Types of Nuclear Reactors in the World
1. POWER REACTOR :
A certain quantity of enriched uranium or plutonium (either in the form of pure metal or in the form of solution in tubes) is used as fuel which is the source of energy in the reactor. As a result of fission (bombardment of nucleus of uranium atom by a neutron) large quantity of heat is generated. The temperature is regulated by control rods made of cadmium. Rate of fission and hence temperature is reduced by Pushing down the rods. The heated liquid (coolant) is passed through a heat exchanger, Water circulated through this heat exchanger is evaporated into steam by this hot liquid. Steam is utilised to run the turbine which in turn drives an electrical generator (alternator) thus producing electrical power. It is found that 1 tonne of uranium can produce as much electricity as that produced by 200 tonnes of coal.
2. GAS COOLED REACTOR (GCR) :
The circuit for GCR is same as that of PWR (Figure) with the following differences.
The reactor core includes a graphite moderator.
• The coolant would be a gas (air, hydrogen, helium or carbon-dioxide).
• Instead of pump a blower or gas compressor is used.
• No pressuriser is needed.
The vessels and piping would be considerably larger because of lighter density of coolant.
The boiler tubes would have extended fins to improve heat transfer rates. Gas pressure would be less than 0.5 bar and the gas could be carbon dioxide.
These are two main types of GCRs namely,
(i) Gas Cooled Graphite Moderated Reactor (GCGM). and
(ii) High Temperature Gas Cooled Reactor (HTGC).
In the first type carbon dioxide is used as coolant and in the second type helium is used. Both types are graphite moderated.
The GCGM uses natural uranium fuel while HTGC reactor employs highly enriched uranium carbide mixed with thorium carbide and clad with graphite. Fig. 6.9 shows Gas cooled reactor. It is known as Hinkley Point GCR.
Advantages :
• high temperature is available with low pressure.
• excellent heat transfer
• high conversion ratio
• superheating of steam is possible
Disadvantages
• leakage problem
• undue thermal stresses
3. LIQUID METAL COOLED REACTOR :
This reactor uses enriched U-235 as fuel element in the form of rods. Coolant is liquid sodium and it circulates at moderate pressure. Boiling point of sodium being 882oC, the coolant can transfer heat at higher temperatures without subjecting it to high pressure. To produce the superheated steam the reactor can be operated at atmospheric pressure. The moderator used in graphite in hexagonal cylindrical form clad with zirconium to prevent absorption of sodium coolant.
With enrichment of fuel upto 93% for the heat output of 20 MW, the electrical power output is 6.4 MW. Similarly for a 1000 MW thermal rating, the net electrical power is 220 MW.
High temperature is available with low pressure. Temperature of sodium at reactor outlet is 510°C. The secondary sodium superheated steam at 438°C.
The liquid Na metal is circulated through primary heat exchanger by centrifugal pumps. The secondary heat exchanger behaves as a steam generator.
Advantages :
• high temperatures available at low pressures
• effective heat transfer
• high conversion ratio
• superheating of steam is possible.
Disadvantages :
• sodium reacts violently with water and is corrosive.
• heat exchanger must be leak proof.
• thermal stresses pose a problem
• intermediate system is necessary to separate active sodium from water.
4. FAST BREEDER REACTOR (FBR) :
Additional neutrons are produced besides heat when U-235 is fissioned. If some U-238 is kept in the same reactor, these additional neutrons convert U-238 into fissible plutonium Pu-239. This substance is capable of maintaining chain reaction. Producing fissible material than that can be consumed is called breeding. About 50% of U-238 in an FBR can be converted into fissible plutonium in a period of about 30 years.
FBR in its simplest form (Figure ) is a small vessel in which the necessary amount of U-235 is used as a core. No moderator is used. Liquid sodium metal is used as a coolant. The central U-235 core is surrounded by a blanket of U-235, which absorbs the excess of neutrons and converts U-238 into plutonium. Part of the plutonium participates in the chain reaction and fissions to produce heat. The reactor core is cooled by liquid sodium metal. Necessary neutron shielding is provided by the use of light water, oil or graphite. Additional shielding for γ-rays is also provided.
Advantages :
• high breeding gain is possible.
• no moderator is required.
• high power density.
• absorption of neutrons is low.
Disadvantages :
• enriched fuel is required.
• neutron flux is high at the centre of core.
• specific power is not as high as in thermal reactors,
• liquid sodium is extremely' corrosive.
• handling of sodium is a major problem as it becomes excessive hot and radioactive.