# Liquid Metal Fast Breeder Reactors (LMFBR) and its Advantages and Disadvantages

1. When uranium U235 is fissioned by slow neutrons it produces heat and an additional neutron. In case a fertile material like U238 is kept in the same reactor surrounding the core of U235, the fast moving additional neutron is absorbed by U238 and converts it into plutonium (Pu239), a fissile material.
2. This man made fuel Pu239 can be used for further fission. Therefore, these type of reactors are important since they not only produce heat but also produce more secondary fissile fuels like plutonium more than fuel consumed in the reactor.
3. This is known as breeding. Similarly, thorium (Th232) can be coqverted into U233 which is also a secondary fissile material.
4. Since India has massive reserves of thorium and limited resources of uranium, development of these fast breeder reactors are important for our country.
5. A fast breeder reactor is shown in Figure. The enriched uranium U235 or Pu239 is kept without a moderator in the reactor core surrounded by the thick blanket of depleted U238. One additional neutron available from fission of U235 is used to convert U238or U233 as secondary fuels.
6. As in the case of sodium-graphite reactor, this reactor also uses two liquid ex coolants in which sodium is used as primary coolant and sodium potassium as seccade coolant. In fast breeder reactors the neutron shielding is provided by the use of boron or graphite In order to protect against gamma radiations a shield is provided made of lead or concrete or of other materials.
Advantages of Liquid Metal Fast Breeder Reactors (LMFBR) :
1. Moderator is not required.
2. Secondary fusible materials by breeding are obtained.
3. Absorption of neutron is slow.
4. Small core is sufficient since it gives high power density compared to other reactors.
Disadvantages of Liquid Metal Fast Breeder Reactors (LMFBR) :
1. Requires enriched uranium as fuel.
2. Thick shielding is necessary against radioactive radiations in primary and secondary circuits as in case of sodium graphite reactors.
3. Neutron flux is high at the centre of the core.