Different Types of Nuclear Reactors with schematic diagram
The main components of Nuclear Power Plant are –
Nuclear Fuel is fissionable element or nuclei. They should undergo nuclear fission by nuclear bombardment. They should be able to sustain the nuclear chain reaction. Examples of Nuclear fuels are U233 , U235 , Pu239 .
The percentage of different Uranium isotopes in the Earth’s crust is
U238 = 99.3% ; U235 = 0.7% and U234 = only trace
Here U235 is most unstable and can sustain chain reaction. And hence it has the name Primary Fuel for Nuclear Power Plant. U233 and Pu239 are produced from Th232 and U238 respectively. And these are called secondary fuels.
In Nuclear reactor because of the chain reactor fast moving neutrons are produced from U235 which are not good for chain reaction to go ahead. So to control the speed of these neutrons by using the nuclei of another element. Fast moving neutrons collide with that material and their speed is reduced. And hence slow neutrons (Thermal Neutrons) are got which are good for reaction. This obstacle for the fast neutrons is called moderator. Graphite, Deuterium Oxide (Heavy water) and Beryllium are generally used as moderator materials.
In the Nuclear Reactor the consumption of fuel and reactor power level depends on the neutron flux on the core. The energy produced in the reactor is generally very high and if the reaction rate is not controlled properly than there is a possibility of damage of the whole structure. And radioactive products can make the plant site contaminated. So to control the power of reactor control rods are used. These control rods can be cylindrical or in the sheet form. These rods are inserted or moved out of the reactor. Their insertion increases the absorption of the neutrons and withdrawal causes less electrons to be absorbed. So insertion damps the reaction. Generally controls Rods are made of Boron or Cadmium.
In the Nuclear Reactor the neutrons unabsorbed try to escape the reactor. Once they move out of the core they are lost. So to these losses should be diminished. So the unabsorbed neutrons are reflected back to the core by using a material in the core surrounding. These are called reflector. Generally Beryllium and Graphite are used for making reflectors.
Reactor vessel is made of strong materials. It contains all the reactor elements like fuel rods, moderators, control rods, shields etc.
This shielding is done to protect men from harmful radiations. From the nuclear reactors harmful alpha particles, beta particles and very dangerous gamma rays are emitted. Thick layers of concretes are used to protect from gamma rays. Thick metal or plastic shielding is enough for alpha and beta to be stopped.
Coolant used in Nuclear reactors for transferring large amount heat produced because of nuclear chain reaction. Coolants should have high boiling point and low melting point. It should have high heat transfer co efficient. Different types of fluids are used as coolants such as light water, heavy water, Helium, Hydrogen, Carbon Dioxide, Liquid Sodium and Potassium etc.
In this type of reactor enriched Uranium is used. In enriched uranium U235 is increased in percentage. Water is used as a coolant. It takes heat and becomes steam and goes to the Steam Turbine. Here water is used as a moderator also.
Here Water is used as coolant and also moderator. Uranium is use as the fuel. Here a pressurizer is used to increase the water pressure to about 1200 psig. It is done to keep the water in the liquid state because in atmospheric condition water will boil at 100 degree Celsius. Pressure enables water to remain in liquid state and as a result it takes more heat. This heated water goes to steam generator where feed water is turned to steam.
Here, Na is used as the primary coolant. Sodium Potassium is used as secondary coolant. As Sodium has a high boiling point it provides high outlet temperatures. This heat can be utilized to produce high temperature steam. As a result the efficiency will also be increased. By using sodium as a primary coolant more electric energy can be produced. Because of low pressure in the primary and secondary coolant circuits inexpensive pressure vessels can be used. But this system has a disadvantage of using sodium. At high temperatures sodium becomes radioactive and reacts with water. That’s why a intermediate Heat Exchanger (IHX) is used. It transfers heat from Sodium to the secondary coolant NaK . This coolant goes to the steam generator heats the feed water to produce steam. Using liquid metal as coolants has certain advantages. They have low melting point and high heat transfer rate.
In this reactor no moderator is used. The primary fuel U235 is surrounded by a blanket of fertile material. This material can be U238 . Here fast electrons are absorbed by U238 which produces Pu239 . Pu239 is sustainable to chain reaction. This reactor also uses two metal cooling system like Sodium Graphite Reactor.
Candu refers to Canadian Deuterium Uranium reactor. It uses 99.8 % heavy water as coolant and moderator. Fuel is passed through the tubes. The heavy water is also passed though the channels surrounding the fuel tubes.
The tubes experience high internal pressure. While running the operation the reactor refueling takes place. The high temperature coolants are then passed to the steam generators which produces steam and run the turbine.