The third facility in the country to immobilise the high level nuclear waste produced by the plants that reprocesses the mixed carbide fuel used in the FBTR, and the thermal power plant at Kalpakkam (MAPS) and other such plants in the country is coming up at Kalpakkam. The immobilisation will be through vitrification.
“The [waste immobilisation] plant will soon become operational,” said Dr. Srikumar Banerjee, Director of BARC, Mumbai.
There are already two facilities, one at Trombay and the other at Tarapur in Maharashtra, which vitrify the waste.
Immobilisation of the high level nuclear waste into a form that will allow for long term storage and disposal is very essential. The immobilisation should also take place in such a manner that no radioactive material gets dispersed from the final product.
High level waste
One way of immobilising the radioactive waste is through vitrification. Vitrification of high level nuclear waste that contains more than 99 per cent of radioactivity associated with the entire fuel cycle involves melting the nuclear waste along with glass frits (preformed glass beads) in a melter through induction.
The nuclear waste is incorporated in the glass. The glass used will be borosilicate.
“Borosilicate glass is good in retaining the radionuclide and has low leaching property,” said Dr. S.D. Misra, Director, Nuclear Recycle Group, BARC.
Vitrification is particularly attractive as the final vitrified glass product is chemically very stable. So the radioactive material cannot be leached and therefore cannot contaminate the ground water or soil.
Vitrification is hence seen as an ideal solution for long-term storage of radioactive waste.
According to a paper published in the Glass Technology journal in 2003, the chemical resistance of glass allows it to remain in a corrosive environment for many thousands of years without any damage. Several glasses are found in nature, such as obsidians (volcanic glass), fulgarites (formed by lightning strikes), microtektites found in the bottom of the Indian Ocean and the Libyan Desert glass in western Egypt.
“Some of these glasses have been in the natural environment for up to 300 million years with very low alteration of only hundredths of a centimetre per million years,” notes the paper. Glass has another advantage.
It has the natural ability to incorporate a vast range of elements including radiocative waste into its structure.
According to Dr. Banerjee, the vitrified waste is cast into blocks and put into stainless steel canisters whose caps are welded.
Three such canisters are in turn put in an overpack. These overpacks are then kept in underground vaults with good ventilation.
“Good ventilation is required to take away the heat produced by gamma rays [that come out from radioactive material that is vitrified],” Dr. Banerjee said. Explaining the need to continuously remove the heat, Dr. Misra said that at about 500 degree C the glass may devitrify and its leachability would increase and its ability to retain radionuclides would reduce.
But will the ventilation not lead to any problems? “The vault will be maintained at negative pressure so that air can only go into the vault. This is to ensure that no air from the vault can go directly into the environment,” Dr. Misra explained. The air from the vault is finally treated before it is let out into the atmosphere.
WIP has provisions to manage low and intermediate nuclear waste as well, noted Dr. Misra.
“The liquid and gaseous discharges to the environment will be bare minimum conforming to the ‘as low as reasonably achievable’ principle commonly known in the nuclear industry,” said Dr. Misra.
Immobilisation and storage in underground vaults form the first two stages of waste management. The final stage will be to dispose of them in a deep geological repository.