Types of Nuclear Reactor

Types of Nuclear Reactors

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.

Components of Nuclear Reactor

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.

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.

Reflector

It is placed round the

core, to reflect back

some of the neutrons

that leak out from core

surface.

FuelUranium-235 is the basic

fuel.

Usually pellets of uranium

oxide (UO2) are arranged in

tubes to form fuel rods.

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.

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

CoolantFunction: - 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

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.

Nuclear Power

Plants in India

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.

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

https://en.wikipedia.org/wiki/Six_factor_formula

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.

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.

Classification of Nuclear Reactor: -1. On the basis of Neutron Energy

a) Fast Reactors

b) Thermal Reactors

2. On the basis of Fuel used

c) Natural fuel

d) Enriched Uranium

3. On the basis of Moderator used

e) Water Moderator

f) Heavy water Moderator

g) Graphite Moderator

h) 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

Pressurized Water Reactor (PWR)

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.


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”.


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.


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.


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.

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

Boiling Water Reactor (BWR)

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.


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.


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.


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).


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

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

Types of Nuclear Reactor

Engineering

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