The fuel has proved its capability to light up a 100 watts bulb continuously for 14,880 hours for 620 days using just one gram of the fuel.

Technically speaking, the carbide fuel had reached the maximum burn-up of 1,55,000 MW days/tonne some time ago without any failure of fuel. Burn-up is the cumulative amount of energy that can be extracted from a unit mass of the fuel.

So the higher the burn-up, the greater is the amount of energy that can be extracted from a given amount to fuel.

Closing the fuel cycle

And in what may be termed as closing the fuel cycle, the IGCAR scientists have successfully reprocessed the spent fuel that has undergone 1,55,000 MW days/tonne burn-up.

Reprocessing is very important as only about 17 atoms per cent of the fuel would have undergone fission to produce energy.

So reprocessing helps to extract the valuable plutonium and separate the fission products from the spent fuel. The reprocessed plutonium is refabricated as fuel.

Reprocessed fuel

It was in 2005 that IGCAR reprocessed the fuel that had undergone 1,00,000 MW days/tonne burn-up.

‘Fast breeder’ reactor, as the name indicates, breeds or produces more plutonium than what it consumes for producing energy. This is what gives it the name “breeder.”

So perfecting the reprocessing technology is a must to extract the unspent plutonium for use in a new reactor.

There are other advantages as well. Reprocessing goes a long way in removing the long lived fission products (actinides) and in the process it makes waste management much easier.

Many challenges

For all its advantages, reprocessing is fraught with many challenges, especially when the fuel has undergone higher burn-ups.

Dissolving the 1 lakh MW days/tonne burn–up fuel with nitric acid, among other steps, was done without much of a problem.

“Our assessment was right about the additional challenges of reprocessing the fuel [that has undergone 1,55,000 MW days/tonne burn-up],” said Dr. Baldev Raj, Director of IGCAR.

“There was more radioactivity, more problems with shielding, the separation process, maintenance and degradation of solvents. But there were no surprises.” An additional challenge came in the form of handling the fuel that has undergone a shorter cooling.

Shorter cooling

“We reprocessed the fuel after just 18 months cooling period. This is comparable to commercial reactors,” he said. It may be recalled that the 1 lakh MW days/tonne burn-up fuel was reprocessed after six years of cooling.

“Today we can say that we don’t see any problems with fuel reprocessing,” he said confidently. So he is thirsting for more challenges. “We would like to reduce the cooling period from 18 to 12 months,” Dr. Raj noted.

Experience gained

While IGCAR has gained much experience and hence confidence in reprocessing the mixed carbide fuel used in the fast breeder test reactor, the PFBR would have a mixed oxide fuel.

So will its experience with carbide fuel help in reprocessing the oxide fuel to be used in the PFBR? “Reprocessing [mixed] carbide fuel is difficult unlike [mixed] oxide fuel. We don’t see any difficulty,” said the Director, sounding confident.

IGCAR is taking up yet another challenge. The scientists are putting the reactor to the litmus test of automatically shutting down at the very first instance of a fuel leakage.

“We want to take one subassembly [that contains the fuel pins] beyond the 1,55,000 MW days/tonne burn-up till the clad [metal casing] fails,” said Dr. Raj.

Scientists feel that they would be able to increase the burn-up by another 9,000 MW days/tonne before the clad gives way. The post irradiation examination (PIE) of the fuel indicates that it has already reached the natural end of its life. But the metal casing (clad) has not given way.

The fuel undergoes continuous fission and it swells. It then comes in contact with the metal casing leading to stress build-up at the casing.

The casing ruptures after some time, unable to accommodate the swelling fuel. “There are specific instruments to detect clad [metal casing] failure. These instruments were checked for their responses recently by putting fuel pins with perforated clad [to simulate failed clad] in the core, and these instruments responded well,” said Dr. P.V. Ramalingam, Director of Reactor Operation and Maintenance Group, IGCAR.

Will the metal casing failure leading to fuel leakage not cause problems? “The clad is the first barrier, the primary sodium system is the second, and the reactor containment building is the last barrier,” said Dr. Ramalingam. “So when the clad fails, it is only the first barrier that would have failed. The fission products released would be contained in the sodium system barrier.”

When this happens, the systems are supposed to detect it and shut down the reactor.

The temperature would then drop and the cracks in the clad would get sealed. “The contamination will be only minimal,” said Dr. Ramalingam.

The mixed oxide fuel for the upcoming Prototype Fast Breeder Reactor (PFBR) and two more new fast breeder reactors to come up at the Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, will be produced within the IGCAR premises. The Fast Reactor Fuel Cycle Facility (FRFCF) to produce the refabricated fuel for the prototype reactor is at its pre-project stage.

The FRFCF, in addition to producing the mixed oxide fuel, will also reprocess the spent fuel coming out of the reactors. This reprocessed fuel will ultimately be refabricated and put back into the reactors.

By April 2009

Dr. Baldev Raj, Director of IGCAR expects the sanction for the facility to come through by April next year. The Rs.2,500 crore facility will be ready for commercial use by 2012.

“The idea is to have the reprocessing, refabrication, and making the sub-assemblies (of the fuel pins) co-located within the reactor site,” said Dr. Raj. “Having it co-located is important from the safety and security point of view; and also of economics.” In the absence of this facility, fuel for the PFBR and two other fast breeder reactors to come up at Kalpakkam will have to be fabricated at BARC and transported all the way from Mumbai.

The focus

Since the focus is on co-location of the fuel fabrication, reprocessing and refabrication facility with the reactors, the other two fast breeder reactors to come up elsewhere in the country by 2020 will have similar co-located facilities.

“It is an ideology to have fuel fabrication co-located wherever the reactor is,” he noted.

