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Sci Tech
Improving the economic viability of nuclear power plants
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IGCAR has been permitted to increase the burn up of its FBTR fuel to 1,04,000 MWd/t. This will help India reach the International benchmark using an indigenous fuel used for the first time in the world, says R. Prasad.
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Availability of porosities and cracks in the fuel would allow the burn up to go beyond one lakh MWd/t.
THE INDIRA Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, has done a Sergei Bubka by bettering its own record. The results of the post irradiation examination (PIE) done at 50,000 MWday/tonne burn up (cumulative amount of energy that can be extracted per unit mass of the fuel) has helped it to raise its ceiling and continue using the same fuel at the fast breeder test reactor till it reaches 1,04,000 MWd/t burn up. The results could not have been better timed than this as the country is on the threshold of adding nearly 20,000 MW nuclear power by the year 2020 using fast breeder nuclear reactors.
This achievement places India in the league of nations to design, fabricate and permitted to reach a burn up exceeding 1,00,000 MWd/t in a fast breeder reactor. That is not all. It assumes special significance as the fuel used has been fabricated indigenously and has a unique composition (plutonium carbide 70 per cent and uranium carbide 30 per cent) used for the first time in the world. More importantly, it is a vindication to their claims amidst doubts cast initially about the viability of the fuel owing to its unique composition. "The achievement is especially laudable as there is no data on this fuel behaviour available in scientific literature," said K.S. Parthasarthy, Secretary, AERB.
"We have so far reached a burn up of 92,000 MWd/t without any clad failure or fuel clad corrosion. This proves that we can increase the burn up still further," said S.B. Bhoje, Director, IGCAR. His optimism is based not on intuition but on hard scientific facts. And if you believe that IGCAR scientists are resting on their laurels then you are mistaken. They are thirsting for more. According to Dr. Bhoje, there are possibilities of increasing the burn up to 1,10,000 MWd/t till the fuel touches the clad (covering the fuel) and causes some straining of the clad.
The permission to increase the burn up has been accorded by the Atomic Energy Regulatory Body based in Mumbai. But the IGCAR scientists were optimistic and confident of reaching the magic figure of one lakh MWd/t long before based on the PIE data obtained after 25,000 MWd/t burn up. In the end, the permission accorded by AERB has proved them right. "The one lakh MWd/t burn up is an international benchmark. And we have successfully reached there," commented Baldev Raj, Director of Materials, Chemical and Reprocessing Group, IGCAR. "It is an index of our expertise to design and fabricate a new fuel and then reach the international benchmark."
Burn up as explained earlier is the cumulative amount of energy extracted per unit mass of the fuel. So higher the burn higher will be the amount of energy extracted from the same quantity of fuel. This will lead to a situation where less fuel is used, the need to fabricate new fuel comes down and the amount of spent fuel to be reprocessed is also reduced to extract a certain amount of energy. In a nutshell, nuclear energy becomes cheaper and economies of scale are achieved with higher burn ups.
One more important achievement is the permission to increase the linear heat rating from the present 320 watt/cm to 400 watt/cm. Linear heat rating is the amount of heat generated by a centimetre length of fuel.
But what limits the burn up limit and why should one not keep increasing the burn up? The fuel contained inside the clad undergoes continuous fission and releases energy when bombarded by neutrons. As the process continues, the fuel releases gases and fission products that are lower in density than the fuel but occupy more volume. This causes volume expansion leading to fuel swelling.
Fuel swelling can be allowed to continue till such time that it does not touch the clad but only occupies the gap provided between the fuel and the clad. When this gap closes with continued (fuel) swelling, the fuel comes in contact with the clad and causes a fuel-clad interaction. Stress build up is seen in the fuel as well as the clad as the interaction starts. The fuel undergoes compressive stress as it is restrained from expanding indefinitely and the clad undergoes tensile stress trying to accommodate the expanding fuel. After some time the clad ruptures unable to accommodate the swelling fuel or the fuel may shrink. If the clad is stronger then the fuel can shrink. Ruptured clad is not safe as the fuel is no longer contained within. And such a sub-assembly (containing the fuel and clad) has to be removed for safety reasons.
