The end of mixed carbide fuel

Twenty years after the Fast Breeder Test Reactor (FBTR) at Kalpakkam in Tamil Nadu went critical, the mixed carbide fuel used in the reactor for the first time in the world had recently reached 1,50,000 MWday/tonne burn-up — the cumulative amount of energy that can be extracted per unit mass of fuel without a single breach in cladding tube that houses the fuel.  This marks the maximum burn-up that the fuel would undergo as the material housing the fuel has reached a low ductility and will not lend itself for further extraction of energy from the fuel. This brings to an end the life of the mixed carbide fuel — 30 per cent uranium carbide and 70 per cent plutonium carbide — in the breeder reactor programme at the Indira Gandhi Centre for Atomic Research (IGCAR).  The 500 MW capacity Prototype Fast Breeder Reactor (PFBR) coming up at Kalpakkam will be fuelled by a mixed oxide fuel — 22-27 per cent plutonium oxide and 78-73 per cent uranium oxide.  Using mixed oxide fuel, a well-proven fuel, in the FBTR could have shrunk the development time of fast breeder reactor technology. The Pokhran adventure in 1974 made this impossible. With the option of importing enriched uranium closed, using mixed oxide fuel to sustain the breeder reaction in the smaller core of the FBTR was not possible.  The choice of mixed carbide fuel was thus forced on the programme despite little knowledge of the fuel characteristics, combined with difficulty in fabricating and reprocessing it.

Though the design of PFBR would not allow the use of carbide fuel, two decades of using the fuel in the FBTR has helped in developing and perfecting other critical components of fast breeder reactor technology. With all the other technological ingredients remaining the same, the Department of Atomic Energy (DAE) would be in a position to choose the fuel  — mixed oxide or metallic — that should be used in future fast breeder reactors.  The final decision will be based largely on factors such as amount of burn-up that needs to be attained, breeding ratio — extra plutonium that a reactor produces compared with what it consumes — and the time taken to produce surplus plutonium for fuelling a new breeder reactor.  While DAE has committed itself to also using metallic fuel in the future, it is yet to develop one; the scientists are far more conversant with mixed oxide fuel.  The target of 1,00,000 MWday/tonne burn-up set by IGCAR scientists before the mixed oxide fuel would be taken out for reprocessing reflects their confidence with mixed oxide fuel. Though PFBR designed for 1,00,000MWday/tonne burn-up is in line with what has been achieved so far in nuclear power reactors globally, the scientists have set themselves the goal of doubling the burn-up in future reactors.  While the lower breading ratio of oxide fuel compared with metallic fuel will mean a longer time to generate sufficient plutonium to start another reactor, the use of metallic fuel will become relevant when the country is poised to build and commission new breeder reactors at a faster pace.