Online edition of India's National Newspaper
Thursday, Oct 10, 2002

About Us
Contact Us
Sci Tech Published on Thursdays

Features: Magazine | Literary Review | Life | Metro Plus | Open Page | Education | Book Review | Business | SciTech | Entertainment | Young World | Quest | Folio |

Sci Tech

Printer Friendly Page Send this Article to a Friend

Fast breeder reactor: Is advanced fuel necessary?

PFBR will use mixed oxide fuel. Use of advanced fuel like nitride or metallic will reduce the time taken to make extra plutonium for a new breeder reactor. Mixed oxide is a proven fuel but has low breeding ratio. The doubling time when advanced fuels are used will be half compared to oxide. It is a question of doubling time versus cost of power production.

THE FAST Breeder Test Reactor (FBTR) at Kalpakkam reaching 100,000MWday/tonne burn up may a great milestone achieved. The Department of Atomic Energy (DAE) is happy but have bigger challenges and taller peaks to conquer. The debate is now on the kind of fuel to be used in future breeder reactors.

"We need to look at advanced fuels like nitride and metallic for the breeder reactors," said Anil Kakkodkar, Chairman of DAE. This comes at a time when the Prototype Fast Breeder Reactor (PFBR) coming up at Kalpakkam is to use mixed oxide fuel.

The FBTR has used mixed carbide fuel of a unique composition. So why use an oxide fuel when all data generated in the FBTR is from a carbide fuel? And now why talk of using advanced fuel when the PFBR is to use an oxide fuel?

Oxide with a fuel ratio of 30 per cent plutonium oxide and 70 per cent uranium oxide is a well-proven fuel world over. The uranium used here is natural uranium (with fissile U {+2}{+3}{+5} comprising only 0.7 per cent). U{+2}{+3}{+5} enriched to 85 per cent was required for FBTR as it has a smaller nuclear core.

As a rule, smaller the core larger is the neutron leakage and vice versa and so to sustain the breeder reaction enriched uranium is mandatory explained S.M. Lee, Director, Safety Research, Health Physics, Information Services, Instrumentation and Electronics group. India had to import enriched uranium if it wanted to go ahead with the FBTR programme using oxide fuel.

"The Pokran adventure made this impossible. So we had to look at other alternatives," said C. Ganguly, Chief Executive, Nuclear Fuel Complex, Hyderabad. Alternatively plutonium rich oxide fuel could have been used but it had its own share of problems. Hence DAE was left with little choice but look at mixed carbide fuel for FBTR.

Here again DAE had to settle for 70 per cent plutonium carbide and 30 per cent (natural) uranium carbide to take advantage of the plutonium availability and to achieve high neutron emission as required by a small core.

Things have changed with PFBR as it has a large core and hence needs no enriched uranium. The debate was whether to use mixed carbide or mixed oxide fuel for PFBR. "We had to settle for oxide fuel due to inherent problems associated with carbide fuel," Dr. Ganguly explained.

Carbide fuel is very difficult to fabricate on a large scale as it is highly pyrophoric and is highly susceptible to oxidation and hydrolysis. Moreover, reprocessing the spent carbide fuel is difficult as it is difficult to dissolve in nitric acid and leaves behind organic complexes. Moreover, carbide fuel has attained a burn up value of just 100,000MWday/tonne. Compare this with nearly 200,000MWday/tonne achieved with oxide fuel in France.

Cost of power production comes down with increased burn up value. Also, oxide fuel is easy to fabricate as it is not phyrphoric and hence needs no inert atmosphere during fabrication. Data on oxide unlike other fuels is available and is very vital for India making a foray into breeder power plants.

Reprocessing possesses no problem either as is the case with carbide fuel. In short, the technology for mixed oxide fuel is very similar to that of uranium oxide used in thermal (nuclear) reactors. "Using oxide fuel will help us to fine tune and perfect other areas of the breeder reactor technology," said S.B. Bhoje, Director, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam.

Breeding ratio

If oxide fuel is better than carbide in all respects then why the debate about using advanced fuels like nitride and metallic in future? The answer is simple. It has to do with breeding ratio. Breeding ratio is the amount of extra plutonium produced in a reactor to start a new one.

Breeder technology works on the principle of self-sustenance and ability to produce more plutonium than it actually consumes to produce power. The breeding ratio is 1.1 in the case oxide fuel while it 1.2-1.3 in the case of carbide and nitride fuels. It is maximum in the case of metallic fuel (1.4-1.5). Hence the doubling time (surplus plutonium produced to start a new reactor) is short in the case of metallic, carbide and nitride fuels compared to oxide fuel.

