Nuclear Fission Process
Nuclear Fission Process
** In 1939,
Hahn and Stassmann discovered that a heavy atomic nucleus as of uranium-235 upon
bombardment by a neutron splits apart into two or more nuclei. U-235 first
absorbs a neutron to form an unstable ‘compound nucleus’. The excited ‘compound
nucleus’ then divides into two daughter nuclei with the release of neutrons and
large amount of energy.
** The
splitting of a heavy nucleus into two or more smaller nuclei is termed nuclear
fission. The smaller nuclei formed as a result of fission are called fission
products. The process of fission is always accompanied by the ejection of
two or more neutrons and liberation of vast energy.
** A given
large nucleus can fission in many ways forming a variety of products. Thus the fission
of U-235 occurs in about 35 ways. Two of these are given below in the form of equations:
** In these
fission reactions, the mass of the products is less than the mass of the
reactant. A loss of mass of about 0.2 amu per uranium atom occurs. This mass is
converted into a fantastic quantity of energy which is 2.5 million times of
that produced by equivalent amount of coal.
Characteristics of nuclear fission
(1)
Upon capturing a neutron, a heavy nucleus cleaves into two or more nuclei.
(2) Two
or more neutrons are produced by fission of each nucleus.
(3)
Vast quantities of energy are produced as a result of conversion of small mass
into energy.
(4) All
the fission products are radioactive, giving off beta and gamma radiations.
Nuclear Chain Reaction
** We know that
U-235 nucleus when hit by a neutron undergoes the reaction:
** Each of the
three neutrons produced in the reaction strikes another U-235 nucleus, thus causing
nine subsequent reactions. These nine reactions, in turn, further give rise to
twenty seven reactions. This process of propagation of the reaction by multiplication
in threes at each fission, is referred to as a chain reaction.
** Heavy unstable isotopes, in general, exhibit a
chain reaction by release of two or three neutrons at each fission. It may be
defined as : A fission reaction where the neutrons from a previous step
continue to propagate and repeat the reaction.
** A chain
reaction continues till most of the original nuclei in the given sample are
fissioned.
** However, it
may be noted that not all the neutrons released in the reaction are used up in propagating
the chain reaction. Some of these are lost to the surroundings.
** Thus for a
chain reaction to occur, the sample of the fissionable material should be large
enough to capture the neutron internally. If the sample is too small, most
neutrons will escape from its surface, thereby breaking the chain. The minimum
mass of fissionable material required to sustain a chain reaction is called
critical mass. The critical mass varies for each reaction. For U-235 fission
reaction it is about 10 kg.
** As already
stated, even a single fission reaction produces a large amount of energy.
** A chain reaction
that consists of innumerable fission reactions will, therefore, generate many
times greater energy.
Nuclear Energy
** A heavy
isotope as uranium-235 (or plutonium- 239) can undergo nuclear chain reaction yielding
vast amounts of energy.
** The energy
released by the fission of nuclei is called nuclear fission energy or nuclear
energy. Sometimes, it is incorrectly referred to as atomic energy.
** The fission
of U-235 or Pu-239 occurs instantaneously, producing incomprehensible quantities
of energy in the form of heat and radiation. If the reaction is uncontrolled,
it is accompanied by explosive violence and can be used in atomic bombs. However,
when controlled in a reactor, the fission of U-235 is harnessed to produce
electricity.
First Chain Reaction
The first
controlled nuclear fission chain reaction, directed by Italian-born American physicist
Enrico Fermi, is captured here in a painting of the event, which took place
under the sports stadium at the University of Chicago in December 1942. The
event was the forerunner of all nuclear reactors.
The Atomic Bomb
** A bomb which
works on the principle of a fast nuclear chain reaction is referred to as the atomic
bomb.
** A design of
such a bomb is shown in Fig. (1):
[1] It contains
two subcritical masses of fissionable material, 235U or 239Pu.
[2] It has a
mass of trinitrotoluene in a separate pocket.
[3] When TNT is
detonated, it drives one mass of 235U into the other. A supercritical mass of the
fissionable material is obtained.
[4] As a result
of the instantaneous chain reaction, the bomb explodes with the release of tremendous
heat energy.
** Temperature
developed in an atomic bomb is believed to be 10 million °C (temperature of the
sun). Besides many radionuclei and heat, deadly gamma rays are released.
