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l          Principle of Nuclear Power Station

 

Ø            Nuclear Fission Process

Few materials are suitable for nuclear fission.  Materials that can easily undergo the fission process are known as “fissile”.  They include uranium 233, uranium 235 and plutonium 239.  Materials that can undergo the fission process, with some difficulties, are known as “fissionable”.  They include thorium 232, uranium 238 and plutonium 240.  Of these 6 nuclides, only thorium 232, uranium 235 and uranium 238 are found in nature.

 

To obtain nuclear energy for commercial use, uranium 235 is the obvious choice for nuclear fission since it is fissile and is reasonably plentiful in nature, even though it makes up only 0.7% in natural uranium, with the remaining being largely uranium 238.

 

In a “nuclear fission”, an incoming neutron hits a uranium 235 atom to form an unstable uranium 236 atom.  It then splits to give two atoms of usually unequal mass, two or three neutrons and much energy.  The neutrons thus produced may hit other uranium 235 atoms to produce further nuclear fission to maintain a “chain reaction”.

 

 

Ø            Thermal Neutron Reactor

Thermal reactor uses uranium having a low enrichment of uranium 235, at 3% - 5%, as a nuclear fuel to produce heat.

 

At low uranium 235 enrichment, it is necessary to slow down a neutron in a process known as “moderation” to increase its probability to initiate a nuclear fission event.  The neutron is known as a “thermal neutron”.  Water (light or heavy) and graphite are suitable material for moderation.

 

Heat produced in the reactor by nuclear fission is taken away by a coolant.  Common coolants for the reactor include water, carbon dioxide and helium.

  

Ø            Fast Neutron Reactor

A Fast Neutron Reactor usually uses a mixture of uranium 235 and plutonium 239 having about 25% enrichment.

 

At this level of enrichment, there is no need to slow down the neutrons before starting the fission process.  Hence the use of “fast neutrons” and there is no need for a “moderator.”

 

The reactor core is more compact than a thermal reactor because the fast reactor has a higher enrichment.  Liquid metal is used as a reactor coolant because it is a very efficient heat transfer medium.

 

Ø            Nuclear Steam Supply System

Heat produced in the reactor is taken away by a coolant often to a heat exchanger to raise steam.

 

The steam is used sometimes for process heat but most often to drive a turbine-generator to produce electricity.

 

Ø            Engineering Safety Features

Engineering safety features are used to maintain a proper level of safety –

 

1.            To control nuclear reaction: inserting rods containing neutron absorbing material such as cadmium, iridium or boron, known as “control rods”, or injecting a neutron absorbing material into the reactor to regulate or even suppress the nuclear reaction.

 

2.            To maintain cooling: reactor cooling is maintained by duplicate engineering cooling systems and/or by engineering features that make use of natural convection phenomenon.

 

3.            To prevent an unintentional leak of radiation: use of reliable physical barriers built to a high standard to contain radioactivity found in the reactor.

 

By using various engineering safety features and by prudent operation according to established procedures, a modern nuclear power station may keep its risk to the public and the environment at a level very much lower than the risks found in our daily experience.