Construction and working principle of Pressurized Water
Reactor (PWR)
Pressurized
Water Reactor (PWR):
Working
principle:
A nuclear power plant differs from a conventional steam
power plant only in the steam generating part. There is no change in the
turbo-alternator and the condensing system.
The nuclear fuel which is at present in commercial use is
Uranium. Heat energy evolved by the fission reaction of one kg of U235
can produce as much energy as can be produced by burning 4500 tons of high
grade coal.
Uranium exists in the isotopic form of U235
which is unstable. When a neutron enters the nucleus of U235, the
nucleus splits into two equal fragments and also releases 2.5 fast moving
neutrons with a velocity of 1.5 × 107 metres / sec producing a large
amount of energy, nearly 200 millions electron-volts. This is
callednuclear‚fission‛.
Chain
reaction
The neutrons released during the fission can be made to
fission other nuclei of U235 causing a ‚chain reaction. A chain
reaction produces enormous amount of heat, which is used to produce steam‛.
The chain
reaction under uncontrolled conditions can release extremely large amounts of
energy causing ‚atomic explosion‛.
Figure: Nuclear fission.
Energy liberated in chain reaction, according to Einstein
Law, is E = mc2, where E = energy liberated, m = mass in grams, c =
speed of light = 3 ´1010 cm/sec.
Out of 2.5 neutrons released in fission of each nuclei of
U235, one neutron is used to sustain the chain reaction, 0.9 neutron
is converted into fissionable material Pu239 and 0.6 neutron is
absorbed by control rod and coolant moderator.
Function of the moderator is to reduce the energy of
neutrons evolved during fission in order to maintain the chain reaction. The
moderators which are commonly used are ordinary water and heavy water.
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