** These play
havoc with life and environment. If the bomb explodes near the ground, it
raises tons of dust into the air. The radioactive material adhering to dust
known as fall out. It spreads over wide areas and is a lingering source of
radioactive hazard for long periods
Little Boy And Fat Man
** Little Boy
was the first nuclear weapon used in warfare. It exploded approximately 1,800 feet
over Hiroshima, Japan, on the morning of August 6, 1945, with a force equal to
13,000 tons of TNT. Immediate deaths were between 70,000 to 130,000.
** Fat Man was
the second nuclear weapon used in warfare. Dropped on Nagasaki, Japan, on August
9, 1945, Fat Man devastated more than two square miles of the city and caused approximately
45,000 immediate deaths.
** While Little
Boy was a uranium gun-type device, Fat Man was a more complicated and powerful
plutonium implosion weapon that exploded with a force equal to 20 kilotons of
TNT.
Nuclear
Reactor
** It has been
possible to control fission of U-235 so that energy is released slowly at a
usable rate.
** Controlled
fission is carried out in a specially designed plant called a nuclear power
reactor or simply nuclear reactor.
** The chief components of a nuclear reactor are:
** The chief components of a nuclear reactor are:
(1) U-235
fuel rods which constitute the ‘fuel core’. The fission of U-235 produces
heat energy and neutrons that start the chain reaction.
(2) Moderator
which slows down or moderates the neutrons. The most commonly used moderator
is ordinary water. Graphite rods are sometimes used. Neutrons slow down by
losing energy due to collisions with atoms/molecules of the moderator.
(3) Control
rods which control the rate of fission of U-235. These are made of boron-10
or cadmium, that absorbs some of the slowed neutrons.
Thus the chain
reaction is prevented from going too fast.
(4) Coolant
which cools the fuel core by removing heat produced by fission. Water used in
the reactor serves both as moderator and coolant. Heavy water (D2O) is even
more efficient than light water.
(5) Concrete
shield which protects the operating personnel and environments from
destruction in case of leakage of radiation
Light-water Nuclear power plant
** Most
commercial power plants today are ‘light-water reactors’.
** In this type
of reactor, U 235 fuel rods are submerged in water. Here, water acts as coolant
and moderator. The control rods of boron-10 are inserted or removed
automatically from spaces in between the fuel rods.
** The heat
emitted by fission of U-235 in the fuel core is absorbed by the coolant. The
heated coolant (water at 300°C) then goes to the exchanger. Here the coolant
transfers heat to sea water which is converted into steam. The steam then turns
the turbines, generating electricity. A reactor once started can continue to
function and supply power for generations.
**About 15 per
cent of consumable electricity in U.S.A. today is provided by light water reactors.
India’s first nuclear plant went into operation in 1960 at Tarapur near Mumbai.
Another plant has been set up at Narora in Uttar Pradesh.
** While such
nuclear power plants will be a boon for our country, they could pose a serious
danger to environments. In May 1986, the leakage of radioactive material from
the Chernobyl nuclear plant in USSR played havoc with life and property around.
** Disposal of
reactor waste poses another hazard. The products of fission e.g., Ba-139 and Kr
92, are themselves radioactive. They emit dangerous radiation for several
hundred years. The waste is packed in concrete barrels which are buried deep in
the earth or dumped in the sea. But the fear is that any leakage and corrosion
of the storage vessels may eventually contaminate the water supplies.
Breeder Reactor
** We have seen
that uranium-235 is used as a reactor fuel for producing electricity. But our limited
supplies of uranium-235 are predicted to last only for another fifty years.
However, nonfissionable uranium-238 is about 100 times more plentiful in
nature. This is used as a source of energy in the socalled breeder reactors
which can supply energy to the world for 5,000 years or more.
** Here the
uranium-235 core is covered with a layer or ‘blanket’ of uranium-238. The neutrons
released by the core are absorbed by the blanket of uranium-238. This is then converted
to fissionable plutonium-239. It undergoes a chain reaction, producing more
neutrons and energy.
** The above
reaction sequence produces three neutrons and consumes only two. The excess neutron
goes to convert more uranium to plutonium-239. Thus the reactor produces or
‘breeds’ its own fuel and hence its name. Several breeder reactors are now functioning
in Europe. However, there is opposition to these reactors because the plutonium
so obtained can be used in the dreaded Hbomb.
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.
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