- Nuclear Reactor Definition - Components of Nuclear Reactor - PWR and its advantages and disadvantages - BWR and its advantages and disadvantages

1. Types of Nuclear
Reactors

2. Nuclear Reactor
 Device designed to maintain a chain
reaction producing a steady flow
of neutrons generated by the fission of
heavy nuclei.
 It is an apparatus in which heat is
produced due to nuclear fission chain
reaction for the generation of the
electricity.

3. Components of Nuclear Reactor

4. Pressure Vessel / Tubes
 Usually a robust steel vessel
containing the reactor core and
moderator/coolant.
 Or it may be a series of tubes
holding the fuel and conveying the
coolant through the surrounding
moderator.

5. Reactor Core
 It consists of fuel elements,
control rods, coolant,
moderator.
 Cores generally have shapes of
right circular cylinders with
diameters ranging from 0.5 to
15 metres.

6. Reflector
It is placed round the
core, to reflect back
some of the neutrons
that leak out from core
surface.

7. Fuel
Uranium-235 is the basic
fuel.
Usually pellets of uranium
oxide (UO2) are arranged in
tubes to form fuel rods.

8. Control Rods
 Usually made up of cadmium or boron.
 Function:
 Slow down the rate of reaction to a
controllable limit . Other wise chain
reaction blows up like a nuclear bomb.
 Raising and lowering the control rods
allow operators to control the rate of the
nuclear reaction.

9. Moderator
 Function: -
To slow down neutrons
from high velocities and hence high energy
level which they have on being released
from fission process so that probability of
neutron to hit the fuel rods increases.
 Main moderator used: -
Water H2O
Heavy water D2O
Graphite
Beryllium

10. Coolant
Function: -
Coolant is used to remove intense heat produced
in the reactor and that heat can be transferred to water in a separate
vessel which is converted into steam and runs the turbine.
 Main coolant used: -
Water H2O , CO2, Hg, He

11. Containment
 The structure around the reactor and associated steam
generators which is designed to protect it from outside
intrusion and to protect those outside from the effects of
radiation in case of any serious malfunction inside.
 It is typically a meter-thick concrete and steel structure.

12. Nuclear
Power
Plants in
India

13. For Starting Reactor
 To start a reactor, a neutron from a source is ejected through
thermal means and the control rods are taken upwards so that the
control rods can not disturb the reaction.
 Hence neutron hits the fuel rods, break it into lighter nuclei,
energy is released, number of neutron keeps on increasing since
K will be greater than 1 for this time period and hence reaction
starts and its rate also increases.
 Hence reaction starts and its rate also increases.

14.  K (the effective neutron multiplication factor) is the average
number of neutrons from one fission that cause another fission.
The remaining neutrons either are absorber in non-fission
reactions or leave the system without being absorbed.
The value of K determines how a nuclear chain reaction
proceeds.
Meaning of K

15. For Maintaining the reaction at constant
level
 When rate of reaction achieves a permissible
value then control rods are inserted between the
fuel rods in such away that K becomes equal to 1.
 Hence the rate of reaction achieves a finite
constant value.

16. For Shutting Down Reactor
 To shut down the reactor either in normal or
emergency conditions, the control rods are
inserted in such away that K becomes less than 1.
 Hence the number of neutrons keeps on
decreasing i.e. rate of reaction decreases, so the
reaction stops after a certain interval of time.

17. Classification of Nuclear Reactor: -
1. On the basis of Neutron Energy
a) Fast Reactors
b) Thermal Reactors
2. On the basis of Fuel used
a) Natural fuel
b) Enriched Uranium
3. On the basis of Moderator used
a) Water Moderator
b) Heavy water Moderator
c) Graphite Moderator
d) Beryllium Moderator
4. On the basis of Coolant used
a) Water cooled reactor (Ordinary
or Heavy)
b) Gas cooled reactor
c) Liquid metal cooled reactor
d) Organic liquid cooled reactor

18. Pressurized Water Reactor (PWR)

19.  Uranium (Uranium – 235) as fuel.
 Chain reaction produces high level of heat.
 Heat tubes pass the heat to the primary cooling system water simply
by contact.
 Primary cooling system is a closed circuit of pressurized water.
 Primary water enters the reactor vessel at 296 °C and exit at 327 °C.
Pressurized Water Reactor (PWR)

20.  The water than passes into a steam generator where it transfers it
heat to a secondary system.
 To ensure that water in the primary system remains in the liquid
state a pressurizer will maintain the constant level of 155 bars.
 Hence the name “Pressurized Water Reactor”.
Pressurized Water Reactor (PWR)

21.  Within the steam generator heat stored in the primary system water
is transferred to a secondary system.
 The heated water enters the bottom of the steam generator transfers
its heat to the secondary system water through the U tubes then
returns to the reactor vessel for a new cycle.
Pressurized Water Reactor (PWR)

22.  Water in the secondary system is heated to boiling temperature turns into
steam and then travels to the turbine set.
 After passing through the turbine the steam is recondensed into liquid
water and return to the steam generator for another cycle.
 Steam pressure from the secondary system drives the generators to
produce electricity.
Pressurized Water Reactor (PWR)

23. Advantages
 Water used as coolant, moderator and reflector is cheap and
available in plenty.
 The reactor is compact and high power density (65 KW/liter).
 Hardly 60 control rods are required in 1000 MW plant.
 Inspecting and maintaining of turbine, feed heaters and condenser
during operation.
 Reducing fuel cost and extracting more energy.

24.  Requires high pressure vessel and high capital cost.
 Thermodynamic efficiency of plant is as low as 20% due to low
pressure.
 Corrosion problems are more severe. Use of stainless steel for
vessel is necessary.
 Fuel recharging requires a couple of months time.
Disadvantages

25. Boiling Water Reactor (BWR)

26.  The core inside the reactor vessel creates heat.
 A steam-water mixture is produced when very pure water (reactor
coolant) moves upward through the core, absorbing heat.
 The steam-water mixture leaves the top of the core and enters the
two stages of moisture separation where water droplets are
removed before the steam is allowed to enter the steam line.
Boiling Water Reactor (BWR)

27.  The steamline directs the steam to the main turbine, causing it to turn
the turbine generator, which produces electricity.
 The unused steam is exhausted to the condenser, where it is condensed
into water.
 The resulting water is pumped out of the condenser with a series of
pumps, reheated, and pumped back to the reactor vessel.
Boiling Water Reactor (BWR)

28.  The reactor's core contains fuel assemblies that are cooled by water
circulated using electrically powered pumps.
 These pumps and other operating systems in the plant receive their
power from the electrical grid.
 If offsite power is lost, emergency cooling water is supplied by other
pumps, which can be powered by onsite diesel generators.
Boiling Water Reactor (BWR)

29.  Boiler water reactor contain between 370-800 fuel
assemblies.
 The cooling water is maintained at about 75 atm (7.6 MPa,
1000–1100 psi) so that it boils in the core at about 285 °C
(550 °F).
Boiling Water Reactor (BWR)

31.  Possibility of radioactive contamination in the turbine
mechanism.
 Safety precautions are needed which can turn out to be costly.
 Wasting of steam and resulting of lower thermal efficiency.
 Only 3% - 5% by mass can be converted to steam per pass.
Disadvantages

32.  Ashvani Shah C&I Reliance. NUCLEAR REACTOR. web. 5 Sep.
2016.
 Abdul Karim et al. NUCLEAR REACTOR AND ITS WORKING.
web. 5 Sep. 2016.
 Nptel Lectures
References