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Sci Tech
Realistic evaluation of materials
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Testing materials in air provides data needed for setting up breeder reactors though these data are not realistic. This lacuna has been filled says R.Prasad. IGCAR scientists have now set up a facility to understand the material charact eristics in the presence of hot flowing sodium.
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First testing of materials for fatigue was done last month at Kalpakkam.
SAFETY REQUIREMENTS make it mandatory to keep petrol and fire as far away as possible. But imagine a situation when petrol and fire have to be kept in close proximity separated by only a few millimetres thick stainless steel pipe.
That is exactly what the scientists at the Indira Gandhi Centre for Atomic Research (IGCAR) have done by using sodium and water at the Fast Breeder Test Reactor at Kalpakkam.
Here sodium is used to extract heat from the nuclear reactor and pass it on to water that gets converted into steam and drives the turbines to generate electricity. Liquid sodium at 550 {+0}C is highly explosive in the presence of air or water.
Materials that come in contact with sodium are subjected to various physical and chemical effects. Of this, creep, fatigue, creep-fatigue interaction and carbon removal from the stainless steel material (decarburisation) and deposition at other places (carburisation) by sodium cause maximum damage to the materials. These effects were traditionally simulated and studied by subjecting the material to (simulated) conditions in air. "The design for the Prototype Fast Breeder Reactor (PFBR) coming up at Kalpakkam was done based on data generated by such studies done in air," said S.L. Mannan, Associate Director, Materials Development Group.
But will studies done in air satisfy the safety requirements under actual working conditions with flowing sodium? "The data generated by testing the material in air meets the basic requirements for setting up breeder reactors," stressed Baldev Raj, Director of Materials, Chemical and Reprocessing Group. Yet scientists cannot get realistic data based on testing materials in air.
This lacuna has now been filled with the scientists at IGCAR setting up an advanced facility for understanding the material characteristics in the presence of hot flowing sodium in an In-Sodium Test Facility (INSOT).
The INSOT has a miniature version of the sodium loop as seen in a breeder reactor. This allows testing materials for various parameters in a realistic environment as encountered in a reactor. The first testing of the materials for fatigue was done last month at this facility.
The achievement gains special significance after many countries unsuccessfully tried simulating the effects of flowing sodium by carrying out experiments in air after exposing the material to sodium. "It is now accepted that such testing cannot simulate the synergistic effects of applied stress and flowing sodium," said K. Bhanu Sankara Rao, Head, Mechanical Metallurgy Division.
Two separate sodium loops have been constructed for creep and fatigue testing. "Two separate loops are required as creep testing takes a longer time unlike fatigue testing," said S. Venugopal, Head of Materials Processing Section. Nearly 500 litres of sodium is present in the INSOT facility. All the components that have gone in to INSOT have been designed and fabricated indigenously.
The INSOT facility will play a major role in choosing and fabricating materials and for assessing the life of reactor components. "If fast breeder reactors are designed only for 40 years of life then this facility may not be of any use," Dr. Raj said, "but when life of reactors is to be extended beyond 40 years then they are based on the actual condition of the components in the reactor." And it is here that INSOT will play a very crucial role.
The research work in flowing sodium would complement the already on-going work on evaluating the mechanical properties like creep, low cycle fatigue and creep-fatigue interaction for the upcoming Prototype Fast Breeder Reactor.
IGCAR has also developed carbon, oxygen and sodium ionisation sensors indigenously. These sensors are a part of INSOT and will also be put in fast breeder reactors to collect data for `condition monitoring' of the reactor components.
With this IGCAR scientists have added one more feather to their cap of indigenously developing components and facility for the breeder reactor.
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