Nuclear Reactor:-
Definition:-
A motor which has a controlled and sustainable nuclear reaction for any of multiple purposes, to make power, to supply neutrons, to induce nuclear reactions, to get ready radioisotope, or to breed fissionable fabric from sure fertile fabrics is called nuclear reactor.
Principle:-
The environment of fissioning nuclei is controlled is such a method that only neutron out of 2.5 neutrons released on the average is used to induce fission in another atom. In this way, the rate of life generation is maintained at a constant level. Thus controlled chain reaction is the principle of nuclear reactor.
Explanation:-
The reactors are divided in to 3 categories on the basis of neutron energy.
v Thermal Reactor
v Fast Breeder Reactor
Thermal reactors use thermal neutrons, whose kinetic life is nearly equal to kinetic life regarding the molecules at space temperature. On the other paw fast breeder reactors use fast neutrons.
Enrichment of uranium:-
Natural uranium contains about 1% (.7%) of 92U235 and 99.3% of 92U238. the rare isotope 92U235 takes the effective component in building up the chain reaction. the nuclear reactor (thermal) uses the uranium as fuel. Natural uranium is a very little grade fuel. First of all natural uranium is enriched by increasing the percentage of 92U235 prior to its use within the nuclear reactor. Enriched uranium is used as nuclear fuel in a nuclear reactor. Enriched uranium means percentage of 92U235 becomes greater than its original percentage in natural uranium.
Components of a Nuclear Reactor:-
A nuclear reactor consists regarding the following parts:
Core:-
This is the greatest important component regarding the nuclear reactor. It contains fuel that is cylinders of Uranium
Moderator:-
The function of moderator is to slow below the neutrons produced during th fission process. As the neutrons produced during the fission process are very fast and are not suitable for producing fission within the reactor fuel.
Manage Rods:-
These rods hold the velocity regarding the fission reaction below control. Usually these are Cadmium or Boron rods, which have the property of absorbing neutrons. By decreased the manage rods into the reactor, the velocity regarding the fission process reduces and shall be stopped when the rods fall in during force failure etc.
Cooling system:-
It consists of pipes along which the coolant is pumped within the shape of gas and water. The coolant (gas) is heated while passing through the core.
Heat exchanger:-
This is the component regarding the nuclear reactor where the heat regarding the nuclear fission is transported to ordinary water. The h2o is converted to steam, which in turn runs the turbine to make electricity.
Biological shield:-
It is a wall of concrete which absorbs radiations emitted due to fission regarding the fuel and protects the workers from their dangerous effects.
Working:-
The fissionable fuel is enriched uranium. The uranium fuel elements are within the shape of thin rods about two meter long and 3 cm in diameter. They can be placed in a moderate known as moderator. The moderator is created up of nuclei of comparable mass to that regarding the neutron and of fabric which does not absorb the precious neutrons. Graphite is usually used a moderator. The fast neutrons produced in a fission process undergo multiple collisions with the nuclei regarding the moderator and are slowed below to thermal range. A slow neutron can induce another fission function within the fuel element and can thus cause a chain reaction.
The life liberated by the fission process yields a huge no. of heat. The heat regarding the system rises or falls as the rate of chain reaction rises or falls. Sequential to maintain a steady rate of chain reaction the heat regarding the system is regulated to a determined price with the help of manage rods of boron or cadmium, which are effective neutron absorbers. These rods shall be moved in and out regarding the reactor from the manage room. If the fission chain starts to leave too fast, the manage rods shall be inserted a bit deeper and neutrons shall be absorbed. As the reaction dies off, the manage rods shall be slightly with drawn.
A charge motor is used to beginning the reactor. The uranium fuel rods are lowered in to fuel channels within the graphite core. The manage rods are then raised slowly and placed at sure position as soon as the chain reaction proceeds at the compulsory rate. The reactor is now spoke about to gone critical and heat is produced steadily.
In a force reactor, a liquid is circulated through the shielded reactor and heat exchanger as shown within the figure. The warm liquid flowing through the heat exchanger vaporizes a more volatile liquid like water, the resulting warm gas or steam below compression drives a turbine. The turbine in turn drives an electric generator.
Draw backs:-
The primary draw return of nuclear reactors is the nuclear waste products. The uranium rods subsequent to usage are converted in to radioactive elements which hold an extended 1/2 life. This waste cannot be drop structure of atom or dumped within the earth. Due to the fact that due to its radio active nature, it shall damage any living present in its vicinity. Although, nuclear life is very cost effective compared to other sources of life but real dispose regarding the waste products is expensive too much.
