Nuclear Reactor | Principle, Construction and Working, Component, Diagram | aurayne

Principle of Nuclear Reactor 

Note a fact of great importance in the fission reactions given in the equations.

The extra neutron(s) is released in the fission process. An average of two and a half neutrons are released per fission of a uranium nucleus. This one is different because some fission events produce 2 neutrons, some 3, etc.

The extra neutrons in turn can initiate fission processes, producing even more neutrons, and so on. This leads to the possibility of a chain reaction, as previously suggested by Enrico Fermi. If the chain reaction is handled appropriately, we can obtain a stable energy output. This happens in a nuclear reactor.  If the chain reaction is uncontrolled, it leads to the production of explosive energy, as in an atomic bomb.

However, there is a hindrance in maintaining a chain reaction, as described here. It is known experimentally that slow neutrons (thermal neutrons) have a higher probability of fission at U-235 than fast neutrons. Also the fast neutrons released in fission would escape instead of causing another fission reaction.

The average energy of the neutron produced in the fission of U-235 is 2 MeV. These neutrons would then escape the reactor without interacting with the uranium nucleus, unless slowed down, unless a very large amount of fissile material was used to sustain the chain reaction.  

What one needs to do is to slow down the fast neutrons by elastic scattering along the light nuclei. In fact, Chadwick's experiments showed that in an elastic collision with hydrogen the neutron almost comes to rest and the proton takes away the energy. This is the same situation when a marble at rest strikes an identical marble head-on. Therefore, in reactors, light nuclei called moderators are provided with fissile nuclei to slow down the fast neutrons.  Commonly used moderators are water, heavy water (D2O) and graphite.  

The Apsara reactor at Bhabha Atomic Research Center (BARC), Mumbai, uses water as a moderator. Other Indian reactors, which are used for power generation, use heavy water as a moderator.

Because of the use of moderators, it is possible that the ratio of the number of fission produced by a given generation of neutrons, K, to the number of fission in the previous generation, may be greater.  

This ratio is called the multiplication factor;  It is a measure of the growth rate of neutrons in the reactor.  For K = 1, the operation of the reactor is said to be critical, which is what we want for steady power operation.  

If K becomes greater than one, the reaction rate and reactor power increase rapidly.  Unless the factor K is brought too close to unity, the reactor will become supercritical and may even explode.

The 1986 Chernobyl reactor explosion in Ukraine is a sad reminder that accidents at a nuclear reactor can be catastrophic.  

The reaction rate is controlled by means of control-rods made of a neutron-absorbing material such as cadmium. In addition to the control rods, safety rods are provided to the reactors, which can be inserted into the reactor when required and the K can be rapidly reduced to unity.

The more abundant isotope U–238 in naturally occurring uranium is non-fissile.  When it captures neutrons, it produces highly radioactive plutonium through these reactions.

Plutonium undergoes fission with slow neutrons.

Nuclear Reactor Working 

The figure shows a diagram of a nuclear reactor based on thermal neutron fission.  The core of the reactor is the site of nuclear fission. This includes fuel elements in suitably manufactured form.  

The fuel can be said to be enriched uranium (that is, containing (U–235) more abundant than naturally occurring uranium). The core contains a moderator to slow down the neutrons. The core is surrounded by a reflector to reduce leakage.

The energy (heat) released in fission is continuously removed by a suitable coolant. A containment vessel prevents radioactive fission products from escaping.  The entire assembly is shielded to prevent harmful radiation from coming out. The reactor can be closed by means of rods (for example, made of cadmium) which have a high absorption of neutrons.  

The coolant transfers heat to a working fluid, And it is carried in the primary circuit to the steam generator, where it flows inside the tubes and its heat is transferred through the walls of the tubes in the steam generator to the secondary circuit. The water in the secondary circuit outside the tubes absorbs this heat and turns into steam under saturated conditions.  

This steam is sent to the turbine where it expands to a low pressure while being directed onto the turbine blades and in doing so transfers its energy to the turbine rotor. The rotor drives an electric generator that generates electrical power.  

The exhaust steam condenses by cooling the water passing through the condenser tubes and in doing so removes the bulk of the heat that cannot be converted to work.  After preheating in the feedwater heating system, the condensate is returned to the steam generator.

Like any power reactor, nuclear reactors generate considerable waste products.  But nuclear waste requires special care for treatment because they are radioactive and dangerous. Elaborate safety measures are needed for both reactor operation as well as for the handling and reprocessing of spent fuel.  

These safeguards are a distinctive feature of the Indian nuclear power programme. A suitable scheme is being developed to study the possibility of converting radioactive waste into less active and short-lived material

Main Component of Nuclear Reactor

1. Nuclear reactor fuel :

Nuclear reactor fuel is a substance that can be broken down by neutrons. A certain mass of nuclear fuel can be taken in the form of tightly closed rods in aluminum containers. The rods are placed in the core of the nuclear reactor and detached by the moderator.  They can be uranium (U-235), Thorium (Th-232), and so on.

2. Moderator :

In the fission of moderator uranium, fast neutrons are released which have very high energy and velocity.  Rather than triggering another nuclear fission reaction, these fast neutrons have a greater tendency to escape.  

Slow neutrons are efficient at triggering nuclear fission reactions.  Thus, moderators such as heavy water, graphite, beryllium oxide are used to slow the fast neutrons to thermal velocity.

3. Control rods : 

Rods of neutron absorbing material such as Cd, B are inserted into the core of the reactor to initiate, stop or control the chain reaction. The rate of neutron production is controlled by adjusting the depth of the control rod.

4. Coolent :

Coolant is a material with high boiling point and high specific heat used to cool fuel rods and moderators. This can remove the large amount of heat produced in the fission process.  

It then transfers this heat to a working liquid like water and produces steam which is then used to drive a turbine and turn a generator.  Heavy water, sodium can be used as a coolant.

5. Steam Generator :

Steam generator Part of the cooling system of pressurized water reactors (PWR & PHWR) where the high-pressure primary coolant that brings heat from the reactor is used in the secondary circuit to generate steam for the turbine. Essentially a heat exchanger like a motor car radiator. The reactor consists of six 'loops', each containing a steam generator.  

Since 1980 more than 110 PWR reactors have replaced their steam generators after 20–30 years of service, more than half of these in the United States.

6. Container Vessel :

The container vessel prevents the escape of nuclear fission products.

7. Shelding :

Shielding fast neutrons and the gamma rays thus generated are highly harmful to the human body. Thus, the reactor core is surrounded by a reactor shield that protects the workers. This reactor shield is a thick concrete wall.

Principle of nuclear reactor | Nuclear reactor working | Main Component of nuclear reactor |

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