Power Plant Engineering

1. The gaseous diffusion method. This method is based on the principle that the diffu-
sion or penetration molecular of a gas with a given molecular weight through a porous barrier is
quicker than the molecules of a heavier gas. Non-saturated uranium hexa-flouride (UF
6
) is used
for gaseous diffusion. The diffusing molecules have small difference in mass. The molecular weight
of U
235
Fs = 235 + 6 × 19 = 349 and that U
238
Fs = 352. The initial mixture is fed into the gap between
the porous barrier. That part of the material which passes through the barrier is enriched product, en-
riched in U
235
Fs molecules and the remainder is depleted product.

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NUCLEAR POWER PLANT
335
2. Thermal diffusion method. In this method (Fig. 10.9) a column consisting of two concentric
pipes is used. Liquid UF
6
is filled in the space between the two pipes. Temperature of one of the pipes is
kept high and that of other is kept low. Due to difference in temperature the circulation of the liquid
starts, the liquid rising along the hot wall and falling along the cold wall. Thermal diffusion takes place
in the column. The light U
235
Fs molecules are concentrated at the hot wall and high concentration of
U
236
Fs is obtained in the upper part of the column.
Liquid
UF
6
Enriched
product
Enriched
product
T = T
1
2
T
2
Magnetic field
region
Fig. 10.9
3. Electromagnetic Method. This method is based on the fact that when ions moving at equal
velocities along a straight line in the same direction are passed through a magnetic field, they are acted
upon by forces perpendicular to the direction of ion movement and the field.
Let
P = force acting on ion e = charge on ion
v = velocity of ion
H = magnetic field strength m = Ion mass
R = radius of ion path P = euH
As this force is centripetal
∴
P = 
2
mv
R
∴
2
mv
R
= evH
∴
R = 
mv
eH
This shows that ions moving at equal velocities but different masses move along ng circumfer-
ences of different radii (Fig. 10.10). Fig. 10.11 shows an electromagnetic separation unit for uranium
isotopes. A gaseous uranium compound is fed into the ion source, where neutral atoms are ionised with
the help of ion bombardment. The ions produced come out in the form of narrow beam after passing
through a number of silos. This beam enters the acceleration chamber. These ions then enter a separation
chamber where a magnetic field is applied. Due to this magnetic field the ions of different masses move
along different circumference.

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336
POWER PLANT ENGINEERING
Ion
source
Accelerating
electrodes
H.V. Power
supply
Magnetic
field
region
Collectors
U
235
U
235
Filament
Light molecule
Heavy moelcule