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NUCLEAR RAECTOR – MAIN PARTS AND THEIR FUNCTIONS

NUCLEAR POWER PLANTS

NUCLEAR RAECTOR – MAIN PARTS AND THEIR FUNCTIONS

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Reactor is that part of nuclear power plant where nuclear fuel issubjected to nuclear fission and the energy released in the process isutilized to heat the coolant which may in turn generate steam or beused in a gas turbine

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The main function of the reactor is to control the emission andabsorption of neutrons. The neutrons emitted as a result of chainreaction are accounted for in the following ways:

Non-fission

absorption

by

control

rod

materials,

moderator

material, fission fragments and impurities

Non-fission absorption by U-238 present in natural uranium toform fissionable Pu-

Fission absorption by U-235, Pu-239 and U-

Escape from the fissionable material

NUCLEAR POWER PLANTS

NUCLEAR RAECTOR – MAIN PARTS AND THEIR FUNCTIONS •^

The size of core, just sufficient to maintain a chain reactionis the “critical size”. It can be brought down by usingenriched uranium as fuel

-^

It is desirable to use core as a cubical or cylindrical in shaperather

than

a^

spherical,

as

it

facilitates

the

refueling

operation

and

simplifies

the

process

of

circulation

of

coolant through the core.

-^

With the configuration, the core has a series of parallel fuelelements in the form of thin plates or small rods, withcoolant

flowing

axially

and

additional

moderator

or

reflector material surrounding the assembly

NUCLEAR POWER PLANTS

NUCLEAR RAECTOR – MAIN PARTS AND THEIR FUNCTIONS •^

For using the reactor to convert the fertile material intofissionable material, the material to be converted should beput around the core so that the neutrons, which otherwisewould escape the core, would be utilized for conversion.

-^

This arrangement also simplifies the process of separationof the converted material during fueling reprocessing. 2)

Moderator

-^

Neutrons produced by the fission process are ejected fromthe nucleus at a very high velocity of about 1.5 x

଻^

m/s

and therefore, have a very large kinetic energy and aretermed as fast neutrons.

NUCLEAR POWER PLANTS

NUCLEAR RAECTOR – MAIN PARTS AND THEIR FUNCTIONS •^

For more effective use in nuclear reactor, it is desirable toslow down fast neutrons to speeds corresponding to thespeed of molecules in a gas at NTP (i.e., to a speed of about2.2 x

ଷ^

m/s). Such neutrons are known as slow or

thermal neutrons.

-^

The absorption properties of U-238 are very much reducedwith

thermal

neutrons.

Thus,

if

natural

uranium

is

bombarded by thermal or slow neutrons, the chain reactioncan be maintained.

-^

This is accomplished with the help of moderator which ismixed with the fissile material in a suitable manner

NUCLEAR POWER PLANTS

NUCLEAR RAECTOR – MAIN PARTS AND THEIR FUNCTIONS •^

Thus, the purpose of moderator in the reactor core is tomoderate

or

reduce

the

neutron

speed

to

a^

value

that

increases the probability of fission occurrence.

NUCLEAR POWER PLANTS