334 POWER PLANT ENGINEERING
10.18 COMPARISON OF NUCLEAR POWER PLANT AND STEAM
POWER PLANT
The cost of electricity generation is nearly equal in both these power plants. The other advan-
tages and disadvantages are as follows :
(i) The number of workman required for the operation of nuclear power plant is much less than a
steam power plant. This reduces the cost of operation.
(ii) The capital cost of nuclear power plant falls sharply if the size of plant is increased. The
capital cost as structural materials, piping, storage mechanism etc. much less in nuclear power plant than
similar expenditure of steam power plant. However, the expenditure of nuclear reactor and building
complex is much higher.
(iii) The cost of power generation by nuclear power plant becomes competitive with cost of
steam power plant above the unit size of about 500 mW.
10.19 Multiplication Factor
Multiplication factor is used to determine whether the chain reaction will continue at a steady
rate, increase or decrease. It is given by the relation,
K =
()P
AE+where K = Effective multiplication factor.
P = Rate of production of neutrons.
A = Combined rate of absorption of neutrons.
E = Rate of leakage of neutrons.
K = 1 indicates that the chain reaction will continue at steady rate (critical) K > 1 indicates that
the chain reaction will be building up
(super critical) whereas K < 1 shows that reaction will be dying down (subcritical).10.20 Uranium Enrichment
In some cases the reaction does not take place with natural uranium containing only 0.71% of
U^235.
In such cases it becomes essential to use uranium containing higher content of U^335. This is
called U^235 concentration of uranium enrichment. The various methods of uranium enrichment are as
- 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.