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Nuclear Fission

Nuclear chemistry deals with the study of the composition of nuclei of atoms, nuclear force, radioactive materials and nuclear reactions.

A nuclear reaction is a process in which either two nuclei or a nucleus of an atom with a subatomic particle like a proton, neutron, or high energy electron from outside the atom, collide and produce a new elements. 

Nuclear reactions are also termed as nuclear decay or radioactive decay. 
They are generally spontaneous nuclear reactions just like spontaneous chemical reaction. Sometime the collision of particles collides cannot change the nuclei, such type if collisions are termed as elastic collision rather than a reaction. 
In general nuclear reactions are two types.
  1. Nuclear Fission: A large atomic nucleus gets decomposed due to bombardment of some subatomic particle forms one or more than one type of small nucleus.
  2. Nuclear fusion: The process of fusion of small nuclei to form bigger nuclei is called as nuclear fusion.

Nuclear Fission and Fusion
Both nuclear reaction release a large amount of energy and have different applications in various field.

 

Nuclear Fission Definition

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In 1934-38 O.Hahn and F. Strassmann observed than when uranium-235 is bombarded with thermal neutrons, it undergoes fission giving barium and krypton as product. Later L.Meitner and O.R.Frisch explained that after the capture of a neutron, the uranium nucleus gets excited and then splits in to two fragments of approximately equal mass.

Hence the splitting of a heavy nucleus into two smaller fragments of approximately equal mass is known as nuclear fission. There is some mass defect observed during fission, the total mass of the products of fission is less than total mass of the neutron and the Uranium-235 atom. The loss of mass appears in the form of energy according to Einstein’s mass energy relation. E = mc2

The nuclear fission of Uranium-235 is as follow:.

92U235 + 0n1 $\to$ 3 0n1 + 36Kr92 + 56Ba141 + ENERGY

Nuclear Fission Diagram

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A general nuclear fission reaction involves the fission of heavy nucleus into small nucleus and active subatomic particles which can further attack on another heavy nucleus to continue the chain reaction.

Nuclear Fission Diagram       
The well known example of nuclear fission is fission of radioactive isotope Uranium -235 due to the bombardment of thermal neutrons to form barium-114 and krypton-92 with three active neutrons.

Example of Nuclear Fission   


Nuclear Fission Equation

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In general in any nuclear fission reaction, a heavy nucleus disintegrated in to one or more small nuclei with some energetic particles. The fission takes place due to the bombardment of some subatomic particles like neutrons. 

Hence the general nuclear fission equation is

Heavy nucleus + neutrons $\rightarrow$ Small nuclei (1) + small nuclei (2)+ neutrons + energy

Nuclear Fission Equation

Just like other chemical reactions, nuclear reaction also follow conservation of mass.

For example the nuclear fission of uranium-235 through the bombardment of thermal neutrons.
92U235 + 0n1 $\rightarrow$ 3 0n136Kr92 56Ba141 + ENERGY
Mass of molecules; 235 +1 = 3x1 + 92 +141 = 236

Examples of Nuclear Fission

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Any nuclear fission equation consist of a parent nuclei with neutrons as reactant and form more than one small daughter nuclei with some number of neutrons and large amount of energy.

For example; the nuclear fission of Uranium-235 with one particle of neutron forms Cesium-140, Rubidium-93 and three neutrons and large amount of energy.

92U235 +0n1 55Cs140+ 37Rb93 +3 0n1 + Energy

Example of nuclear fission. Some other examples of nuclear fission equations are as follow.

92U235 + 0n1 56Ba141+ 36Kr92 +3 0n1 + Energy
92U235 +0n1
56Ba137+ 36Kr97 +3 0n1 + Energy
92U235 +0n1
92U236 54Xe140+38Sr94 +2 0n1 + Energy

Nuclear Fission Energy

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Some Points about Nuclear Fission Energy has Given Below:-
  1. Nuclear fission releases a large amount of energy which can be calculated from the binding energies of the nuclei as well as from the mass defect.
  2. The nuclear fission of uranium-235 release around 215 MeV of energy.
  3. The binding energy of Uranium -235 lies around 7.6 MeV per nucleon while in the case of products of fission lies close to 8.5 MeV per nucleon.
  4. While the formation of uranium nucleus from its nucleons can release 235 x 7.6 MeV of energy and the formation of nuclei of products of fission can release about 235 x 8.5 MeV of energy.
  5. Hence the energy released during the fission of the nucleus of uranium of the mass number 235 will be equals to 235 (8.5-7.6) MeV = 211.5 MeV.
  6. Thus the fission of an atom of releases 211.5 MeV of energy and one mole of uranium-235 released 235 x 6.022 x 1023 MeV = 20.41 x 109 kJ of energy.
  7. Hence the energy released during the nuclear fission is million times greater than any chemical reaction.

