A nuclear reactor is a system where a controllable nuclear fission chain reaction can be maintained. The principal parts of reactor are
1. The core:
This is the main part containing the nuclear fuel. The solid fuel material is fabricated into various small shapes – plates, pellets, pins etc., which are usually put together and called as sub-assemblies or bundles.
Fuel elements: A reactor core may contain from tens to hundreds of these fuels sub-assemblies, held in a fixed geometrical pattern.
The moderator is a material that has the ability to slow down neutrons quickly and which at the same time has little tendency to absorb neutrons. Moderator is used in thermal reactor to slow down the neutrons as the fuel has high fission cross-section for low energy neutrons.
Materials used as moderators include ordinary water, heavy water, graphite, beryllium and certain organic compounds.
The moderator should be well distributed within the fuel zone or core. In some reactors the fuel materials and moderator materials are intimately mixed together.
The reflector reduces the leakage of neutrons by reflecting back the neutrons escaping from the core. The same material used for moderator can be used for the reflectors in the case of thermal reactors.
In the fast reactors where fast neutrons are utilized for fission, nickel, molybdenum and stainless steel reflectors are used.
4. The Cooling System:
This system removes the heat released from the reactor core. It consists of pipes through which the coolant is pumped. When passing through the reactor cores, the coolant picks up the heat, transfers the heat to another working medium through a heat exchanger and then returns to the reactor. Gases, heavy and light water, and liquid metals such as sodium, lithium, potassium etc., can serve as coolants. In a reactor, we must be able to control the amount of heat produced. The heat produced depends upon the number of fissions taking place per second in the reactor, which in turn depends upon the number of neutrons present in the reactor.
5. The Control System:
The control system is designed to control the number of neutrons, thus control the rate of the chain reaction and power level.
This system includes a number of devices, sensing elements that measure the number of neutrons in the reactor, control rods – containing strong neutron absorbers such as cadmium or boron, and other devices to regulate the position of the control rods. These neutron absorbing control rods when lowered into the reactor absorb the neutrons to reduce the neutron population and when raised allow the rise in number of neutrons. It is also possible to control a reactor by increasing or decreasing its size. (Increasing the size reduces the leakage of neutrons and vice versa). Hence some reactors are controlled by varying the level of moderator. In the heavy water moderated reactors like Candu, a combination of moderator level control and neutron absorber rods are used.
6. Protective shield:
The fission reaction is accompanied by emission of radiation like α, β and γ. Exposure to these radiations is dangerous. In order to protect the persons working near the reactor from these harmful radiations the reactor is enclosed in steel and concrete which are capable of stopping these radiations. This arrangement of protection is called Radiation shielding.
Types of Reactors:
Nuclear reactor may be classified according to the velocities of the neutrons which cause fission as thermal reactors and fast reactors.
Reactors are classified according to secondary features as the type of moderator, coolant, etc.
Reactor classification based on purpose:
The essential ingredient of a nuclear fuel is a fissionable material i.e. a substance that readily undergoes fission when struck by neutrons.
Fissile and Fertile:
The only naturally occurring nuclear fuel is uranium which contains two isotopes of U-238(99.3%) and U-235(0.7%). Out of this U-235 only is fissionable by slow neutrons and is called fissile fuel. The remaining isotope U-238 can be converted into a fissionable substance called plutonium (Pu-239) when bombarded by neutrons. Hence U-238 is called Fertile.
Thorium-232 is also fertile material and can be similarly converted into a fissile material called U-233 by nuclear reaction.
The artificial method of increasing proportion of U-235 to U238 is called as enrichment. So enriched uranium fuel means fuel having a higher fissionable content than that of natural uranium. The advantages of enriched fuel are
-Physically smaller reactors can be built.
-More energy can be extracted from the fuel before it is replaced.
-Many of the reactors use slightly enriched uranium. In the BWR type of reactor slightly enriched uranium is used. In our FBTR enriched Uranium is used along with Plutonium.
Other parts of reactor are
Coolants, Heat-exchangers, Pumps, Electrical systems, Reactor control system etc.
Some of the typical reactors used in India for generation of electrical power are:
Boiling water reactor (BWR):
In this reactor the enriched uranium oxide is used as the fuel and light water is used as the coolant and moderator. This is a thermal reactor. The water is circulated by a pump and the water boils in the reactor vessel itself. The steam produced is fed directly to turbine. In BWR, the steam is generated in the core itself.
The reactor pressure vessel has to be strong and is enclosed in concrete containment vessel to prevent hazard from the failure of the pressurised circuit.
The exhaust steam from turbine is condensed and the condensate is sent back to the reactor core through a feed pump. Another pump is used for recirculating the coolant in the reactor vessel before converting to steam.
In Tarapur Atomic power station two BWR’s are used for power generation of 210MWe each.
Pressurized water reactor (PWR):
PWRs keep water under pressure so that it gets heat, but does not boil. Water from the reactor and the water in the steam generator that is turned into steam never mix. In this way, most of the radioactivity stays in the reactor area.
CANDU type reactor:
The series of power reactors developed in Canada are called as “CANDU” reactor in short for Canada-Deuterium-Uranium.
Fast Breeder Test Reactor:
This is test reactor. Fast neutrons cause the fission. The fuel burn-up or depletion is offset to some extent by breeding (i.e. by converting fertile to fissile material). Hence it is named as Fast Breeder Test Reactor (FBTR).
Three Stage Nuclear Power Programme
Nuclear Reactors in India