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.
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.
See the India section of www.world-nuclear.org
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GEAR
Experimental Breeder Reactor I was created in 1950.
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You can't compare and contrast nuclear reactors and breeder reactors, any more than you can compare a lion with a mammal. A lion is one example of many mammals; a breeder reactor is just one example of many types of nuclear reactor.