THE HINDU dated 23.09.2004

    THE INDIRA Gandhi Centre for Atomic Research (IGCAR) at Kalpakkam has successfully reached another milestone when it completed the reprocessing of the mixed carbide fuel that had undergone 25,000 MW day/tonne burn up. Reprocessing spent nuclear fuel is a crucial step in closing the fuel cycle.
    Dissolution of the first batch of ten pins that had undergone 25,000 MW day/tonne burn up in nitric acid was completed in March this year. Dissolution of the irradiated mixed carbide fuel is the first step in fuel reprocessing. The pins cut to nearly 30 mm size are put in concentrated nitric acid for 24 hours at boiling temperature.
Fuel dissolution
    A complete dissolution was considered a big success as mixed carbide fuels can pose problems. Dissolution of plutonium rich mixed carbide fuel is difficult. Moreover the fuel poses an additional problem with the formation of complex organic compounds during dissolution. Despite these potential problems, a complete dissolution achieved had made the use of the electrolysis process redundant.
The dissolved fuel along with the undissolved fission products is taken through solvent extraction procedure to separate out uranium and plutonium. Fission products do not get dissolved in the solvent and hence this facilitates the separation of the fuel from the fission products. Recovery of fuel as high as 99.99 per cent is achievable through this process.
    Separation and purification are very important as some amount of uranium can be found in plutonium but not vice versa. Yet, there is an upper limit of one per cent uranium that is allowed in plutonium. However, the purity levels of plutonium should be very high if the final reprocessed fuel is ever going to be used for purposes other than civilian use. So establishing the technology to get pure reprocessed plutonium cannot be ignored. "But this is purely and solely for technology demonstration purpose," emphasized R. Natarajan, Associate Director, Reprocessing Group, IGCAR.
There are compelling reasons to have uranium free of plutonium though. Unlike plutonium, refabrication of uranium is not done in a leak proof environment. So any presence of plutonium in uranium will pose a great health hazard.
    Having completed one set of 25,000MWd/t sample, IGCAR scientists have taken a second batch of samples that have undergone 25,000MWd/t burnup for reprocessing. Plans are to take up a sample of 50,000MWd/t for reprocessing by November and 1 lakh MWd/t burnup sample by January next year. "Challenges will go up many fold when we take up a 50,000MWd/t sample for reprocessing," said Dr. Baldev Raj, Director, IGCAR. While it had taken nearly eight months to complete the first reprocessing cycle, it is planned to shrink this to just one month subsequently.
    It is not without reason that a second batch of 25,000MWd/t samples is being reprocessed. "If at all we have to modify a process, it is easier to do so with a lower burnup sample. Essentially this run is to reconfirm the data," the Director said. "The instrumentation and hardware are robust. Any change will be in the process only."
Reprocessing PFBR fuel
    The fuel to be used in Prototype Fast Breeder Reactor (PFBR) will be a mixed oxide fuel. Though all results from FBTR are on mixed carbide fuel, quite a few steps will be the same for reprocessing the PFBR fuel. "Expect for the first few steps, rest of the processes will be the same," Dr. Raj explained. According to him, dissolution is easier in the case of mixed oxide fuel used in PFBR compared to mixed carbide fuel. One of the reasons is the high concentration of plutonium in mixed carbide fuel that makes dissolution difficult. This is not the case with PFBR fuel.
Cooling period
    But the biggest challenge will come from reprocessing PFBR fuel that has undergone higher burnups. Burnups in the order of 2 lakh MWd/t can be expected in the case of PFBR fuel compared to just 1 lakh in the case of mixed carbide fuel.
    The challenge becomes double fold when reprocessing higher burnup PFBR fuel that has undergone only short cooling periods. While the carbide fuel had undergone few years of cooling period, the targeted cooling period for mixed oxide fuel is just 240 days. Rationale behind having shorter cooling periods is simple. "Shorter cooling period means faster rate at which reused fuel is put to good use," Dr. Raj elucidated.
    The one big advantage with longer cooling period is that radioactivity comes down with time. And lesser radioactivity by itself solves many problems. In nuclear energy everything seems to have valid reasons.
    Take the case of fixing the cooling period as 240 days. The period (240 days) is to coincide with the discharge cycle of the fuel when part of the fuel is discharged. With everything being of a closed loop nature, the need to have everything synchronised becomes paramount.

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