Since the FRFCF is expected to be operational only by 2012, two years after the PFBR goes critical, the fuel for the first two cores would be fabricated by the Advanced Fuel Fabrication Facility (AFFF) at BARC, Mumbai. This is because changing the fuel in the core, called a campaign, needs to be done once in 8 months in the case of the PFBR.

Hence the facility coming up at Kalpakkam will only refabricate the reprocessed fuel for PFBR. However, it would be able to fabricate the fuel for the two new reactors coming up within the same premises at Kalpakkam.

More demanding

“The technology to refabricate a plutonium based fuel is much more demanding and challenging,” said Dr. Raj, “as it [the fuel] will still have some radioactivity.”

The FRFCF facility, according to him, will be much more complex and sophisticated compared with the Hyderabad based Nuclear Fuel Complex.It may be remembered that the mixed carbide fuel for the FBTR was fabricated by the BARC facility and not the NFC, Hyderabad. Scientists at IGCAR already have some experience refabricating mixed oxide fuel on a pilot scale. A demonstration fuel reprocessing plant is expected to be ready by 2010.

The current burn-up target for the mixed oxide fuel in the PFBR is 1 lakh MW days/tonne. The scientists at the Indira Gandhi Centre for Atomic Research (IGCAR) at Kalpakkam are striving to increase this by 50 per cent to 1,50,000 MW days/tonne.

The rationale

The rationale behind increasing the burn-up is to get more energy from a unit mass of the fuel. But more importantly, the scientists want to increase the time between two fuel handling campaigns from 8 to 12 months. Increasing the burn-up would give them more time between fuel campaigns.

This is analogous to increasing the time interval between fuel refills of a motor vehicle by improving its fuel efficiency by 50 per cent so that a litre of fuel lasts longer.

With 50-60 sub-assemblies containing 217 fuel pins per sub-assembly taken out during each fuel changing campaign, fuel reprocessing and refabrication have to be completed by the time the next fuel campaign is due.

The 8 months cooling period poses an additional challenge.

“Future fuel handling campaigns will be at 12-month interval. So if we develop the technology to reprocess a 12- month-cooled fuel, then we are like online,” said Dr. Baldev Raj, Director of IGCAR.

Target 2011

IGCAR is working on an advanced oxide fuel that would help have a burn-up of 1,50,000 MW days/tonne. The Director is confident that they would be able to achieve this by 2011.

Incidentally, the advanced oxide fuel will have no changes in the fuel composition.

Unlike the mixed carbide fuel, it is the clad (metal that cases the fuel) and wrapper that set the limit for achieving higher burn-ups. So improving the steel used in making the clad and wrapper would make it an advanced oxide fuel.

The shift

“We are working on changing the composition and thermo-mechanical treatment of the steel used for making the clad and wrapper,” said Dr. Raj, “so the shift is from fuel to metallurgy for achieving a higher burn-up.” He is even confident of increasing the burn-up to 2,00,000 MW days/tonne by 2015.

Fast breeder reactors (the sixth reactor onwards) that would come up after 2020 will be 1,000 MW and not 500 MW reactors. And they would have metallic fuel and not mixed oxide fuel.

The entire fuel core of the Fast Breeder Test Reactor (FBTR) would be changed to metallic fuel by 2017. “We are getting ready to irradiate advanced metallic fuel in FBTR by 2010,” said Dr. Baldev Raj, Director of the Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam.

Pilot facility

A facility to fabricate, reprocess and refabricate metallic fuel would come up at IGCAR and become operational by 2014. IGCAR is first setting up a pilot reprocessing facility at its complex.

The need to go in for a metallic fuel is understandable. The ability to put up new fast breeder reactors depends on the amount of plutonium available. Breeder technology, as the name indicates, produces or “breeds” more plutonium than it consumes for producing energy.

Self-sustaining

Hence fast breeder reactors are not only self-sustaining but have the capability to produce extra plutonium to start new reactors.

The rate at which surplus plutonium is produced depends on the fuel used in a fast breeder reactor. In the case of mixed carbide fuel used in the FBTR, the extra plutonium produced would be 0.2-0.3 (breeding ratio being 1.2-1.3); it would be just 0.1 (breeding ratio being 1.1) in the case of oxide fuel.

However, in the case of metallic fuel the plutonium gain would be 0.3-0.5. The breeding ratio is 1.3-1.5.

“We are aiming at a breeding ratio of 1.5 against 1.3 which is the norm,” said Dr. Raj.

Faster doubling time

The higher the breeding ratio, the faster would be the doubling time — (time taken to produce surplus plutonium to start a new reactor).

So the doubling time would be the least in the case of metallic fuel and the most in the case of oxide fuel.

“We would have at least 15 years’ advantage to get the necessary fuel [plutonium] for another 1,000 MW reactor,” said Dr. Raj.

The plutonium content of 20-25 per cent in metallic fuel would be the same as in carbide or oxide fuel.

“But there are no lighter elements like oxygen or carbon to absorb the neutrons [in metallic fuels],” explained Dr. Raj on why metallic fuels have a higher breeding ratio and hence faster doubling time.

The reprocessing of metallic fuel will be very different from other fuels.

“The reprocessing technology is very difficult,” he said, “it will be a pyro-metallurgical route.”

Though the FBTR fuel core would be changed from carbide to metallic, it would not be possible to do the same with the Prototype Fast Breeder Reactor (PFBR) or the two reactors coming up at Kalpakkam.

Design constraint

“We can’t put metallic fuel in these reactors as the design does not permit it,” he said, “though it is a desired situation.”

But he and the scientists at Kalpakkam have not given up on the idea. “We will continue to explore ways of doing this,” he said, sounding optimistic.