Liquid Metal Fast Breeder Reaction (LMFBR):-
Definition:-
A kind of reactor which breeds itself as well as uses liquid metal for transformation of heat in to heat exchanger is called LMFBR.
Explanation:-
U235 being fewer abundant cannot be sued for ever like a fuel. So a method out was wanted to use the abundant isotope like a fuel within the nuclear reactor. This should increases the life span regarding the present Uranium reserves by 100.
Breeding process:-
In this kind of reactor fast neutrons are used to convert 92U238 in to 94Pu239 as follows:
92U238 + on1 → 93Np239 + β-
93Np239 → 94Pu239 + β-
94Pu239 shall be used as fuel as efficiently as 92U235 but the only difference is that fast neutrons can carry out the following fission process.
on1 + 94Pu239 → X + Y + 2on1 + 200 MeV
Where X and Y are fission fragments. As it is emitting 3 neutrons per fission, that is why it shall be used for nuclear bombs as well.
The Liquid Metal Coolant:-
The 94P239 breeder reactor is called LMFBR due to the fact that a liquid metal like sodium is used like a coolant instead of water, which slows below the the neutrons and hinders the process of breeding. Sodium is solid at space heat but becomes liquid at 98C and is that is why a good heat transfer liquid.
Working:-
The core regarding the reactor (the region containing the fissionable material) is created up of 15 to 30% U235 that is surrounded by blanket of U238. since fast neutrons are more efficient in converting U238 to 94Pu239, there is no need for a moderator to slow below the neutrons produced within the fission. Cadmium rods manage the chain reaction within the blanketed core, contained in a primary vessel. The flow diagram regarding the sodium coolant and the intermediate heat exchanger are shown within the fig. the feed h2o circulating within the outer heat exchanger is converted in to steam to drive a turbine, which shall operate an electric generator to make electricity.
Advantages and Disadvantages:-
In comparison to thermal reactors, the following are the advantages and disadvantages of breeder reactors:
Advantages:-
v The life span regarding the present uranium. Uranium sources are increased by 100 times by creating use of 92U238 instead of 92U235.
v The 92U238 left over while enrichment of 92U235 for thermal reactor shall be used.
v Ores with a very tiny concentration of 92U235 that is useless for conventional reactors shall be used for the as blanket of 92U238 in LMFBR.
v Life produced is of little price due to readily availability of 92U238(with out enrichment).
Disadvantages:-
v Sodium explodes if exposed to space or water. That is why any leakage shall be dangerous.
v The coolant used in LMFBR and the heat exchanger is opaque. That is why it is not likely to look inside the core, what is going on there while repairing during maintenance.
v The high life neutron creates sodium radioactive because:
0n1 + 11Na23 → 11Na24 + γ-ray ( T1/2 = 15 hrs)
To be safe, it is advised to wait at fewest twenty 1/2 lives that is 300 hours or 12.5 days. So it is important to wait 12.5 days subsequent to shutting below the reactor sequential to carry some repair work.
v Long switching off the reactor, increases the price significantly and also reliability of LMFBR.
v Highly riched sodium coolant can cause a serious accident in LMFBR and then there is a chance that 94Pu239 should be emitted in to atmosphere which can result in devastating results.
Uses:-
Nuclear reactors shall be used for following purposes:
v Nuclear reactors are widely used as sources of best power. They can be gradually replacing the more conventional life sources for the production of electricity. Karachi nuclear force plant (KANUP) is an example of converting nuclear life to electrical energy. Reactors force some ocean going ships and submarines.
v Reactors are used to supply neutrons, which are of immense importance in nuclear studies and research. Neutron diffraction is a powerful tool for studying crystals structure.
v Reactor are used to get ready a huge variations of artificial radioisotopes which are widely used in industry, health related diagnose and therapy, and agriculture. In fact the tracer isotope techniques are widely used for the learn of many processes in biology, chemistry, physics, archaeology and other branches of science.
v Production of plutonium that is used in atomic bombs like a fuel.
v Nuclear reactors shall be used to make fissionable fabric fro sure non fissionable but fertile materials.
v It is used for the production of atomic life for non-residential and peaceful purposes.
Thorium Breeder Reactor:-
The above difficulties with LMFBR have led to h2o moderated slow neutron breeder reactor creating use of 90Th 232to breed 92U238in spite of decreased breeding ratio. Such a breeder should be safe and hold a fewer concentrated core, thus avoiding well-being problems with LMFBR.
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