The next post irradiation examination will be done when the fuel reaches 1,00,000 MWd/t burn up. "This will tell us about the characteristics of fuel-clad interaction," Dr. Bhoje indicated. If the fuel is creeping it will not exert pressure on the clad even if it is touching it. But if it has hardened then it can rupture the clad. On the other hand if the clad is stronger then it can cause the fuel to shrink.
Testing a new
fuel composition
The next task for the IGCAR scientists is to test a new fuel composition. "We would change the composition from 70:30 (plutonium carbide and uranium carbide ratio) to 55:45. The enrichment of the fuel with plutonium will come down for the reactor to become critical as the size of the core (where the fuel is kept) becomes bigger. The thermal conductivity will improve and the temperature of the fuel will reduce as the plutonium content is decreased. Hence one can also expect higher burn ups when this happens. The fuel fabrication is done by the Advanced Fuel Fabrication plant at BARC, Mumbai.
Will maintaining the plutonium content at 70 per cent even in a big reactor help extract more heat and hence more power? "No, it does not," said S. Govindarajan, Head of Core Engineering and Component Handling division. When the plutonium content in the fuel is more than what is required then the number of neutrons produced will also be in excess to sustain the chain reaction. Hence more control rods to absorb the extra neutrons need to be added. Neutrons so absorbed are wasted and is not in tune with the breeder technology. "In the end more plutonium is used while equivalent power is not produced," Dr. Govindarajan said.
To produce 40 MW power the amount of plutonium carbide used will be 55 per cent and it be further reduced to 25 per cent when 500 MW power is to be produced. Smaller the core higher should be amount of plutonium used. Hence nuclear bombs contain nearly 90 per cent of plutonium while a 1000 MW nuclear power plant would contain cores having only 18 per cent plutonium. In other words, as the amount of plutonium used is reduced the core size increases and the reactor size becomes bigger. The advantages of larger reactors are the reduction in the leakage of neutrons and increase in nuclear fission.
Already the fast breeder test reactor contains nearly 10 subassemblies with the new fuel composition. "These fuel have already undergone 30,000 MWd/t burn up and we would do post irradiation examination once we reach 50,000 MWd/t burn up," Dr. Bhoje said. And any future replacement of the subassembly at FBTR will contain only the new fuel.
There is no stopping the scientists now. New targets, new goals and tougher challenges are nothing new to these scientists who have developed the fast breeder test reactor from the scratch using indigenous technology. Jai vigyan.
Highlights of PIE results
Post irradiation examination (PIE) was carried out on the fuel sub assembly. Visual examination and dimensional measurements were carried out on the subassembly as well as on the fuel pins extracted from it. Non-destructive examinations were carried out on the pins. Fission gas analysis, metallography of the fuel pins, micro structural analysis and hardness measurements on the clad were also carried out.
Visual examination of the subassembly and the pins indicated their good health. Dimensional measurements revealed no significant deformation either on the hex-can or the pins.
Detailed PIE was carried out on ten fuel pins. Eddy current testing did not indicate any defect in the fuel pin clad. X-Radiography also did not indicate any abnormalities in the fuel pins. Neutron radiography of the fuel pins was carried out for the first time. Pellet to pellet gaps were discernable at the end of the fuel column. There was no evidence of any abnormality or redistribution of actinides in the neutron radiographs.
Fission gas extracted from the fuel pins was analysed. The total gas release into the plenum was found to be within the expected limits.
Metallography of the fuel pins revealed that no restructuring of the fuel has occurred. The fuel microstructure shows a reduction in the porosities when compared to the fuel examined after a burn-up of 25,000 MWd/t. The fuel could undergo further swelling before fuel-clad interaction can exert stresses on the clad. No indication of carburisation was found. Hardness measurements did not reveal any significant change.
"The results show that the fuel has performed well. Absence of any significant deformation on the hex-can and the clad tubes and availability of porosities and cracks in the fuel to accommodate further swelling indicate that higher burn-up can be achieved," said K.V. Kasiviswanathan, Head of PIE and Head of Remote Handling Section, Metallurgy group.
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