CLASSIFICATION OF NUCLEAR REACTORS
Nuclear reactors are classified based on different criteria.
The Criterion and Type of reactor is explained below.
1. Neutron Energy - Fast reactors, Intermediate reactors, thermal reactors.
2. Type of fuel used – Plutonium - 239 and Uranium – 238; Uranium – 238 and Thorium – 232; Uranium – 235 and Uranium – 238
3. Type of fuel enrichment - Natural fuel, Enriched fuel
4 Fuel reproduction characteristic - Non-regenerative, Regenerative breeder
5. Type of moderator (core-moderator) used - Graphite; light water; heavy water; beryllium; organic liquid
6. Moderator arrangement – Homogeneous, Heterogenous
7. Type of coolant used - Light water; heavy water liquid metal; organic liquid gas.
8. Functions, applications and construction -
(i) Research reactors
Ex. Isotope production, Materials testing, As neutron source.
(ii) Plutonium production
(iii) Power reactors
(a) Stationary
Ex. package reactors
(b) Mobile reactors
Ex. naval, aircraft reactors, etc
Reactors based on Neutron Energy:
In fast reactors the fission is caused by fast neutrons and in thermal reactors it is caused by slow neutrons. Initially all the neutrons are fast when emitted in a reactor, but their speed is reduced by using moderators in slow reactors.
Fast reactors employ any atomic fuel; they are small and compact reactor core is made of any structural material. But they require high fuel loading; heat transfer and cooling pose a big problem on account of high power density (thermal power to core volume ratio).
Slow reactors require a relatively low fuel loading; their breeding ratio (ratio of moderator vol. to fuel volume) is not so high they have low power densities and hence cooling problem is simpler. The main drawback with them is that it is almost impossible to breed them with any fuel except Uranium - 233. Because of low power density, they have relatively large volume and are harder to shield.
Reactors based on Fuel used :
Main nuclear fuel occuring in nature is uranium of which 0.7% is fissible U - 235 and 99.3% is fertile U - 238 isotope, Two more possible fuels, Pu - 239 and U - 233 are produced artificially from fertile U-238 and U-232 respectively. Hence U - 235, Pu - 239 and U - 233 are called fissible materials. They are used for power generation in reactors called Burner reactors, On the otherhand, U - 238 and U - 232 are called fertile materials. They can be converted into fissible materials inside a nuclear reactor called converter reactor.
If the fertile material is converted into fissible material similar to the one initially supplied to the core, the reactor is known as Breeder reactor. Thus, if natural uranium is the fuel and thorium is converted into U - 233, of if plutonium is the fuel and U - 239 is converted into plutonium, the process is called breeding, and the reactor in which breeding takes place is called 'breeder reactor. A breeder reactor is also one in which the fertile (natural), material in converted into fissible material at a rate more than that at which the fissible material is consumed.
Reactors based on Coolant used :
Example : Water cooled reactor, Gas cooled reactor, Liquid metal cooled reactor etc.
1. In water cooled reactors water is used both as a coolant and moderator. The water may be ordinary one i,e., light water or heavy water (D20). Light Water Reactor (LWR) has a high hydrogen concentration. It has good thermal properties. However, LWR suffers a drawback that water is corrosive; it must be highly pressurized in order to operate at moderate temperatures. LWRs are again of two types, Pressurized Water Reactor (PWR) and Boiling Water Reactor (BWR).
In the PWR water is pressurized to about I 50 atm. The hot water from the reactor flows to a steam generator where its heat is transferred to the feed water to generate steam. The coolant then flows from the steam generator to circulating pump which pumps it back to the reactor. Steam is produced at relatively low pressure, and temperature, i.e., 75 bar and 300°C.
Nuclear reactors: BWR is the simplest form of nuclear reactor. The condensate from the condenser is circulated through the reactor. It is evaporated into steam which leaves the reactor from its top and enters the turbine.
2. In Gas Cooled Reactor, air, hydrogen, helium or carbon dioxide is used as coolant.
3. In Liquid Metal Cooled Reactors such as Sodium Graphite Reactor, sodium is circulated through the reactor core as a coolant. The heat in the sodium is transferred to NaK liquid metal which in turn gives up its heat to generate steam.