Nuclear Fission: Definition, Properties, Examples, Applications

– In this subject, we will discuss the Nuclear Fission (Definition, Properties, Examples, Applications)

Nuclear Fission: Definition, Properties, Examples, Applications

Nuclear Fission Process


– In 1939, Hahn and Stassmann discovered that a heavy atomic nucleus 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 a 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 the 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 that produced by the equivalent amount of coal.

Nuclear Fission: Definition, Properties, Examples, Applications

Properties 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 the conversion of small mass into energy.

(4) All the fission products are radioactive, giving off beta and gamma radiation.

Nuclear Chain Reaction

– We know that the 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 the 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 such 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.

Nuclear Fission: Definition, Properties, Examples, Applications

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.

Nuclear Fission: Definition, Properties, Examples, Applications

The Atomic Bomb

– A bomb that works on the principle of a fast nuclear chain reaction is referred to as an atomic bomb.

– The 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.

The temperature developed in an atomic bomb is believed to be 10 million °C (the temperature of the sun).

– Besides many radio nuclei and heat, deadly gamma rays are released.

– These play havoc with life and the environment.

– If the bomb explodes near the ground, it raises tons of dust into the air.

– The radioactive material adhering to dust is 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 the 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 a nuclear reactor.

– 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, which 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 a 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

Nuclear Fission: Definition, Properties, Examples, Applications

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 a 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 the 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 percent of consumable electricity in the 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 environment.

– 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.

– However, 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 so-called 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 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 H-bomb.

Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolor edition.

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