Types of Nuclear Reactors

Types of Nuclear Reactors

1. Pressurised Water Reactor (PWR) :

• It is similar to the reactor we have studied (Refer Figure. A). The arrangement of PWR is shown in figure A.
• The fuel used in PWR is enriched Uranium. Water under pressure is used as both moderator and coolant. The water does not boll when it is in the reactor core because it is circulated very far (under pressure). The desired pressure is maintained on water by pressure equalizer. So water absorbs the heat generated due to chain reaction and transfers this heat to steam circuit or heat exchanger. In heat exchanger and biller, steam is generated. This steam is passed on to the turbine, so it starts rotating and mechanical input is given to alternator which generates the electricity. The steam after doing useful work is passed on to the condenser where it is condensed and recirculated in heat exchanger.
• Since, the water (coolant) actually passes through the reactor core, it becomes radioactive therefore the heat exchanger also need shielding along with reactor.

Figure (A)

2. Boiling Water Reactor (BWR) :

• This type of reactor uses enriched Uranium as a fuel. Water is used as both coolant and moderator. The uranium elements are arranged in a particular lattice form inside a pressure vessel which contains water.

Figure (B)

• The heat energy released during fission is absorbed by the water and it is converted to steam in the vessel itself. The steam is passed on to the turbine, so it starts rotating and mechanical Input is given to the alternator which generates the electricity. The steam after doing useful work is passed on to the condenser where it is condensed and is then recirculated in the reactor.

3. Advanced Gas Cooled Reactor

• A gas cooled reactor employes a gas as coolant Instead of water. A heat exchanger is necessary.
• Even though a gas is Interior to water so far as heat transfer properties are concerned, it offers number of advantages which do not exist with water. However, because of its poor heat transfer qualities, a large quantity of gas is required for circulation. 
• This results in increased consumption of auxiliary power. Not withstanding this a gas cooled reactor is inherently safer than a water cooled reactor. Other advantages are : Less severe corrosion problems and capability for utilization of natural uranium as a fuel. 
• Two principle classes of gas cooled reactors are :

1. Gas cooled graphite modulated (GCGM) reactor.
2. High temperature gas cooled (HTGC) reactor.

• The first uses carbon dioxide as a coolant while the second uses hellum as coolant.

4. Fast Breeder Reactor :

• A fast breeder reactor is a small vessel in which the necessary quantity (corresponding to critical mass) of enriched uranium or plutonium is kept without a moderator. The vessel is surrounded by a fairly thick blanket of depleted fertile uranium.
• The fertile material absorbs neutrons from the fissile material and gets converted into fissile material. The reactor core is cooled by liquid metal. Use can be converted to Pun (or Th₂₃ to U₂₃₃); the latter can be used in other thermal or fast breeder reactors.
• In fast breeder reactors, neutron shielding is provided by the use of boron; light water, oil or graphite. Gamma-ray shielding is effected by lead, concrete, concrete with added magnetite or barium etc.

Figure (C)

• A fast breeder reactor is shown in Figure C.

Boiling Water Reactor (BWR)

Advantages of Boiling Water Reactor :

1. Heat exchange circuit is eliminated, hence cost is reduced.
2. As water is allowed to boil inside the reactor, the pressure Inside the reactor vessel is considerably low, hence reactor vessel can be made much lighter reducing its cost considerably.
3. The BWR cycle is more efficient than PWR.
4. The metal surface temperature is lower than in the case of a PWR.
5. A BWR is more stable than a PWR.

Disadvantages of Boiling Water Reactor :

1. There is a possibility of radioactive contamination in the turbine mechanism should there occur any failure of fuel elements. Hence more safety measures are required.
2. There is wastage of steam resulting in lowering of thermal efficiency on part load operation.
3. Since the power density of a BWR is nearly half than PWR the size of vessel will be considerably large in comparison to that of PWR.
4. Boiling of water on the surface of fuel allows the possibility of burn out of fuel. 
5. 5. It cannot meet sudden Increase in load.

PWR Reactor

 Advantages of PWR Reactor :

1. It is relatively compact in size compared with other types.
2. There is a possibility of breeding plutonium by providing a blanket of U.
3. The reactor has a high power density
4. As heat exchanger is used, containment fission products is possible.
5. An inexpensive substance (light water) can be used as a moderator-cum-coolant-cum reflector.
6. It responds to supply more power when the load increases.

Disadvantages of PWR Reactor :

1. Moderator remains under high pressure, a strong pressure vessel is required.
2. Expensive cladding material is required to prevent corrosion.
3. There is heat loss due to use of heat exchanger.
4. More elaborate safety devices are required.

Fast Breeder

Fast Breeder Reactor Advantages :

1. More efficient use of uranium is made as compared to thermal reactor.
2. It might be possible to produce Puz » s from natural uranium,
3. There is more latitude in selection of structure material.
4. Fast reactors are characterized by small volumes and compactness.

Fast Breeder Reactor Disadvantages :

1. There is a high specific power in small volume, which makes heat removal difficult.
2. They are more sensitive to changes in dimension and geometry as compared to thermal reactors. 
3. The technique of handling sodium/ potassium coolant is not much developed.
4. Large amount of enriched fuel (U32 235 or Pu90239 m93233) are required, which is quite expensive.