One of the main characteristics studied is the permeability of the medium; the quality of Kolar rock was not found to be good
The search for a possible final resting place or a deep geological repository for storing the high level radioactive waste generated during reprocessing of spent fuel has been going on for the past few decades and may well continue for some more time. The defunct Kolar gold mine was studied for its suitability as early as the 1980s. The mine was one of the many sites studied by the Indian nuclear establishment.
That the mine was found to be unsuitable to become a repository is common knowledge. “The quality of rock was not good,” said Dr. Sekhar Basu, Director of the Bhabha Atomic Research Centre (BARC), while explaining to this Correspondent the reason why the study did not progress further. But studying the suitability at such great depths helped the scientists. “It gave us the opportunity to work at such great depths. It gave us the experience of studying underground,” noted Dr. Basu. Several other non-nuclear related studies were carried out by other scientists, including a study on neutrinos.
Some countries like France have chosen clay deposits as the perfect place to house a repository. The choice of clay deposits was based on the fact that the clay contained water that was millions of years old. “This is a proof of the impermeability of clay,” he said.
While trying to identify a possible site, one of the main characteristics that is studied is the permeability of the medium. As the October 28 Op Ed published in The Hindu (‘Our policy is to reprocess all the fuel put into a nuclear reactor’) had pointed out, the nuclear establishment is currently studying the granite belts to locate a possible repository site.
The affidavit filed in the Supreme Court by the Nuclear Power Corporation of India Ltd stresses this point. “Current efforts within the Indian geological repository programme are directed towards granite-based URL [under ground research laboratory].”
Another issue that has been raised time and again is the current capacity of the reprocessing facilities to handle the spent fuel from the Kudankulam nuclear power plants. The fundamental issue that the spent fuel coming from these reactors cannot be reprocessed by the existing reprocessing facilities has gone unnoticed. The Light Water Reactors (LWR) come under safeguards and hence the spent fuel from these power plants cannot be reprocessed by the existing reprocessing facilities.
“A new facility to reprocess the spent fuel from these reactors is needed,” he said. “Reprocessing technology is being developed for Light Water Reactor spent fuel.”
One of the important differentiators of spent fuel generated by LWRs and Pressurised Heavy Water Reactors (PHWR) boils down to the amount of time the fuel is present in the reactor before it becomes spent fuel and has to be reprocessed. Unlike the PHWR that use natural uranium, the reactors at Kudankulam use enriched uranium.
In the case of enriched uranium, the amount of energy that can be extracted from a unit mass of fuel is much more. Scientists call this as burn-up. In the case of PHWR, the burn-up is 7,000MWd per tonne. Compare this with enriched uranium that has a burn-up of 50,000 MWd per tonne. Hence the amount of electrical energy that can be extracted from a unit mass of enriched uranium is nearly seven times more than that from natural uranium.
More the burn-up, the higher the amount of electrical energy extracted from a unit mass of fuel, which in turn means the fuel stays in the reactor for a longer period. As a chain reaction, the number of times refuelling is required reduces, and the frequency with which the spent fuel has to be reprocessed becomes lesser. And the amount of spent fuel to be stored is reduced for a given unit of electrical energy extracted.
According to Dr. Basu, the need for refuelling a part (of the fuel) would arise only about one and half years after the power plant starts generating power. Compare this with PHWRs, where refuelling is needed once or twice a day.
Other processes like vitrification and storage of the vitrified waste in intermediate storage systems for 30 plus years further prolong the time taken before the waste has to be finally kept in repositories.