The doubling time becomes paramount when the country goes on an overdrive to commission new reactors. Hence the stress on using advanced fuels. But Dr. Bhoje feels otherwise. "The PHWR and the new reactors coming up would be able to provide plutonium to commission breeder reactors of 40,000MW capacity. Will we be able to commission reactors in time to utilize even this plutonium? he asked.

He cites the example of coal. Nearly 50 billion tones of coal are available for mining and hundreds of thermal plants can be commissioned to utilize them. But we are not in a position to do so. Similar will be the situation with breeder reactors where more plutonium will be produced using advanced fuel than will be utilized.

"The disadvantage of the lower breeding ratio with oxide fuel is not very critical for the next 30-40 years," Dr. Bhoje said. "We must look at survival first then think about growth. I personally think that we may not use nitride or metallic fuel for breeder programme." According to him the availability of technology, producing power at cheaper cost, gaining expertise and earning public acceptance are vital to breeder programme. And using oxide fuel does just that.

The carbide fuel used in FBTR achieved a burn up of 100,000MWday/tonne. A commendable burn up indeed but at less than 400 watts/cm linear heat rating for most part. A commercial reactor has to have a linear heat rating of 700-800 watts/cm. "We may not be able to achieve this with the present carbide fuel," Dr. Bhoje stressed. The end result — lower burn up when using carbide fuel. "It is not economical to have lower burn up. Also the fabrication and reprocessing would increase the cost of power production," he added.

But the long-term strategy does call in for fuels with shorter doubling time. Plans are to have a few subassemblies with nitride fuel in PFBR. This will help study the fuel behaviour firsthand and collect relevant data to help commission reactors with this fuel in future. Dr. Ganguly is convinced that advanced fuels have to be used at the earliest. But will the reprocessing of the advanced fuels consume more time than oxide fuels? If that be so the advantage of shorter doubling time is compromised.

Using advanced fuel in lieu of oxide fuel can halve doubling time. "But is the country prepared to commission breeder reactors at the same rate? Dr. Bhoje wondered. If not then producing more plutonium and paying more cost for power produced using advanced fuel may not be to India's advantage he felt.

Finally optimising certain parameters like increasing the pin diameter, increasing the density of oxide fuel from 82 to 88 per cent and reducing the stainless steel thickness (which absorbs neutrons) will help increase the breeding ratio to 1.2-1.25 and reduce the doubling time. "Already Russia has been able to shrink the doubling time to ten years and U.S. to 15 years," Dr. Bhoje said.

Metallic fuel uses 70 per cent enriched U{+2}{+3}{+5} (weapon grade). This makes it a costly fuel. Moreover, high swelling is seen limiting the burn up to 10,000MWd/t.

The Angonne National Laboratory in the U.S. has achieved higher burn up of 100,000MWd/t by using low density (70 per cent) fuel inside the clad. On swelling the fuel instead of touching and cracking the clad actually becomes soft due to the presence of high porosity. This has made fuel swelling a non-issue.

Nitride and carbide belong to the same family and have similar thermophysical and thermodynamic properties. It is more stable than carbide asthere are no higher nitrides unlike in the case of carbide.

But nitride fuel is not a panacea either. It has its own share of drawbacks too. It is more stable and easily soluble in nitric acid unlike carbide fuel and hence causes fewer problems at the reprocessing stage.

Also, it is susceptible to oxidation and hydrolysis and is pyrophoric though less in magnitude than carbide fuel. But the biggest drawback with nitride is N{+1}{+4} produces C{+1}{+4} a beta emitter. And large quantities of C{+1}{+4} can posse problems at the reprocessing stage. Enriching N{+1}{+4} to N{+1}{+5} can be done but again involves additional expenditure.

"Some trade off is needed," admits Dr. Ganguly. "In science one needs to be very aggressive and advanced fuel is the solution in the long term if India is serious about nuclear power and want to increase the number of breeder reactors." And the debate goes on.

R. Prasad

Printer friendly page  
Send this article to Friends by E-Mail

Sci Tech

Features: Magazine | Literary Review | Life | Metro Plus | Open Page | Education | Book Review | Business | SciTech | Entertainment | Young World | Quest | Folio |

The Hindu Group: Home | About Us | Copyright | Archives | Contacts | Subscription
Group Sites: The Hindu | Business Line | The Sportstar | Frontline | Home |

Comments to :   Copyright 2002, The Hindu
Republication or redissemination of the contents of this screen are expressly prohibited without the written consent of The Hindu