The main disadvantage is the positive temperature coefficient of reactivity.
You may mean FBR - this stands for Fast Breeder Reactor
Dounreay PFR (Prototype Fast Reactor)
Kalpakkam has both Thermal reactor (Madras Atomic Power Station) and Fast Breeder Reactor -FBTR and PFBR (Under construction). In MAPS (thermal reactor) it is Heavy Water (D2O) Which acts as a coolant as well as moderator, where as in Fast Breeder Test Reactor (FBTR) and Prototype Fast Breeder Reactor (PFBR) Sodium (Na) is used as coolant. Since there should not any moderators for fast reactors D2O will not be used as coolant in fast reactors.
Thermal Breeder Reactors use moderators but Fast Breeder Reactors don't use moderator.
See the India section of www.world-nuclear.org
Moderator is not used in case of fast breeder reactor because there is no need to slow down neutron energy. Nuclear fission takes place at high energy of neutrons.
GEAR
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A type of reactor called a "Fast Breeder" reactor.
Experimental Breeder Reactor I was created in 1950.
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