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:

(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.