The same amount of energy can be calculated by using mass defect concept.

92U235 + 0n1 $\rightarrow$ 56Ba141+ 36Kr92 +3 0n1 + Energy

Mass of reactants, 235.044 +1.009 = 236.053 a.m.u.
Mass of products, 91.905 + 140.908 +3.027 = 235.840 a.m.u
Hence the mass defect, ∆M = 236.053-235.840 = 0.231 a.m.u.
Energy released 0.231 a.m.u. x 931.5MeV = 198.41 MeV which is very close to 211.5 MeV.

Energy from Uranium Fission
Form of energy released Amount of energy released(MeV)
Kinetic energy of two fission fragments 168
Immediate gamma rays
7
Delayed gamma rays
3-12
Fission neutrons
5
Energy of decay products of fission fragments
---
Gamma rays
7
Beta particles
8
Neutrons
12
Average total energy released
215MeV

Another example if nuclear fission is the fission of Uranium-236 which produced forms uranium-235 by neutron capture, in to Cesium-137 and Rubidium-95 releases around 191.1 MeV of energy.

92U235 + 0n1 $\rightarrow$ 92U236 $\rightarrow$ 55Cs137+ 37Rb95 +4 0n1 + Energy



Nuclear Fission Energy

Nuclear Fission Chain Reaction

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Nuclear fission is an important nuclear reaction not only because it accompanied by the release of an enormous amount of energy but also because the nuclear reaction resulting from the capture of neutrons which formed as a fission product during the reaction. The neutrons thus released may cause fission of other nuclei of Uranium-235 and set up a chain reaction.

Once initiation required for the reaction, but after that the fission process would be self sustaining with a continuous release of energy. This concept is looking pretty good for the production of large amount of energy, but actually that does not happen.

This is because of the loss of neutrons in different side reactions like

  1. Neutrons lost in some non-fission reactions with parent nuclei.
  2. Neutrons escape from the system as such.

Hence in order to secure a self-sustaining chain reaction in a fission process, it is necessary to produce a certain number of neutrons, must be at least equals to the number of neutrons involve in fission and non-fission process plus the number of neutrons escapes from the system. Hence we have to increases the surface area of parent nuclei so area by volume ratio decreases, as neutron can escape only through the surface of nuclei so this will decreases the loss of neutrons.

The size of the reactant material which permits the escape of the neutrons to such an extent that at least one neutron is definitely left behind per fission is known as critical size and the corresponding mass is known as critical mass. If the mass of fissionable material is less than critical mass, the fission reaction would not occur and that system is said to be in sub-critical state. If mass is more than critical mass, than only fission reaction would occur and system known as super-critical state.

Nuclear Fission Chain Reaction


For example: The critical mass for Uranium-235 is 1 kg for an aqueous solution of uranium salt containing 90% of Uranium-235.

Nuclear Fission Reactor

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Nuclear fission reactors are device used to produce large amount of energy which can be utilized for some good purpose. These reactors based on the nuclear fission reaction and generally used uranium and polonium isotopes for nuclear fission reaction.

Nuclear Fission Reactor 

There are several types of nuclear reactor made up of some general components like

Fuel
  • Generally Uranium and polonium used as a basic fuel in nuclear fission reactors.
  • Uranium is taken as pellets of uranium oxide (UO2) which are arranged in tubes to form fuel rods.
Moderator
  • There must be some material to decrease the speed of neutron produced as fissionable product.
  • These materials used to slow down the speed of neutrons are called as Moderators.
  • Generally heavy water and graphite rods serve as a good moderator in reactors.
  • Other moderators are helium at 100 atm and 1273 K, beryllium at high temperature, sodium at 773 to 873 K used in breeder     reactor or BeF+ ZrF4  used in gas-cooled reactors.
Control rods
  • As name implies, these rods used to control the fission reactions.
  • They are made up of some neutron-absorbing material like cadmium, hafnium or boron.
  • During nuclear fission reaction, they are inserted or withdrawn from the core to control the rate of reaction.
Coolant
  • Coolant is a liquid or gas which circulates through the core to transfer the heat from core to coolant.
  • Water is a best primary coolant which forms steam after absorbing heat from the core.
Pressure vessel or pressure tubes

These are series of robust steel holding the fuel and conveying the coolant through the moderator.

