FBTR: mixed carbide fuel reaches natural end of life

Published in The Hindu on December 1, 2005

Twenty five years since the Fast Breeder Test Reactor (FBTR) based at Kalpakkan went critical, the indigenously developed mixed carbide fuel used in the reactor has reached a peak burn-up of 1,50,000 MWday/tonne a week ago.

“With this we have come to an end of life for this particular fuel,” said Dr. Baldev Raj, Director of Indira Gandhi Centre for Atomic Research (IGCAR). “The story ends now.” And with a happy note.

Burn-up is the amount of energy that can be extracted from a given quantity of fuel. Higher the burn-up, lesser will be the fuel used. It also means that lesser fuel would need to be reprocessed and lesser radioactive waste generated.

Great achievement

This marks the landmark achievement of the nuclear scientists who had worked against all odds. The end of life for the carbide fuel marks the tremendous confidence that the scientists have gained in the last two decades.

There was a compulsion to use carbide fuel — a first of its kind in the world — in the face of restrictions imposed on India for importing enriched uranium. By taking the burn-up of mixed carbide fuel to 1,50,000 MWday/tonne, the scientists have proved a point — restrictions can at most delay but never prevent the indigenous development of technology.

“We had initially set a target of 25,000 MWday/tonne burn-up when we started the programme,” he noted. “There was little data on the fuel and uncertainty was high.”

The main concern then was not on the structural part (of the cladding material used for housing the fuel) but on the fuel itself and the way it would interact with the cladding. That highlights the long way the scientists have come.

Every time the target burn-up was reached, the IGCAR scientists raised the bar. So why not now?

“The cladding and wrapper material have reached a very low ductility of 3 per cent. It was more than 50 per cent when we started,” he explained. “Anything that is brittle is prone to failure.” And they are not game for it.

There has been no pin failure. There has been no leak of the fuel or fission material either. “The structural integrity is good. But as a precautionary method we would not like to take it to higher burn-ups,” Dr. Raj highlighted.

Deformed cladding

Even at 1,00,000 MWday/tonne burn up the fuel had touched the cladding; it is expected that the cladding would have deformed much more now as the fuel continuously expands when undergoing higher burn-up. If the ductility is not as low as predicted from experimental data and models, chances are that the fuel would be taken to higher burn-up. “But the chances are remote,” he said candidly.

Reprocessing of the carbide fuel that has undergone 1,50,000 MWday/tonne burn-up will be undertaken after 6-8 months.

They have now trained their guns on another challenge set by the Department of Atomic Energy — using metallic fuel in future nuclear power plants (The Hindu, November 2004). “Metallic fuel is not new. But reprocessing technology is not that well developed,” he explained. He is confident that fuel fabrication will pose little challenge.

Metallic fuel has good breeding ratio (the amount of plutonium produced being higher than what is consumed while producing power), shorter doubling time (the amount of plutonium produced to be used as fuel in another power plant), and less radiation exposure at the fabrication stage.

Ambitious target

“We are ambitious to reach a burn-up of 1,50,000-2,00,000 MWday/tonne with metallic fuel,” he noted. “The limitation will arise only from the cladding and wrapper.”

Efforts are already on to find a material that will enable the scientists to go beyond the target set by adjusting the composition of the material and thermo-mechanical treatments of the cladding and wrapper. One subassembly containing 37 pins of mixed oxide fuel has already undergone 55,000 MWday/tonne burn-up in FBTR.

It may be recalled that the 500MW Prototype Fast Breeder Reactor (PFBR) coming up at Kalpakkam will be fuelled with mixed oxide fuel.

With advanced structural material being used in cladding and wrapper, Dr. Raj is confident that one subassembly containing mixed oxide fuel that is put in the FBTR would be taken out only after it completes 1,00,000MWday/tonne burn-up.