength Range:  from 4000 to 7800 Angstroms.

Production: radiated from ionized gases and incandescent bodies.

Properties:  ehibit reflection, refraction, interference, diffraction, polarization, photoelectric effect.

Application: used in LASER technology.

Infrared radiation:

Wavelength Range:  from 7800 to 0.001 meter.

Production: by hot bodies.

Properties:  heating effect on thermopiles and bolometer. Exhibit refelction, refraction, and photographic emulsion.

Application: used in industry, astronomy & medicine etc.

Hertzian waves:

Wavelength Range:  from 0.001 to 1 meter.

Production: by spark discharge, by electronic devices such as Klystron & magnetron.

Properties:  reflection, refraction & diffraction. Produces spark in gaps of receiving circuits.

Application: used in radar and Masers and also to reveal finer details of atomic and molecular structure.

Radio waves:

Wavelength Range:  from 1 to 10000 meter.

Production: oscillating circuits and electronic devices.

Properties:  reflection, refraction & diffraction.


Application: used in television and radio broadcast system.





 

What is Loss of coolant accident in a Nuclear Reactor?

A loss of coolant accident (LOCA) is a mode of failure for a nuclear reactor ; if not managed effectively, the results of a LOCA could result in reactor core damage.

Every Nuclear power plant's Emergency Core Cooling System(ECCS) exists specifically to deal with LOCA.

When coolant flow is reduced, or lost altogether, the nuclear reactor's emergency shutdown is designed to stop fission chain reaction. However due to radioactive decay the nuclear fuel will continue generate a significant amount of heat.

The decay heat produced by reactor shutdown from full power is initially equal to about 5% to 6% of thermal rating of reactor.

If all independent cooling trains of ECCS fail to operate as designed, this heat can increase the fuel temperature to the point of damaging the reactor.