Steam generator

It is a part of the cooling system where water used as a primary coolant and produce steam for the turbine. 

Containment

It is a meter-thick concrete and steel structure around the reactor core which is designed to protect core from outside intrusion as well as to protect those outside from the effects of radiation in case of any malfunction inside.

On the basis of different coolant, moderator and fuel, nuclear fission reactor can be different types.

For example,

Reactor types
Capacity in thousands of megawatts (GWe)
Fuel
Coolant
Moderator
Pressurized water reactor (PWR) 251.6 Enriched UO2
Water Water
Boiling water reactor (BWR)
86.4 Enriched UO2   Water  Water
Pressurized heavy water reactor 'CANDU' (PHWR)
 
24.3 Natural UO2
CO2 Graphite
Gas-cooled reactor (AGR & Magnox) 10.8 Natural U (metal), enriched UO2  CO2 Graphite
Light water graphite reactor (RBMK) 12.3 Enriched UO2 Water Graphite
Fast neutron reactor (FBR) 1.0 PuO2 and UO2
Liquid sodium none
Other
0.05 Enriched UO2
Water  Graphite

Advantages of Nuclear Fission

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Adavantages of N  uclear Fission has Given Below:-
  1. In nuclear reactor the nuclear chain reaction is carried out in a controlled manner and liberated heat is converted into electricity.
  2. The fuel used in nuclear reactor is relatively expensive and available in trace amounts but fuel used in very little quantity to produce such a large amount of energy.
  3. For example; about 28gm of Uranium releases the same amount of energy as the energy released by 100 metric tons of coal.
  4. The nuclear fission is not contributes in global warming or other pollution effects which mainly associated with fossil fuel combustion.
  5. Because of small quantity required for fuel in nuclear reactors, it can be easily transported.
  6. The large heat produced in nuclear reactors can be used to produce cheap electricity which can be further used for other benefits.
  7. The waste product formed during the fission process is very less and nuclear reactors are very reliable source of energy.
  8. The nuclear reactor can be active for approx. 40 to 60 years.

Advantages and Disadvantages of Nuclear Fission

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Advantages and Disadvantages has given :-


Advantages Disadvantages
1 Produce a large amount of energyNuclear power reactor are very compact and capital cost of building is very high
2
Little amount of fuel required
Reactors require high security
3
Less amount of waste product formed Small leakage from reactor can cause lethal effect
4
No contribution in global warming and pollution
Fuel used in reactors is very expensive and present in trace amount in nature
5
Maintenance and running costs are relatively low
Possibility of uncontrolled nuclear fission can cause nuclear fallout

Uses of Nuclear Fission

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  • Nuclear fission energy appears promising for space propulsion applications and in the generation of high mission velocities with less reaction mass. This is because of production of large amount of energy, around 106 times more than any chemical reactions and can be used for the current generation of rockets.
  • Nuclear fission is generally used in radioisotope thermoelectric generators for space mission.
  • Just like conventional thermal power stations which generate electricity by using the thermal energy released from burning fossil fuels, nuclear power plants follow the conversion of the energy released from nuclear fission in nuclear reactor. 
  • The heat produced during the nuclear fission removes from core by using coolant and used to generate steam which drives a steam turbine. These steam turbines are connected to a generator to produce electricity.
Nuclear Fission
  • Nuclear technology is widely used in diagnostics and radiation treatment like radiation therapy for cancer.
  • Nuclear fission is used to produce some less common radioisotopes like cesium-137 (Cs-137) by using uranium-235 which is used in photographic sources.
  • Nuclear fission energy is also use as a power source for propelling submarines and some type of surface vessels. 
  • Another application of nuclear fission reactors is their high neutron fluxes which can be used for studying the structure and properties of materials.
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