Stage 1
- Pressurized Heavy Water Reactors are based on natural uranium that contains 99.3% U-238 and 0.7% U-235.
- U-235 being fissile sets up the chain reaction and U-238 being non-fissile gets converted to Pu-239 as a byproduct (spent fuel) which is in turn used in the Fast Breeder Reactors in the 2nd stage.
Stage 2
- Fast Breeder Reactors are primarily based on Plutonium.
- Further, the breeder reactors use a mix of Plutonium-239 produced in the 1st stage and U-238 which is abundantly found on Earth to produce Plutonium inside the reactor.
- Since the amount of Plutonium produced inside the reactor is more than that initially used, it is called the Breeder reactor.
- In order to increase the probability of neutrons hitting U-238, these reactors do not use a moderator to slow down neutrons and thus they are called Fast Breeder Reactors.
- Once the Plutonium-239 is completely used, Thorium is introduced in the reactor to convert it into U-233 which will be used in the 3rd stage.
Stage 3
- Thermal Breeder Reactors use U-233 which is produced in the 2nd stage using thorium-232.
- Thorium is also not radioactive and thus not fissile.
- Since these reactors also produce U-233 out of Thorium-232, they are called breeder reactors.
- India has abundant thorium reserves in the form of monazite sand, and therefore the 3rd stage is critical for India’s nuclear energy basket.